The Department of Physics
Physics and Engineering Seminar Series - Spring 11

(All talks are at 1:00 PM in Room S-3-126 unless otherwise noted.)

Wednesday, Feb. 2nd     Cancelled due to weather! -- Rescheduled for 11th Feb

Solving the Mysteries of Life Using Single Molecule Biophysics
Mara Prentiss
Harvard University

Abstract: In order to reproduce, chromosomes must pair. Thus, like chromosomes must fine each other and align in sufficiently close proximity that they can exchange basepairing. This must occur despite the strong negative charge on the DNA. Previously, it had been assumed that proteins solved this problem, but recent work has shown that in the absence of proteins long double stranded DNA sequences can efficiently find and join with a sequence matched partner even in the midst of far more prevalent non-sequence matched competitors. This effect had been predicted by theory, but some major qualitative results depart from the theoretical predictions.



Wednesday, Feb. 9th

FLIM Techniques
Beniamino Barbieri
Iss Medical Inc.

Abstract: Increasingly sophisticated microscopy systems have allowed the expanding use of Fluorescence Lifetime Imaging Microscopy or FLIM as a tool to determine FRET efficiency in protein and cell studies. Using advanced designed 4-channel confocal imaging systems and specially designed software, it is possible to acquire data in a short time thereby increasing photostability in samples. Both single photon and multiphoton approaches are discussed as well as both time correlated single photon counting (TCSPC) and frequency domain techniques are differentiated.


Friday, Feb. 11th     note special day!   -- rescheduled from Feb 2nd

Solving the Mysteries of Life Using Single Molecule Biophysics
Mara Prentiss
Harvard University

Abstract: In order to reproduce, chromosomes must pair. Thus, like chromosomes must fine each other and align in sufficiently close proximity that they can exchange basepairing. This must occur despite the strong negative charge on the DNA. Previously, it had been assumed that proteins solved this problem, but recent work has shown that in the absence of proteins long double stranded DNA sequences can efficiently find and join with a sequence matched partner even in the midst of far more prevalent non-sequence matched competitors. This effect had been predicted by theory, but some major qualitative results depart from the theoretical predictions.



Wednesday, Feb. 16th

Analysis of cold-atom interferometer with optical beam splitting and recombination
Ebubechukwu Ilo-Okeke
WPI

Abstract: Since the demonstration of interference by Bose-Einstein condensates (BECs), there have emerged different advanced techniques for realizing atom interferometers using BECs. Despite the advances made to coherently manipulate the atomic BECs in atom interferometers, the unwanted phases created by the inter-atomic interactions and spatial phase distortions limit the fringe contrast observed in experiments. This talk focuses on a waveguide atom interferometer that relies on the use of off-resonant laser pulses to split a cloud of BEC into two clouds that travel along different paths and are then recombined using the same laser pulses. After recombination, the BEC in general populates both the cloud at rest and the moving clouds. The relative number of atoms in each of the clouds gives information about the relative phase shift accumulated by the two moving clouds during the interferometric cycle. I will be discussing how the phases affect the population of atoms found after recombination and ways to optimize the operation of the interferometer.



Wednesday, Feb.
23rd:

Rescheduled for Friday (see below)



Friday, Feb. 25th   note special day!

Understanding the Role of Redox Dynamics in Breast Cancer Using Quantitative Biophysical Imaging Methods
Loling Song
Harvard Medical School

Abstract: The mitochondrion, where cellular energy is produced, has a role in both cell growth and cell death. Reactive oxygen species (ROS) are a byproduct of the energy-generating process and the level of ROS has been implicated in cancer and in cancer drug resistance. Using a redox-sensitive green fluorescent protein-based biosensor and a number of quantitative fluorescence methods, I examine the dynamic changes in mitochondrial redox potential in normal mammary epithelial cells and in those transformed with the "insulin growth factor 1 receptor" cancer gene. The two-photon excited fluorescence lifetime imaging method, based on time-correlated single photon counting, can make dynamic and quantitative assessments of oxidized and reduced species in mitochondria in living cells. Fluorescence ratiometric measurements in microscopy provide semi-quantitative support and flow cytometry confirms our findings on large cell populations. I will discuss our latest results on the redox dynamics between normal and oncogene-transformed cells and the potential new insights linking cancer genes and redox regulation.


Wednesday, Mar. 2nd

Locomotion of C. elegans in Structured Environments
Trushant Majmudar
Courant Institute of Mathematical Sciences

Abstract: Undulatory microorganisms like soil-dwelling worms and spermatozoa must navigate complex fluidic environments filled with obstacles, and so must deal with hydrodynamic effects and geometrical constraints. I study this class of problems via experiment and numerical simulation of the soil-dwelling nematode C. elegans as it swims and pushes through arrays of micro-pillars. The worms show a number of locomotory modes, depending on the array spacing, and we find that interactions with the pillars can allow them to achieve much higher speeds when compared to simple swimming. These results may have significant impact on the foraging behavior of the worm in its natural environment. We complement the experimental approach with hydrodynamic simulations of an actuated flexible swimmer moving through obstacles. The simulations not only reproduce much of the observed behavior qualitatively, but also allow us to disentangle the effects of geometric constraints and hydrodynamics quantitatively. We find that what appears as the result of sensing and behavior can sometimes be explained through simple mechanics.


Friday, Mar. 4th   note special day!

Single Molecule Biophysics Using Nanopores and Nanodevices
Slaven Garaj
Harvard University

Abstract: Nanopore-based devices are versatile platform for single molecule detection and analysis, and hold promise for ultrafast, inexpensive DNA sequencing. A DNA molecule, electrophoretically driven through a nano-scale pore, is highly confined and linearized within the nanopore constriction, allowing for separate parts of the DNA molecule to be interrogated in succession. In this talk, I will present several nanopore-based biomolecular sensors we have developed recently. In particular, I will focus on nanopores fabricated in atomically-thin graphene membranes, which were used to detected conformation of individual DNA molecules. Graphene's effective thickness in water was measured to be less then one nanometer, indicating - in collusion with theoretical modeling - that the graphene nanopore is intrinsically capable of discerning between sub-nanometer features along the DNA molecule. Excellent electrical conductivity of the graphene opens up a possibility to use a graphene nanopores concomitantly as electrical sensors and as electrodes for controlling the DNA translocation dynamics.


Wednesday, Mar. 9th

Mechanical Influences in Biological Systems: Helicobacter pylori Pathogenesis
and Cell-Matrix Interactions in 3D Tumor Models

Jonathan Paul Celli
Harvard Medical School

Abstract: The study of human diseases, whether driven by microbial infection, malignancy, or otherwise, traditionally focuses on molecular and genetic characterization of the underlying biology. This paradigm, which has undeniably yielded enormous therapeutic successes, nevertheless overlooks provocative physical interactions that may be equally critical drivers of disease pathogenesis and therapeutic outcomes. Here I will discuss two biological systems where interactions between cells (bacterial and mammalian) and their mechanical microenvironment have important implications. First, I will focus on the ulcer-causing bacterium, Helicobacter pylori, which must swim through a protective viscoelastic mucus gel in the human stomach to infect its host. I will make the case that contrary to conventional thought, H. pylori does not generate sufficient motor torque to bore its way through the mucus. I will present rheology, microrheology and imaging studies collectively demonstrating that motility is achieved through a surprising interaction between the pH-dependent sol-gel transition of gastric mucin and H. pylori's biochemical mechanism for pH-regulation allowing it to survive in the stomach. Taken together this presents a picture of a dynamic system in which the bacterium directly influences the rheology of the host mucus, which in turn determines the motility of the bacterium. In the second portion of the talk I will extend this theme to an exploration of human epithelial cancers, for which tumor growth behavior and treatment response are critically influenced by cell-matrix interactions. Cancer cells grown on a compliant extracellular matrix spontaneously undergo coordinated co-migration and assembly events to form heterogeneous multicellular 3D nodules resembling micrometastatic disease in vivo. This behavior, which is suppressed when the same cells are grown on a rigid plastic substrate, leads to a reproducible bimodal lognormal size distribution with important and contrasting implications for response to treatment both with carboplatin, a traditional chemotherapeutic agent, and photodynamic therapy (PDT), in which photophysical mechanisms lead to generation of cytotoxic radicals. Going forward I will present a vision bringing together tools and concepts from microrheology with the quantitative analysis of tumor growth and treatment response introduced here, in conjunction with 3D tumor models built on synthetic matrices with tunable rheology. This dialogue between biophysics and biological modeling comprises a new platform to elucidate key interactions between tumor cells and microenvironment mechanics, to provide a more comprehensive understanding of tumor growth and to inform more effective therapeutic strategies.


Thursday, Mar. 10th   note special day!

Chromosome Distribution and Membrane Curvature Localize Cell Division Machinery in Escherichia coli
Jaan Mannik
Kavli Institute of Nanoscience, Delft University of Technology

Abstract: Micro- and nanofluidic devices combined with high-resolution quantitative imaging offer a variety of new possibilities to study single cells down to a nanometer-scale resolution. Here, I describe a study where we rely much on these new technological advances in elucidating the molecular mechanisms of bacterial cell division. We investigate how Escherichia coli bacteria are able reliably and accurately position their cell division proteins, i.e. the divisome, in normal and in irregularly shaped phenotypes. We use squeezed E. coli in shallow nanofabricated channels (J. Mannik et al., Proc. Natl. Acad. Sci. U.S.A. 106 (2009) 14861) as an irregularly shaped phenotype and compare these cells to their normal rod-shaped counterparts. We study the roles of two molecular systems in this process - one consisting of MinCDE proteins and the other of the bacterial chromosome. We find that while Min proteins are effective in excluding cell division at the poles of rod-shaped bacteria, this inhibitory system becomes chaotic in more complicated cell shapes. Instead, we observe that localization of the divisome is highly anticorrelated with the complex pattern of chromosome distribution in squeezed E. coli cells. We also show that in addition to chromosomal distribution the curvature of a cell's plasma membrane is instrumental in the fine scale positioning of the divisome. We find that linear divisome complexes form on the circumference of the squeezed cells so that their line curvature is maximized.


Monday, Mar. 14th   note special day!   Rescheduled from Wednesday

Protein-based Biomacromolecules -
The Physics Bridging Biological and Polymer Science

Xiao Hu
Tufts University

Abstract: Protein-based biomacromolecules, a recent booming research field, have increasing applications in biomaterials, controlled drug release and delivery, biosensors, biophotonics, nano-biotechnology, and tissue regeneration medicine. Unlike traditional small biomolecules, long chain proteins can form variable soft materials such as thin films, hydrogels, microspheres, nanoparticles, nanofibers, or composite materials, similar to the plastic polymers. And they can perfectly interact with human tissues without potential toxicity and immunoreactions. This talk will first discuss how to understand theses proteins from the perspective of biophysics and polymer physics, such as the methods to reveal their self-assembly mechanisms and dynamic structural transitions. A natural biomacromolecule, beta-sheet crystal rich silk fibroin proteins from silkworms, will be used as an example. The second part of this talk will focus on how to apply the above biophysical theories and predictions to produce biomaterials with controlled structure and morphology. A temperature controlled water vapor annealing technique, as an example, will be used to demonstrate how to physically control the crystal structure of biomacromolecules, which results in control of mechanical, thermal, and biological properties in protein-based biomaterials including their stem cell responses. This topic, as a physics "bridge" covering theories and methods between the modern synthetic polymer science and the developing biological science, would benefit multiple scientific areas and extend the studying regime of modern soft condensed matter physics.


Wednesday, Mar. 16th:

Colloqium is rescheduled for two days earlier



Wednesday, Mar. 23rd:

Week of the APS March Meeting (No seminar)



Wednesday, Mar. 30th
At special time -- 2 pm

Quantum chaos, random matrix theory, and the Riemann zeta-function
Paul Bourgade
Harvard University

Abstract: Evidence for deep connections between number theory and random matrix theory has been noticed since 1972, when H. Montgomery and F. Dyson realized that the repulsion of the zeros of the Riemann zeta function is of fermionic nature, like eigenvalues of large random matrices. In this talk, we will review these historical analogies, involving for example quantum chaology, as well as some recent developments concerning the extreme gaps in spectra of some random operators.


Wednesday, Apr. 6th

Femtosecond Infrared Spectroscopy on Biomolecules:
Why do Proteins Need to Respond on Fast Timescales?

Shyamsunder Erramill
Boston University

Abstract: Femtosecond spectroscopy has established that the photoactive proteins respond on sub-picosecond time-scales following the absorption of the photon. We have observed that the peptide backbone of proteorhodopsin responds with a time constant of ~ 500 fs (Amsden et al, 2007). Proteorhodopsin functions as a proton pump with a reaction cycle time of about 20 ms. Is there a fundamental reason for the protein to respond so fast, when the energy stored is not needed by the cell until ~ 20 ms later? The first part of the answer has to do with the physics of the small chromophore in the large protein: the response rate is set by the electron-phonon interaction timescale, which depends only on fundamental physical constants.
  Ultrafast response is observed in semiconductors, and inorganic materials as well. But why does the protein encasing the chromophore respond fast? Experiments on vibrational energy relaxation in water done by Chieffo et al suggest that energy begins to be lost irreversibly on timescales of ~ 140 fs, due to the creation of entropy. Taken together the femtosecond experiments suggest an intriguing answer: the protein needs to respond so fast because, if it didn't, the energy would be lost as heat to the aqueous medium. Thus the dynamic properties of water, and the Second Law of Thermodynamics, ultimately determine the need for ultrafast protein response.
  This leads to the following speculative suggestion: Only those photoactive proteins that exhibit ultrafast dynamics are selected by evolution, since only such proteins can overcome the competition with water. If the suggestion is right, it provides a new way of thinking about how ultrafast dynamics of biomolecules are a factor in biomolecular evolution. Support for the research from the NSF, DOD and NIH is gratefully acknowledged.


Friday, Apr. 8th   note special day!

Tiny Technologies and Medicine
Sangeeta Bhatia
MIT, HHMI, and BWH

Abstract: Our laboratory studies how micro- and nanoscale systems can be deployed to understand, diagnose, and treat human disease. In this talk, I will describe our progress in two application areas: liver disease and cancer. In the area of hepatic tissue engineering, we are developing microtechnology tools to understand how ensembles of cells coordinate to produce tissues with emergent properties in the body. We have used this understanding to fabricate human microliver tissues in both '2D' and '3D' formats that enable us to study the pathogenesis of human drug-drug interaction, drug-induced liver disease, and viral infection. In the area of cancer, we are developing nanotechnology tools to meet the challenge of delivering cargo into the tumor microenvironment where transport is dominated by diffusion. Our strategy is to design nanotechnologies which emulate nature's mechanisms of homing, activation, and amplification to deliver cytotoxic drugs, imaging agents, and siRNA to tumors. Thus, using nature as a guide, we are establishing a framework for building systems from micro- and nanoscale components that function collectively to treat human disease.


Wednesday, Apr. 13th
Special Room: Healey Library, Lower Level (LL) Room 3507 (Media Auditorium)

Optical platforms for diagnostics, protein microarrays and single virus detection
Bennett Goldberg
Boston University

Abstract: Over the past ten years, many techniques have developed through engineering and physics that have fundamentally changed the landscape of multiplexed sensing and diagnosis, genomic analysis, and point-of-care technologies for resource limited settings. A good example is the development of zero-mode waveguides with nano-wells that have since become Pacific Biosciences with the fastest and highest throughput sequencers available. In this talk I will describe our efforts using simple common path interferometry combined with polymer chemistry and forward models to build and develop a robust platform for optical detection and imaging. Called Interferometric Reflection Imaging System, or IRIS, the platform has been used to make high-throughput protein microarrays that are sensitive, real-time, and quantified. IRIS has been also developed to image single nanoparticles and viruses, accurately determining both size and shape. I will discuss applications for diagnosis of renal disease and as an allergy chip, and demonstrate detecting H1N1 and Dengue fever virus in a compact system appropriate for resource limited settings.


Wednesday, Apr. 20th
Special Room: Healey Library, Lower Level (LL) Room 3507 (Media Auditorium)

The Structure and Dynamics of Economic Complexity
Cesar A. Hidalgo
MIT

Abstract: With billions or products, billions of people, and trillions of interactions, the world economy is one of the most outstanding complex systems to have ever emerged. Can we use complexity science to improve our understanding of a system of such paramount complexity? In this lecture I summarize recent research that uses networks to describe, characterize and understand differences in the productive structure of nations. First, I show how the complexity of an economy can be quantified by looking at the structure of the network connecting countries to the products they export and that countries tend to approach a level of income which is dictated by the complexity of their economies. Then, I show how development is constrained by a projection of this network into the space of products, or Product Space, by demonstrating empirically that the evolution of countries comparative advantage is constrained by the structure of this network. I conclude by presenting a simple model that can account for some of the stylized facts that arise from this network description of the world economy and show how coordination problems constraint the development of poor countries and cause increasing returns to economic diversity.


Friday, Apr. 22nd   note special day!

Energy, Information, and Maxwell's Demon
Kurt Jacobs
UMass Boston

Abstract: The laws of thermodynamics limit the degree to which energy in the form of heat can be converted into a soure of energy for powering vehicles (work). But these laws assume that the amount of information available to the observer is also limited. A demon who can learn microscopic details can extract more work, and this appears to contradict the second law of thermodynamics. I will begin by explaining how entropy is directly related to information, and discuss how the acquisition of additional information will allow one not only to extract more useful work, but also how this fits within the second law of thermodynamics. The moral of the story is that there is no free lunch, not even for a demon chef.


Wedensday, Apr. 27th   Note: Prof. Renugopalakrishnan's talk is rescheduled for next semester

Supersymmetry, Reflectionless Scattering, and the Sine-Gordon Equation
Andrew Koller
UMass Boston

Abstract: Scattering without reflection at all energies is a very unique phenomenon. Imagine, for instance, a piece of glass that perfectly transmits waves of every wavelength -- it is not obvious that such a construction is even possible. Yet, reflectionless scattering was discovered over fifty years ago in the form of such classical reflectionless dielectrics, and equivalent reflectionless quantum-mechanical potentials. It was later understood that these reflectionless potentials have a deep link to free space via the algebra of quantum mechanical supersymmetry (QMSUSY). This supersymmetric connection to free space explains their reflectionless nature. Furthermore, it was discovered that these potentials lead to soliton solutions of the Korteweg-de Vries (KdV) equation when they are used as initial states of the KdV field. In this talk, I will discuss a family of reflectionless Hamiltonians, namely, Akulin's Hamiltonians, that parallels the connection between supersymmetry, reflectionless scattering, and integrable nonlinear partial differential equations. Akulin's Hamiltonians are 2x2 matrix differential operators whose reflectionless nature, like the reflectionless quantum-mechanical potentials, is explained by a supersymmetric connection to free space. Furthermore, Akulin's Hamiltonians can be used to generate soliton solutions of the sine-Gordon equation. Our findings also explain why a set of specific laser pulse shapes do not invert the population of a two-level atom, regardless of the value of the laser detuning.


Friday, Apr. 29th   note special day!

Adaptive Measurements for Profit and Pleasure
Howard M. Wiseman
Griffith University

Abstract: The theory of adaptive measurements at the quantum limit goes back almost 4 decades [1], but the last few years have seen a burst of experimental activity in the area (e.g. [3] [4] [5] [8]). In this talk I will briefly review the field. I will first define what adaptive measurements are, and then cover how they let one

        1. do some things better [6,2] [3] [7,5] [6]
        2. do some things much better [7] [8]
        3. do some things perfectly [1,9] [10,6] [7] [11]
        4. do some things uniquely [4,12] [13].

Then for the second part of the talk I will concentrate on recent work in the last category: How many bits does it take to track an open quantum system? [13].

[1] S. J. Dolinar, "An optimum receiver for the binary coherent state quantum channel," MIT Res. Lab. Electron. Quart. Progr. Rep. 111, pp. 115-120, 1973.
[2] M. A. Armen, J. K. Au, J. K. Stockton, A. C. Doherty, and H. Mabuchi, "Adaptive homodyne measurement of optical phase," Phys. Rev. Lett. 89, 133602, 2002.
[3] B. L. Higgins, D. W. Berry, S. D. Bartlett, H. M. Wiseman, and G. J. Pryde, "Entanglement-free Heisenberg-limited phase estimation," Nature 450, pp. 393-396, 2007.
[4] Robert Prevedel et al. "High-speed linear optics quantum computing using active feed-forward", Nature 445, 65-69, 2007.
[5] T. A. Wheatley et al. "Adaptive Optical Phase Estimation Using Time-Symmetric Quantum Smoothing" Phys. Rev. Lett. 104, 093601, 2010.
[6] B. L. Higgins, B. M. Booth, A. C. Doherty, S. D. Bartlett, H. M. Wiseman, and G. J. Pryde, "Globally optimal quantum control for mixed-state discrimination," Phys. Rev. Lett 103, 220503, 2009.
[7] H. M. Wiseman, "Adaptive phase measurements of optical modes: Going beyond the marginal Q distribution," Phys. Rev. Lett. 75 pp. 4587-4590, 1995.
[8] D. Berry and H. M. Wiseman, "Adaptive phase measurements for narrowband squeezed beams", Phys. Rev. A 73, 063824, 2006.
[9] R. L. Cook, P. J. Martin, and J. M. Geremia, "Optical coherent state discrimination using a closed-loop quantum measurement," Nature 446, pp. 774-777, 2007.
[10] A. Acõn, E. Bagan, M. Baig, Ll. Masanes, and R. Munoz-Tapia, "Multiple copy 2-state discrimination with individual measurements," Phys. Rev. A 71, 032338, 2005.
[11] R. B. Griffiths and C.-S. Niu, "Semiclassical Fourier transform for quantum computation," Phys. Rev. Lett. 76, pp. 3228-3231, 1996.
[12] R. Raussendorf and H. Briegel, "A One-Way Quantum Computer", Phys. Rev. Lett. 86, 5188-5191, 2001.
[13] R. Karasik and H. M. Wiseman "How many bits does it take to track an open quantum system?", Phys. Rev. Lett. 106, 020406, 2011.


Wednesday, May. 4th     No seminar today



Wedensday, May 11th

Quantum Phases in Ultracold Atomic and Molecular Lattice Systems
Barbara Capogrosso-Sansone
ITAMP,
Harvard-Smithsonian Center for Astrophysics, and Harvard Physics Department

Abstract: Ultracold atoms in optical lattices have proved to be versatile and highly controllable systems with many interesting applications and with the possibility of simulating condensed matter models. They allow studying of ultracold collisions and few-body physics, and realize many-body quantum phases. Ultracold polar molecules have instead only recently become experimentally available and they promise to open exciting new research directions due to their more complex structure and to the long-range, anisotropic dipole-dipole interaction. In this talk I will address quantum phases realized by ultracold dipolar systems. I will show that various solid and supersolid phases can be stabilized by dipolar interactions. When mixtures or multi-layered systems are considered, pairing can be realized. The results presented, based on exact quantum Monte-Carlo simulations, provide guidance for experimental realization of such phases.


Physics and Engineering Seminar Series - Fall 10

(All talks are at 1:00 PM in Room S-3-126 unless otherwise noted.)

Wednesday, Sept. 15th:

Integrability and Vortex Dynamics: The best of both worlds
Shabnam Beheshti
Rutgers

Abstract: In the past three decades the concept of integrability has transformed the study of mathematical physics, from modeling shallow water waves (KdV) to determining the structure of 2D gravity (sine-Gordon). More recently, certain solutions to the sinh-Poisson equation appear as equilibrium states of vortices in fluids and plasmas. We give a basic introduction to this beautiful integrable equation in the study of vortex dynamics, reaching into inverse scattering theory, statistical mechanics, complex analysis, and number theory.



Wednesday, Sept. 22nd:

Example of a Quantum Anomaly in the Physics of Ultracold Gases
Maxim Olshanii
UMass Boston

Abstract: In this presentation, we show that for the two-dimensional harmonically trapped Bose gas, the Pitaevskii-Rosch dynamical symmetry becomes weakly broken under quantization. The very same symmetry remains intact for the unitary three-dimensional gases. We interpret this effect as a quantum-mechanical symmetry breaking---otherwise known as quantum anomaly---, give a detailed quantitative analysis of its empirical manifestations, and suggest a detailed experimental scheme for its detection.
In collaboration with Helene Perrin and Vincent Lorent (U Paris-Nord, France)



Wednesday, Sept. 29th:

Thermalization of Interacting Fermions and Delocalization in Fock Space
Clemens Neuenhahn
Ludwig-Maximilians-UniversitŠt MŸnchen and
Friedrich-Alexander-UniversitŠt Erlangen-NŸrnberg Institute for Theoretical Physics II

Abstract: We investigate the onset of 'eigenstate thermalization' and the crossover to ergodicity in a system of 1D fermions with increasing interaction. We show that the fluctuations in the expectation values of the momentum distribution from eigenstate to eigenstate decrease with increasing coupling strength and system size. It turns out that these fluctuations are proportional to the inverse participation ratio of eigenstates represented in the Fock basis. We demonstrate that eigenstate thermalization should set in even for vanishingly small perturbations in the thermodynamic limit.



Wednesday, Oct. 6th:

Non-equilibrium steady state of sparse systems and billiards
with vibrating walls

Doron Cohen
Ben-Gurion University of the Negev

Abstract: A resistor-network picture of transitions is appropriate| for the study of energy absorption by weakly chaotic or weakly interacting driven systems. Such "sparse" systems reach a novel non-equilibrium steady state (NESS) once coupled to a bath [1]. In the stochastic case there is an analogy to the physics of percolating glassy systems, and an extension of the fluctuation-dissipation phenomenology is proposed. In the mesoscopic case the quantum NESS might differ enormously from the stochastic NESS, with saturation temperature determined by the sparsity. The theory might apply to the analysis of energy absorption by Billiards with vibrating boundaries [2,3]. If the billiard were strongly chaotic, and the collisions with the walls were regarded as uncorrelated, the rate of heating would be determined by the "Wall formula", which is analogous to the "Drude formula" in the theory of electrical conductance.

[1] D. Hurowitz and D. Cohen, arXiv:1007.0766 (2010).
[2] A. Stotland, D. Cohen and N. Davidson, Europhysics Letters 86, 10004 (2009).
[3] A. Stotland, L.M. Pecora and D. Cohen, arXiv:1005.4207 (2010).


Wednesday, Oct. 13th

Computational Microscopy
Charles A. DiMarzio
Northeastern

Abstract: Advances in lasers, computers, and chemistry have revitalized the field of microscopy. Techniques such as quantitative phase imaging, optical coherence tomography, confocal microscopy, and various types of nonlinear microscopy have enabled imaging inside highly scattering media that characterize most biological specimens. The W.M.Keck Three Dimensional Fusion Microscope, a multi-modal microscope with seven modes on a single stage, provides images based on different contrast mechanisms offering unique insights into the interior of seemingly opaque specimens. However, extracting all the information from multiple modes requires understanding the propagation of light in complicated heterogeneous media. In computational microscopy, models of light propagation are used to generate synthetic images in order to better understand the fundamental limits on performance of the various techniques. Through the development of progressively more complex models and comparison of synthetic images to experimental ones it is possible to develop hardware and algorithms to approach these fundamental limits. This talk will present some results of computational microscopy along with experiments on embryos, skin, and lung.



Wednesday, Oct. 20th:

Quantum Transport in Nanoscale Devices using Wigner Distribution Function
Ramarao Inguva
East West Enterprises Inc.

Abstract: We summarize recent developments in the use of Wigner Distribution function method for quantum transport in nano scale devices. The method will be illustrated with the example case of transport across a single tunneling barrier.



Wednesday, Oct.
27th:

Quantifying Quantum Correlations in Light-Harvesting Complexes
Sai Vinjanampathy
UMass Boston

Abstract: Biological systems have been of recent interest for the role that quantum correlations may play for functionality or in evolution. One such biological phenomenon under study is the photosynthesis of certain organisms, for instance low light adapted green sulfur bacteria. The Fenna-Matthews-Olson (FMO) protein complex is a biological light harvesting complex that is found in such systems. It has drawn considerable attention as a template to understand the role of quantum correlations. Many measures exist that can be employed to characterize quantum correlations. One such measure, quantum discord, captures all non-classical correlations that are present in the system. I will introduce quantum discord and present results quantifying quantum discord in FMO complexes.


Friday, Oct.
29th:
Note:  ---- Unusual Day! ----

Coherent Atomtronics Transport with Localized States
Kunal Das
Kutztown University

Abstract: Atomtronics is the new science of creating circuits, devices and new quantum materials with trapped ultracold atoms instead of electrons. The progress of this field critically hinges upon ability to conduct controlled transport of atoms in the presence of different potential structures, including optical lattices and superlattices. We consider certain mechanisms involving time-varying potentials that lead to controlled transport in varied trapping potentials and lattice structures. Specifically, we demonstrate transparent and selective transport of a single species in a superlattice containing two interacting atom species.

In the study of electron transport processes the primary focus is on the current, where the exact location of individual electrons is not always relevant; therefore this has a natural description in terms of extended states, where electrons are assumed to be in momentum eigenstates. Atoms, on the other hand, are naturally created and manipulated in localized states, and many potential applications are dependent upon knowing the location of the atoms. We use this to advantage to propose a novel approach to conducting transport related experiments with ultracold atoms, where localized atomic wavepackets are manipulated, leading to results that can be made indistinguishable from electron transport experiments.


Wednesday, Nov. 3rd:

How to Succeed at Non-Academic Physics, or "From Clown to Magician"
Paul Burstein
Skiametrics Inc.

Abstract: How do you make a transition from university physics to solving important real problems that range from vexing to critical? Why are physicists [almost] uniquely qualified to make such a transition? What kind of institutions bridge these worlds? How do you go about creating them, if you can't find one? How do you make a living while really enjoying yourself? One or two examples of some projects that worked out [for me] will be presented.


Wednesday, Nov. 10th:

Type Ia Supernovae
Robert Fisher
UMass Dartmouth

Abstract: In this talk, I will discuss the biggest nuclear-powered explosions in the universe -- an amazing stellar explosion known to astronomers as "Type Ia supernovae." Type Ia supernovae are so incredibly bright that they can outshine the combined stellar light of an entire galaxy, and be visible across enormous distances in the cosmos. Yet, each Type Ia supernova event has very nearly the same intrinsic brightness regardless of where or when in the universe it exploded. Consequently, they provide us with standard candles which have enabled precision cosmological measurements of Hubble's constant and of the acceleration parameter of the universe, and led directly to the existence of a mysterious new type of energy, which has been termed "dark energy." I will discuss recent work which has begun to unravel the mystery of these remarkable explosions, and for the first time, provided a self-consistent theoretical explanation of the mechanism underlying their detonation. My talk will include stunning, award-winning visualizations of supercomputer simulations of Type Ia supernova explosions, which are based upon rigorous physics, and have helped to elucidate the complex nature of the explosion.



Wednesday, Nov. 17th
Counter-Synchronous Sloshing in Free Containers
Andrzej Herczynski
Boston College

Abstract: The sloshing wave motion in a container partially filled with liquid and free to move horizontally will inevitably cause the container to accelerate. A variety of complicated rectilinear motions can arise depending on the initial conditions. In the simplest case, the container oscillates in a periodic fashion about a fixed point, out-of-phase with the liquid oscillating within. This counter-synchronous sloshing, at the fundamental frequency as well as in higher modes, is investigated using linearized potential flow theory for containers of symmetric shapes which are amenable to analytical solutions: rectangular boxes, circular cylinders, 90-degree wedges and cones. Experimental results, obtained using containers filled with water and supported on a low-friction cart, are compared to the theoretical predictions.



Wednesday, Nov. 24th:

Nonlinear Optical Studies of Nanomaterials and Applications for Optical Limiting and Medicine
Ivan Kislyakov
St.Petersburg University of Information Technology

Abstract: Nonlinear behavior of materials based on carbon nanoparticles, fullerenes, nanotubes and astralens as well as dielectric-metal and semiconductor-metal plasmonic nanostructures were studied and exploited for applications in optical power limiting. New type of singlet oxygen photosensitizers on the base of solid-phase fullerene-containing structures was studied and its efficiency in application of virus inactivation in protein solutions was demonstrated. Results on luminescence and singlet oxygen photosensibilization properties of endogenous porphyrins for photoluminescent diagnostics and photodynamic therapy of cancer tumors will be presented.



Wednesday, Dec. 1st:

2=1: Teaching the gentle art of lying
Sanjoy Mahajan
Olin College

Abstract: Even talented students struggle with fundamental concepts in mathematics and physics. They cannot reason with graphs and have no feel for physical magnitudes. Their instincts are Aristotelian; in their gut they believe that force is proportional to velocity. With such handicaps in mathematical and physical reasoning, they can learn only by rote. I'll discuss these difficulties and how the art of approximation can improve our teaching. Students then cannot conceal misconceptions behind mathematical agility, and can instead enjoy estimating the size of raindrops or the distance that birds and 747's can fly without refueling.


Wednesday, Dec. 8th:

Quantitative Phase-Contrast Microscopy Using In-line Digital Holography
Bhargab Das
UMass Boston

Abstract: Optical imaging has been the most commonly used method of investigation in medicine, biology, material science etc., and various related technologies have been developed over the past few decades. Numerous biological samples, including live cells, are generally transparent under visible-light illumination and behave essentially as phase objects. Their phase structure can be made visible by use of spatial phase filtering, as is done in Zernike's phase contrast (PC) method, or by interferometric technique such as Nomarski's differential interference contrast (DIC) method. Both of these techniques are used in commercial microscopes to render the phase structure visible by transforming the phase information into intensity distribution. However the phase information provided by these techniques is only qualitative. Furthermore, these techniques entangle the phase and amplitude information and so have limitations when both phase and amplitude information is required. Digital holographic microscopy (DHM) is becoming increasingly popular due to its ability to provide simultaneous amplitude and quantitative phase information of biological specimens. It is a non-destructive, full-field, and label-free imaging technique. We present a novel in-line DHM technique which enables simultaneous acquisition of two interferograms in order to increase the acquisition rate. The technique utilizes full spatial bandwidth of the camera and requires only two interferograms recorded at two different planes, which can be recorded simultaneously using two sensor arrays. Thus the hologram acquisition time can be significantly shortened. This increased acquisition rate together with the improved reconstruction capability of the current technique may find applications in biomedical research enabling visualization of rapid dynamic processes at the cellular level.



Physics and Engineering Seminar Series - Spring 10

(All talks are at 1:00 PM in Room S-3-126 unless otherwise noted.)

Wednesday, Feb. 3rd:

Quantum State Transfer with Superconducting Circuits
Frederick Strauch
Williams College

Abstract: Superconducting circuits are artificial atoms that can be wired up into complex structures. I will present a theoretical proposal to use such circuits to implement perferct quantum state transfer between nodes of a hypercube network of phase qubits (quantum bits), including the effects of decoherence, disorder, and higher-order couplings. I will also describe recent work on parallel state transfer using networks of coupled resonators (harmonic oscillators). These examples of novel quantum transport in artificial solids have many applications for quantum computing.



Wednesday, Feb. 10th:
Time change: 2 - 3 pm ---- CANCELLED DUE TO WEATHER! ----

Statistical Mechanics of Complex Networks
Ginestra Bianconi
Northeastern

Abstract: Complex networks describe a large variety of interacting complex systems. In the last ten year there has been an impressive progress in understanding the universal properties of networks, and to describe their evolution and robustness. In this talk I present relevant examples of fundamental concepts and phenomena in statistical mechanics such as the Bose-Einstein condensation or the definition of entropy, that play a crucial in order to extract information from complex networks and predict their behavior.



Wednesday, Feb. 17th:

Quantum Criticality and the Cuprate Superconductors
Subir Sachdev
Harvard

Abstract: I will begin with a simple introduction to the theory of quantum criticality, as applied to experiments on certain insulating antiferromagnets. I will then survey the phenomenology of the cuprate high temperature superconductors, and show how ideas from quantum criticality have helped explain or predict the results of a number of recent experiments. The applications to the cuprates focus attention on key problems associated with the criticality of Fermi surfaces in two dimensions which remain unresolved. I will conclude with a brief discussion of how these open problems are being addressed by the AdS/CFT correspondence discovered in string theory.



Wednesday, Feb. 24th:

Granular Gases under Driving with Rare but Powerful Energy
Wenfeng Kang
UMass Amherst

Abstract: Granular gases in two-dimensions are studied by using event-driven molecular dynamics simulations. Stationary state is attained by rare injection of large amounts of energy to balance the dissipation due to collisions. We find that under extreme driving, with the injection rate much smaller than the collision rate, the velocity distribution has a power-law high-energy tail. The numerically measured exponent characterizing this tail is in excellent agreement with predictions of kinetic theory over a wide range of system parameters. We conclude that driving by rare but powerful energy injection leads to a spatially homogeneous gas and constitutes an alternative mechanism for agitating granular matter. In this distinct nonequilibrium steady- state, energy cascades from large to small scales. Our simulations also show that when the injection rate is comparable with the collision rate, the velocity distribution has a stretched exponential tail.



Wednesday, Mar. 3rd

Maxwell's Demon
Kurt Jacobs
Umass Boston

Abstract: I will describe the problem of Maxwell's demon, and how it can be understood within the 2nd law of thermodynamics. I will also explain how Maxwell's demon can be analysed for quantum systems, and show how much work can be extracted from a system by the demon.



Wednesday, Mar. 10th:

The Application of Monte Carlo Methods in Radiotherapy
Joao Seco
Harvard Medical School

Abstract: Monte Carlo techniques, are used heavily in radiotherapy to solve the problem of particle transport and energy deposition within the patient. In the case of radiotherapy, electrons pose the largest problem for physicists---the problem to understand their energy deposition and their motional behavior. Since electrons are produced in nearly all forms of particle interactions, understanding their behavior becomes critical in photon, electron and proton therapy. The present talk gives an overview of the Monte Carlo techniques and how they are used in radiation therapy.



Wednesday, Mar.
17th:

Spring Break (No seminar)



Wednesday, Mar. 24th:

Where does the optical enhancement come from with metal nanoparticles?
Greg Sun
UMass Boston

Abstract: There has been a great deal of interest in exploring the modification of optical properties of nano-scaled, optically-active objects that are placed in the close proximity of metal nanoparticles. Understanding of these phenomena have been mostly based on numerical methods that are time consuming and often times unsatisfactory in revealing the physical mechanisms at work and providing optimization route for improvement. In this talk, I will present a simple yet rigorous analytical model that adequately describes what have been observed in various experiments with clear underlining physics. Specifically, I examine and the enhancement of optical absorption, electro- and photo-luminescence, and explain the luminescence quenching effect. Using the examples of silver or gold nanospheres embedded in gallium nitride, we establish limits of enhancement and present an algorithm for optimization. I further analyze the enhancement by the coupling of nanoparticles that are the closely spaced.



Wednesday, Mar. 31st:

Computational Aspects of the Unitary Question
Alfred Noel
UMass Boston

Abstract: The Representation Theory Group consisting of Steven Jackson, Alfred No‘l, Gerard Koffi and Lily Silverstein works at the intersection of Representation Theory of Groups and Invariant Theory. The group contributes to the current international research on the computation of the Unitary Dual via collaboration with the "Atlas of Lie Groups and Representations project" and through other projects. In this talk, I will explain some of the ideas that lie behind the determination of the character table of the split real form of the exceptional group E_8 by the Atlas project. If time permits, I will make some connections with Physics. A certain mathematical maturity is all that is required.



Wednesday, Apr. 7th:

Imaging Universal Conductance Fluctuations in Mesoscopic Graphene
Mario Borunda
Harvard

Abstract: Graphene provides a fascinating testbed for new physics and exciting opportunities for future applications based on quantum phenomena. At sufficiently low temperatures and small size scales, the diffusive transport of electrons through graphene becomes coherent, leading to universal conductance fluctuations (UCF). I will broadly describe joint theoretical and experimental efforts aimed at understanding coherent transport in graphene devices. We employ a nanoscale probe that can access the relevant length scales: the tip of a liquid-He-cooled scanning probe microscope (SPM) capacitively couples to the graphene device below creating a movable scatterer for electron waves. By scanning the tip over a device, we map these conductance fluctuations vs. scatterer position. We find that the conductance is highly sensitive to the tip position, producing fluctuations of the conductance of e^2/h when the tip is displaced by a distance comparable to half the Fermi wavelength. These measurements are in good agreement with detailed quantum simulations of the imaging experiment, and demonstrate the value of a cooled SPM for probing coherent transport in graphene.



Friday, Apr. 9th   (note unusual day!)

Statistical Mechanics of Complex Networks
Ginestra Bianconi
Northeastern University

Abstract: Complex networks describe a large variety of interacting complex systems. In the last ten year there has been an impressive progress in understanding the universal properties of networks, and to describe their evolution and robustness. In this talk I present relevant examples of fundamental concepts and phenomena in statistical mechanics such as the Bose-Einstein condensation or the definition of entropy, that play a crucial in order to extract information from complex networks and predict their behavior.



Wednesday, Apr. 14th:

Quantum Nucleation with Emphasis on a Superheated Liquid
Leon Gunther
Tufts

Abstract: TBA



Wednesday, Apr. 21st:

Theory of Resistor and Impedance Networks: A New Formulation
Fred Wu
Northeastern

Abstract: The resistance between arbitrary two nodes in a resistor network is obtained and formulated in terms of the eigenvalues and eigenfunctions of the Laplacian matrix associated with the network. Explicit formulas for two-point resistances are deduced for regular lattices in one, two, and three dimensions under various boundary conditions including that of a Moebius strip and a Klein bottle. The emphasis is on lattices of finite sizes. The extension to impedance networks is discussed. We also deduce new summation and product identities.



Wednesday, Apr. 28th:

Exploring New Frontiers of Quantum Optical Science
Mikhail Lukin
Harvard

Abstract: In this talk we will discuss recent developments involving a new scientific interface between quantum optics, many body physics, nanoscience and quantum information science. Specific examples include quantum manipulation of individual spins and photons using atom-like impurities in diamond and control of light-matter interactions using sub-wavelength localization of optical fields. Novel applications of these techniques ranging from novel approaches to quantum computation at room temperature to implementation of quantum optical networks and nanoscale magnetic sensing will be discussed.



Wednesday, May 5th:

From Classical Mechanics to General Relativity: Vignettes of Integrability
Shabnam Beheshti
Rutgers

Abstract: In the past three decades, the theory of solitons has had a rich and varied impact on mathematical physics, from appearing as equilibrium states of vortices in fluids or plasmas to determining Killing vectors and consequently the structure of two-dimensional gravity. We introduce the sine-Gordon equations $\psi_{xx} +/- \psi_{yy} = sin \psi$ (as well as the associated sinh-Poisson equations $\psi_{xx} +/- \psi_{yy} = sinh \psi$, telling the story of this family of solvable PDES through several concrete examples from classical mechanics, fluid dynamics and gravitation. Time permitting, open questions in each topic will also be explored.



Wednesday, May 12th:

Phase-Space Representation of Quantum Dynamics
Anatoly Polkovnikov
Boston University

Abstract: In this talk I will describe how quantum dynamics can be represented through a combination of deterministic motion in classical phase space and quantum jumps. This represantation is a perturbative expansion of dynamics in quantum fluctuations. I will discuss corpuscular (Newtonian) and wave (Gross-Pitaevskii) classical limits and will demonstrate direct analogy between them. I will also show how such concepts as Wigner function, Weyl ordering, Moyal product, Bopp operators and others naturally emerge from the Feynmann's path integral representation of quantum evolution in the Heisenberg representation. I will illustrate the general approach with specific examples.



Friday, May 14th:   (note unusual day!)

Non-Linear Optics with Atomic Vapors
Michael Moore
Michigan State University

Abstract: Collective recoil effects in atomic vapors can lead to highly correlated photon pair emission in optically thick samples. I will describe recent experimental advances and give a microscopic quantum interpretation that allows us to understand the fundamental limits on the pair-correlations, pair-production rate, and the one-photon line-width. I will also show results from detailed numerical simulations based on nonlinear rate/coherence equations, and describe some proposed innovations that could lead to pair emission rates several orders of magnitude higher than has been achieved with nonlinear crystals.



Physics and Engineering Seminar Series - Fall 09

(All talks are at 1:00 PM in Room S-3-126 unless otherwise noted.)

Wednesday, Sept. 9th:

Optical Biomedical Sensors
Xingwei Wang
UMass Lowell

Abstract: Biomedical sensors are of particular interest in genetics, pathology, criminology, pharmacogenetics, food safety, and many other fields. Problems associated with the established technique include the bulky size, high equipment costs, and time-consuming algorithms. Therefore, such systems are limited to research laboratories and difficult to be applied for in-vivo situations. In this talk, I will introduce the concept of biosensors and give an example of a label-free optical biosensor. In addition, I will talk about a medical sensor – a miniature blood pressure sensor for cardiologist uses.



Wednesday, Sept.
16th:

Physics for All: From Special Needs to Olypiads
Arthur Eisenkraft
Recipient of the 2009 Robert A. Millikan Medal, awarded by the AAPT
COSMIC, UMass Boston

Abstract: Can a physics course be considered “real” physics if all students feel welcome? Physics First and Physics for All have become a success story for thousands of students in urban, suburban and rural districts. Simultaneously, the International Physics Olympiad and other competition programs have raised the expectation of what the most highly motivated students can achieve. Many physics educators are exploring ways to set higher goals for our most gifted students while also providing physics instruction to those students previously excluded from our physics classes. Great novels and symphonies are accessible to people of different backgrounds and levels of expertise. Both the literary scholar and the casual reader can enjoy Steinbeck’s Grapes of Wrath. We should develop teaching strategies that enable us to share an understanding of physics with all students – because everyone deserves a peek at the wondrous workings of our universe.



Wednesday, Sept. 23th:

“Having your cake and seeing it too” - Quantum phase transition and quantum criticality of ultracold atoms in optical lattices
Cheng Chin
University of Chicago

Abstract: The Bose-Hubbard model describes one of the simplest realizations of a quantum phase transition, a phase transition that occurs even at zero temperature. Near the phase boundary (critical point), quantum criticality, resembling that of Ising-type magnetics in higher dimensions, is expected to emerge with a full universal behavior. In particular, fluctuations and correlations are expected at all length scales.
  Our observation of atomic density profiles in optical lattices provides a powerful tool to determine all relevant thermo-dynamical quantities, as well as density fluctuations and density-density correlations. I will describe our efforts to identify the superfluid-Mott insulator phase boundary, to extract quantum fluctuations and correlations, and also discuss the prospects to identify and characterize quantum criticality and universality based on trapped quantum gases in an optical lattice.



Wednesday, Sept.
30th:

Exact Relations for Quantum-Mechanical Few-Body and Many-Body Problems
with Short-Range Interactions

Felix Werner
UMass Amherst

Abstract: We derive relations between large-momentum behavior of the momentum distribution, short-distance behavior of correlation functions, derivatives of the energy, and regular part of the wavefunction (defined through the behavior of the wavefunction when two particles approach each other). Some of these relations are generalisations of the ones obtained in [1, 2, 3]. Applications are presented at the unitary limit: derivatives of the energy are computed analytically and compared to numerics for three particles, and exact relations are compared to existing fixed-node Monte-Carlo data for the unitary Fermi gas.

[1] M. Olshanii and V. Dunjko, Phys. Rev. Lett. 91, 090401 (2003).
[2] S. Tan, Ann. Phys. 323, 2952 (2008); ibid. 323, 2971 (2008).
[3] R. Combescot, F. Alzetto and X. Leyronas, Phys. Rev. A 79, 053640 (2009).



Wednesday, Oct.
7th:

Probing Quantum Systems Through Quenches: an "Exciting" Approach
Claudia De Grandi
Boston University

Abstract: The challenge of understanding the behaviour of many-body systems out-of-equilibrium has motivated lately theoretical studies as well as interesting experiments. The knowledge of how a quantum system responds to an external perturbation is a useful tool to get insight into the specific properties of the system, and moreover to be able to manipulate it in an optimal way for applications like quantum computing. I will present the analysis of the scaling properties of some observables for a system driven away from equilibrium at the quantum critical point through an adiabatic or a sudden quench. The general results are then presented for the specific case of the one-dimensional sine-Gordon model, showing the connection of this model with experimental realizations in cold atoms experiments.



Wednesday, Oct.
14th:

Spatiotemporal Pattern Formation in Reactive Microemulsions
Irving Epstein
Brandeis University

Abstract: I will look at pattern formation in a reverse microemulsion consisting of nanometer diameter droplets of water containing the reactants of the Belousov-Zhabotinsky oscillating chemical reaction dispersed in oil. This system allows the physical structure (size, spacing) of the droplets and their chemical composition to be controlled independently, allowing one to generate a remarkable variety of stationary and moving patterns, including Turing structures, ordinary and antispirals, packet waves and spatiotemporal chaos. I will show examples of patterns and discuss two alternative mechanisms by which they may arise, one involving diffusion of different species at very different rates, the other via cross diffusion, whereby gradients in the concentration of one species influence the rate of diffusion of another species. I will also look briefly at a related system consisting of much larger (100 micron) drops in structured microfluidic arrays in 1D and 2D.



Wednesday, Oct.
21st:

Quantum Computation for Chemistry
Alan Aspuru-Guzik
Harvard University

Abstract: It is expected that chemical simulations will be one of the early application of early quantum computers. The simulation of the exact (within a basis) electronic structure and dynamics of mesoscopic quantum systems such as molecules or materials would benefit from an exponential speedup over classical computer algorithms. About 100 quantum bits are required to beat current classical computers. I will summarize the efforts of the theoretical chemists and quantum information scientists in this area. Another area of application of quantum computers would be to problems that are related to statistical mechanics, such as finding the low-energy conformations of random heteropolymers: a problem of interest to the Protein Folding community. I will talk about the recent advances and opportunities in this area. Finally, I will concluded with the description of an early experimental quantum computing calculation for chemistry that was recently completed.



Wednesday, Oct.
28th:

Multimodal Microscopy for Imaging of Brain Oxygen Delivery
and Consumption in Small Rodents

Sava Sakadzic
Harvard Medical School

Abstract: Intravital imaging of brain oxygen delivery and consumption is of key importance for understanding a wide range of clinical conditions such as stroke, head injury, and Parkinson’s disease. In addition, better understanding of hemodynamic responses will improve interpretation of the data obtained with fMRI, which is currently driving a revolution in our understanding of normal and diseased brain. Advanced optical microscopy methods are central to new discoveries in cerebrophysiology through their ability to measure multiple physiological processes with subcellular resolution. Here, we combine in a single instrument optical techniques such as two-photon microscopy, Doppler optical coherence tomography, and confocal microscopy and use them to investigate cerebrophysiology in small rodents. While Doppler optical coherence tomography allows fast three dimensional scans of blood flow and vasculature anatomy, two-photon microscopy provides subcellular resolution images based on variety of endogenous and exogenous contrast agents. We use two-photon microscopy to obtain anatomical maps of microvasculature, blood velocity and oxygenation, tissue oxygenation, structural and functional information about neuronal and glial cells, and to image cellular metabolic markers such as NADH. We investigate the effects of hypoxia and functional activation on cellular respiration by measuring the NADH fluorescence, and influence of blood volume changes on the measured NADH signal. Finally, a newly developed oxygen-sensitive phosphorescence dye, with enhanced two-photon excitation cross section, allowed us for the first time to significantly increase imaging depth, image oxygenation in various vascular compartments including capillaries, and perform measurement of oxygen partial pressure in cortical tissue.



Wednesday, Nov.
4th:

Information Thermodynamics
Masahito Ueda
The University of Tokyo

Abstract: The second law of thermodynamics presupposes a clear-cut distinction between the controllable and uncontrollable degrees of freedom by means of macroscopic operations. The cutting-edge technologies in quantum information and nano-science seem to force us to abondon such a notion in favor of the distinction between the accessible and inaccessible degrees of freedom. In this talk, I will discuss the implications of this paradigm shift by focusing on how the second law of thermodynamics can be generalized in the presence of a feedback control.



Wednesday, Nov.
11th:

Veteran's Day (No seminar)



Wednesday, Nov.
18th:

How Many States Does a Thermal Bath Need?
Kurt Jacobs
UMass Boston

Abstract: This will be a (largely) blackboard talk on work that Vanja Dunjco and I have recently  completed. 



Wednesday, Nov.
25th:

Quantum and Classical Aspects of Cold Atoms
Stephen Choi
UMass Boston

Abstract: Cold atoms -- atoms with very low kinetic energy -- are now routinely produced in labs around the world. Cold atoms obey quantum mechanics, since atoms are of microscopic size. It is by now well established that, when subjected to pulses of optical lattices, cold atoms can be mapped onto a quantum kicked rotor (QKR). It is also known that a QKR can exhibit classical behavior if one makes a continuous measurement of the position of the atoms. In this work,it is shown that by restricting the measurement to a bare minimum form (a stroboscopic measurement) one can unravel the effect of quantum measurement on the QKR analytically, and make some progress towards understanding how the transition from quantum to classical behavior arises. In the second part of the talk, a discussion of the semiclassical dynamics of a Bose-Einstein condensate -- cold atoms behaving coherently at even lower temperatures -- is provided.



Wednesday, Dec.
2nd:

Some New Results on Structural and Dynamical Aspects of Networks
Bala Sundaram
UMass Boston

Abstract: Both the structure and function of many complex systems can be described in terms of networks consisting of nodes and links between them. The nodes can be either individual components or sub-systems and the strength of the connections between these can be fixed or variable. In recent years, there has been a great deal of interest in structural aspects of networks and the exploration of dynamics on these topologies is rapidly expanding. The talk will focus on our recent work on a statistical view of the construction or growth of networks as well as some related results on spatio-temporal dynamics.



Wednesday, Dec. 9th:

The Physics and Device Applications of Intersubband Transitions in
Wide-Bandgap Nitride Semiconductors

Roberto Paiella
Boston University Photonics Center

Abstract: Intersubband transitions in semiconductor quantum structures offer unique opportunities for the development of new device concepts and applications in optoelectronics. In particular, extensive work with As-based quantum wells in the past several years has led to the creation of an entirely new class of light emitters and photodetectors (i.e., quantum-cascade lasers and quantum-well infrared photodetectors), currently providing unparalleled performance at mid-infrared wavelengths. In this talk I will review our recent work aimed at extending the spectral range and functionality of intersubband devices using novel materials systems - most notably GaN-based quantum wells. These heterostructures can accommodate intersubband transitions at near-infrared fiber-optic-communication wavelengths, and we have used them to demonstrate all-optical switching with ultrafast (sub-picosecond) response times and concomitantly low switching energies. Furthermore, we have demonstrated optically pumped intersubband light emission from GaN/AlN quantum wells at the record short wavelength of about 2 mm. Finally, the potential of nitride semiconductors in the area of THz quantum-cascade sources, related to their characteristically large optical phonon energies, will also be discussed.



Wednesday, Dec. 16th:

The Memory of Initial Conditions in Incompletely-Chaotic Quantum Systems
Vladimir Yurovsky
Tel Aviv University

Abstract: A system of two atoms in a circular, transversely harmonic waveguide in the multimode regime is analyzed. While showing some signatures of the quantum-chaotic behavior, the system fails to reach a complete quantum chaos, even when the interaction between the atoms is infinitely strong. A relaxation from an initial state leads to a final state which is different from a thermal equilibrium; this state retains some memory of the initial conditions. The results inspire a more general theory of relaxation in incompletely-chaotic systems of which our system is a particular case.



Physics and Engineering Seminar Series - Spring 09

(All talks are at 1:00 PM in Room S-3-126 unless otherwise noted.)

Day change: Monday, May 18th:

Picture Perfect: Persuasion, Politics and Prejudice
Surrounding the Scientific Image, 1800 – 2009

Eric Heller
Harvard

Abstract: Over the years, some physical scientists and mathematicians have eschewed diagrams, declaring them as unnecessary crutches for weaker minds. They preferred formal mathematical argument. In recent years, computers have made possible images of stunning clarity and pedagogy, winning over most (but not all) skeptics, and a great following among students and the public. Does the use of imagery amount to dumbing down the discipline? Is there such a thing as proof by image alone? Are iconic images good for science? A thread through the science of classical and quantum waves leads us through the prejudices, successes and failures of the scientific image from 1800 to the present day.



Wednesday, May 13th:
Time change: 2 - 3 pm

Tunneling and Coherence in Few and Many Body Systems
Eric Heller
Harvard

Abstract: This talk will be an overview of situations, such as high resolution resonance spect- roscopy, where the perspective of tunneling in many degrees of freedom (especially dynamical tunneling) is very useful. The asymptotic nature of many body wave functions in the tunneling regime is interesting and some results are known and will be discussed. Coherent and incoherent tunneling regimes will also be mentioned.



Wednesday, May 6th:

Photonics and Plasmonics in the Mid-Infrared
Dan Wasserman
Photonics Center, UMass Lowell

Abstract: The mid-infrared (mid-IR) spectral range (3-30µm) has become a burgeoning and dynamic field of research both for fundamental exploration and as well as for more applied research in health and the environment, security and defense, communication, and sensing. Much of the recent interest in the mid-IR can be directly attributed to the rapid advances in mid-IR sources, specifically the quantum cascade laser (QCL). As the QCL continues to develop, fundamental challenges such as wavelength range, wall-plug efficiency, tunability, and surface emission, are faced. In addition, while the subfield of mid-IR photonics centered around emitters (and to a lesser extent, detectors) has experienced phenomenal growth of late, the mid-IR does not boast the “optical infrastructure” of more established wavelengths, such as the telecom and visible ranges. I will discuss recent work incorporating quantum dots into cascade-like heterostructures as a possible avenue towards improved efficiency and surface emitting mid-IR sources. In addition, I will discuss recent results in the field of mid-IR plasmonics and how novel plasmonic devices can not only serve as a bridge between electronics and photonics, but how they may also form the basis of a novel class of electro-optic devices, augmenting the mid-IR optical infrastructure. Finally, I will demonstrate how plasmonic structures can be applied to mid-IR sources, in an effort to develop plasmon-enhanced emission from intersublevel transitions in 3D nanostructures. In all, I will demonstrate that while only ~27µm wide, the narrow width of the mid-IR spectral range width belies its capacity for exciting new research and applications, both fundamental and applied.



Wednesday, April 29th:

Controlling the Shot Noise of a Quantum Point Contact
via Coupling to a Mechanical Resonance

Alex Rimberg
Dartmouth College

Abstract: A canonical one-dimensional system, the quantum point contact (QPC) has been extensively studied for almost twenty years. Shot noise in QPCs, which is subtle probe providing information about the system’s quantum nature, has been measured for more than ten. Nonetheless, this simplest of quantum systems continues to surprise. We have recently performed the first broadband frequency resolved measurements of shot noise in a radio frequency QPC. Our measurements reveal a remarkable frequency dependence entirely absent from over two decades of theoretical investigation. Our data suggest a piezoelectric mediated feedback loop in which shot noise drives resonant mechanical vibrations of the sample that in turn create correlations in the tunnelling of electrons. The feedback loop concentrates the initially white noise in a narrow band around the sample’s resonant frequency. This allows us to not only clearly observe the faint shot noise signal but also to engineer the spectrum to suppress the shot noise in an approximately 1 MHz detection bandwidth. As the ultimate sensitivity and quantum mechanical backaction of a QPC charge sensor is determined by its shot noise, our ability to control the shot noise spectrum likely has important implications for how closely a QPC sensor can approach the quantum limit for charge detection. Our results also suggest application to extremely sensitive displacement detection as we show that our ability to measure the weak shot noise signal corresponds to sensing vibrations at the sample edge of only a few angstroms.



Wednesday, April 22nd:

Diagrammatic Monte Carlo: What Happens to the Sign Problem?
Nikolai Prokof'ev
UMass Amherst

Abstract: Feynman diagrams are the most celebrated and powerful tool of theoretical physics usually associated with the analytic approach. I will argue that diagrammatic expansions are also an ideal numerical tool with enormous and yet to be explored potential for solving interacting many-body systems. The current Monte Carlo scheme is based on direct simulation of Feynman diagrams for the proper self-energy up to some high order. Though the original series based on bare propagators are sign-alternating and often divergent one can determine the answer behind them by using two strategies (separately or together): (i) using proper series re-summation techniques, and (ii) introducing renormalized propagators which are defined in terms of the simulated proper self-energy, i.e. making the entire scheme self-consistent. The first results for the resonant Fermi gas and the Fermi-Hubbard model at U/t=4 away from half-filling prove that this approach is working.



Wednesday, April 15th:

Quantum Manipulation of Mechanical Motion Using a Single Spin Qubit.
Peter Rabl
ITAMP (Institute for Theoretical Atomic, Molecular and Optical Physics)
Harvard-Smithsonian Center for Astrophysics and Harvard Physics Department

Abstract: Micro- and nano-mechanical systems have recently attracted a lot of interest, mainly due to the experimental progress in opto-mechanical cooling schemes and the expected demonstrations of quantum features with macroscopic objects in the near future. However, so far ground state cooling has not yet been achieved and different techniques will be require for the preparation and detection of non-trivial quantum superpositions of motion. In this talk I will describe a system where the quantized motion of nano-mechanical resonator is magnetically coupled to a spin qubit associated with a solid state impurity, e.g a nitrogen-vacancy defect in diamond. Under realistic conditions the Zeemann shift per quantum of motion can exceed both the intrinsic spin coherence time and the motional dephasing rate of high-Q mechanical resonators and the spin becomes strongly coupled to mechanical motion in analogy to the strong coupling regime of cavity QED. I will first show how this regime can be accessed in a practical setting by a careful preparation of dressed spin states which eliminate fast dephasing of the spin due to interactions with the nuclear spin bath. Optical preparation and readout techniques for the spin states then allow quantum ground state cooling as well as the generation and detection of arbitrary superpositions of motional states. In the second part of the talk I will extend the discussion to a whole spin-resonator array, where the motion of electrostatically coupled resonators serves as a quantum bus which mediates effective spin-spin interactions over long distances. I will discuss the implementation of basic spin entangling operations in this setup and the influence of magnetic and mechanical noise on the resulting gate fidelities. Finally, I will describe potential applications of these ideas as a universal and scalable coupling scheme for spin based quantum computers.



Wednesday, April 8th:

Modelling with Quantum Gases
Tilman Esslinger
ETH Zurich

Abstract: The experimental realization of dilute quantum gases marks a change in the approach to many-body phenomena in condensed matter physics. The research in this field had mostly been stimulated by the surprise observation of intriguing phenomena, such as superfluidity or superconductivity. These findings then stimulated the search for theoretical models to provide satisfying explanations. Research in quantum gases follows a very different route, since the experiments aim to realize phenomena predicted by existing concepts of quantum many-body physics, such as Bose-Einstein condensation or superfluidity in the BEC-BCS crossover. A highly interesting situation appears, when the physics of the underlying model is not understood in its entirety. This is the case for the two-dimensional Hubbard model which is assumed to support d-wave superfluidity. Here, we will discuss an experiment in which we have realized a fermionic Hubbard model using a repulsively interacting two-component Fermi gas in an optical lattice. We observe the formation of a Mott insulator which is signalled by a drastic suppression of doubly occupied lattices sites, a strong reduction of the compressibility inferred from the response of double occupancy to atom number increase, and the appearance of a gapped mode in the excitation spectrum. A quantitative comparison with the Fermi-Hubbard model in the atomic limit provides a characterization of the Mott insulator [1]. Perspectives for quantum simulation will be discussed. A different approach to quantum simulation is the comparison of two experimental realization of the same Hamiltonian. In this context we have shown in a recent experiment that a Bose-Einstein condensate inside an ultrahigh-finesse optical cavity can be mapped to a generic cavity-optomechanical system [2], as realized in gravitational wave detectors and micromechanics.

[1]: R. Jördens, N. Strohmaier, K. Günter, H. Moritz, T. Esslinger,
       A Mott insulator of fermionic atoms in an optical lattice, Nature 455, 204 (2008).
[2]: F. Brennecke, S. Ritter, T. Donner, T. Esslinger,
       Cavity Opto-Mechanics with a Bose-Einstein Condensate, Science 322, 235 (2008).



Wednesday, April 1st:

A High-Sensitivity Diamond Magnetometer with Nanoscale Resolution
Paola Cappellaro
ITAMP (Institute for Theoretical Atomic, Molecular and Optical Physics)
Harvard-Smithsonian Center for Astrophysics and Harvard Physics Department

Abstract: Isolated electronic spins in the solid-state have been recently proposed as sensitive magnetic sensors [1,2]. This novel approach to magnetometry is enabled by the good coherence properties of electronic qubits, such as the spins associated with Nitrogen-Vacancy (NV) centers in diamond, as well as by advanced techniques for their coherent control. The key feature of this solid-state magnetometer is the possibility to confine the sensing spins into a crystal of nanometer size that can be brought extremely close to the magnetic field source, thus achieving high spatial resolution. Our experiments showed the potential for the resulting magnetic sensor to provide an unprecedented combination of high sensitivity and spatial resolution. The ultimate sensitivity limit is set by the interaction of the spin sensor with its environment and in particular the nuclear and electronic spin bath. As an outlook, I will discuss how engineering, controlling or harnessing the environment can lead to better sensitivity, even beyond the standard quantum limit. Finally, I will outline several exciting applications of these novel magnetic sensors in areas ranging from bio- and materials science to fundamental physics and single electronic and nuclear spin detection.

[1] J. M. Taylor, et al. Nature Phys. 4, 810-816 (2008).
[2] J. R. Maze, et al. Nature 455, 644 - 647 (2008).



Wednesday, March 25th:

Remarks on Neurodynamics: Spikes, Spike Synchronization, and
Spike-Timing-Dependent Plasticity

Michael Bukatin
MetaCarta Inc. and Brandeis University

Abstract: This is a review talk consisting of three parts. I will start with reviewing a variety of selected recent results. In the second part of the talk, I will discuss the transition from models of biological neural systems based on firing rates of neurons to models based on exact timing of spikes, and on such phenomena as spike synchronization and neural oscillations, synchronicity detection, and spike-timing-dependent plasticity. This transition has so far had a rather limited impact on the progress in artificial neuromoprphic systems and on our understanding of higher cognitive functions. In conclusion, I'll discuss some of the issues involved here.



Wednesday, March 11th:

Noise Spectroscopy in Amorphous Indium Oxide
Steve Arnason
Umass Boston

Abstract: As a result of the correlations between electrons, electron glasses show enhanced fluctuations in conductance with a 1/f  frequency dependence. We are studying the time dependence of the fluctuation spectra as the system relaxes towards equilibrium after a discontinuous change in chemical potential. Our measurements are taken on FET structures where the conductance channel is fabricated from amorphous Indium Oxide. Changing the potential on the gate electrode allows us to change the chemical potential and we measure the resistance of the conductance channel as a function of time. In addition to the fluctuations there is a slow, logarithmic relaxation of the channel conductance. Because of this slow relaxation our signal is not stationary, calling into question the application of Fourier transform based analysis techniques. One approach to coping with this problem is the subtraction of the slowly varying background before the calculation of the Fourier transforms, so called detrended fluctuation analysis. This talk presents results on simulations of this technique as applied to computer generated signals with characteristics similar to our actual data. The frequency dependence of the fluctuation spectra is imperfectly preserved but can be similar to the actual fluctuation spectra within certain bounds of analysis parameters.




-- Rescheduled due to snow --

Wednesday, March 4th:

Leveraging Dynamics to Probe Delicate Physical Systems
JM Geremia
University of New Mexico

Abstract: In a variety of physical settings, ranging from laser cooled atoms for quantum information theory to the optical fields used for telecommunications to the intermediates of a biochemical network, it is desirable to observe the state of the system while disturbing it to the least extent possible. Yet, the standard approach to measurement in most physical contexts is quite disruptive: making projective measurements on a quantum system that thereby destroy its state, attaching fluorescent tags to biomolecules which consequently change or kill enzymatic activity on those species, and so on.

In this talk, I will discuss a general approach to observing physical systems based on continuous measurements and state estimation theory. Such an approach can enable particularly non-invasive measurements, even in practice, as it relies upon inference of the system’s state based on how the system evolves in time, rather than by direct manipulation of the system. An attractive aspect of this general approach is its broad applicability to a variety of physical and biochemical systems. I will discuss ongoing experiments in the precision detection of magnetic fields and plans for the quantitative detection of untagged biochemical intermediates in situ.



Day change: Friday, Feb 27th:
Time change: 1:30 pm

Statistical Properties of Granular Materials
Jie Zhang
Duke University

Abstract: Granular materials, of which sand and soil are only two examples, are ubiquitous in nature. The physics of granular material is very rich and has important implications for many industrial and geophysical processes. They behave very differently from the ordinary forms of matter: solids, liquids or gases because of the three important aspects: the existence of friction, kBT is effectively zero, and for moving grains, the inelastic nature of their collisions. In this talk, I will first review novel uses of photoelastic techniques to investigate different granular phenomena. Next I will focus on an experiment to study the evolution of structure and stress for a system of 2D photoelastic particles that is subjected to multiple cycles of pure shear. Throughout this process, the system continuously changes its behaviors from the soft, fluidlike to the rigid, solidlike or vice versa. Despite the great complexity, we find the system can be well described using a single state variable: the average contact number, Z.



Wednesday, Feb 25th:

Optical Imaging for Biomedical Applications
Chandra Yelleswarapu
UMass Boston

Abstract: Photonic materials such as photopolymers and liquid crystals show many intrinsic nonlinear optical properties even at very low input powers. The molecular reorientation of these molecules can be manipulated by the wavelength, intensity, and polarization of the incident light enabling us to change amplitude, phase, polarization, and index of refraction of the incident light. Over the years, we studied the nonlinear optical properties of various photonic materials and simultaneously exploited these properties for wide variety of nanophotonics and biophotonics applications such as all optical switching, modulation, power limiting for laser eye protection, medical image processing for early detection of breast cancer, and slow and super luminal light. We are currently focusing on using some of the nonlinear optical properties for biomedical imaging. A novel Fourier phase contrast microscope (FPCM) was developed exploiting monochromaticity, intensity and phase coherence of the laser beam and photo-induced birefringence of liquid crystals. Further, a common path multimodal optical microscopy (CMOM) system is demonstrated for imaging amplitude, phase, and fluorescence features of biological specimens using a single optical path, single light source, and single camera with no requirement of image registration. In this talk, I will present FPCM and CMOM systems with applications to study biological processes such as live cell dynamics. I will also discuss future directions of my research in optical imaging and sensing for biomedical and defense applications.



Day change: Friday, Feb 20th:
Time change: 1:30 pm

Generating Optical Near-Fields From Afar via Absorbance Modulation
Rajesh Menon
MIT
LumArray Inc.

Abstract: Diffraction limits the resolution of far-field optics to approximately half the wavelength of illumination [1]. The high-spatial-frequency components of an optical field, which carry high-resolution spatial information, exist only in the near-field. They decay exponentially, and hence are not propagated to the far-field. Techniques that exploit the optical near-field such as contact photolithography and near-field optical scanning microscopy can overcome this limitation, but at the complexity of maintaining the distance between the source of the near-fields and the substrate to exquisite precision. Furthermore, in the case of contact photolithography, defects on the photomask pose a serious practical concern. In this presentation, I will describe absorbance modulation as a technique to overcome these limitations, and achieve deep sub-wavelength resolution with only far-field optical elements.

Absorbance modulation generates optical near fields in the far-field by combining photochemistry with structured illumination [2]. A thin film of photochromic molecules, referred to as the absorbance-modulation layer (AML), is illuminated with a bright spot at one wavelength, L1, and simultaneously with a ring-shaped spot at another wavelength, L2. The photochromic molecules are designed such that upon absorbing photons at L1, they transform from an opaque state to a transparent state. This transformation is reversed by absorption of photons at L2. Under these circumstances, the dynamic equilibrium generated via these reversible transformations results in a sub-wavelength transparent “aperture” in the vicinity of the central node of the L2 ring-shaped spot. The spatial extent of this aperture is no longer limited by diffraction, but primarily by the photochromic parameters of the molecules and the ratio of the intensities at the two wavelengths [3]. Photons at L1 penetrate this aperture forming an optical nanoscale probe. By placing the AML atop a substrate, and scanning the substrate relative to the far-field optics, one can achieve nanopatterning or nanoscopy. In this presentation, I will describe these experiments, the associated theory, extension to two dimensions using dual-wavelength diffractive lenses (dichromats) [4,5], and application to high-throughput nanoscale imaging.

[1] E. Abbe, Arch. f. Mikroscop. Anat. 9, 413 (1873).
[2] R. Menon, et al., Phys. Rev. Lett. 98, 043905 (2007).
[3] R. Menon, et al., J. Opt. Soc. Am. A, 23(9), 2290 (2006).
[4] R. Menon, et al., J. Opt. Soc. Am. A 26(2), 297 (2009).
[5] H-Y. Tsai, et al., Opt. Lett. 33(24), 2916 (2008).



Wednesday, Feb 18th:

Metal-Insulator Transition in Thin Film Vanadium Dioxide
Dimitry Ruzmetov
Harvard University

Abstract: The phenomenon of metal-insulator phase transition in strongly correlated electron systems is one of the focus areas of research in condensed matter physics. The interest is partly motivated by the potential of the materials exhibiting a metal-insulator transition to be used in novel electronics and electro-optic applications as switches or memory elements. There is also considerable interest in understanding the fundamental science behind the correlated electron behavior responsible for striking material property changes such as a metal-insulator transition and colossal magnetoresistance. Vanadium dioxide has received special attention because of the substantial scale of the metal-insulator transition in this material, the fact that the transition temperature is near room temperature (67˚C), and extremely fast optical switching upon the transition (~100 fs). I will present the results of our study of the energy band structure of vanadium dioxide (VO2) thin films by means of x-ray absorption and photoemission spectroscopy across the metal-insulator transition (MIT). The results are related to the electron transport parameters, stoichiometry, and morphology of the films. The analysis allows us to conclude on the importance of the electron correlations, details of the atomic coordination changes in the material for different film morphologies and across MIT, and the nature of electron transport. Our optical studies of VO2 films revealed record high switching of the mid-infrared reflectance upon MIT and provided the evidence of the percolative nature of the phase transition. I will also present our recent results on the electron transport properties of VO2 across MIT in the presence of a high (12 Tesla) magnetic field.



Day change: Friday, Feb 13th:
Time change: 1:30 pm

Atom Interferometry with Guided Thermal Atoms
Alexey Tonyushkin
Harvard University

Abstract: Recent growth of the atom interferometry field is being driven by the wide array of its possible applications in precision measurement of the fundamental physical constants, and for sensing of inertial effects. Inertial sensing such as rotation was one of the first and one of the most practically important demonstrated applications for atom interferometers. Many believe that cold atom-based interferometer for rotational sensing – a device called a gyroscope – can be both compact and highly sensitive. In my talk, I review various types of atom interferometers and show that cold thermal atoms in a magnetic guide are well suited for atom interferometry. I will also talk about our recent implementation of a quantum kicked rotor, a system whose classical counterpart exhibits chaos. I will discuss the applications of our quantum kicked rotor to accurate measurements of gravitational acceleration and atomic recoil frequency as well as to study a quantum-classical correspondence principle.



Wednesday, Feb 11th:

Learning About Order From Noise
Eugene Demler
Harvard University

Abstract: The probabilistic character of measurement processes is one of the most fascinating aspects of quantum mechanics. In many-body systems quantum noise can reveal the non-local correlations and multiparticle entanglement in the underlying states. In this talk I will review recent theoretical and experimental progress in applications of the quantum noise analysis to the study of many body states of ultracold atoms in optical lattices and fluctuating low dimensional condensates.



Wednesday, Feb 4th:

Reactions and Thermalization Due to Non-Integrability in
Quasi-One-Dimensional Ultracold Gases

Vladimir Yurovsky
Tel Aviv University

Abstract: Atom waveguides that tightly confine a motion of ultracold particles in two transverse directions [1] have been realized recently in elongated atomic traps, two-dimensional optical lattices, and atomic chips. Quasi-one-dimensional Bose gases can be approximately described by the Lieb-Liniger-McGuire model with energy-independent zero-range atom-atom interactions. This model is a rare example of integrable many-body systems. It has an exact Bethe-ansatz solution characterized by non-diffractive scattering, where the atoms can exchange their momenta, but the asymptotic momentum set remains unchanged. A consequence of integrability was observed in the quantum Newton’s cradle experiment at Penn State [2], performed on an array of quasi-one-dimensional Bose gases out of equilibrium in a two-dimensional optical lattice. The momentum distributions are not observed to change with time as a result of thermalization. Reflection and dissociation in atom-diatom collisions and three-atom association are forbidden within the Lieb-Liniger-McGuire model and can appear when the integrability is lifted [3]. Another possible observable effect of non-integrability is the stabilization of broad Feshbach dibosonic molecules in atom waveguides [4]. Non-integrability can also lead to thermalization due to a change of the asymptotic momentum set in three-atom elastic collisions, as well as due to general diffractive scattering. Thermalization was observed in recent Penn State experiments when the lattice depth is decreased, although the collision energy remains substantially below the transverse waveguide frequency. The thermalization can be related to tunneling of atoms between adjacent waveguides.

[1] V. A. Yurovsky, M. Olshanii, and D. S. Weiss, Adv. At. Mol. Opt. Phys., 55, 61, (2008).
[2] T. Kinoshita, T. R. Wenger, and D. S. Weiss, Nature, 440, 900, (2006).
[3] V. A. Yurovsky, A. Ben-Reuven, and M. Olshanii, Phys. Rev. Lett, 96, 163201, (2006).
[4] V. A. Yurovsky, Phys. Rev. A 77, 012716, (2008).

Physics and Engineering Seminar Series - Fall 08

(All talks are at 1:00 PM in Room S-3-126 unless otherwise noted.)

Wednesday, Dec 10th:

Anthropic Predictions in the Landscape
Delia Schwartz-Perlov
Tufts

Abstract: A beautiful feature of inflation is that it is generically eternal giving rise to the "multiverse". Developments in string theory have also led to the complementary world view that the fundamental laws of physics admit a vast array of possible solutions. In such models there are of order 10500 different solutions/vacua with each vacuum state representing a possible type of bubble universe governed by its own low-energy laws of physics. In the context of the multiverse, some physical parameters that were once thought of as fundamental universal parameters may simply be ``local'' environmental parameters. The most famous example of one such parameter, is that of the cosmological constant. Anthropic considerations have led to successful predictions for the cosmological constant. However these predictions have hinged on the assumption of a flat prior distribution which depends on unknown details of the landscape and the dynamics of eternal inflation. I will discuss the calculation of the prior within a toy string theory “discretuum” proposed by Bousso and Polchinski (BP model) and also within the toy landscape model of Arkani-Hamed, Dimopolous and Kachru (ADK model).



Wednesday, Dec 3rd:

The Threshold for Chaos and Thermalization in The One-Dimensional Mean-Field Bose-Hubbard Model
Amy Cassidy
UMass Boston

Abstract: We study the threshold for chaos and its relation to thermalization in the 1D mean-field Bose-Hubbard model, which in particular describes atoms in optical lattices. We identify the threshold for chaos, which is finite in the thermodynamic limit, and show that it is indeed a precursor of thermalization. Far above the threshold, the state of the system after relaxation is governed by the usual laws of statistical mechanics.



Wednesday, Nov 26th:

The Quantum-to-Classical Transition and the Emergence of Chaos
Justin Finn
UMass Boston

Abstract: Closed quantum systems do not exhibit chaos by virtue of the fact that Schrödinger’s equation is linear. However, the emergence of classical motion, and thus chaotic dynamics, is explained by quantum measurement theory. When a quantum system is continuously observed, the dynamics becomes nonlinear, and as long as the action of the system is reasonably large compared to Planck’s constant, the smooth trajectories of the equivalent classical dynamics are produced by the quantum system. The question of how the Lyapunov exponent, the key measure of chaos, changes as the system goes through the transition is a topic of current research interest. We have calculated the range of Maximal Lyapunov Exponents for the Driven, Damped Duffing Oscillator system through this transition for a case where it exhibits chaotic behavior in the classical regime as well as the transition region near the classical limit, and for a sufficiently damped variation of the system where chaos is absent in these same regions.



Wednesday, Nov 19th:

Creating Mesoscopic “Schrödinger Cats” in a Nano-Mechanical Resonator
Kurt Jacobs
UMass Bsoton

Abstract: We show that a nano-resonator can be prepared in mesoscopic-superposition states ("Schrodinger Cats") merely by monitoring a qubit sensitive to the square of the resonators position. This works for thermal initial states, and bypasses the need for a third-order nonlinearity. The required coupling can be generated using an open-loop control protocol, obtained with optimal control theory, or an off-resonant interaction. We simulate the complete preparation process, including environmental noise. Our results indicate the power of open-loop control for state-engineering and measurement in quantum nano-systems.



Wednesday, Nov 12th:

The Feynman-Mensky Approach to Stroboscopic
Quantum Nondemolition Measurements

Roberto Onofrio
Dartmouth College

Abstract: I will review the restricted path integral approach to stroboscopic quantum measurements and applications to harmonic and anharmonic oscillators. I will then discuss the case of the SQUIDs dynamics relevant for testing temporal Bell inequalities.



Wednesday, Nov 5th

Subcellular Surgery and Nanosurgery
Eric Mazur
Harvard

Abstract: We use femtosecond laser pulses to manipulate sub-cellular structures inside live and fixed cells. Using only a few nanojoules of laser pulse energy, we are able to selectively disrupt individual mitochondria in live bovine capillary epithelial cells, and cleave single actin fibers in the cell cytoskeleton network of fixed human fibro-blast cells. We have also used the technique to micromanipulate the neural network of C. Elegans, a small nematode. Our laser scalpel can snip individual axons without causing any damage to surrounding tissue, allowing us to study the function of individual neurons with a precision that was not achievable before.



Date: Wednesday, October 29th:
Time: 1 - 3 pm
Location: Point lounge (3rd Floor), Campus Center 

Like a Work of Shakespeare: Reality and Metaphor in Modern Physics
Richard Wolfson
Middlebury College

Abstract: Scholars of the humanities thrive on metaphor. So do the rest of us, in our everyday use of language. In fact, metaphors often shape our perception and understanding of reality. Surely, though, science is beyond the ambiguities of metaphorical language. But no! In fact, modern physics abounds with metaphor. The very language we use to describe reality at the atomic level itself affects what we observe. And a host of physical phenomena, from the wave-particle duality to the recently discovered Bose-Einstein condensate, and on to such far-out ideas as parallel universes and time travel, all admit metaphorical description or outright links to concepts from literature and the humanities. This talk will explore metaphorical connections between modern physics and the humanities. The talk is aimed at an audience of nonscientists and scientists together. I've sometimes given it as a joint event sponsored by departments of Physics and English.



Wednesday, October 22nd

Dynamically Error-Corrected Gates for Accurate
Quantum Control and Computation

Lorenza Viola
Dartmouth College

Abstract: Achieving accurate control over quantum dynamics is a long-sought goal in a variety of quantum physics, chemistry, and engineering settings as well as in quantum information processing applications. Scalable quantum computation in realistic devices, in particular, requires that precise control can be implemented efficiently in the presence of both decoherence and operational errors. In this talk, I will present a constructive procedure for designing robust unitary gates on an open quantum system without encoding or measurement overheads. Our results allow for a low-level error correction strategy solely based on Hamiltonian control under realistic power and bandwidth constraints, and may prove instrumental to reduce implementation requirements for fault-tolerant quantum computing architectures. Illustrative examples will be discussed throughout.



Wednesday, October 15th:

The Feynman-Mensky Approach to Continuous Quantum Measurements
Roberto Onofrio
Dartmouth College

Abstract: I will discuss the restricted path integral approach to continuous quantum measurements of position and energy, and its relevance for monitoring nonlinear quantum systems and two-level systems respectively.



Wednesday, October 8th:

Trapped Cold Atoms with Resonant Interactions
Felix Werner
UMass Amherst & Ecole Normale Supérieure

Abstract: Near a Feshbach resonance, cold atoms are strongly interacting because the scattering length diverges. Moreover the interactions are short ranged, which allows to model them by a zero-range pseudopotential.
  We solve the 3-body problem for an infinite scattering length in an isotropic harmonic trap. For bosonic particles, we find two types of eigenstates: universal states which only depend on the oscillation frequency of a particle in the trap, the particles' mass, and Planck's constant; and efimovian states which also depend on a 3-body parameter, similarly to the 3-body bound states in free space discovered by Efimov. In an experiment, we predict that the universal states are long-lived, which is unusual for bosonic atoms. This lifetime is determined by the coupling between universal and efimovian states induced by the non-zero range of interactions.
  In the N-body case, we find that the hyperradius, a collective degree of freedom describing the global size of the gas, is separable. We determine the dependence of the many-body wavefunctions on the hyperradius. We deduce a relation on the thermal fluctuations of the gas size.
  Finally we obtain several generalized virial theorems. For the trapped resonant Fermi gas, we deduce that the trapping potential energy has an inflexion point at the unitary limit if the inverse scattering length is varied adiabatically.



Wednesday, October 1st:

Confinement-Induced Resonances
Vanja Dunjco
UMass Boston

Abstract: Confinement-induced resonances (CIRs) arise when particles scatter in the presence of an external potential that spatially confines their motion. The scattering depends on several parameters, such as the free-space interatomic coupling constant, the strength of the confining potential, and the scattering energy. For some combinations of values of these parameters, the scattering cross-section reaches the unitary limit: this is CIR. The talk will start with a toy model that exhibits a scattering resonance similar to a CIR. Like systems with CIRs, the toy model naturally leads to an effective theory with a reduced number of degrees of freedom. In the second part of the talk, we will present the most important examples of CIRs discovered to date, and conclude by discussing the future of the field.



Wednesday, September 24th:

Microscopic Diagonal Entropy, Heat, and the Laws of Thermodynamics
Anatoly Polkovnikov
Boston University

Abstract: Entanglement entropy, which is a measure of quantum correlations between separate parts of a many-body system, has emerged recently as a fundamental quantity in broad areas of theoretical physics, from cosmology and field theory to condensed matter theory and quantum information. The universal appeal of the entanglement entropy concept is related, in part, to the fact that it is defined solely in terms of the many-body density matrix of the system, with no relation to any particular observables. However, for the same reason, it has not been clear how to access this quantity experimentally. In this talk we shall present a universal relation between entanglement entropy and the fluctuations of current flowing through a quantum point contact (QPC) which opens a way to perform a direct measurement of entanglement entropy. In particular, by utilizing space-time duality of 1d systems, we relate electric noise generated by opening and closing the QPC periodically in time with the seminal S = 1/3 log L prediction of conformal field theory.



Wednesday, September 17th:

Quantum Noise as an Entanglement Meter
Leonid Levitov

Massachusetts Institute of Technology
Cambridge, MA

Abstract: Entanglement entropy, which is a measure of quantum correlations between separate parts of a many-body system, has emerged recently as a fundamental quantity in broad areas of theoretical physics, from cosmology and field theory to condensed matter theory and quantum information. The universal appeal of the entanglement entropy concept is related, in part, to the fact that it is defined solely in terms of the many-body density matrix of the system, with no relation to any particular observables. However, for the same reason, it has not been clear how to access this quantity experimentally. In this talk we shall present a universal relation between entanglement entropy and the fluctuations of current flowing through a quantum point contact (QPC) which opens a way to perform a direct measurement of entanglement entropy. In particular, by utilizing space-time duality of 1d systems, we relate electric noise generated by opening and closing the QPC periodically in time with the seminal S = 1/3 log L prediction of conformal field theory.



Wednesday, September 10th:

Tracking Dust
Jason Ralph

Department of Electrical Engineering and Electronics
University of Liverpool

Abstract: Complex (dusty) plasmas - consisting of micron-sized grains within an ion-electron plasma - are a unique vehicle for studying the kinematics of microscopic systems. Athough they are mesoscopic, they embody many of the major structural properties of conventional condensed matter systems (fluid-like and crystal-like states) and they can be used to probe the structural dynamics of such complex systems. Modern state estimation and tracking techniques allow complex systems to be monitored automatically and provide mechanisms for deriving mathematical models for the underlying dynamics - identifying where known models are deficient and suggesting new dynamical models that better match the experimental data. This talk explores how tracking and state estimation techniques can be used to explore and control important physical processes in complex plasmas: such as phase transitions, wave propagation and viscous flow.



Wednesday, September 3rd:

Rapid Measurement of Quantum Systems Using Feedback Control 
Joshua Combes

School of Science
Griffith University

Abstract: In this talk I will show how feedback control can be used during a measurement to increase the speed at which the measurement extracts information. In particular, I will describe a a feedback protocol that increases the speed at which a measurement extracts information about a d-dimensional system by a factor that scales as d2. By generalizing this algorithm it can also be applied it to a register of n qubits and in this case achieves an improvement that scales as n.
 

Physics and Engineering Seminar Series -Spring 08

(All talks are at 1:00 PM in Room S-3-126 unless otherwise noted.)

Wednesday, May 21st:

Several Approaches to Understanding Persistent Patterns in Time-Periodic
Advection-Diffusion Problems
Andrew C. Poje

Physics Department
City University of New York

Abstract: The characterization of the behavior of a passive, diffusing scalar advected by a prescribed, smooth velocity field is of primary importance in applications ranging from micro-fluidic mixers to global climate dynamics. Above and beyond its obvious practical importance, the 'scalar turbulence' exhibited by solutions of the linear advection-diffusion equation serves as a simple phenomenological model for the nonlinear dynamics of the velocity field itself.

Despite the linearity of the governing equation, much remains unknown about the behavior of solutions to the advection-diffusion equation, even in the relatively simple case of time-periodic planar flows and their associated maps. In this talk, we will review some well-known results and then concentrate on three related approaches to understanding the formation of persistent patterns, often called strange eigenmodes, of the scalar field. These approaches involve (1) the introduction of a relatively novel measure (χ2) which allows us to identify the dynamical stages involved in the emergence of these patterns and the scaling laws governing their overall decay rates, (2) a formal averaging procedure for integrable flows which transforms the time dependent problem to a time-independent system with renormalized velocity and diffusivity and (3) numerical identification of the Green's Function for the full, time-periodic advection-diffusion equation. 



Wednesday, May 14th:

Interferometric UV Lithography: From Optical Fibers to Biomembranes
Anup Sharma

Physics Department
Alabama A&M University

Abstract: Interferometric Lithography is used to produce sub-micron scale periodic structures in glass optical fibers as well as biomembranes and other substrates of biological interest. Normal as well as slanted Bragg-gratings fabricated by this technique within the core of optical fibers are fabricated and used for health monitoring/distributed sensing in carbon-polymer composite structures. Interferometric UV lithography is also used to produce microarrays on phospholipids bilayer membranes as well as other polymer substrates. Phospholipids are one of the major constituents of biological cell membranes and lipidic films have found extensive uses as simple models of cell membranes. Model biomembranes are known to be excellent platforms for biosensing.

The experiments described here were motivated by the promise of interferometric techniques to extend membrane photolithography to produce nanometer scale features. Holographic gratings and microarrays are recorded in azo-dye (NBD)-labeled phospholipid thin films using 244 nm UV light. Diffraction efficiency of these gratings shows extreme sensitivity to humidity and can increase reversibly by two orders of magnitude in air, which is saturated with water vapor. This effect is related to the unique characteristics of phospholipid molecules to undergo hydration-dependent structural reorganizations and self-assembly. Using established techniques, diffraction characteristics of the membrane-grating can be made sensitive to molecules recognized by a biological probe immobilized on the bilayer. Interferometric lithography can give much finer engineering of biomembranes than can be accomplished with use of masks. This has the potential for high-density biosensor designs.



Wednesday, May 7th:

Dynamics of Coupled Cellular Oscillators 
Premananda Indic

University of Massachusetts Medical School
Worcester, Massachusetts

Abstract: I will discuss how circadian rhythms are generated in the mammalian brain, how such rhythms can break down in certain pathological conditions, and new methods for quantifying and restoring circadian rhythmicity. I will show that biological tissues composed of a population of coupled cellular oscillators can exhibit a variety of complex rhythmic behaviors. Based on theory of coupled oscillators we derive conditions for collective synchronization and desynchronized states of a network and apply this theoretical formulation to understand the dynamics of cellular oscillators in the suprachiasmatic nucleus – a circadian clock located in the hypothalamus of the mammalian brain. Our analysis reveals different salient characteristics of the biological clock that can help to understand the control of circadian rhythmicity and its restoration in pathological conditions such as jet lag and disorders of the sleep/wake cycle.



Wednesday, April 30th:

Plasmonic Enhancement of Spontaneous Emission Efficiency
Greg Sun
University of Massachusetts, Boston

Abstract: A great deal of interest has been directed towards the issue of using surface plasmons for enhancing the efficiency of spontaneous emission (fluorescence). With all the results reported, it is not easy to predict what degree of improvement one can obtain in a given material system – there are too many parameters influencing the overall luminescence efficiency and it is far from clear whether the schemes involved in experiments are actually optimal for the given emitters and collection optics. We have developed a comprehensive theory aimed at answering two simple questions: what is the optimum configuration of the surface Plasmon structure for an emitter with a given radiative efficiency in a given light-collection geometry, and what, if any, is the improvement that such apparatus will offer? In this talk, I will present results of optimal plasmonic enhancement that is provided by either a metal sheet or an array of metal nanoparticles that are deposited on an emitting device.



Wednesday, April 23rd:

Magnetic and Magnetoelectric Properties of Jahn-Teller Crystals
Michael D. Kaplan
Simmons College, Boston

Abstract: I will start with a brief introduction to the use of spin chains for quantum communication. Following this, I will describe some methods to perfect this communication using certain reasonbable strategies when one uses chains in their ferromagnetic ground state. I will then briefly point out the use of antiferromagnetic spin chains for the same purpose. Next, I will discuss the use of bosons hopping freely in a one dimensional lattice to generate entanglement between the ends of the lattice. I will also describe the generation of entanglement from the ground state of a chain of qudits coupled by purely exchange interactions.



Wednesday, April 2nd:

Quantum Communication and Entanglement Distribution Through
Spin Chains and Allied Systems

Sougato Bose
University College, London

Abstract: I will start with a brief introduction to the use of spin chains for quantum communication. Following this, I will describe some methods to perfect this communication using certain reasonbable strategies when one uses chains in their ferromagnetic ground state. I will then briefly point out the use of antiferromagnetic spin chains for the same purpose. Next, I will discuss the use of bosons hopping freely in a one dimensional lattice to generate entanglement between the ends of the lattice. I will also describe the generation of entanglement from the ground state of a chain of qudits coupled by purely exchange interactions.



Wednesday, March 12th:

What's New in Quantum Nano-Electro-Mechanical Systems*
Kurt Jacobs
University of Massachusetts, Boston

Abstract: I'll present highlights from the recent Gordon conference on Quantum NEMS.
(*The title is a statement, not a question!) 



Wednesday, March 5th:

Plasmonic-Electronic Transduction
Robert Peale
University of Central Florida

Abstract: Nanophotonics is an active research area in which bound and tightly-confined electromagnetic waves on metallic surfaces, so called surface plasmons, play a major role. Plasmonic integrated circuits have been suggested, but with plasmons excited and detected using free-space radiation that requires bulky external optics. Direct electronic generation and detection of plasmons on surfaces and in two dimensional conductors would eliminate the need for external optics. Interaction between plasmons and electron transport is known at long-wave IR (LWIR) and THz frequencies, but not in the optical range where most nanophotonics research is concentrated. Hence, a new set of materials and devices must be developed for efficient generation, confinement, propagation, and detection of 2D low-frequency plasmons. Our investigations on the direct interactions between 2D plasmons and electron transport, relevant to future plasmonic transceivers and networks, will be presented.



Wednesday, February 13th:

Thermalisation and its Mechanism in Closed Quantum Systems
Maxim Olchanii
University of Massachusetts, Boston

Abstract: Time dynamics of isolated many-body quantum systems has long been an elusive subject, perhaps most urgently needed in the foundations of quantum statistical mechanics. Only very recently experimentalists have begun performing detailed studies of this matter. In generic systems, one expects the nonequilibrium dynamics to lead to thermalization: a relaxation to states where the values of macroscopic quantities are stationary, universal with respect to widely differing initial conditions, and predictable through the time-tested recipe of statistical mechanics. The relaxation mechanism is not obvious, however; for example, dynamical chaos cannot play the key role as it does in classical systems since quantum evolution is linear. That new rules could apply to isolated quantum systems was underscored by recent studies suggesting that statistical mechanics may give wrong predictions for their relaxation. Here we demonstrate that a generic quantum many-body system does relax to a state well-described by standard statistical mechanical prescription. Moreover, we show that time evolution itself plays a merely auxiliary role and that thermalization happens instead at the level of individual eigenstates, as first proposed by M. Srednicki. Due to this eigenstate thermalization scenario, thermalization joins the list of processes that, like scattering, seem intrinsically temporal, and yet in quantum mechanics effectively become time-independent problems.
 

Physics and Engineering Seminar Series - Fall 07

(All talks are at 1:00 PM in Room S-3-126 unless otherwise noted.)

Wednesday, November 28th:

Random Waves and Quantum Statistical Mechanics
Eric J. Heller
Harvard University

Abstract: The Berry hypothesis. that  eigenstates of classically ergodic  systems are locally given by random superpositions of plane waves of fixed kinetic energy may be an alternative formulation of quantum statistical mechanics.  By systematically incorporating boundary conditions as constraints we move closer to the question of statistical mechanics, which must deal with constraints.  We derive the quantum version of the equivalence of ensembles in terms of unfamiliar asymptotic limits of Bessel functions. In the end, the ambiguity of whether the Berry hypothesis is a prescription for quantum statistical mechanics comes down to the question of degrees of freedom which are frozen out by quantization and finite temperature. However quantum statistical mechanics has no statistical answer for this either - one must solve the Schroedinger equation.



Wednesday, November 21st:

Cold Atoms and Out of Equilibrium Quantum Dynamics
Anatoli Polkonikov

Boston University

Abstract: Recent experimental progress in cold atom physics allows us to get nearly isolated many-particle systems with tunable interactions. This opens new possibilities to experimentally study quantum dynamics far from equilibrium. In particular, one can now address such fundamental problems of condensed matter physics as thermalization, role of integrability, decoherence and quantum measurements in many-body interacting systems, etc. Such experiments also motivate a lot of theoretical attention to these problems.

In this talk I will first review some recent experiments in quantum dynamics of cold atoms. Then I will concentrate on a particular example of slow (adiabatic) dynamics and will discuss how quantum adiabatic theorem is related to the thermodynamic adiabatic theorem of statistical physics. I will argue that in low-dimensional gapless systems the thermodynamic adiabatic theorem can fail in two different ways. I will illustrate general statements with specific examples of tuning a system across a quantum phase point and slow dynamics in weakly interacting superfluids. If time permits, I will also describe another example of slow dynamics in moving superfluids. I will present a non-equilibrium phase diagram and show that it qualitatively depends on the dimensionality of the system. The predictions of this work are in excellent agreement with recent experiments.



Wednesday, November 14th:


Matter-waves Coherence Loss and Revival Induced by an
Atom-optics Kicked Rotor

Alexya Tonyushkin

Harvard University

Abstract: We implemented an atomic kicked rotor in a de Broglie wave interferometer. The dynamics of the matter-wave's coherence is investigated by looking at the echo signal of the interferometer. The overlap of a reference interferometer signal and its perturbed version is analogous to a fidelity amplitude measurement of our interferometer. We directly observed that at quantum resonances the fidelity decay saturates at a finite value after just a small number of kicks. We discuss the applications of the scheme to high precision measurements and quantum simulations.



Wednesday, November 7th:


Supersolid State of Matter
Boris Svistunov
University of Massachusetts – Amherst

Abstract: The discovery of the phenomenon of supersolidity in helium, a non-dissipative transport of He-4 atoms through He-4 crystals, is an amazing recent achievement in the low-temperature physics.
While some of the mechanisms of supersolidity in He-4, such as superfluid grain boundaries and superfluid screw dislocations, are now established at the level of ab initio simulations, a microscopic interpretation of the phenomenon of non-classical rotational inertia originally observed by Kim and Chan is still lacking, and actually gets even more controversial in light of recent experiments by other groups.



Wednesday, October 31st:
Time: 4 pm
Left handed light
Srinivas Sridhar
Northeastern University

Abstract: We discuss negative refraction at microwave and optical frequencies in 1D and 2D metallic and dielectric photonic crystal media. Here negative refraction is due to the anomalous dispersion characteristics of the medium. In negative index media, the fields and the wavevector obey a left-handed relationship. The experiments show that materials with tailor-made negative or positive refractive indices over broad spectral ranges can be designed and fabricated. We have also demonstrated that negative refraction leads to some novel optical elements for imaging, such as flat lenses and focusing by plano concave lenses. A general theory of imaging by a flat lens without optical axis has been developed, leading to specification of the characteristics required of the flat lens metamaterial. Features of images formed using negatively refracting optical elements, including sub-wavelength resolution, are discussed. Potential applications for imaging and communications at microwave and optical frequencies are discussed.

Work supported by the National Science Foundation and the Air Force Research Laboratories, Hanscom.



Wednesday, October 24th:

New Frontiers in Trapping of Ultra-Cold Atoms and Molecules
Mark G. Raizen
University of Texas at Austin

Abstract: The method of laser cooling has opened the door to low temperature physics of dilute gases.  Despite the great success of this method, it has been limited to a small set of atoms in the periodic table. I will describe in this talk new approaches to trapping of atoms and molecules that do not require lasers and are hence applicable to most atoms.  I will then describe how the trapped atoms can be further cooled in an experimental realization of Maxwell's demon.  Applications of these methods to atom optics and to fundamental tests will be described.



Wednesday, October 17th:

Quantum Simulation
Markus Greiner
Harvard University

Abstract: Recent advances in ultracold atom research have opened a new research field in which fundamental questions of modern condensed matter physics can be addressed with ultracold quantum gases. I will discuss such quantum simulations of condensed matter systems, present our concept of a quantum gas microscope and report about first results with our new apparatus.



Thursday, October 11th:

Monte Carlo Simulations of Transport Properties in Nanostructures
Rob Kelsall
The University of Leeds


Abstract:  Stochastic (Monte Carlo) methods are well suited to the simulation of particle trajectories in a diverse range of media.  This approach offers numerous advantages over continuum models:  a high degree of microscopic physical detail can be obtained; non-equilibrium transport phenomena can be treated without approximation, and transient responses can be obtained without significantly extra computational effort.  In this presentation, the application of Monte Carlo methods to a range of nanostructured systems will be described, including simulations of electronic transport in multilayer semiconductor devices, optoelectronic devices, and conducting organic layers.  Results from multiphysics algorithms, in which Monte Carlo trajectory simulations are self-consistently coupled with electrostatic (Poisson) and thermal (heat diffusion) solutions, will be presented.   Application of Monte Carlo methods to phonon transport simulations will also be discussed.



Wednesday, October 10th:

How do we know what they know?
Arthur Eisenkraft
COSMIC, University of Massachusetts, Boston

Abstract: Assessment is a crucial component of good instruction.  Advances in the cognitive and measurement sciences allow us to take a fresh look at the way in which we assess student understanding.  Theoretical approaches to assessment can inform practical applications in the classroom to enhance student performance and success in courses.  A review of recent literature and some concrete recommendations for course assessments will be discussed.



Wednesday, October 3rd:

Dimensionality and dynamics in the behavior of C. elegans
Greg Stephens
Princeton University

Abstract: A major challenge in analyzing animal behavior is to discover some underlying simplicity in complex motor actions.  Here we show that the space of shapes adopted by the nematode C. elegans is surprisingly low dimensional, with just four dimensions accounting for 95% of the shape variance, and we partially reconstruct `equations of motion’ for the dynamics in this space. These dynamics have multiple attractors, and we find that the worm visits these in a rapid and almost completely deterministic response to weak thermal stimuli.  Stimulus-dependent correlations among the different modes suggest that one can generate more reliable behaviors by synchronizing stimuli to the state of the worm in shape space. We confirm this prediction, effectively ``steering’’ the worm in real time.



Wednesday, September 26th:

WKB Propagation of Gaussian Wavepackets
Raul Vallejos
Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, Brasil


Abstract: We will analyze the semiclassical evolution of a Gaussian wavepacket in a chaotic closed system using standard time-dependent WKB theory (no complex trajectories). We will show that the Wigner function develops the structure of a classical filament plus quantum oscillations, with phase and amplitude being determined by geometric properties of an evolving classical manifold. We also will discuss the extension of the theory to open systems in the Lindbladian context.



Wednesday, September 19th:

Thermalization in closed quantum systems: open problems and suggested solutions
Maxim Olchanii
University of Massachusetts, Boston

Abstract: I will begin with a list of open questions on the foundations of statistical mechanics of closed quantum systerms.  Among them are: (1) how can linear quantum dynamics provide chaos necessary for thermalization, and (2) which constraints should be used in building an appropriate thermodynamical ensemble, given that in a quantum system with nondegenerate spectra all integrals of motion are functionally dependent on the Hamiltonian?

To answer these questions, we perform an ab initio numerical analysis of a system of hard-core bosons on a lattice and show that these controversies can be resolved via the Eigenstate Thermalization Hypothesis [Srednicki, 1994] and its further generalizations for integrable systems.  According to this hypothesis, in quantum systems thermalization happens at the level of individual eigenstates, but hidden initially by coherences between them.

In course of time evolution the thermal properties become revealed through (linear) decoherence.  This answeres the question (1). Our numerical experiemnts with integrable lattices partially resolve the controversy (2), indicating that dunctional independence between the integrals of motion plays no role in quantum thermodynamics and should probably be superseded by functional independence between their quantum expectation values.