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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.
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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.
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|
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.
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|
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.
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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.
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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.
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