Physics and Engineering Seminar Series - Fall 09

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

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

Day change: Monday, May 18th:

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

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:

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:

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:

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:

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:

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:

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:

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:

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.

Wednesday, March 4th:

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

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, k_^BT 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:

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

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:

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

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:

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

Anthropic Predictions in the Landscape
Delia Schwartz-Perlov
Tufts

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

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

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

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

Subcellular Surgery and Nanosurgery
Eric Mazur
Harvard

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

Dynamically Error-Corrected Gates for Accurate
Quantum Control and Computation
Lorenza Viola
Dartmouth College

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

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

Confinement-Induced Resonances
Vanja Dunjco
UMass Boston

Physics and Engineering Seminar Series -Spring 08

Interferometric UV Lithography: From Optical Fibers to Biomembranes
Anup Sharma
Physics Department
Alabama A&M University

Dynamics of Coupled Cellular Oscillators 
Premananda Indic
University of Massachusetts Medical School
Worcester, Massachusetts

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

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

Quantum Communication and Entanglement Distribution Through
Spin Chains and Allied Systems
Sougato Bose
University College, London

Physics and Engineering Seminar Series - Fall 07

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

Cold Atoms and Out of Equilibrium Quantum Dynamics
Anatoli Polkonikov
Boston University

Matter-waves Coherence Loss and Revival Induced by an
Atom-optics Kicked Rotor
Alexya Tonyushkin
Harvard University

Left handed light
Srinivas Sridhar
Northeastern University

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

Quantum Simulation
Markus Greiner
Harvard University

Rob Kelsall
The University of Leeds

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

University of Massachusetts, Boston