Seminars
Spring 2012 Seminars
All Physics Seminars are held in S-3-126 at 1:00pm unless otherwise noted
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February 1
Indubala Satija
George Mason University and National Institute of Standard and Technology
Topological Insulators
Abstract: Topological insulators are unconventional states of matter that are insulating in the bulk but conduct along the boundaries. Quantum Hall and its close cousin, quantum spin Hall states are two prime examples of such states that arise even in systems of non-interacting fermions. Quarter century since the discovery of quantum Hall effect, the subject of topological insulators has reemerged as a new frontier, discovering novelties in the single particle band theory and providing new impetus to constantly evolving quantum many body physics of strongly correlated systems. Ultracold atoms due to their unprecedented controllability, are versatile tool to explore such exotic states of matter. After introducing the subject of topological insulators, I will discuss a rather counterintuitive result that quantum Hall states can be detected in momentum distribution of atoms, an everyday observable in cold atom laboratories [1]. I will present a simple scheme for creating quantum spin Hall states using SU(2) gauge fields [2] in cold atom systems and show a variety of novel quantum phase transitions between topological and normal insulating phases. My talk will also touch upon the subject of incommensurate flux where a a novel manifestation of topology emerges in the spatial profiles of the states as topology gets intertwined with self-similar spatial pattern encoding topological fingerprints at all length scales.
References
[1] "Chern Numbers hiding in Time of Flight Images", Zhao, Bray-Ali, Williams, Spielman and Satija, Phys Rev A, 84, 063629, (2011).
[2] "Realistic Time-Reversal Invariant Topological Insulators with Neutral Atoms", Goldman, Satija, Nikolic, Bermudez, Martin-Delgado, Lewenstein, Spielman, Phys Rev Lett, 105, 255302 (2010).
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February 8
(Three 20 minute talks)
Stephen Choi
UMass Boston
Optimized sympathetic cooling of atomic mixtures via fast adiabatic strategies
Abstract: The talk will explore the extent to which frictionless cooling techniques may be useful in sympathetic cooling of Fermi gases. It is argued that optimal cooling of an atomic species may be obtained by means of sympathetic cooling with another species whose trapping frequency is dynamically changed to maintain constancy of the Lewis-Riesenfeld adiabatic invariant, which in turn determines the temporal-profile of the changing frequency. An important motivating factor is that an usually undesired feature of these techniques, i.e., the fact that the atomic cloud does not increase its phase-space density and therefore its degeneracy, turns into a crucial asset when viewed from the perspective of maintaining the gas in the nondegenerate regime, thus making it an optimal coolant. Advantages and limitations of this cooling strategy are discussed, with particular regard to the possibility of cooling Fermi gases to a deeper degenerate regime. We also show that the links between the suggested method and quantum squeezing.
Xiaoting Wang
UMass Boston
Ultraefficient Cooling of Resonators: Beating Sideband Cooling with
Quantum Control
Abstract: There is presently a great deal of interest in cooling high-frequency micro- and nano-mechanical oscillators to their ground states. The present state of the art in cooling mechanical resonators is a version of sideband cooling, which was originally developed in the context of cooling trapped ions. Here we present a method based on quantum control that uses the same configuration as sideband cooling-coupling the resonator to be cooled to a second microwave (or optical) auxiliary resonator-but will cool significantly colder. This is achieved by applying optimal control and varying the strength of the coupling between the two resonators over a time on the order of the period of the mechanical resonator. As part of our analysis, we also obtain a method for fast, high-fidelity quantum information transfer between resonators.
Sai Vinjanampathy
UMass Boston
The approach to typicality in many-body quantum systems
Abstract: The recent discovery that for large Hilbert spaces, almost all (that is, typical) Hamiltonians have eigenstates that place small subsystems in thermal equilibrium, has shed much light on the origins of irreversibility and thermalization. Here we present numerical evidence that many-body lattice systems generically approach typicality as the number of subsystems is increased, and thus provide further support for the eigenstate thermalization hypothesis. We will present our results that indicate that the deviation of many-body systems from typicality scales as an inverse power of the number of systems, and we compare this with the equivalent scaling for random Hamiltonians.
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February 15
(two talks: 20min- and 35min-long)
Zaijong Hwang
UMass Boston
Origins of bright soliton transparency to Bogoliubov quasi-particles
Abstract: Bogoliubov quasi-particles can pass through a one-dimensional bright soliton without reflection at all energies[D. J. Kaup, Phys. Rev. A {\bf 42}, 5689 (1990)]. Reflectionless properties of this kind usually originate from a supersymmetric structure of the corresponding Hamiltonian[E. Witten, Nucl. Phys. B {\bf 188}, 513 (1981); C. V. Sukumar, J. Phys. A {\bf 18}, 2917 (1985)]. However, we give a strong indication that in this case, the mathematical mechanism enabling full spectrum transparency of a scattering object does not fall into any of the conventional paradigms.
Maxim Olshanii
UMass Boston
Integrability versus Thermalizability in Isolated Quantum Systems: a Simple Relevant Geometric Structure in the Space of Quantum Observables
Abstract:The purpose of this presentation is to assess the status of our understanding of the transition from integrability to thermalizability in isolated quantum systems. In Classical Mechanics, the boundary stripe between the two is relatively sharp: its integrability edge is marked by the appearance of finite Lyapunov's exponents that further converge to a unique value when the ergodicity edge is reached. Classical ergodicity is a universal property: if a system is ergodic, then every observable attains its microcanonical value in the infinite time average over the trajectory. On the contrary, in Quantum Mechanics, Lyapunov's exponents are always zero. Furthermore, since quantum dynamics necessarily invokes coherent superpositions of eigenstates of different energy, projectors to the eigenstates become more relevant; those in turn never thermalize. All of the above indicates that in quantum many-body systems, (a) the integrability-thermalizability transition is smooth, and (b) the degree of thermalizability is not absolute like in classical mechanics, but it is relative to the class of observables of interest. In accordance with these observations, we propose a concrete measure of the degree of quantum thermalizability, consistent with the expected empirical manifestations of it. As a practical application of this measure, we devise a unified recipe for choosing an optimal set of conserved quantities to govern the after-relaxation values of observables, in both integrable quantum systems and in quantum systems in between integrable and thermalizable.
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Friday February 24 *unusual day*
Eric R. May
U Michigan
Two Sides of the Same Coin: Virus Maturation from Multiple Perspectives
Abstract: In this talk, I will discuss multiscale computational approaches to understand the large-scale conformational and morphological changes, which viral capsids undergo as part of their viral life cycle. The primary motivation has been to understand the maturation process of the bacteriophage HK97, during which the virus transitions from a non-infectious spherical particle into an infectious particle with a faceted, polyhedral shape. We have approached this problem from both the mechanical (continuum elasticity) and thermodynamic (energy landscapes) viewpoints, hence the multiple perspectives. During the mechanical part of the talk, I will discuss our theoretical and methodological advances in computing elastic properties of viral capsids from equilibrium fluctuations, and discuss the relationship between these calculations and experimental measurements. I will also discuss how the shape behavior of capsids can be understood in terms of a buckling transition for a thin shell. I will move on to non-equilibrium studies and discuss dynamical aspects of the HK97 maturation transition, including symmetry-breaking features and the underlying free-energy landscape governing these dynamics. By combining these viewpoints and employing a variety of simulation techniques at different resolution levels, one can begin to obtain a thorough understanding of viral processes, which otherwise would not be accessible through straight-forward simulation methods.
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February 29
No Colloquium: March Meeting
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Monday March 5 *unusual day*
Rahul Kulkarni
Virginia Polytechnic
Stochastic modeling of gene expression and its regulation by small RNAs
Abstract: One of the fundamental problems in biology is understanding how phenotypic variations arise in individuals. Phenotypic variation is generally attributed to genetic or environmental factors. However, in several important cases, phenotypic variations are observed even among genetically identical cells in homogeneous environments. Recent research indicates that such `non-genetic individuality' can arise due to intrinsic stochasticity in the process of gene expression. Correspondingly there is a need to develop a framework for quantitative modeling of stochastic gene expression and its regulation. Of particular interest is modeling of regulation by small RNAs, which is often a critical component of cellular processes such as development, differentiation and cancer.
In this talk, I will discuss approaches developed by my group that lead to new analytical results for stochastic models of gene expression. In biologically relevant limits, we develop a mapping to queueing theory to derive exact results for general models of stochastic gene expression. Focusing on specific regulatory mechanisms, we propose and analyze a comprehensive model for regulation by small RNAs. The results obtained provide new insights into the role of small RNAs in fine-tuning the noise in gene expression. I will conclude with a discussion of future research projects building on the framework developed.
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March 7
Yang-Yu Liu
Northeastern U
Taming Complexity --- The mathematics of network control
Abstract: The ultimate proof of our understanding of natural or technological systems is reflected in our ability to control them. Although control theory offers mathematical tools for steering engineered and natural systems towards a desired state, a framework to control complex self-organized systems is lacking. Here we develop analytical tools to study the controllability of an arbitrary complex directed network, identifying the set of driver nodes with time-dependent control that can guide the system’s entire dynamics. We apply these tools to several real networks, finding that the number of driver nodes is determined mainly by the network’s degree distribution. We show that sparse inhomogeneous networks, which emerge in many real complex systems, are the most difficult to control, but that dense and homogeneous networks can be controlled using a few driver nodes. Counterintuitively, we find that in both model and real systems the driver nodes tend to avoid the high-degree nodes.
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Friday March 9 *unusual day*
Tao Hu
Howard Hughes Medical Institute
Non-linear Predictive Coding as a Model of Early Sensory Processing
Abstract: Early stages of sensory systems face the challenge of compressing information from a large number of receptors onto a much smaller number of projection neurons, a so called communication bottleneck. To make more efficient use of limited bandwidth, compression may be achieved using predictive coding, whereby predictable, or redundant, components of the stimulus are removed. In the case of the retina, Srinivasan et al. (1982) suggested that feedforward subtraction of a linear prediction generated from nearby receptors implements such compression, resulting in biphasic center-surround receptive fields. However, inhibition often operates in a feedback manner and with non-linear input output transformations, considerably complicating the dynamics of such circuits. Here, we solve the transient non-linear recurrent dynamics of a generic early sensory circuit in response to a step-like stimulus. We show that interneuron activity in time constructs progressively less sparse but more accurate representations of the stimulus, thus providing a powerful theoretical framework to understand the dynamics of early sensory processing in a variety of physiological experiments. More generally, our results demonstrate that highly non-trivial computations, at the forefront of modern signal processing, can be mapped onto a concrete neuronal circuit.
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March 14
Spring Break
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March 21
Karl K Berggren
MIT
Counting Single Photons with Superconducting Nanostructures
Abstract: Superconducting nanowire single-photon detectors are promising candidates for single-photon detection systems that require high efficiency, low noise, good timing accuracy, and sensitivity across a wide range of optical wavelengths. They may enable applications in quantum optics, laser radar, space communications, and integrated-circuit evaluation. However, these detectors also have interesting electrical, thermal, and optical properties at the nanoscale that influence their performance, and can be exploited to improve the device efficiency, particularly in the infrared. In this seminar, we will describe the device operation, what we know of the device physics, and describe how these devices can be used in applications. Examples will include a variant of a new device architecture that provides improved infrared sensitivity and readout signal to noise ratio, as well as a nano-antenna design that enhances the optical absorptance of the device.
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March 28
Ioannis Kominis
U Crete
A Bird's Eye View of Spin Coherence
Abstract:Radical-ion pair reactions play a fundamental role in photosynthesis and are also understood to underlie the avian magnetic compass mechanism. It was recently shown that these biochemical reactions exhibit a host of quantum coherence phenomena, thus providing a promising and very exciting link between biological systems and quantum information science. We will review the recent progress in understanding the fundamental quantum dynamics of radical-ion-pair reactions and elaborate on the possible repercussions of the new findings for the rapidly emerging field of quantum biology.
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April 4
Emanuele Dalla Torre
Harvard U
Noisy quantum phase transitions: an intuitive approach
Abstract: Equilibrium thermal noise always destroys the scale invariance of quantum critical points, by the introduction of a finite thermal length. What are the effects of non-equilibrium scale-invariant noise? In two recent papers[1-2] we have considered the specific case of 1/f charge noise by means of a novel analytic real-time RG approach. In this talk I will present the model, show its relevance to experiments with solid state devices and cold atomic systems, and describe the RG results. In addition, I will explain how these results can be understood by simpler arguments based on circuit theory and fluctuation-dissipation relations.
REFERENCES:
[1] E.G. Dalla Torre, E. Demler, T. Giamarchi, E. Altman, ``Quantum critical states and phase transitions in the presence of non-equilibrium noise'', Nature Physics 6, 806 (2010)
[2] E.G. Dalla Torre, et al. ``Dynamics and universality in noise driven dissipative systems'', arXiv: 1110.3678 (PRB 2012)
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April 11
Douglas Mason
Harvard U
Extending the Flux Operator Using Husimi Projections
Abstract: A common tool among physics in the transport community is the probability flux operator, but connecting this operator to measurement has not received much attention. We have extended the definition of flux by using coherent states, rendering it both measurable and infinitely more useful. For instance, we can use our extended definition to study closed systems and the classical dynamics of individual quantum states, and then connect these dynamics to resonant states interacting with an environment. This analytical technique, based on semi-classics, brings new tools to bear on wavefunction analysis and visualization.
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April 18
Bala Sundaram
UMass Boston
Persistent Patterns and Mixing in Mixed Phase Spaces
Abstract: In fluid dynamics the term advection means the transport of material by the flow and the role of chaotic dynamics in assisting the mixing or homogenization process is of broad interest with a wide range of applications. These dynamics provide a mechanism to produce concentration gradients on finer scales which are then readily removed through a diffusive mechanism. However, anomalies in this process were predicted and experimentally reported in periodically stirred dynamics where patterns are seen to persist, despite stirring, for very long times. The talk will discuss a framework for the emergence of these patterns in terms of a simple measure. Within this description, scaling laws related both to the formation and persistence of patterns are shown to emerge. These are correlated with the details of the Lagrangian or particle dynamics; in particular, the non-uniform hyperbolicity of the phase space. A modified Floquet analysis is also used to address this issue from a spectral perspective. Finally, if time permits, the fact that in even dimensions the fluid problem is identical to classical probability density dynamics will be used to link our results to those in problems in quantum chaos.
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April 25
Luca D'Alessio
Boston University
Phase Transitions in repeatedly driven interacting systems: aperiodic vs periodic driving
Abstract: When we do work on a system, we transfer energy. Repeatedly adding a small amount of energy to the system by a cyclic process can result in a large energy transfer and cause the system to undergo a phase transition. I will show that: 1) when the driving is aperiodic or periodic with period longer than the relaxation time, the final energy distribution of the system displays two qualitatively different regimes with a continuous second order like transition between them. 2) When the driving is periodic with period shorter than the relaxation time new physics emerge. The system can display two new types of phase transition each one related to the breakdown of a particular perturbation theory.
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May 2
Suzanne Pittman
Harvard U
Few-body Systems Under Harmonic Confinement with Disorder
Abstract: We study the effect of single-body disorder on few interacting particle systems subject to isotropic harmonic confinement. By approximating disorder with a single-body random potential described by a GOE Random Matrix and interparticle interactions with a s-wave pseudo-potential, we analyze the degree of mixing realized in systems composed of bosons, fermions, and distinguishable particles. We specifically calculate the system's time-average flow and energy exchange between particles. Using these measures, we find that the degree of localization depends on the
relative strength of the random potential relative to the interparticle interaction, which can best be understood through the competing time scales of each process and the system's breaktime.
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May 9
Chris Amey
UMass Boston
The Influence of Lagrangian Structures on Fluid Mixing
Abstract: We look at the behavior of two-dimensional flows described by the advection diffusion equation, where the underlying advection is described by classical Lagrangian dynamics. While fluid systems corresponding to uniformly chaotic or completely regular phase spaces are well understood, mixed phase space systems have non-uniform Lagrangian structures which complicate their behavior. We show that the dominant behavior in mixed phase space systems depends on competition between diffusion and local stable structures. We also show that, in the small diffusivity limit, mixed systems exhibit universal diffusive scaling.
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Fall 2011 Seminars
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September 21
Mikhail Zvonarev
Harvard University
Mobile impurity dynamics in one-dimensional quantum gas
Abstract: The talk is about the dynamics of the quantum particle (impurity) immersed into the sea of quantum particles of different type (hosts) in one spatial dimension (1D). A necessity of the collision events for the particles to exchange their positions in the 1D geometry favors the transfer of the impurity momentum to the hosts. Other specific feature of 1D, integrability, does the opposite. Which of these effects wins is an open question we address using the combination of Bethe Ansatz, Luttinger Liquid, and numerics. We focus on the range of parameters relevant for Cambridge and Innsbruck experiments.
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September 28
Lincoln Carr
Colorado School of Mines
The Nonlinear Dirac Equation
Abstract: A very famous equation that describes, for example, both atomic Bose-Einstein condensates (BECs) and light in fiber optics, is the nonlinear Schrodinger equation (NLSE). We present the relativistic generalization of the NLSE, the Nonlinear Dirac Equation (NLDE). The NLDE can be realized in atomic BECs in a honeycomb optical lattice, utilizing the Dirac point in the band structure, just like graphene. The effective "speed of light" in graphene is the Fermi velocity ~ c/300, where c is the speed of light in a vacuum. In atomic BECs the effective speed of light is mm to cm per second, 10 orders of magnitude slower than c! Moreover, we show that the nonlinearity in the NLDE gives rise to a zoo of vortices and solitons; the analog of such solutions in everyday experience are hurricanes and tsunamis, respectively. In the NLDE, unlike in the ocean or the earth's atmosphere, phase as well as amplitude is key to supporting vortices and solitons, and there are even more exotic solutions such as skyrmions and half-quantum vortices. Lastly, we touch briefly on a more advanced topic, the relativistic linear stability equations, the relativistic generalization of the Boguliubov-de Gennes equations used to describe lowest order quantum fluctuations around the mean field of a BEC.
References:
L. H. Haddad and L. D. Carr, "Relativistic linear stability equations for the nonlinear Dirac equation in Bose-Einstein condensates," Europhys. Lett. v. 94, p. 56002 (2011)
L. H. Haddad and L. D. Carr, "The Nonlinear Dirac Equation in Bose-Einstein Condensates: Foundation and Symmetries," Physica D: Nonlinear Phenomena, v. 238, p. 1413 (2009)
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October 05
David Pritchard
Green Professor Physics, MIT;
Developer of Mastering Physics
What Are Students Learning, From What Activity, and Is That OK?
Abstract: What knowledge are students learning? What instructional activities are they learning it from? What student habits are helpful or detrimental to learning? What learning do they remember at graduation? We must be able to answer these questions to have an informed discussion about our educational process. Our recent research, including educational data mining, begins to answer these questions. I will also describe a software tutor that really helps students learn.
Finally I shall address a key question - what do we really want students to learn? Given that most teachers want students to become more expert-like, I will describe a pedagogy, “Modeling Applied to Problem Solving” that has achieved this result. These studies were done in introductory college physics.
For a preview of the research see http://relate.mit.edu
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October 12
Vladan Vuletic
MIT
Squeezing quantum noise to improve precision measurements
Abstract: Atomic clocks, the most sensitive instruments ever made by mankind, are now approaching a mindboggling absolute stability of 10-17. The performance of the best instruments is limited by the quantum noise in the final readout measurement of the clock, a situation referred to as the standard quantum limit. This limit arises from the projection postulate when applied to an ensemble of independent particles, i.e. it arises from single-particle quantum mechanics. I will discuss how quantum mechanically correlated (entangled) states of the many-body system can be used to overcome the standard quantum limit, and how we generate such states in an ensemble of laser-cooled atoms using light.
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October 19
Martin Zwierlein
MIT
A Little Big Bang: Ultra-cold Fermi Gases at the Quantum Limit
Abstract: TBA
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October 26
Susanne Yelin
ITAMP, Harvard-Smithsonian Center for Astrophysics
Nonlinear Quantum Optics in Interacting Media
Abstract: Polar molecules have commanded a lot of interest lately, now existing at ultra-cold temperatures and being hailed as the next best option for quantum information and simulation. I will, in particular, discuss their use for single-photon nonlinear optics, thus using their interaction properties. The same interaction, dipole-dipole interaction, also lies at the heart of superradiant phenomena. Applications of superradiance include diverse systems as polar molecules, Rydberg atoms, and solid state materials.
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November 02
Charles Keese
Applied BioPhysics, Inc.
The use an impedance-based biosensor to monitor cell behavior
Abstract: In Electric Cell-substrate Impedance Sensing (ECIS) cells are cultured upon small gold electrodes whose impedance is measured with a weak AC signal. When cells attach and spread on these electrodes, their insulating membranes constrain the current, forcing it to flow beneath and between the cells. This results in impedance changes that can be readily measured and used to quantify cell behavior. The importance of measuring complex impedance at different AC frequencies from 0.1 to 100 kHz to understand changes in cell behavior will be discussed.
Data from the biosensor will also be presented showing its ability to measure the dynamics of cell-surface interactions, cell motility and the permeability of cell layers. Another feature to be described is the instrument’s ability to apply invasive electric fields to either introduce membrane impermeable compounds or to electrically kill cells to accomplish an automatic cell migration measurements.
The ECIS analytical approach for cell biology offers scientists an alternative to the microscope for acquiring data regarding cell behavior in real time.
Charles R. Keese is President and cofounder of Applied BioPhysics, Inc. While he was a staff scientist at the General Electric Research and Development Center, his studies with Ivar Giaever (1973 Noble Prize in Physics) resulted in the discovery and development of the ECIS technology.
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November 09
Walter Buchwald
Solid State Scientific Corporation, Nashua NH
Quantum Dots from Quantum Wells: Towards Integrated Quantum Optics
Abstract: Essential to the field of quantum optics is the ability to generate, detect and manipulate single photons. In order to move quantum optics research from optical tables to the nano-photonic chip scale, the use of semiconductor quantum dots has often been proposed. Although the traditional methods of quantum dot growth are known to produce high quality three dimensional electron confinement, dot to dot variations in bound state transition energies and the difficulties with the deterministic placement of these dots has historically motivated research into other methods of quantum dot formation. The modification of quantum well systems by either etching, or, through the use of externally applied bias potentials, are typical approaches which offer the benefit of the deterministic placement of a quantum dot but at the expense of either dot quality or complicated fabrication procedures. This talk presents experimental evidence that the inherent surface depletion region, common to all semiconductor systems, has the potential to be utilized for the formation of a deterministically placed quantum dot from a quantum well system. Depending on design, the lateral confinement offered by this surface depletion controlled method is sufficient to offer room temperature operation of this long wavelength system. As will be discussed, this method is expected to find use in integrated quantum optics scenarios where the ease in which such a system could be integrated into waveguides and cavities could be exploited. Other applications will also be discussed.
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November 16
Mario Borunda
Harvard University
Transport in Graphene: Ballistic, Anomalous, or Diffusive?
Abstract: attracted great scientific interest due to its Dirac-like energy spectrum. Its structure consists of a layer of carbon atoms arranged in a honeycomb lattice, which represents a conceptually new class of materials that are only one atom layer thick. Recent experiments achieved extremely high mobilities in suspended graphene leading to ballistic transport in the sample. In this talk, I will first review some general aspects of transport in graphene. We study implications of diffusive and ballistic transport in graphene devices and ask what could be possible signatures of "anomalous" (or superdiffusive) transport. Experimental setups to differentiate between these stochastic processes are discussed.
Research done in collaboration with Holger Hennig & Rick Heller.
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November 23
Venkatesan Renugopalakrishnan, B. Barbiellini, H. L. Tuller, P. Somasundaran, M. Chin, C. King, D. Moser
Harvard, MIT, Columbia, Northeastern Universities, Children's Hospital Boston
Engineering a Robust Artificial Photosynthetic System via Efficient Quantum Coherence
Abstract: At the heart of every photovoltaic device is a mechanism for photon-to-electrical transduction. Ideally, the device should capture light across a broad spectrum, and efficiently transfer this energy to the electrons. Photosynthesis is a collection of the most advanced and efficient systems nature has crafted to convert solar energy into an electrical potential and again into chemical compounds for energy storage. Bio-solar cells (BSSC) and bio-fuel cells represent the emerging frontier in green energy sources. In recent years Renugopalakrishnan et al., 2009 has reported several advances in the feasibility of bacteriorhodopsin (bR) as bio-photosensitizer in excitonic solar cells. His Lab has engineered mutants of bR to enhance thermo-stability and to favor charge separation. There are at least key components which implement the energy transfer step : (a) a hybrid mixture of engineered protein bR and quantum dots (QD) to absorb and trap the photon energy, (b) a thin layer of gold (Au) to produce the ballistic electrons and recycle the dye.
Current DSSC designs have achieved efficiencies of over 10%, but make use of expensive, toxic dyes (e.g. Ru based ) and use a reactive liquid electrolyte, leading to sealing and aging/degradation issues in the design of solar panels. Recently McFarland and Tang, 2003 proposed a design, which removes the need for the liquid electrolyte.. The McFarland-Tang device uses a thin Au layer between the dye and the semiconductor substrate with energy transferred from the dye to Au by direct electron injection, leading to the production of ballistic electrons in the Au. The Au layer also acts as a source of electrons to regenerate the dye, thus playing the role of the liquid electrolyte in earlier designs. McFarland and Tang suggested that electron injection into the TiO2 occurs by ballistic transport through the gold film. The Ti/TiO2/Au/Dye photovoltaic device shows a photovoltage of about 650 mV; the photocurrent is, however, unacceptably low (?A/cm2). This is partly a result of small active area; the light is harvested by only a (sub)-monolayer of dye molecules with limited effort to optimize the dye absorption spectra.
Modifying the McFarland-Tang device
One of the principal objectives of our research is to design the morphology of the BSSC cell in order to increase the active surface and to optimize the effectiveness of charge transfer from bR-QD to Au, and to maximize the production of useful ballistic electrons in the Au layer. Furthermore the McFarland-Tang device uses a traditional dye as the photo-absorber; the second key novelty is the use of bR-QD as the photon energy absorber/trap, Li R..,2007. The main challenge is to alter the electronic, thermal and optical properties of this hybrid system, composed of a mixture of quantum dots (CdSe nanoparticles) and bacteriorhodopsin (bR) placed on a thin layer of Au above a semiconductor substrate (for example titanium dioxide), in order to mimic the high efficiency of natural photosynthetic systems. A fundamental understanding of the interactions at the bio-nano interface [Somasundaran 2009, Audette 2011] is vital for the successful design of the quantum coherent photovoltaic cell.
Integrating biological building blocks with inorganic materials such as CdSe QDs, gold nanofilms and titanium dioxide nanotubes, open new vistas for solar energy technology.
Audette, G.F. et al. (2011). MaterialsToday.
Li R., C. M. Li, H. Bao, Q. Bao, V.S. Lee, (2007). Applied Physics Letters 91: 223901.
McFarland, E. W. and J. Tang (2003). Nature 421: 616.
Thavasi, V. et al.(2009). Journal of Nanoscience and Nanotechnology 9: 1679.
Somasundaran P. et al. (2009). Nature Materials, 8: 543.
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November 30
Takuya Kitagawa
Harvard University
The non-equilibrium dynamics of one dimensional quasi-condensates;
the first experimental observation of prethermalization in
interference patterns
Abstract: The understanding of thermalization dynamics in many-body systems is one of the most important problems in many areas of physics, which has implications in the advance of technology such as precision measurements. Yet, the study of non-equilibrium systems presents outstanding challenge in both theory and experiments due to the lack of tools to analyze and characterize these systems.
In this talk, we study, as an example of many-body dynamics, the dynamics of interference patterns between split one dimensional condensates; a single quasi-condensate is split into two at the beginning, and subsequently, the interference patterns between these two quasi-condensates are observed after free evolution of them for time t. In order to study the strong quantum fluctuation of one dimensional dynamics, here we employ a novel method of full distribution function (FDF) of interference patterns. By obtaining the analytic expression of FDFs, we demonstrate the occurrence of prethermalization in this dynamics, where the interference patterns of non-equilibrium, dephased states become indistinguishable from those of thermal equilibrium states. The theoretical prediction of prethermalization above is confirmed by recent experiments by J. Schmiedmayer's group, and I will present the comparison between the theory and experiments, which shows a good agreement.
The observation of prethermalization phenomena suggests an alternative scenario to the naive thermalization story, and opens perspectives for further study of this physics. Moreover, the method of full distribution function developed in our theory is a powerful tool to study non-equilibrium physics, and extensions to other systems are promising future direction.
References: T. Kitagawa, et al Phys. Rev. Lett. 104, 255302 (2010), New J. Phys. 13 073018 (2011).
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December 07
Elena G Strekalova
Boston University
Effect of hydrophobic confinement on the hypothesized
liquid-liquid critical point of water
Abstract: We often think of water as a typical liquid because of its ubiquity in our lives. However, do we know enough about water?
The thermodynamic behavior of water is very complex and anomalous when compared to simple liquids. For example, water presents a density anomaly, i.e., a decrease in density upon isobaric cooling. Water anomalous behavior is more pronounced at subfreezing temperatures and numerous theoretical and experimental studies are directed towards developing a coherent thermodynamic and dynamic framework for understanding supercooled water.
The liquid-liquid phase transition hypothesis arose from molecular dynamics studies on the structure and equation of state of supercooled bulk water and has received some support. Below the hypothesized second critical point, the liquid phase separates into two distinct liquid phases: a low-density liquid (LDL) phase at low pressures and a high-density liquid (HDL) at high pressure. Bulk water near the hypothesized liquid-liquid critical point is a fluctuating mixture of molecules whose local structures resemble the two phases, LDL and HDL. These enhanced fluctuations influence the properties of liquid bulk water, thereby leading to anomalous behavior.
To this day, the experimental study of supercooled bulk water is hampered by the homogeneous nucleation of the crystal. Recently water confined in nanoscopic structures has attracted interest because nucleation can be delayed. Confined systems have a tremendous relevance also for current biological advances. In particular, the study of water behavior in the presence of apolar interfaces helps understanding such phenomena as self-assembling of micelles, membrane formations and protein folding.
Here we ask: what is the effect of the hydrophobic confinement on the hypothesized liquid-liquid critical point of water? Using Monte Carlo simulations, we study a coarse-grained model of a water layer confined in a fixed disordered matrix of hydrophobic nanoparticles. In addition, we study Jagla ramp potential particles confined in ordered and disordered matrices of fixed hard spheres using discrete molecular dynamics simulations.
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December 14
Nir Bar-Gill
Harvard University
Spectroscopy of composite solid-state spin environments for improved metrology with spin ensembles
Abstract: For precision coherent measurements with ensembles of quantum spins the relevant Figure of Merit (FOM) is the product of polarized spin density and coherence lifetime, which is generally limited by the dynamics of the spin environment. Here, we apply a coherent spectroscopic
technique to characterize the dynamics of the composite solid-state spin environment of Nitrogen-Vacancy (NV) centers in room temperature diamond; and thereby realize NV FOM values that are an improvement of three orders of magnitude over other NV-diamond results, which is almost an order of magnitude larger than previously achieved in any room temperature solid-state spin system and is within an order of magnitude of the state-of-the art atomic system. We also identify a new mechanism for suppression of electronic spin bath dynamics in the presence of a nuclear spin bath of sufficient concentration. This suppression could inform efforts to further increase the FOM for solid-state spin ensemble metrology and collective quantum information processing.
