# Upcoming Seminars

# Upcoming Seminars

__Spring 2015 Seminars__

__Spring 2015 Seminars__

All Physics Seminars are held in S-3-126 at 1pm on Thursday, unless otherwise noted.

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**Thursday, Jan 29, 1:00pm**

**Charles Reichhardt
Center for Nonlinear Studies and Theoretical Division
Los Alamos National Laboratory **

__The Dynamics of Active Matter Particles on Disordered Landscapes: Jamming, Clogging, and Avalanches __

There has been tremendous growth in studying nonequilibrium systems in which the individual units are internally driven and are self-mobile. Such dynamics can effectively describe certain biological systems such as run-and-tumble bacteria or crawling cells, as well as non-biological systems such as self-driven colloids or artificial swimmers. These systems are now being grouped into a new class of matter called active matter. They exhibit a wealth of novel nonequilibrium behaviours, such as clustering, flocking, and phase separation. In non-active systems there are numerous examples of collections of interacting particles that can be driven over random and periodic substrates such as vortices in type-II superconductors, colloids on periodic optical lattices, models of friction, and particle flows through porous media. Here we examine the dynamics of active matter systems interacting with random or periodic substrates. We show that active particles can exhibit a number of distinct dynamical phases when moving or driven over random or periodic substrates. We find that in some cases, increasing the activity or the propulsion of the particles can increase the transport across the substrate; however, there are also regimes in which increased activity can lead to enhanced pinning or jamming of the systems. For non-active systems we show that in the presence of random arrays the flow and density become highly heterogeneous. For intermediate activity the flow becomes uniform, and at high activity the flow once again becomes heterogeneous leading to strong clogging effects. In the dense particle limit, we find that active matter systems can show interesting motion of dislocations, grain boundaries and other topological structures distinct from purely thermally driven systems or systems with external drives.

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**Thursday, Feb 5, 1:00pm**

**Paola Cappellaro
MIT **

__Quantum control strategies for imaging and spectroscopy __

Quantum control techniques have proven effective to extend the coherence of qubit sensors, thus allowing quantum-enhanced sensitivity at the nano-scale. The key challenge is to decouple the qubit sensors from undesired sources of noise, while preserving the interaction with the system or field that one wishes to measure. In addition, tailoring the sensor dynamics can help reveal temporal and spatial information about the target.

In this talk I will show how we can use coherent control of quantum sensors to reconstruct the arbitrary profile of time-varying fields, while correcting the effects of unwanted noise sources. These control techniques can be further used to reveal information about classical and quantum noise sources. For example, they can achieve high frequency resolution, thus allowing precise spectroscopy and imaging of the spatial configuration of a spin bath.

I will illustrate applications of these strategies in experimental implementations based on the Nitrogen-Vacancy center in diamond.

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**Thursday, Feb 12, 1:00pm**

**Gene Wayne
Boston U **

__The Nonlinear Schrödinger Equation as an approximation for water waves and normal forms for partial differential equations __

In 1968 V.E. Zakharov derived the Nonlinear Schrödinger equation as an approximation to the 2D water wave problem in the absence of surface tension in order to describe slow temporal and spatial modulations of a spatially and temporarily oscillating wave packet. I will describe a recent proof that the wave packets in the two-dimensional water wave problem in a canal of finite depth can be accurately approximated by solutions of the Nonlinear Schrödinger equation and discuss the relationship of this approximation to questions about normal forms for PDE's.

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**Thursday, Feb 19, 1:00pm**

**Lea Santos
Yeshiva University **

__Relaxation Process of Isolated Quantum Systems with Interacting Particles __

We study the evolution of isolated finite interacting quantum systems after an instantaneous perturbation. Both clean and disordered one-dimensional systems are analyzed. In the first case, we show three scenarios in which the probability for finding the initial state later in time (survival probability) decays nonexponentially, often all the way to saturation. The decay is Gaussian in systems with two-body interactions in the limit of strong global perturbation; it involves a Bessel function for evolutions under random matrices; and it approaches the fastest decay as established by the energy-time uncertainty relation when a very strong local perturbation is applied. In the disordered case, we report the observation of a power-law decay of the survival probability near the many-body localization transition. We provide numerical evidence suggesting that the exponent of this decay is related to the multifractal structure of the eigenstates through the so-called correlation dimension.

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**Thursday, Feb 26, 1:00pm**

**Jacob Khurgin
Electrical and Computer Engineering, Johns Hopkins U **

__How small can one make a semiconductor laser? __

Recently semiconductor lasers based on metal dielectric structures promising sub-wavelength operation have generated a lot of interest. Yet it is not clear exactly how small semiconductor laser can be made without sacrificing performance. In this talk recent experimental and theoretical results will be reviewed and t a theory outlining the fundamental limits of how small can the nano-laser actually be will be presented. Comparison with state of the art semiconductor lasers, such as VCSELâ€™s in terms of threshold, efficiency, linewidth and modulation speed will be made.

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**Thursday, Mar 5, 1:00pm**

**Pavel Krapivsky
Boston U **

__A Kinetic View of Statistical Physics __

I will try to illustrate the essence of non-equilibrium statistical physics, the branch of science which is not so much the study of a speciï¬c subject per se, but rather a collection of ideas and tools that work for an incredibly wide range of problems. I will exemplify some of the key insights that have emerged from the analysis of far-from-equilibrium behaviors. As in other interacting many-particle settings, the large size often plays an advantageous, rather than deleterious, role in leading to simple collective properties. I'll focus on diffusion-reaction systems and I'll also discuss a connection with non-equilibrium spin systems.

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**UNUSUAL DAY, TIME, PLACE**

**Tuesday, Mar 10, 2:00pm, ISC Conference Room 1200**

**Part of the Quantum Science and Technology Seminar series**

**Oscar Viyuela
U Complutense de Madrid **

__Quantum, Topological and Dissipative __

In this seminar I will introduce the Uhlmann geometric phase as a tool to characterise density matrices of 1D and 2D topological insulators and superconductors. We achieve this goal by constructing new topological invariants called Topological Uhlmann numbers.

Since this phase is formulated for general mixed quantum states, it provides a way to extend topological properties to finite temperature situations. New effects appear such as the existence of critical temperatures, novel thermal-topological transitions in models with high Chern numbers, breakdown of the usual bulk-edge correspondence, ...

Moreover, as the Uhlmann phase is an observable itself, we analyse potential measurement schemes that could be applicable to current experimental setups like cold atoms in optical lattices.

[1] 2D Density-Matrix Topological Fermionic Phases: Topological Uhlmann Numbers, O. Viyuela, A. Rivas, M.A. MartÃn-Delgado, Phys. Rev. Lett 113, 076408 (2014).

[2] Uhlmann Phase as a Topological Measure for One-Dimensional Fermion Systems, O. Viyuela, A. Rivas, M.A. MartÃn-Delgado, Phys. Rev. Lett 112, 130401 (2014).

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**Thursday, Mar 12, 1:00pm**

**Panayotis Kevrekidis
UMass Amherst **

__TBA __

TBA

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**UNUSUAL DAY, TIME, PLACE**

**Friday, Mar 13, 3:00pm, ISC Conference Room 1200**

**Part of the Quantum Science and Technology Seminar series**

**Jan-Åke Larsson
Linköping University **

__Introduction to contextuality and what it can teach us about quantum-mechanical systems __

Contextuality is a strange property in quantum systems. The concept originally comes from Ernst Specker's philosophical speculations, whether it is possible to know the answers of all possible questions, or the answer to a question would depend on other questions asked at the same time. In a quantum-mechanical system, this translates to the question of whether it is possible to predict the outcome of a measurement, independent of which set of commuting measurements are co-performed (the measurement context), or if the outcome would depend on the context.

Kochen and Speaker showed in 1967 that for quantum systems of dimension 3 or higher, the answer is the latter: the outcome must depend on the context. In this talk, we will have a close look at the reason to initially believe that the outcome would be independent of the context, or "noncontextual", and then continue to explicitly show that it must be dependent, or contextual. We will first use the original setup, but also look at some more recent ways to prove similar statements.

These results will then be used to learn more about properties of quantum systems, like lower bounds on memory requirements for the systems, or lower bounds on their quantum dimensionality. Finally, we will look at performed experiments that attempt to test and teach us about these properties, and some shortcomings of them, together with a recently proposed remedy.

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**Thursday, Mar 19**

**NO SEMINAR
SPRING BREAK **

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**UNUSUAL DAY, TIME, PLACE**

**Tuesday, Mar 24, 2:00pm, ISC Conference Room 1200**

**Part of the Quantum Science and Technology Seminar series**

**Ken Funo
U Tokyo **

__Quantum nonequilibrium equalities with absolute irreversibility __

Nonequilibrium equalities such as fluctuation theorems and Jarzynski equalities have attracted a great deal of interest in the field of nonequilibrium statistical mechanics, since they give general insights into thermodynamic quantities in nonequilibrium processes irrespective of details of individual systems. For example, the Jarzynski equality relates work done on the system in a nonequilibrium process to the equilibrium free-energy difference, and the detailed fluctuation theorem relates the ratio of the forward and backward (time-reversed) path-dependent probabilities to the entropy production, which measures the irreversibility of the process.

It is known that the Jarzynski equalities are inapplicable to such cases as free expansion and feedback control involving projective measurements because in these cases there exist those forward paths with vanishing probability that have nonvanishing corresponding backward probabilities, and therefore the ratio between these probabilities (or the entropy production) is not well defined. We shall call such processes absolutely irreversible and derive nonequilibrium equalities by taking into account the dissipation due to this irreversibility. I will show an example of the quantum piston model to illustrate the obtained equalities.

Finally, I will briefly show the quantum nonequilibrium equalities when we consider Maxwell's demon, i.e., including measurement and feedback control processes. In this case, the effect of absolute irreversibility can be used to quantify the efficiency of the feedback control and the effect of the irreversibility of the memory due to the measurement process.

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**Thursday, Mar 26, 1:00pm**

**David Gelbwaser
Harvard U **

__Quantum Heat Machines __

Quantum heat machine models are an alternative approach to a wide range of problems: from fundamental questions as the validity of the thermodynamic limits at quantum scales to practical applications related to cooling and to the minimum possible achievable temperature in several systems (e.g. the cooling of spin bath by a NV center probe). During this talk, I will review some examples of quantum heat machines in order to answer the above questions and reveal the thermodynamic aspects of different quantum resources.

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**UNUSUAL DAY, TIME, PLACE**

**Tuesday, Mar 31, 2:00pm, ISC Conference Room 1200**

**Part of the Quantum Science and Technology Seminar series**

**Adolfo del Campo
UMass Boston **

__Nonexponential decay of a many-body quantum system __

Understanding the decay dynamics of unstable isolated quantum systems is of relevance to a wide variety of fields ranging from foundations of physics to statistical mechanics and cosmology. At the single-particle level, as a function of time, observables such as the fidelity decay or survival probability, generally evolve through three sequential stages: short-time deviations from exponential decay, a regime with a well-defined decay rate, and power-law deviations at long times. These deviations arise due to the possibility that the decay products reconstruct the initial state.

In this talk I will describe how the above understanding carries over many-body systems, for which the decay dynamics is poorly understood even in the absence of interactions. In addition, I shall report work in progress concerning the exact decay dynamics of an interacting many-body system. It will be shown that the strength of interactions determines the power-law exponent governing the long-time fidelity decay. This observation challenges the understanding that at arbitrarily long times particles are far apart and cease to interact. We reconcile this observation by noticing that the power-law decay is associated with state reconstruction, which justifies the persistence of the role played by interactions at arbitrary long expansion times.

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**Thursday, Apr 2, 1:00pm**

**Ariel Amir
Harvard U **

__Getting into shape: how do cells control their geometry? __

Microorganisms such as bacteria and budding yeast are remarkably successful in accurately self-replicating themselves within several tens of minutes. How do cells decide when to divide? How do they control their morphology? I will show how ideas from statistical mechanics and materials science can help answer these questions. In particular, I will show how a stochastic model of cell size control, combined with single cell data, can be used to infer a particular strategy for cell size control in bacteria and budding yeast, and how the theory of elasticity can be utilized to understand the coupling of mechanical stresses and cell wall growth in bacteria.

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**Thursday, Apr 9, 1:00pm**

**Marcus Cramer
Ulm U **

__Local Equilibration and Thermalization in Quantum Lattice Systems __

The emergence of thermal states will be discussed in the setting of quantum quenches and canonical typicality. Two recently obtained lemmata, one on the local closeness of states and one on the rate of convergence in the quantum central limit theorem, allow us to make previous results more explicit, to prove the local equivalence of micro- and macrocanonical ensembles, and to obtain a sufficient condition for thermalization after a quantum quench.

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**UNUSUAL DAY, TIME, PLACE**

**Tuesday, Apr 14, 2:00pm, ISC Conference Room 1200**

**Part of the Quantum Science and Technology Seminar series**

**Marcus Cramer
Ulm U **

__Scalable Quantum State Tomography __

Recent contributions in the field of quantum state tomography have shown that, despite the exponential growth of Hilbert space with the number of qubits, tomography of quantum systems may still be performed efficiently by tailored reconstruction schemes. We discuss some of these (certifiable) scalable methods to reconstruct pure and mixed states. Scalability relies on the fact that only local information about the state is required.

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**Thursday, Apr 16, 1:00pm**

**Bulbul Chakraborty
Brandeis U **

__Physics of Sand: Emergent Behavior in the Macro World __

Diversity in the natural world emerges from the collective behavior of large numbers of interacting objects. The origin of collectively organized structures over the vast range of length scales from the subatomic to colloidal is the competition between energy and entropy. Thermal motion provides the mechanism for organization by allowing particles to explore the space of configurations.

This well-established paradigm of emergent behavior breaks down for collections of macroscopic objects ranging from grains of sand to asteroids. In this macro-world of particulate systems, thermal motion is absent, and mechanical forces are all important. We lack understanding of the basic, unifying principles that underlie the emergence of order in this world. In this talk, I will explore the origin of rigidity of granular solids, and present a new paradigm for emergence of order in these athermal systems.

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**UNUSUAL DAY, TIME, PLACE**

**Tuesday, Apr 21, 2:00pm, ISC Conference Room 1200**

**Part of the Quantum Science and Technology Seminar series**

**Jordan Horowitz
UMass Boston **

__â€œQuantum Fluctuation Theorems for Completely-Positive, Trace-Preserving Mapsâ€ __

Fluctuation theorems have proven to be useful tools in understanding thermodynamic irreversibility in small fluctuating systems. However, generalizing them to quantum thermodynamic processes has been challenging. In this talk, I will discuss a general fluctuation theorem for a large class of completely-positive, trace-preserving quantum maps that incorporates as all known quantum fluctuation theorems in a single and simplified framework.

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**Thursday, Apr 23, 1:00pm**

**Julian Sonner
MIT **

__From black hole dynamics to (quantum) quenches __

AdS/CFT duality, one of the most exciting recent conceptual developments in theoretical physics, relates ordinary quantum field theories to theories of quantum gravity. We can therefore use our understanding of gravity to further our understanding of field theory and its applications, and vice versa. In this talk I will describe how this leads to insights into the physics of strongly correlated matter, paying particular attention to nonequilibrium dynamics. In the context of holographic duality this leads us to the study of black holes and their dynamical properties, and we will explore how their universal relaxation dynamics gives insights into the equilibration dynamics of strongly-coupled quantum field theories.

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**Thursday, Apr 30, 1:00pm**

**Israel Klich
U of Virginia **

__Spin jams, tilings and exotic entropy scaling in a frustrated magnet __

When spins are regularly arranged in a triangular fashion, the spins may not satisfy simultaneously their antiferromagnetic interactions with their neighbors. This phenomenon, called frustration, may lead to a large set of ground states and to exotic states such as spin ice and spin liquid. Here we report a novel approach to an old open problem, motivated by puzzling behavior of certain magnetic systems such as SCGO. We show how a clean system governed by simple Heisenberg interactions freezes into a glass like state induced by quantum fluctuations, in contrast to the usual mechanisms for classical spin-glass which rely on the presence of disorder. At the heart of the effect is an unusual scaling of the number of local minima, with a scaling extensive in the boundary length rather than the volume. We establish these properties by a combination of tools and mappings, leading to a problem of counting tessellations. Recent experimental evidence for the spin jam scenario will also be presented.

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**UNUSUAL DAY, TIME**

**Friday, May 1, 2:00pm**

**Steve Hagen
U Florida **

__Worlds within worlds: Bacterial circuitry for sensing and processing environmental cues __

Natural genetic competence (â€œcompetenceâ€) is a transient state in which a bacterium is able to take up DNA from its environment and thus acquire new traits. Competence is central to the virulence of pathogens such as Streptococcus mutans, a bacterium that inhabits the human mouth and etches away tooth enamel. Bacteria regulate competence and virulence in response to numerous environmental cues that include local physical and chemical conditions as well as signals received from other cells. The environmental information that is processed by organisms like S. mutans is very complex: S. mutans reside in physically and chemically heterogeneous, multispecies communities (biofilms) characterized by interspecies competition and cooperation as well as stressful extremes, such as locally intense acidity (pH <5), depleted oxygen, and feast-or-famine nutrient availability.

We are using microfluidic methods to define and control the S. mutans environment so that we can disentangle the gene regulatory circuits that control competence and virulence. Our studies reveal a world of complex phenomena in these circuits, from stochasticity and fluctuations to bistability and bimodality. These behaviors can be understood in terms of layers of circuits characterized by nonlinearity, feedback, switching â€“ all precisely tuned to environmental inputs. The result is an oddly intricate relation between the cellâ€™s environment (input) and its behavior (output): the more finely one controls the environment of the cell, the more complexity one finds in the organismâ€™s response.

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**Thursday, May 7, 1:00pm**

**Mathieu Beau
UMass Boston **

__Quantum dynamics and diffraction of matter waves __

Quantum dynamics is very active ï¬eld of research with many applications such as tunneling decay, atom number state preparation, cold atoms, atomic optics and more speciï¬cally diï¬€raction of matter waves. In the past few years there was a revival of interest in the slit diï¬€raction experiment because of modern technologies which allow to tackle new challenges [R.Bach et. al., New J. Phys. 15, 033018 (2013)], [S.Frabboni et. al., Am. J. Phys. 79 (6), 615-618 (2011)]. However, giving the full quantum propagator of the multi-slit system is a theoretical problem which remain unsolved and give rise to controversy. The diï¬ƒculty lies in taking into account the quantum ï¬‚uctuation along the propagation axis of the incident wave. The motion along this direction is usually assumed to be classical. There is a series of articles (see [R. Sawant et. al., Phys. Rev. Lett. 113, 120406 (2014)] and the references therein) dealing with this problem via the Feynman path integral approach, but this is not satisfactory. With Prof. T. Dorlas, we have recently published an article where we derived a consistent solution of the multi-slit problem using Greenâ€™s function approach, and also gave quantum corrections in the semi-classical regime. Our approach is deeply related to the diï¬€raction in time introduced by M. Moshinsky in 1952 and mainly developed over the last two decades. In this talk I would like to give an overview of the diï¬€raction problem in the framework of the quantum dynamics ï¬eld, and then present and discuss our recent results.

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