Thursday, Feb 11, 1:00pm
Neutrino oscillation experiments performed throughout the latter half of the twentieth century have yielded valuable information on the nature of neutrino masses and mixings. The evidence gathered has provided the first positive evidence for physics beyond the standard model. Currently, a new suite of precision experiments will come online to provide greater insight into the physics and significance of neutrino mass. This talk will review our current state of knowledge on neutrino masses, and how new experiments---specifically classical experiments using beta decay---will complement that knowledge in years to come.
Thursday, Feb 18, 1:00pm
NO SEMINAR THIS WEEK
Unusual Day, Time, Place
Tuesday, Feb 23, 1:00pm
Part of the Quantum Science and Technology Seminar Series
Jesus del Pozo Mellado
UMass Boston, Chemistry Department
Scaling Laws and Bulk-Boundary Decoupling in Transport Phenomena
When driven out of equilibrium by a gradient, fluids respond by developing a non-trivial, inhomogeneous structure according to the governing macroscopic laws. Here we show that such structure obeys strikingly simple scaling laws arbitrarily far from equilibrium, provided that both macroscopic local equilibrium and constitutive equations hold (i.e. Fourier's or Fick's laws). Extensive simulations of hard disk fluid confirm the scaling laws even under strong gradients, implying that constitutive equations remains valid in this highly non-linear regime, with putative corrections absorbed into non-linear transport coefficient functionals. In addition, our results show that the scaling laws are robust in the presence of strong finite-size effects, hinting at a subtle bulk-boundary decoupling mechanism which enforces the macroscopic laws on the bulk of the finite-sized fluid. This allows one to measure the marginal anomaly of the heat conductivity predicted for hard disks.
Thursday, Mar 3, 1:00pm
NO SEMINAR THIS WEEK
Wednesday, Mar 9, 1:00pm (ISC 1200)
Bioantenna in a cavity: quantum engineering of energy transfer in living systems
Photosynthetic organisms are spectacular examples of nature's engineering capabilities. To optimize the efficiency of solar light-harvesting, plants and photosynthetic bacteria developed intricate molecular antennas known as light-harvesting complexes (LHCs). These antennas absorb photons and convert the energy to molecular electronic excitations -- excitons. Then, the excitons are used for very efficient directional energy transfer on 10-100 nanometer length scales. Understanding exciton dynamics in LHCs at the microscopic level can provide us with new principles for the design of artificial sun-powered systems. In this talk, I will describe how carefully designed nanostructures can be used to probe and modify energy transfer in LHCs of photosynthetic bacteria. I will begin with an overview of the general concepts and challenges of excitation dynamics in natural LHCs. Green sulfur bacteria will be discussed as a model system. I will then describe an example where optical cavities accentuate response signals from LHCs and will explain how this effect can be used to modify the energy transfer paths in living organisms.
Thursday, Mar 10, 3:30pm (ISC 1200)
The uncertainty principle and the future of secrecy
Heisenberg's uncertainty principle captures the essence of quantum mechanics, boiling the formalism down to a single, simple law. This talk will take you on a journey through the history of the uncertainty principle. This includes its recent reformulation in terms of entropy and information, and more recently, an exotic generalization where the observer may be entangled to the particle being measured . Surprisingly, a recent breakthrough in our fundamental understanding of the uncertainty principle came when we unified it with another basic principle known as wave-particle duality . Finally, I will discuss a technological application of the uncertainty principle for secret communication. With the threat of quantum computers on the horizon, society will need a new method of encrypting communication, and this is where "quantum cryptography" comes in. While evaluating the performance of quantum cryptographic protocols is theoretically challenging, I will discuss my new numerical approach to addressing this problem .  Coles et al. Physical Review Letters. 108 (2012)  Coles et al. Nature Communications. 5 (2014)  Coles et al. arXiv:1510.01294 (2015)
Thursday, Mar 17, 1:00pm
Tuesday, Mar 22, 3:30pm (ISC 1200)
Los Alamos National Laboratory
Optimal control of nonequilibrium quantum evolution: quantum simulations, cooling, and nonadiabatic braiding
Nonadiabatic unitary evolution with tailored time-dependent Hamiltonians can prepare many-body quantum states with various desired properties. We discuss novel application of optimal control to quantum simulations, cooling beyond the current state of the art, and beating the barrier of adiabaticity in the braiding of non-Abelian anyons. We show that highly nonadiabatic "bang-bang" protocols can serve as a powerful resource in quantum technologies.
Wednesday, Mar 23, 1:00pm (ISC 1200)
University of Maryland
Dynamical phase transitions in quantum driven-dissipative systems
Many-body systems with both coherent dynamics and dissipation constitute a rich class of models which are nevertheless much less explored than their dissipationless counterparts. The advent of numerous experimental platforms that simulate such dynamics poses an immediate challenge to systematically understand and classify these models. In particular, nontrivial many-body states emerge as steady states under non-equilibrium dynamics. In this talk, I use a systematic approach based on the Keldysh formalism to study nonequilibrium phases and phase transitions in such models. I show that an effective thermal behavior generically emerges as a result of dissipation, and the universal behavior including the dynamics near the steady state is described by a universality class in equilibrium thermodynamics. In the end, I will also discuss possibilities that go beyond the paradigm of an effective thermal behavior.
Thursday, Mar 24, 1:00pm
Monday, Mar 28, 1:00pm (ISC 1200)
Order, Equilibrium, Memory: The search for a quantum hard drive and new laws for non-equilibrium quantum statistical mechanics
The three concepts of the title are closely related in statistical mechanics and condensed matter. Order is an organisational principle that gives a material its macroscopic properties.
Equilibration is the most common phenomenon in Nature. There is no other law that is stronger than the one that imposes systems to eventually reach equilibrium.
On the other hand, memory is the property of staying away from equilibrium. If one has to store information, one has to keep a system away from equilibrium for long times.
All these concepts can be illustrated by the functioning of a device that we use every day: the Hard Drive. In this talk, I will tackle the following question: can one build a Quantum Hard Drive?
In this talk, I show that the existence of exotic phases of the matter called topological phases - which do not rely on the symmetry breaking mechanism - provides a mechanism for keeping quantum states alive for a long time, and therefore to build a quantum hard drive. This kind of order relies on a topological pattern of entanglement. I will address the problem of the survival of Topological Order at finite temperature and after a quantum quench. Time permitting, I will discuss some novel routes towards a stable quantum memory, including many-body localization.
Thursday, Mar 31, 3:30pm (ISC 1200)
Power-law Decays and Thermalization in Isolated Many-Body Quantum Systems
I will discuss the short- and long-time dynamics of isolated many-body quantum systems. At short times, the decay of the survival probability of the initial state can be very fast, even faster than exponential when the system is strongly perturbed out of equilibrium. At long times, however, the evolution of any quantum system with a bounded spectrum slows down and shows a powerlaw decay. The value of the powerlaw exponent depends on the properties of the spectrum, structure of the initial state, and number of particles that interact simultaneously. An exponent greater than or equal to 2 indicates that the energy distribution of the initial state is ergodically filled and that the system will therefore thermalize.
Thursday, Apr 7, 1:00pm
Physics Department and Center for Interdisciplinary Research in Complex Systems, Northeastern U
Compression and Lubrication of Salt Free Polyelectrolyte Microgel Particles in Highly Compressed Suspensions by Counterion Osmotic Pressure
The compression of polyelectrolyte microgel particles in a salt-free highly compressed colloid due to osmotic pressure outside of the particles due to counterions located there is studied for a model based on a quasi-analytic solution of the Poisson-Boltzmann equation and a model for the gel elasticity based on counterion osmotic pressure inside the particles and polymer elasticity (of entropic origin). It is found that for particles of radius of the order of a tenth of a micron, the counterion osmotic pressure should play a significant role in the compression of the particles, especially particles which do not have a corona (i.e., nonlinked polymer chains attached to their surface). The presence of a corona of monomer density smaller than that of the core of the microgel reduces the contribution of the osmotic pressure due to counterions outside of the microgel. It is also demonstrated that counterion osmotic pressure outside the particles can provide a significant contribution to the lubrication of the interface between the particles and a surface along which the compressed colloid is made to slide, for sufficiently slow velocities.
Thursday, Apr 14, 1:00pm
Time dilation in quantum interference - from the quantum twin paradox to decoherence
We consider how gravitational time dilation affects low-energy quantum systems. In a quantum version of the â€œtwin paradoxâ€ a clock is brought in superposition of being at two different gravitational potentials. We show that time dilation induces entanglement between internal degrees of freedom and the center-of-mass, which can be probed in optical or matter-wave interferometry. In addition, we derive that time dilation causes decoherence of all composite quantum systems. Our results show that the interplay between quantum theory and general relativity offers novel phenomena and that such a regime can be accessed in quantum optical experiments.
Thursday, Apr 21, 1:00pm
Jonathan Mboyo Esole
Geometric Engineering with Elliptic Fibrations
I will explain how the geometry of elliptic fibrations is used in string theory and M-theory to model gauge theories. I will also review how hyperplane arrangements naturally appear in these questions as a tool to describe the Coulomb branch of gauge theories.
Thursday, Apr 28, 1:00pm
A mathematical framework for the transmission of oscillations and information among neuron populations
Oscillatory electrical activity is ubiquitous in the brain at a wide range of frequencies, each with different behavioral correlates in different brain regions. The "communication though coherence" hypothesis states that coordination between oscillating populations can dynamically facilitate or prevent information flow between them. This hypothesis links two often separate fields of mathematics -- dynamical systems and information theory -- and raises questions in both fields. On the dynamical systems side, what is the appropriate mathematical framework in which to understand the emergence and stability of this coordination between strongly interacting oscillatory populations? On the information theory side, how can we describe and quantify transfer of information among a network of spiking populations in dynamically variable conditions? I will share my thesis work on the first question, involving a search for invariant manifolds in the state space of a rhythmic neuronal system, and my more recent work on the second, in which I formalize the notion of communication between inhomogeneous (e.g., periodically forced) point processes.
Thursday, May 5, 1:00pm