|USC Quantum Information - Condensed Matter Physics - Biophysics Seminars|
For more information contact Stephan Haas (213) 740-4528.
Population annealing: massively parallel Monte Carlo simulations and efficient estimation of the density of states
Lev Barash (HSE Tikhonov Moscow Institute of Electronics and Mathematics)
Population annealing is a promising recent approach for Monte Carlo simulations in statistical physics, in particular for the simulation of systems with complex free-energy landscapes. It is a hybrid method, combining importance sampling through Markov chains with elements of sequential Monte Carlo in the form of population control. The particular advantage of this approach is that it is fully scalable up to many thousands of threads. We present a highly optimized implementation of the population annealing algorithm on graphics processing units and obtain speed-ups of several orders of magnitude as compared to a serial implementation on CPUs. Our code includes implementations of some advanced algorithmic features that have only recently been suggested, such as the automatic adaptation of temperature steps and a multi-histogram analysis of the data at different temperatures. We discuss behavior of population annealing for different systems undergoing continuous and first-order phase transitions. A technique based on population annealing enhanced with a multi-histogram analysis is introduced that allows calculation of the density of states. Its performance for spin glasses is analyzed. Its ability to overcome the pitfalls of other entropic samplers is demonstrated, resulting in some cases in large scaling advantages that can lead to the uncovering of new physics. The new technique avoids some inherent difficulties in established approaches and can be applied to a wide range of systems without relevant tailoring requirements. Benchmarking of the studied techniques is facilitated by the introduction of several schemes that allow us to achieve exact counts of the degeneracies of the tested instances.
Sep 20, 2pm
The Weighted Constraint Satisfaction Problem: Applications in Physics and Materials Science
Satish Kumar Thittamaranahalli (USC ISI)
In this talk, I will present my work on the Weighted Constraint Satisfaction Problem (WCSP), a fundamental combinatorial problem of interest in many research communities (albeit under different names). A WCSP is characterized by a set of system variables, a set of interaction potentials between subsets of the system variables, and the goal of having to find the minimum cost state of the system. Machine learning and inference algorithms based on the WCSP framework have numerous applications in AI, computational physics, theoretical computer science, operations research, and robotics, among many other research areas. Despite the generality of the WCSP, I will show that it is reducible to a specific problem in graph theory, called the minimum weighted vertex cover (MWVC) problem. This reduction throws new light on how to kernelize and efficiently solve large optimization problems, how to exploit structure and do transfer learning in combinatorial problems, how to convert k-body interactions to 2-body interactions, how to formulate constrained optimization problems on a quantum annealer, and how to do physics-based machine learning, among other things. I will also talk about my ongoing collaborative work with USC’s physics and materials science departments: (a) Understanding hysteresis in combinatorial optimization problems; and (b) Imputing missing data and detecting anomalies in MoS2 CVD growth and electron microscopy images.
Sep 27, 2pm
Neutrinos and electrons/positrons: The building elements and catalysts of our Universe
Constantinos G. Vayenas (Department of Chemical Engineering, University of Patras & Division of Natural Sciences, Academy of Athens)
The formation of composite particles, such as protons and/or other hadrons from their constituent quarks, known as baryogenesis or hadronization, appears to have many similarities with catalytic chemical synthesis. In this talk, I will discuss similarities between these two processes and provide thermodynamic and kinetic considerations. This catalytic role in baryogenesis is played by electrons and positrons, and it is discussed in the context of the Standard Model (SM) and the recently derived Rotating Lepton Model (RLM). The latter is constructed based on a Bohr-type model of rotating neutrinos, where gravity is the attracting force but where special relativity is also used for the corresponding masses. The model predicts masses and other properties of hadrons with an astonishing precision of 1 percent without the use of any adjustable parameters. According to RLM the catalytic role of electrons and positrons is due to their gravitational mass. This leads to an acceleration of neutrinos to ultrarelativistic velocities, such that the relativistic neutrino mass reaches the quark mass, the neutrino gravitational mass reaches the Planck mass and the gravitational attraction reaches the value of the Strong Force. The result is gravitational confinement in circular orbits with radius size of the order of fm. We will discuss the implications of these findings regarding the nature of the Strong and Weak Forces, as well as similarities and differences from classical catalysis in chemical and biological systems, and in which the catalytic action results from electrostatic interactions. Some potential aspects of power generation via controlled baryogenesis will be also discussed.
Oct 4, 2pm
Parity-Time-symmetric optics, extraordinary momentum and spin in evanescent waves, optical analog of topological insulators, and the quantum spin Hall effect of light
Franco Nori (RIKEN)
This talk provides a brief overview to some aspects of parity-time-symmetric optics, extraordinary momentum and spin in evanescent waves, optical analog of topological insulators, and the quantum spin Hall effect of light. 1. Parity-Time-Symmetric Optics Optical systems combining balanced loss and gain provide a unique platform to implement classical analogues of quantum systems described by non-Hermitian parity-time (PT)-symmetric Hamiltonians. Such systems can be used to create synthetic materials with properties that cannot be attained in materials having only loss or only gain. We report PT-symmetry breaking in coupled optical resonators. We observed non-reciprocity in the PT-symmetry-breaking phase due to strong field localization, which significantly enhances nonlinearity. In the linear regime, light transmission is reciprocal regardless of whether the symmetry is broken or unbroken. We show that in one direction there is a complete absence of resonance peaks whereas in the other direction the transmission is resonantly enhanced, which is associated with the use of resonant structures. Our results could lead to a new generation of synthetic optical systems enabling on-chip manipulation and control of light propagation. 2. The quantum spin Hall effect of light: photonic analog of 3D topological insulators. Maxwell's equations, formulated 150 years ago, ultimately describe properties of light, from classical electromagnetism to quantum and relativistic aspects. The latter ones result in remarkable geometric and topological phenomena related to the spin-1 massless nature of photons. By analyzing fundamental spin properties of Maxwell waves, we show that free-space light exhibits an intrinsic quantum spin Hall effect -surface modes with strong spin-momentum locking. These modes are evanescent waves that form, for example, surface plasmon-polaritons at vacuum-metal interfaces. Our findings illuminate the unusual transverse spin in evanescent waves and explain recent experiments that have demonstrated the transverse spin-direction locking in the excitation of surface optical modes. This deepens our understanding of Maxwell's theory, reveals analogies with topological insulators for electrons, and offers applications for robust spin-directional optical interfaces.
Oct 11, 2pm
Using phase-coherent transport to uncover new electronic phenomena in quantum materials
Nicholas Breznay (Harvey Mudd College)
Understanding the driving mechanisms for quantum materials - whether strongly correlated, strongly disordered, or characterized by nontrivial band topology - often derives from precise studies of electronic phase coherence. Among other impacts, long electronic spin and phase-coherence lifetimes result in weak localization or (in the presence of strong spin-orbit coupling) antilocalization. These transport phenomena can be used to spectroscopically probe complex materials, including the interface between perovskite oxide materials, and phase-change chacogenide glasses. First, we examine thin-film bilayers of BaPbO3 on BaBiO3, an oxide system that (like LAO-STO) exhibits superconductivity at the interface. The bilayer films exhibit quantum interference phenomena below 100 K and show an enhanced dephasing rate over a broad range of temperatures (2-50 K). Increased dephasing may be linked to instabilities that have been proposed to drive unconventional superconductivity in the bismuthates and demonstrates that the BPO-BBO interface hosts a complex electronic system. Second, we find an unexpected spin sensitive hopping conductivity in the phase change material SnSb2Te4. Here, an isotropic magnetoconductance arises from disruption of spin correlations that inhibit hopping transport, a recently hypothesized 'spin memory' effect whose occurrence signals that the spin plays a previously overlooked role in the disorder-driven transition between weak and strong localization in spin-orbital materials.
Oct 18, 2pm
The sign problem, non-stoquasticity and everything in between
Itay Hen (USC ISI)
The sign problem is a key challenge in quantum many-body simulations, encapsulating our inability to properly understand many important quantum phenomena in physics, chemistry and the material sciences. Despite its centrality, the sign problem is often not very well understood. I will present the results of several studies that aim at elucidating various aspects of the problem including the circumstances under which the problem emerges, its relation to the concept of non-stoquasticity, new paths towards a potential resolution of the problem and its manifestation in quantum algorithms.
Dec 11, 2pm
Nicole Younger Halpern (ITAMP)