|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.
Nov 15, 2pm
On-chip quantum photonics: A step closer
Jifei Zhang (USC)
On-chip photonic quantum optical circuits are highly sought to enable compact quantum information processing systems. Their realization however demands: (1) spatially ordered and spectrally uniform single photon source (SPS) arrays, and (2) the integration of such SPS arrays with light manipulating units (LMUs) in scalable architectures that control and manipulate emitted photons at single and a few photon level. Guided by this holistic view, in this talk I will present my work on these two synergistic fronts: For the SPS array, I will report on the realization of a new class of on-chip integrable spatially ordered and spectrally uniform InGaAs quantum dots (QDs) as SPSs with single photon emission purity >99% and spectral non-uniformity down to unprecedented 1.8nm (< 2meV) in 5x8 array distributed over 1000 square microns. Strikingly, several pairs of the as-grown QDs emit within 300 micro eV, within the range of established on-chip local tuning methodologies. Realization of such SPS arrays has provided the platform on which co-designed LMUs can be fabricated to create on-chip controlled interference and entanglement between photons from distinct known sources at long separations.
For implementing the LMUs, I will discuss our proposed new paradigm that uses a collective Mie-like resonance of co-designed dielectric metastructures based upon interacting dielectric nanoresonators to provide all five required light manipulating functions of enhancement of the SPS emission rate, control on emission directionality, guiding, splitting and recombining. The spectral width of a collective Mie resonance relaxes the strict requirement of spectral matching between the LMU and SPSs. This new paradigm enables the on-chip scalable realization of photon interference and entanglement between photons from distant sources, a step that moves the status closer to realizing quantum photonic optical circuits.
Dec 6, 2pm
Quantifying the incompatibility of quantum measurements
Yorgos Styliaris (USC)
Incompatibility in quantum mechanics is famously captured through uncertainty relations. In this talk, we adopt the perspective that incompatibility can be fundamentally understood in terms of simulation of measurements by other measurements. From this viewpoint, we introduce an operationally motivated ordering over quantum measurements which, in turn, gives rise to families of monotones, i.e., scalar quantifiers that preserve the ordering. Our approach allows establishing a connection between entropic uncertainty relations, quantum coherence, and measurement simulatability via the mathematical tool of multivariate majorization.
Dec 12, 2pm
MBL-mobile: Many-body-localized engine
Nicole Younger Halpern (ITAMP)
Physical systems from coffee to black holes to superconducting qubits thermalize. Yet many-body-localized (MBL) systems do not under their intrinsic dynamics. The athermality of MBL, we propose, can be harnessed for thermodynamic tasks. We illustrate this ability by formulating an Otto engine cycle for a quantum many-body system. The system is ramped between a strongly localized MBL regime and a thermal (or weakly localized) regime. The difference between the energy-level correlations of MBL systems and of thermal systems enables mesoscale engines to run in parallel in the thermodynamic limit, enhances the engine's reliability, and suppresses worst-case trials. We estimate analytically and calculate numerically the engine's efficiency and per-cycle power. The efficiency mirrors the efficiency of the conventional thermodynamic Otto engine. The per-cycle power scales linearly with the system size and inverse-exponentially with a localization length. This work introduces a thermodynamic lens onto thermalization and localization in quantum many-body systems.
Jan 17, 2pm, MCB 102
Environmental regulation, benefits and costs of bacterial motility
Victor Sourjik (Max Planck Institute Marburg)
Microorganisms possess diverse mechanisms to regulate investment into individual cellular processes according to their extra- and intracellular environment. How these regulatory strategies reflect the inherent tradeoff between the benefit and cost of resource investment remains largely unknown, particularly for many cellular functions that are not immediately related to growth. I will present our recent work on understanding the physiological importance and environmental regulation of motility and chemotaxis of Escherichia coli, one of the most complex and costly bacterial behaviors. I will particularly focus on the regulation of motility as a function of bacterial growth rate. Our work showed that in poor nutritional conditions bacteria increase their investment in motility in proportion to the selective advantage provided by chemotaxis. Thus, bacteria appear to pre-invest resources into the motile behavior in proportion to the anticipated benefit that can be provided by chemotaxis when gradients of secondary nutrients are either introduced in their environment or created by bacterial communities through excretion and consumption of metabolites.
Jan 24, 2pm
Condensation and interactions of molecules on nanoparticles
Michal Farnik (J. Heyrovsky Institute of Physical Chemistry, Prague)
Clusters represent a bridge between individual molecules and bulk. Their investigations provide a detailed molecular-level understanding of bulk properties and complex processes in condensed matter. Our versatile cluster beam apparatus (CLUB) in Prague allows for a large variety of experiments with clusters and nanoparticles: adsorption of molecules; photoionization mass spectrometry; electron ionization and attachment; photodissociation and velocity map imaging including IR-UV pump-probe experiments, etc. Some of these experiments will be introduced in the talk. The first part will concentrate on molecular adsorption in relevance to aerosol particle formation in the atmosphere. The second part will show how intermolecular reactions are affected by the cluster medium. Using examples, we will demonstrate how the investigated phenomena are relevant to atmospheric processes, to astronomy and astrochemistry, and even to surface-assisted technologically relevant processes such as FEBID (focused electron beam induced deposition).
Jan 31, 2pm
From nanotech to living sensors: unraveling the spin physics of biosensing at the nanoscale
Clarice Aiello (UCLA)
Substantial in vitro and physiological experimental results suggest that similar coherent spin physics might underlie phenomena as varied as the biosensing of magnetic fields in animal navigation and the magnetosensitivity of metabolic reactions related to oxidative stress in cells. If this is correct, organisms might behave, for a short time, as “living quantum sensors” and might be studied and controlled using quantum sensing techniques developed for technological sensors. I will outline our approach towards performing coherent quantum measurements and control on proteins, cells and organisms in order to understand how they interact with their environment, and how physiology is regulated by such interactions. Can coherent spin physics be established – or refuted! – to account for physiologically relevant biosensing phenomena, and be manipulated to technological and therapeutic advantage?
Feb 21, 2pm
Mercehdeh Khajavikhan (USC EE)
Feb 28, 2pm
Photonics in complex system
Chia Wei Hsu (USC EE)