USC Physics Seminars

                                   Quantum Information – Condensed Matter – Biophysics




These seminars are scheduled on Fridays at 2:00pm, and the location is SSL 150, unless otherwise noted. Some of the seminars will be held jointly with UCLA and CALTECH.
For more information contact Lorenzo Campos Venuti (condensed matter) or Ben Reichardt (quantum information). Biophysics seminars are held the first Friday of each month and are indicated in blue.
UPC seminar location: Ahmanson Center for Biological Research, ACB 238. Location for remote viewing of HSC seminars: ACB 238
HSC seminar location: Herklotz seminar room, Zilkha Neurogenetic Institute HSC.  Location for remote viewing of UPC seminars: Herklotz



Friday October 26, 2pm SSL 150

Ron Naaman (Weizmann Institute of Science)
Chiral Molecules and the Electron’s Spin: From Spintronics to Enantio-Separation

Spin based properties, applications, and devices are commonly related to magnetic effects and to magnetic materials. However, we found that chiral organic molecules act as spin filters for photoelectrons transmission, in electron transfer, and in electron transport. 
The new effect, termed Chiral Induced Spin Selectivity (CISS),, was found, among others, in bio-molecules and in bio-systems. It has interesting implications for the production of new types of spintronics devices, and on electron transfer in biological systems. Recently we found that charge polarization in chiral molecules is accompanied by spin polarization. This finding shed new light on enantio-specific interactions and it opens the possibility to construct novel methods for enantio-separation.



Friday November 9, 2pm SSL 150

Angelo Lucia (Caltech)
Size-Driven Quantum Phase Transitions


The standard approach to mathematically describe phase transitions in classical and quantum many-body models is to consider sequences of increasing size and then study their limit, which is usually called the "thermodynamic limit". Nonetheless numerical simulations and real world experiments can only consist of a finite number of particles, from which is sometimes possible to extrapolate information about what happens in the limit.  While this approach has been generally very successful, it has been shown recently that some properties of the thermodynamic limit are undecidable, i.e. there cannot exist an algorithm which predicts them. This implies that very exotic behaviour can appear: as an example, I will show how to construct 2D quantum spin models which appear to be classical at finite sizes but will reveal their nature of topological models at system sizes inaccessible for all practical purposes.



Friday November 16, 2pm SSL 150
Claudia Ojeda-Aristizabal (California State University, Long Beach)
Thin film C60: a new available block for the building of van der Waals heterostructures

C60, the organic molecule made of sixty carbon atoms arranged in a truncated icosahedron, has brought a lot of excitement to condensed matter physicists, chemists and material scientists since it appearance in 1985. The combination of the buckyball structure with important electron-electron and electron-phonon interactions, bring unique properties not seen in ordinary non-molecular crystalline materials, such as superconductivity at relatively high temperatures when doped with alkaline metals. In its bulk form, C60 arranges itself in a face-centered cubic (fcc) lattice (one C60 molecule centered at each lattice site), while in its thin film form it is deposited in layers corresponding to the (111) direction of the bulk, forming a triangular lattice. As an initial approximation one would think that C60’s electronic structure is dominated by the electronic interactions within a single molecule. Instead, we have found that long range interactions between the molecules have a profound effect shaping the electronic structure of the material. In this talk, I will show angle resolved photoemission spectroscopy experiments and density functional calculations that support this claim. I will also discuss an electronic device made of a van der Waals heterostructure composed of a thin film crystalline C60, graphene and hexagonal boron nitride.
C. Ojeda-Aristizabal*, E. J. G. Santos* et. al.  ACS Nano 11, 4686 (2017).
D. W. Latzke*, C. Ojeda-Aristizabal* et. al. Submitted (2018)
D. W. Latzke*, C. Ojeda-Aristizabal* et. al. In preparation (2018)



Friday November 30, 2pm SSL 150

Andreas Bill
(California State University, Long Beach)
Intertwined states in Superconducting-Magnetic Hybrid Nanostructures

Modern developments in spintronics and quantum computing rely on nanoscale devices where the role of interfaces between materials with different physical properties is dominant. Typically, none of the competing ground states is established and the resulting electronic state is intertwined with new features. We discuss multiple facets of intertwined states in superconducting-magnetic hybrid nanostructures with localized or itinerant magnetism. Superconducting pair correlations, magnetization and variations of the charge density are determined self-consistently. Josephson currents through various inhomogeneous magnetic multilayers are calculated. We provide an overview of possible states, how the symmetry of pair correlations is strongly modified by tunable magnetic inhomogeneities and discuss their measurable signature in the Josephson current. We also show how electronic phase separation and the competition of magnetic and superconducting orders occur in itinerant ferromagnets.

Support from the National Science Foundation (DMR-1309341) is gratefully acknowledged.



Friday December 7, 2pm SSL 150
Takahiro Sagawa (University of Tokyo)
Second law and eigenstate thermalization in isolated quantum many-body systems

In recent years, the fundamental mechanism of thermalization of isolated many-body quantum systems has attracted renewed attentions, in light of quantum statistical mechanics, quantum information theory, and quantum technologies. In particular, it has been recognized that the eigenstate thermalization hypothesis (ETH) plays a crucial role in understanding the mechanism of thermalization, which states that even a single energy eigenstate is thermal if the system is quantum chaotic.
In this talk, I will discuss our recent results on the second law of thermodynamics for pure quantum states [1].   In our setup, the entire system obeys unitary dynamics, where the initial state of the heat bath is not the Gibbs ensemble but a single energy eigenstate.   Our proof is mathematically rigorous, and the Lieb-Robinson bound plays a crucial role.   In addition, I will talk about our numerical result on large deviation analysis of the ETH [2], which directly evaluates the number of athermal energy eigenstates and validates the ETH.    Our results would reveal a general scenario that thermodynamics emerges purely from quantum mechanics.
[1] E. Iyoda, K. Kaneko, and T. Sagawa, Phys. Rev. Lett. 119, 100601 (2017).  
[2] T. Yoshizawa, E. Iyoda, and T. Sagawa, Phys. Rev. Lett. 120, 200604 (2018).  



Friday February 1, 2pm SSL 150
Masoud Mohseni (Google)
Approximate optimization and sampling with tensor networks


We introduce a deterministic physics-inspired classical algorithm to efficiently reveal the structure of low-energy spectrum for certain low-dimensional spin-glass systems that encode optimization problems. We employ tensor networks to represent Gibbs distribution of all possible configurations. We then develop techniques to extract the relevant information from the networks for quasi-two-dimensional Ising Hamiltonians. In particular, for the hardest known problems devised on Chimera graph known as Deceptive Cluster Loops, for up to 2048 spins, we we find 10^{8} of the high quality solutions in a single run of our algorithm. To this end, we apply a branch and bound strategy over marginal probability distributions by approximately evaluating tensor contractions. Our approach identifies configurations with the largest Boltzmann weights corresponding to low energy states. Moreover, by exploiting local nature of the problems,  we discover spin-glass droplets geometries. This naturally encompasses sampling from high quality solutions within a given approximation ratio which is #P hard. It is thus established that tensor networks techniques can provide profound insight into the structure of disordered spin complexes, with ramifications both for machine learning and noisy intermediate-scale quantum devices. At the same time, limitations of our approach highlight alternative directions to establish quantum speed-up and possible quantum supremacy experiments.


Friday February 15, 2pm SSL 150
Matthew Gilbert (Stanford)
The Non-Hermitian Chern Insulator

During the past decade, the landscape of condensed matter physics has been dominated by a desire to understand the topological nature of materials and the observable consequences that are associated with the presence of topology. To this end, topological band theory has provided a foundation that has allowed for the definition of topological invariants, or conserved quantities that do not change based on adiabatic changes to the system parameters. Nonetheless, each of the systems that has been considered to this point, have relied on the fact that the system under consideration is both closed and Hermitian. Recent work has extended topological band theory to open, non-Hermitian Hamiltonians but little is understood about how non-Hermicity alters the topological quantization of associated observables. In this talk, we will begin to address these problems by examining the non-Hermitian Chern insulator where we focus on changes in observables and its relation to our current understanding about non-Hermitian topological band theory.



Friday March 22, 2pm SSL 150
Philip Stamp (University of British Columbia, Caltech)
Entanglement correlators and Quantum Ising systems

Physical quantities are typically discussed in terms of 1-particle or 2-particle correlations. However there is now an urgent need to go beyond this - in quantum information processing and elsewhere - to deal with multipartite entanglement. The solution of this problem leads to some fascinating results. In particular:     
(i) One can derive equations of motion for a set of 'entanglement correlators', which define the multipartite correlation and entanglement between a set of N qubits - these are in general coupled to some environment, which causes decoherence and relaxation in the dynamics.     
(ii) These equations can then be applied to 'quantum Ising systems', which not only describe quantum information processing, but also a large variety of experimental systems. One can make - in some cases - quite striking predictions. 
In this talk I give an overview of the theory, and also show how it can be applied to several solid-state systems, including the rare earth LiHoxY1-xF4, molecular magnet systems like Fe8 or Mn12, and to superconducting networks. A key feature is the quantitative treatment of decoherence in these systems.




Friday March 29, 2pm SSL 150
Zachary Levine (Yale)
Disordered Protein Folding and the Thermodynamic Determinants of Amyloid Behaviors

A longstanding guiding principle in molecular biophysics is that the amino acid sequence of a protein determines its three dimensional fold, and that structure is inherently linked to function. However, nearly a third of proteins encoded in the human proteome contain intrinsically disordered regions, and lack a discernible tertiary structure. Many of these proteins, often referred to as intrinsically disordered proteins (or IDPs), can in fact adopt multiple transient conformations in order to carry out a wide-variety of functions, challenging the classical structure-function paradigm. Dysregulation of IDPs can also lead to the pathological accumulation of cytotoxic amyloids and fibrils, thereby contributing to degenerative disorders such as Alzheimer's Disease and Type II Diabetes. Characterizing the structures and functions of IDPs has historically been an onerous task, in part because transient states are difficult to capture in experiments. As a result, molecular simulations have emerged as a powerful tool for interpreting and augmenting experimental measurements of IDPs from first-principles. In this talk I will summarize my work using molecular modeling to characterize and modulate IDP behaviors in a wide variety of physiological and pathological environments. These studies reconcile individual protein behaviors with macroscopic experimental observations using protein ensembles that populate vast free energy landscapes. I will also discuss new and ongoing work to design peptide aptamers that mitigate the symptoms of cancer and Alzheimer’s Disease through common approaches that target heterogeneous amyloid behaviors. Discerning subtle differences between physiological and pathological protein behaviors is essential for understanding and exploiting the biophysics of amyloids, and can inform next-generation disordered biomaterials that require integration between state-of-the-art theoretical and experimental techniques.
 


Friday April 5, 2pm SSL 150
Lorenzo Campos Venuti (USC)
Ergodicity, eigenstate thermalization, and the foundation of statistical mechanics in quantum and classical systems

Boltzmann ergodic hypothesis is a possible way to explain the emergence of statistical mechanics in the classical world. In quantum mechanics instead, the Eigenstate Thermalization Hypothesis (ETH) is generally considered to be a possible route to thermalization. The notion of ergodicity itself is less clear in the quantum world and often it is simply taken as a synonym for thermalization. Here I will show, in an elementary way, that when quantum ergodicity is properly defined, ETH is in fact equivalent to the latter. In turn ergodicity is equivalent to thermalization thus implying equivalence between thermalization and ETH. This is a result that already appeared in [De Palma et al., Phys. Rev. Lett. 115, 220401 (2015)] but becomes particularly clear in this context. I will also show that it is possible to define a classical analogue of ETH which is implicitly assumed to be satisfied when constructing classical statistical mechanics. Classical and quantum statistical mechanics are built according to the familiar standard prescription. This prescription, however, is ontologically justified only in the quantum world.



Friday May 3, 2pm SSL 150
Kavan Modi (Monash University, Melbourne)
Quantum stochastic processes: A complete theory for non-Markovian quantum phenomena

In science we often want to characterise dynamical processes to identify the underlying physics, predict the future states, or exercise control over the system. If state of the system at any time depends only on the state of the system at the previous time-step and some predetermined rule then the dynamics are characterised with relative ease. For instance, the dynamics of quantum mechanical systems in isolation is described in this way. However, when a quantum system repeatedly interact with an environment, the environment often ’remembers’ information about the system's past. This leads to non-Markovian processes, which depend nontrivially on the state of the system at all times during the evolution. Such dynamics are not, in general, be easily characterised using conventional techniques. Indeed, since the early days of quantum mechanics it has been a challenge to fully describe non-Markovian processes. Here we will show, using operational tools from quantum information theory, how to fully characterise any non-Markovian process [1]. This newly developed framework allows us to build an unambiguous criteria for quantum Markov processes [2]; extend the notion of Markov order to quantum systems [3,4]; cast master equations in terms of operational elements, i.e., CPTP and higher order maps [5]; and show that our framework constitutes the theory for quantum causal modelling [6]. Finally, using these tools we expose non-Markovianity in IBM's five qubit computer [7,8].

[1] Phys. Rev. A 97, 012127 (2018)
[2] Phys. Rev. Lett. 120, 040405 (2018)
[3] arXiv:1810.10809 to appear in Phys. Rev. A (2019)
[4] arXiv:1805.11341 to appear in Phys. Rev. Lett. (2019)
[5] Quantum 2, 76 (2018)
[6] arXiv:1712.02589 (2017)
[7] arXiv:1901.05223 (2019)
[8] arXiv:1902.07980 (2019)

 


Friday May 10, 2pm SSL 150
Sabrina Maniscalco (University of Turku)
Bosonic Complex Quantum Networks: What, when and why

In this talk I will present some perspectives on the “what? when? why?” questions in the title by looking at Hamiltonian models describing complex networks of quantum harmonic oscillators. I will first show that such systems are very useful for investigating the properties of open quantum systems, namely quantum systems interacting with an environment. This framework considers one of the nodes as the open system and the other nodes of the network as part of the environment. I will show that, changing the properties of the network, it is possible to engineer ad hoc open quantum dynamics by modifying the spectral density of the environment. This is particularly relevant in connection to quantum technologies where understanding and modelling environmental noise is crucial to realise robust and scalable commercial quantum devices. With a change in perspective to the complementary view point, the node forming the open quantum system can be seen as a local probe from which one can extract certain properties of the network. Remarkably, we show that global properties can be mapped into the time evolution of the probe hence, measuring the latter one, one can extract them. I will focus in particular on the ability to measure the connectivity of the network by local probing. Finally, I will discuss schemes for efficient and robust energy and entanglement transfer across complex quantum networks. I will argue that, independently of whether or not these systems exist in Nature, the ability to engineer them experimentally has great relevance to both fundamentals of quantum mechanics and applications such as quantum technologies.














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