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

September 2, 2pm SSL150
Daniel Ben-Zion (UCSD)

SPT and SET physics from decorated string nets

It has been understood for some time now that quantum entanglement plays a central role in classifying phases of matter. Patterns of long range entanglement can distinguish two phases even when symmetries cannot; this is called topological order. One interesting question is: what possibilities are there in systems with both topological order and a global symmetry? On the other hand, even in the absence of topological order it is occasionally possible to have two phases which preserve all symmetries yet are nonetheless distinct. This phenomena is known as a 'symmetry protected topological' phase. In this talk, I will give a few explicit examples of models both with and without topological order, emphasizing the interplay with global symmetries. These models have the feature that domain walls are decorated with auxiliary degrees of freedom which arrange themselves into particular interesting states.

September 7, 2pm SSC319
Si-Hui Tan (CQT Singapore)

An encryption scheme for malleable quantum data with information-theoretic security

We introduce an approach to homomorphic encryption on quantum data. Homomorphic encryption is a cryptographic scheme that allows evaluations to be performed on ciphertext without giving the evaluator access to the secret encryption key. Random operations from an finite abelian unitary group chosen using an encryption key chosen uniformly at random perform the encryption, and operations that lie within the centralizer of the encryption group perform the computation. Since the latter operations commute with any evaluation in the encryption group by definition, applying the inverse of the encryption decrypts the evaluated state, and the decryption key depends only on the encryption key. We show that the group of operations that can be used for computation is isomorphic to a unitary group of a large dimension. Moreover our scheme is information theoretically secure, that is, given orthogonal inputs to our sceme, the evaluator can only extract some amount of classical information that is exponentially suppressed via the Holevo quantity. For a specific encoding, we show that our scheme is able to hide a constant fraction of bits that can be made arbitrarily close to unity.

October 14, 2pm SSL150
Fariborz Nasertorabi (Bridge Institute/USC)

Structure Biology Center at bridge  enables USC research labs to extend their research into structure biology

Structural biology center (SBC) has been established to enable all biology labs at USC to extend their research into the field of structural Bioloy. Here at Bridge Institute we have executed an unique center that can efficiently help labs to progress into structural biology at a minimal cost and without  any prior knowledge in structural biology.  We at the center will work with labs at USC to solve the structure of their protein of interest alone or in complex with ligands/drugs, DNA, RNA or protein. We can additionally do ligand search for the highest stabilizer of the targeted protein.  
During my talk, I will walk you through how the center is set up and how USC labs can best benefit from the center in their research.

October 21, 2pm SSL150
Gerd Bergmann (USC)

Introduction into Topological Semiconductors

Topological insulators are, at low temperatures, insulating in the bulk, but at the surfaces or edges they are conducting. Due to strong spin-orbit coupling the electron spin and momentum are locked. In the simplest cases the spin is oriented perpendicular to the momentum.  As a consequence a current carried by these states carries a net spin and introduces a spin polarization at the surface. For two-dimensional topological insulators one obtains one-dimensional edge states. A current along these edges is protected by spin-momentum locking and time-reversal symmetry and should be free of dissipation. It shows a spin quantum Hall effect. The 2016 Nobel prizes in Physics for Thouless and Haldane are mainly given for their pioneering work in introducing the concept of topology in solid state physics. An overview of the physics of topological insulators will be given.

October 28, 2pm SSL150
Mohseninia (Sharif University of Technology)
Quantum non-Abelian Potts model and its exact solution

We introduced the one-dimensional non-Abelian Potts model, which is the generalized form of the one-dimensional Potts model to the case where the symmetry group is a non-Abelian finite group. In this talk, I make a rather detailed study of its properties. I determine the complete energy spectrum, i.e., the ground states and all the excited states with their degeneracy structure and also the partition function which means that the model is an exactly solvable model. Finally I determine the entanglement properties of the ground states and show that this model has a quantum nature, that is its spectrum of energy eigenstates consists of entangled states.
In the same way that the Ising and Potts models have led to a large number of applications in physics, it may also be the case that the non-Abelian case, in view of its richer structure, may find such applications.

Mohseninia, R., & Karimipour, V. (2016). Quantum non-Abelian Potts model and its exact solution. Physical Review B93(3), 035127.

November 11, 2pm SSL150

Barry Sanders (IQST,Caltech)
Machine Learning for Quantum Control

Quantum control is valuable for quantum technologies such as high-fidelity quantum gates, adaptive quantum-enhanced metrology, and ultra-cold atom manipulation. Although both supervised and reinforcement learning are used to optimize control parameters in classical systems, quantum control for parameter optimization is mainly achieved via gradient-based greedy algorithms. However, greedy algorithms can yield poor results for quantum control, especially for highly constrained large-dimensional quantum systems. We employ differential evolution algorithms to circumvent the stagnation problem of non-convex optimization, and we average over the objective function to improve quantum control fidelity for noisy systems. To reduce computational cost, we introduce heuristics for early termination of runs and for adaptive selection of search subspaces. Our implementation is massively parallel and vectorized to reduce run time even further. We demonstrate our methods with two examples, namely quantum phase estimation and quantum gate design, for which we achieve superior fidelity and scalability than obtained using greedy algorithms.

November 18, 2pm SSL150
Fabio Traversa (UCSD)

Practical Realization of Memcomputing Machines with Self-Organizing Logic Gates

Memcomputing is a novel computing paradigm based on (non-von Neumann) architectures that employ interconnected memory cells capable of using the collective state of the network to compute.
The computational power of these machines can be substantially increased if we embed in the network some extra information related to the problem to solve. We call this feature "information overhead’’. By taking advantage of this embedded information we can exponentially reduce the complexity of many problems such as the Non-deterministic Polynomial (NP) ones. In fact, the latter ones can be solved by a memcomputing machine with only polynomial resources (in time, space, energy).
A practical realization of these machines can be obtained by employing “self-organizing logic gates’’, i.e., logic gates that can accept inputs from all terminals, including the conventional output terminals and self-organize to satisfy their logic propositions. I will show how to use these gates to solve specific NP problems with polynomial resources. These novel logic gates and circuits can be realized with available nanotechnology components and are scalable.

December 9, 2pm SSL150
David Beratan (Duke University)

January 27, 2pm SSL150
Victor Martin-Mayor (Complutense University of Madrid)
Why are hard-to-solve problem instances so hard?

The difficulty of solving the small problem instances that one can submit to DW2 processors is very heterogeneous, ranging from easy to impossible [1,2]. This difficulty is controlled (to a large extent) by a physical effect named "temperature chaos" (TC). The best quantitative description of TC is provided by the performance of the Parallel Tempering, a Markov-chain Monte Carlo algorithm, on the system under study. TC controls the performance of simulated annealing and that of Selby's algorithm as well. It is only natural to guess that TC points to some topological features of the instance's energy landscape. In this talk, we try to unearth these topological features that underlie computational hardness by borrowing ideas from the Physics of supercooled liquids and glasses. We consider two ensembles of problem instances. On the one hand, our "control experiment" instances are  usual problem instances filtered according to their degree of TC. On the other hand, we consider quantum-protected instances, namely an ensemble of problem instances designed to increase the robustness of the Quantum Annealing algorithm, yet just as hard to solve with classical algorithms as  the "control experiment" instances. As it could be expected, DW2 is found to perform better on the ensemble of quantum-protected instances. However, we succeed in finding an "energy-landscape predictor" that quantitatively explains the  performance of DW2 both for the standard and the quantum-protected instances[3].

[1] V.M-M. and Itay Hen, Scientific Reports 5, Article number: 15324 (2015).
[2] Jeff Marshall, V.M-M. and Itay Hen, Phys. Rev. A 94, 012320 (2016).
[3] Mohammad Amin,  Evgeny Andriyash, Jeff Marshall, Itay Hen and V.M-M., manuscript in preparation.

January 27, 2pm UPC
Andre Kosmerlj, Ph.D. (Princeton)
Aggregation of Proteins: Growth of Glucagon Fibrils and Bacterial Growth

February 3, 2pm SSL150
Eli Kapon (Ecole Polytechnique Fédérale de Lausanne (EPFL) Switzerland)
Quantum Photonics with Ordered Quantum Dot and Quantum Wire Systems

Quantum wire (QWR) and quantum dot (QD) systems offer means for tailoring the electronic structure of semiconductors thanks to multi-dimensional quantum confinement.  By placing them in confined photonic structures (waveguides, cavities) it is possible to tailor light-matter interaction via the introduced modifications in the density of states of excitons and photons.  We review the technology of ordered QWR and QD structures grown by metallolrganic vapor phase epitaxy on patterned substrates and their integration with photonic components.  Tailoring exciton wavefunctions, controlling their recombination dynamics, and observing cavity quantum electrodynamic effects in the integrated structures are described.  Applications in quantum information technology and ultralow threshold lasers are discussed.

Eli Kapon received his Ph.D. in physics from Tel Aviv University, Israel in 1982. He then spent two years at the California Institute of Technology, Pasadena, as a Chaim Weizmann Research Fellow, and then nine years at Bellcore, New Jersey, as member of technical staff and District Manager. Since 1993 he has been Professor of Physics of Nanostructures at the Swiss Federal Institute of Technology in Lausanne (EPFL), where he heads the Laboratory of Physics of Nanostructures. In 1999-2000 he was a Sackler Scholar at the Mortimer and Raymond Sackler Institute of Advanced Studies in Tel Aviv University, Israel.  During that period he helped establishing the Tel Aviv University Center for Nanoscience and Nanotechnology and served as its first Director from 2000 to 2002.  In 2001 he founded the start up BeamExpress, serving as its Chief Scientist. His research interests include quantum- and nano-photonics, low-dimensional semiconductors, and vertical cavity semiconductor lasers. Prof. Kapon is Fellow of the Optical Society of America, the Institute of Electrical and Electronics Engineers, and the American Physical Society of America, a recipient of a 2007 Humboldt Research Award, and a Photonics Society Distinguished Lecturer for 2105-2017.


February 8, 2pm HSC

From Ion Selectivity to Drug Design in Transmembrane Proteins

February 17, 12:00 SSC 319
Abhinav Prem (Boulder, Colorado)
Multiply quantised vortices in fermionic superfluids: angular momentum, unpaired fermions, and spectral asymmetry

We compute the orbital angular momentum $L_z$ of an $s$-wave paired superfluid in the presence of an axisymmetric multiply quantised vortex. For vortices with winding number $|k| > 1$ we find that in the weak-pairing BCS regime, $L_z$ is significantly reduced from its value $\hbar N k/2$ in the BEC regime, where $N$ is the total number of fermions. This deviation results from the presence of unpaired fermions in the BCS ground state, which arise as a consequence of spectral flow along the vortex sub-gap states. We support our results analytically and numerically by solving the Bogoliubov-de-Gennes equations within the weak-pairing BCS regime.

Thursday March 9, 4pm EEB 123
Peter Shor (MIT)
Capacities for Quantum Communication Channels

In 1948, Shannon discovered his famous formula for the capacity of a communication channel. This formula does not apply, however, to channels with significant quantum effects. For quantum channels, the question of capacity is much more complicated, as there are different capacities for sending classical information and for sending quantum information. We will discuss the capacities of quantum channels, and survey the historical development of the subject.
Peter Shor received a B.S. in Mathematics from Caltech in 1981, and a Ph.D. in Applied Mathematics from M.I.T. After a one-year postdoctoral fellowship at the Mathematical Sciences Research Institute in Berkeley, he took a job at AT&T Bell Laboratories, and stayed at AT&T until 2003.  In 2003, he went to M.I.T., where he is the Morss Professor of Applied Mathematics.

March 10, 2pm SSL150
Christian Majenz (Caltech)
Quantum supremacy with random circuits

Quantum supremacy has been a trending topic in the last years, with experimentalists eager to try problems on their small scale quantum computing devices which are likely hard for a classical computer as a benchmark, however useless they may be in practice. In this talk, I will review the development of theoretical quantum supremacy results with random circuits and explain the main ingredients and difficulties of supremacy results for sampling problems. Finally I will present some initial results towards a clearer framework for supremacy with random circuits.

Tuesday April 4, 2pm EEB 248
Matthew J. Gilbert (Urbana-Champaign)
Topological Energy Transduction

Within the CMOS architecture, the interconnected devices may either be categorized as an “active” device, which produces energy in the form of a current or a voltage, or a “passive” device, which stores or maintains energy in the form of a current or voltage. The societal demand for smaller sized electronic devices, such as computers and cellular phones, with improved functionality has forced not only the sizes of the constituent components of CMOS information processing technology to rapidly shrink, but for the operational frequencies to increase. While it has been possible to reduce the size of active CMOS devices, passive devices have not seen the same reduction in size. Of the passive devices (e.g. resistors, capacitors and inductors) used in CMOS technologies, the circuit element that consumes the most area on a circuit board while simultaneously finding the least success in miniaturization is the inductor. In this talk, we will present a novel method for energy transduction that utilizes the interplay between magnetism and topology on the surface of a newly discovered materials, referred to as time-reversal invariant topological insulators, to create a paradigmatically different inductor. Using a novel self-consistent simulation that couples AC non-equilibrium Green functions to fully electrodynamic solutions of Maxwell's equations, we demonstrate excellent inductance densities up to terahertz frequencies thereby providing a potential solution to an eminent grand challenge.

Wednesday April 5, 2pm KAP 209
Matthew J. Gilbert (Urbana-Champaign)
Unconventional Superconductivity in Topological Heterostructures The search for materials and systems that exhibit unconventional superconductivity, or superconductivity beyond the canonical s-wave pairing as predicted in BCS theory, is one of the most active areas within condensed matter physics. This effort has been reinvigorated by the interesting properties inherent to a new class of materials that possess topological phases. A topological phase is unique in that it does not break any of the underlying symmetries of the system and cannot be described by a local order parameter. In other words, the inherent properties of the system cannot be changed by adiabatic shifts in materials parameters unless the system passes a quantum critical point associated with a phase transition. More recently, this search has taken on additional significance due to the fact that systems that possess unconventional superconductivity may enable a new type of fault tolerant quantum information processing that may significantly increase computing power when compared to traditional information processing. In this talk, I will discuss the appearance and signatures of unconventional superconductivity and review some of the most prominent systems that have been predicted to exhibit unconventional superconductivity. In particular, I will focus on heterostructures containing s-wave superconductors and proximity-coupled 3D time-reversal invariant topological insulators. I will explain some of the experimentally relevant conditions that must be satisfied in order to observe the features of unconventional superconductivity and conclude by examining the potential for finding unconventional superconductivity in emergent topological materials such as semimetals and crystalline insulators.

March 31, 2pm UPC
Xiaojiang Chen, Ph.D. (USC)
DNA Remodeling and Modification By Nucleic Acid Transaction Enzymes: Helicases and Deaminases

April 28, 2pm UPC
Sima Setayeshgar, Ph.D. (Indiana)

Tuesday May 2, 2pm SSL150
Rosa Di Felice (USC)
Theory of proteins and nucleic acids on inorganic surfaces

Proteins naturally recognize other proteins; combinatorial assays show that they can also recognize inorganic surfaces, but the specificity mechanisms are still elusive. Understanding such mechanisms would enable control of the structures, which is crucial to attain bio-nanotechnology applications. Nucleic acids have been proposed for molecular electronics and molecular computing. To be exploited as a material in nano-scale devices, DNA should resist surface contact (e.g., substrates, electrodes). It is known that the height of dsDNA on a mica substrate, measured with an atomic force microscope, is less than 50% of the nominal diameter in solution. Unfolding of the native helix is a hypothesis, but no experimental methods can tackle the atomic structure resolution of DNA on a surface. In my presentation, I will illustrate a long-term plan to investigate these systems by computational strategies, founded on electronic structure theory. Full quantum simulations of the target systems are hindered by the large size and complexity. Indeed, most density functional theory (DFT) electronic structure codes are suitable for either inorganic materials and surfaces (plane wave basis sets and gradient-corrected functionals) or molecules (Gaussian/atomic basis sets and hybrid functionals with a tunable fraction of exact exchange). On the other hand, classical molecular dynamics (MD) simulations are biased by the precision of the interatomic force field to evolve Newton’s equations of motion: in particular, when this project started (~2007), bio-inorganic interface force fields were in their very infancy and no implementation compatible with bio-oriented software existed. Because of its relevance and thorough characterization, the Au(111) surface was chosen as a prototype substrate to develop DFT-based interface force fields. The interface force field should take into account the polarization of the metal surface due to point charges in its proximity: this effect was mimicked by replacing each Au atom on the top plane with a rigid dipole free to rotate. Chemisorption effects were described by fitting force field parameters to DFT calculations of amino acid side chains and DNA bases. Van der Waals and stacking effects were tuned on experimental data for alkyl chains and aromatic molecules, respectively. The developed force fields were tested on case studies and then applied to amyloid-β-42@Au(111) and DNA-15-mers@Au(111). I will discuss the methodology assessment, results obtained for these systems and the implications for my future research plans.

Friday May 5, 2pm SSL150
Orianna Bretschger (J. Craig Venter Institute, Aquam)
Bioelectrochemical systems and electrogenic microbiomes

Microbial extracellular electron transfer is a process whereby microbes can respire solid-phase electron acceptors, or use solid-phase electron donors as an energy source. The energy derived from extracellular electron transfer into, or out of, a cell is used to facilitate growth and other microbial functions. Microbial extracellular reactions have been shown to drive geochemical cycling in soils, sediments and water columns; and these processes can also be applied to biotechnology applications for energy recovery and wastewater treatment using bioelectrochemical systems.
Several mechanisms have been proposed for how extracellular electron transfer may happen within model organisms such as Shewanella oneidensis MR-1 and Geobacter sulfurreducens. While studying model organisms has led to a deeper understanding of extracellular electron transfer mechanisms, microbes rarely thrive as monocultures and prefer to live in diverse microbial communities. To better understand how extracellular electron transfer may occur and be regulated within mixed microbial communities, we have developed a stimulus-induced metatranscriptomic approach to characterize the dynamic responses of key genes associated with electron transfer reactions in complex consortia. Results suggest a high diversity of microbes able to catalyze extracellular electron transfer reactions; and preferential syntrophic relationships between taxa that arise as a function of specific surface potentials.
Building on these results, we have designed microbial fuel cell systems to select for ‘electrogenic’ microbes that can convert the chemical energy bound in waste organics into direct electricity. Pilot-scale installations of these systems for the treatment of swine waste and human waste have shown that microbial fuel cells can provide a sustainable option for improved sanitation and water recycling.

Friday June 2, 2pm SSL 150
Justin Dressel (Chapman)
Continuous measurement of transmon qubits: state-dragging and stabilization using the quantum Zeno effect

Monitoring the energy levels of a superconducting transmon qubit continuously in time via the homodyne measurement of a microwave field has recently become possible in experiment. These measurements allow the reconstruction of stochastic quantum state trajectories that show competition between unitary Hamiltonian dynamics and non-unitary collapse dynamics. I review how to efficiently model such a measurement process, highlighting realistic inefficiencies and calibration difficulties, and discuss several recent achievements made in collaboration with the Siddiqi laboratory at UC Berkeley. I focus in particular on the use of the quantum Zeno effect for dynamical tasks. For example, actively rotating the measurement basis can effectively drag a collapsed qubit state along with the changing basis. Similarly, adding feedback control of a unitary Rabi drive creates nontrivial attracting points in state space to which a qubit may be stabilized for on-demand preparation.

Friday June 9, 2pm SSC 319
Ben Reichardt (USC)
Fault-tolerant quantum computation with few qubits

Reliable qubits are difficult to engineer, but standard fault-tolerance schemes use seven or more physical qubits to encode each logical qubit, with still more qubits required for error correction.  We give space-efficient methods for fault-tolerant error correction and computation.  For example, in a system with fewer than 20 qubits total, we can protect and compute fault tolerantly on seven encoded qubits.  Seven qubits suffice to protect one encoded qubit.  A main technique is to use gadgets to catch correlated faults.  The procedures could enable testing more sophisticated protected circuits in small-scale quantum devices. 
Joint work with Rui Chao.  arXiv:1705.02329 and 1705.05365. 

Monday June 12, 2pm EEB 248
Costas Courcoubetis (ESD, SUTD, Singapore)
Drivers, Riders and Service Providers: The impact of the sharing economy on Mobility
(Joint work with S. Benjaafar and H. Bernhard)

Ride sharing, the practice of sharing a car such that more than one person travels in the car during a journey, is often heralded as a more sustainable alternative of private transportation. It is widely believed that ride sharing through sharing economy platforms will significantly reduce congestion in populated urban areas. We introduce a model in which individuals may share rides for a certain fee, paid from the rider(s) to the driver through a ride sharing platform. Collective decision making is modelled as an anonymous non-atomic game with a finite set of strategies and payoff functions affine in the individuals’ types that include their utility for using private transportation and their income. We demonstrate that equilibria in this game may be represented as convex partitions of the two dimensional type space and are unique for almost all parameter combinations. With this model we study how congestion and ownership are affected through the introduction of a ride sharing platform to a population of given characteristics. In particular, we examine whether the potential reduction in congestion widely expected is actually attainable once monetary incentives are introduced that affect both the behaviour of users and the price choices of the platform.

We find that when car costs are low, casual ride sharing (P2P) will dominate the ride sharing market. When car costs are high, professional ride sharing (B2C) will dominate. Focusing on a monopolist revenue maximizing platform we encounter some paradoxical phe- nomena: For example, increasing car ownership costs as a measure to curb traffic volume might yield counter-intuitive outcomes: an increase in traffic volume, ownership and platform revenue coupled with a decrease in welfare. Comparing a revenue- with a welfare-maximizing platform we find that when cars are cheap the two platform objectives may be aligned. When cars are expensive, a revenue maximizing platform tends to induce an equilibrium with strictly worse welfare and strictly higher congestion compared to the welfare optimum. This suggests that in such a setting, a monopolist platform would need to be regulated more strictly to avoid socially undesirable outcomes.
Prof. Costas A Courcoubetis was born in Athens, Greece and received his Diploma (1977) from the National Technical University of Athens, Greece, in Electrical and Mechanical Engineering, his MS (1980) and PhD (1982) from the University of California, Berkeley, in Electrical Engineering and Computer Science. He was MTS at the Mathematics Research Center, Bell Laboratories, Professor in the Computer Science Department at the University of Crete, Professor in the Department of Informatics at the Athens University of Economics and Business, and since 2013 Professor in the ESD Pillar, Singapore University of Technology and Design where he heads the Initiative for the Sharing Economy and co-directs the new ST-SUTD Center for Smart Systems. His current research interests are economics and performance analysis of networks and internet technologies, sharing economy, regulation policy, smart grids and energy systems, resource sharing and auctions. Besides leading a large number of research projects in these areas he has also published over 100 papers in scientific journals and conferences. He is co-author with Richard Weber of “Pricing Communication Networks: Economics, Technology and Modeling” (Wiley, 2003).

Friday June 16, 2pm SSC 319
Martin M. Müller (Université de Lorraine)
Squeelix Elastica
Biological filaments are typically modeled as semiflexible chains. In recent years it was found that this picture risks to be far too simple. In particular, microtubules and actin filaments are not necessarily straight but can adopt the shape of a helix when no external forces and torques act on them. When confined to a plane, such as the plane of observation of an in vitro experiment, the helical filament becomes a "squeelix". The resulting ground states are circles, waves or even spirals depending on the material parameters.
In my talk I will present these shapes together with a stability analysis. We will see that conformational quasi-particles are generated which interact with each other. A theoretical study based on these quasi-particles allows to quantify the different energies that are involved in shape generation. The results can potentially be used by experimentalists to determine material parameters of such helical filaments.

October 27, 2pm HSC
Ralf Langen, Ph.D. (USC)

November 17, 2pm UPC
Emily Liman, Ph.D. (USC)


Januar 2015 - July 2016

January-August 2015

More seminars


USC | Department of Physics and Astronomy
How to get to USC