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
Razieh
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.

Biography
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

2017 BIOPHYSICS SEMINAR SERIES Van Ngo, Ph.D. (Calgary)
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.


March 10, 2pm SSL150
Christian Majenz (Caltech)
TBA



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)
TBA


October 27, 2pm HSC
April 28, 2pm UPC
Ralf Langen, Ph.D. (USC)
TBA

November 17, 2pm UPC
April 28, 2pm UPCEmily Liman, Ph.D. (USC)
TBA
 


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