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).
Fall 2014

November 21 (2014), 2pm SSL 150
Nikhil Malvankar (UMass)
Seeing is Believing: Imaging Charge Flow along Bacterial Proteins Reveals A Novel Mechanism of Biological Electron Transport

Electron flow in biologically proteins generally occurs via tunneling or hopping mechanism and the possibility of electron delocalization or metal-like conductivity has been considered previously impossible. In this colloquium, I will present our recent studies on protein nanofilaments, pili, secreted by a common soil microorganism Geoabacter sulfurreducens, that challenge this long-standing belief. Using a scanning probe microscopy-based nanoscopic approach to visualize charge propagation in native biomolecules, we have found out that pili propagate charges in a manner similar to metallic carbon nanotubes [1]. Conductive pili enable Geobacter to export electrons outside their body to carry out respiration [2] and cell-to-cell electron exchange [3] over several micrometers using direct electrical connections. I will discuss our fundamental studies as well as potential applications and bioelectronic devices using this new class of electronically functional proteins.

[1] Malvankar et al. Nature Nanotechnology, (2014) DOI: 10.1038/NNANO.2014.236
[2] Malvankar et al. Nature Nanotechnology, 6, 573 (2011)
[3] Summers, Fogarty, Leang, Franks, Malvankar and Lovley. Science, 330, 1413 (2010)
Speaker url:

December 5 (2014), 2pm SSL150
Angelo Bassi, University of Trieste
Models of spontaneous wave function collapse: what they are and how they can be tested

There are few proposals, which explicitly allow for (experimentally testable) deviations from standard quantum theory. Models of spontaneous wave function collapse (collapse models) are among the most-widely studied proposals of this kind. The Schroedinger equation is modified by including nonlinear and stochastic terms, which describe the collapse of the wave function in space. These spontaneous collapses are “rare” for macroscopic systems, hence their quantum properties are left almost unaltered. On the other hand, collapses become more and more frequent, the larger the object, to the point that macroscopic superpositions are rapidly suppressed. I will briefly review the main features of collapse models. Then I will present promising experimental tests, ranging from cosmological observations, to matter-wave interferometry, to optomechanics, to spectroscopy.

December 12 (2014), 2pm SSL150
Bill Kaminsky (MIT)
Diabatic Quantum Computation – Why it Might Not Really Pay to Go for the “A”?

In this talk, we argue that the time has come to consider how well an adiabatic quantum computer approximates problems when it degrades into following not its instantaneous ground state, but rather the diabatic continuation of its initial ground state. In other words, we argue the time has come to ask:
How low an energy does one achieve in the spectrum of the final, problem Hamiltonian if one is going sufficiently fast that at every avoided crossing one takes upper branch rather than the lower branch, but not so fast that one induces any notable excitation away from avoided crossings?
Moreover, we argue that the time has come to ask this question not merely because there are a mounting bodies of theoretical, numerical, and experimental evidence that an adiabatic quantum computer generically cannot supply an exponential speedup on exactly solving instances of NP-hard problems while there is a quite paltry body of evidence on how much it can speedup approximation of such instances to high accuracy. Rather, we argue the time has come because we actually possess tools --- namely, high-order perturbation theory around the initial Hamiltonian extended by Hermite-Padé interpolation at avoided crossings --- with which we should be able to make substantial progress on question of how good an approximation the diabatic continuation of the initial ground state provides. In closing, we also discuss progress on understanding how fast an adiabatic quantum computer can approximate NP-hard problem instances when it faces the practical constraint of operating at a constant temperature that cannot shrink as the instance sizes grow. Specifically, we describe a stand-alone result about a moment-based method to upper-bound how many energy eigenstates are within O(1) energy of a given energy eigenstate in the lowest part of the spectrum at a generic point in an adiabatic quantum computer’s interpolation.

January 30 (2015), 2pm SSL150

Andrew King (D-Wave)
Looking and not looking at analog control error in quantum annealing processors

D-Wave quantum annealing processors are subject to transient and systematic analog control errors that at current levels are well understood and can be reduced through technological advances.  In the meantime, we seek a meaningful study of these processors that takes this into account.  We therefore seek input classes and performance metrics whose sensitivity to control error is minimal (i.e. not looking at error).  In this talk I will present our error model and discuss how it affects performance on various input classes (i.e. looking at error).  This motivates the study of time to epsilon, measuring performance via approximate solution whose excitation from the ground state scales based on our error model.  I will then discuss methods for reducing error sensitivity in input classes, which might give us a better look at the dynamics of the platform.

February 4 (2015), 2pm SSC 319
Victor Martin-Mayor (Universidad Complutense Madrid)
Quantum versus Thermal annealing, the role of Temperature Chaos

The "D-Wave Two" machine presumably exploits quantum annealing effects to solve optimization problems. One of the preferred benchmarks is the search of ground-states for spin-glasses, one of the most computationally demanding problems in Physics. In fact, the "Janus" computer has been specifically built for spin-glasses simulations.  Janus has allowed to extend the time scale of classical simulations by a factor of 1000.  Whether D-wave's quantum annealing achieves a real speed-up as compared to the classical (thermal) annealing or not is a matter of investigation.  Difficulties are twofold. On the one hand, the number of q-bits (476), although a World record, is still small. On the other hand, the 476 q-bits are disposed in a particular topology (the chimera lattice), where hard-to-solve instances are extremely rare for a small system.  However, our work with Janus has taught us about a relevant physical effect: temperature chaos. Given a large enough number of q-bits, rough free-energy landscapes should be the rule, rather than the exception.  Therefore, the meaningful question is: how well quantum-annealing performs in those instances displaying temperature-chaos?  For a small number of q-bits, temperature-chaos is rare but fortunately not nonexistent. In the talk, we explain how our previous experience with Janus is allowing us to find chaotic instances for a small chimera lattice. The performance of both thermal annealing and quantum-annealing (D-wave) will be assessed over this set of samples.

February 20 (2015), 2pm SSL150

Paul Brumer (Toronto)

March 27 (2015), 2pm SSL150
Ilya Krivorotov (UC Irvine)

April 10 (2015), 2pm SSL150
Yaroslav Tserkovnyak (UCLA)

April 17 (2015), 2pm SSL150
Guanghou Wang (Nanjing University)
Clustering effect on topological transport of Cu-doped Bi2Te3 crystals

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