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USC Quantum Information and Condensed Matter Physics Seminars |
For more information contact Stephan Haas (condensed matter) (213) 740-4528, or Daniel Lidar (quantum information) (213) 740-0198.
David Boyd (Caltech)
Plasmon Assisted Optofluidics
Micron scale fluidic systems play host to a number of physical phenomena that have attracted both fundamental and applied interest. This presentation will discuss a recently demonstrated all-optical method for manipulating fluid and performing chemical reactions in a micro-fluidic circuit. Our approach makes use of the plasmon resonance in an array of nanoscale metal structures that are incorporated into the channel to produce localized heating of the fluid when illuminated by a low power laser near the resonant frequency of the particles. At low laser fluxes, the local photothermal energy allows for a host of functionality including pumping, interphase mass-transport, and sample pre-concentration, and at increased laser fluxes, the heating allows for heterogeneous catalysis. We will discuss our results as well as the advantages, challenges, and potential applications of the technique.
Sept. 18
Ricky J. Sethi (UCR)
The Data Driven Hamiltonian Monte Carlo
Motion and image analysis are both important for activity recognition in video. We present a new approach that extends the Hamiltonian Monte Carlo (HMC) to allow us to simultaneously search over the combined motion and image space in a concerted manner using well-known Markov Chain Monte Carlo (MCMC) techniques. For motion analysis in video, we use tracks generated from the video to calculate the Hamiltonian equations of motion for the systems under study, thus utilizing analytical Hamiltonian dynamics to derive a physically significant HMC algorithm which can be used for activity analysis. We then use image analysis to help explore both the motion energy space and the image space by integrating the Hamiltonian energy-based approach with an image-based data-driven proposal to drive the HMC, thereby yielding a Data Driven HMC (DDHMC). We reduce the enormity of the search space by driving the Hamiltonian dynamics-based MCMC with image data in this DDHMC. We also develop the reverse algorithm, which uses motion energy proposals to search the image space. Experimental validation of the theory is provided on the well-known USF Gait and Weizmann datasets. While HMC has been used in other contexts, this is possibly the first paper that shows how it can be used for activity recognition in video taking into account the image analysis results and using the physical motion information of the system. In addition, the DDHMC framework has potential application to other domains where statistical sampling techniques are useful, as we outline in the section on future work.
Sept. 25
Vivek Aji (UCR)
New Route to Superconductivity: Local Criticality and Time Reversal Violation
The discovery of High Temperature superconductors has given rise to a number of new paradigms in condensed matter physics for the past two decades. Among competing ideas that attempt to explain the phenomena, time reversal violation has gained prominence due to recent experimental observations. In this talk I will present evidence from polarized neutron scattering data that reveal the existence of magnetic order in the pseudo-gapped phase. Quantum disordering of this state results in a quantum critical point which is unconventional. The critical fluctuations are local in space and power law in time. This phase transition is driven by the proliferation of a new topological defect. The coupling of electrons to the quantum fluctuations leads to an attractive interaction in the d-wave channel providing a novel mechanism for superconductivity.
Oct 2
Eric Cohen Samulon (Stanford)
Magnetic Phases in the frustrated spin dimer compound Ba3Mn2O8
Ba3Mn2O8 is a spin-dimer compound based on pairs of S=1 3d2 Mn5+ ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero-field. Interactions between dimers broaden the triplet and quintuplet bands such that application of a magnetic field leads to multiple states marked by long range order above characteristic critical fields. In this talk I will present results of magnetization, heat capacity, magnetocaloric effect and torque magnetometry measurements of single crystal samples which reveal a complex phase diagram containing at least three distinct ordered states across the triplet and quintuplet regimes. Much of the phase diagram can be understood in terms of an effective spin ½ Hamiltonian containing only the lowest energy states (|0,0> & |1,1> and |1,1> & |2,2>, referred to the dimer states, for the singlet-triplet and triplet-quintuplet regimes respectively). Two distinct ordered states are observed in the singlet-triplet regime, which can be ascribed to the delicate interplay between single ion anisotropy and antiferromagnetic interdimer exchange on the frustrated triangular lattice. Finally, I will present new results on the effects of disorder created by doping nonmagnetic V5+ ions into Ba3Mn2O8.
Oct 9
Paolo Zanardi (USC)
Unitary equilibration after a quantum quench
Closed quantum systems evolve unitarily and therefore cannot converge in a strong sense to an equilibrium state starting out from a generic pure state. Nevertheless for large system size one observes temporal typicality. Namely, for the overwhelming majority of the time instants, the statistics of observables is practically indistinguishable from an effective equilibrium one. In this paper we consider the Loschmidt echo (LE) to study this sort of unitary equilibration after a quench. We draw several conclusions on general grounds and on the basis of an exactly-solvable example of a quasi-free system. In particular we focus on the whole probability distribution of observing a given value of the LE after waiting a long time. Depending on the interplay between the initial state and the quench Hamiltonian, we find different regimes reflecting different equilibration dynamics. When the perturbation is small and the system is away from criticality the probability distribution is G aussian. However close to criticality the distribution function approaches a double peaked, "batman-hood" shaped, universal form.
Oct 23
Alioscia Hamma (Perimeter Institute)
How sound can protect quantum memory: The Toric-Boson model
We discuss the existence of stable topological quantum memory at finite temperature. At stake here is the fundamental question of whether it is, in principle, possible to store quantum information for macroscopic times without the intervention from the external world, that is, without error correction. We study the toric code in two dimensions with an additional bosonic field that couples to the defects, in the presence of a generic environment at finite temperature: the toric-boson model. Although the coupling constants for the bare model are not finite in the thermodynamic limit, the model has a finite spectrum. We show that in the topological phase, there is a finite temperature below which open strings are confined and therefore the lifetime of the memory can be made arbitrarily (polynomially) long in system size. The interaction with the bosonic field yields a long-range attractive force between the end points of open strings but leaves closed strings and topological order intact.
Oct 27
David Parker (Naval Research Laboratory)
Tests of order parameter symmetry in the superconducting iron arsenides
The iron arsenide superconductors have been extensively investigated since the original discovery by Kamihara early in 2008, with maximum Tc's exceeding 50 K. Despite this, the most basic questions such as pairing symmetry and mechanism remain highly controversial. In this talk, I propose two methods of ascertaining this order parameter symmetry. The first is based upon the phase-sensitive Josephson interferometry, which was applied to the cuprates with great success, ultimately identifying the d-wave nature of these materials. The pnictides present great challenges to such experiments because many pairing states proposed for the pnictides are equivalent in the a and b directions, so that unlike in the cuprates, ab-plane corner junctions will not be useful. Yet one can find ways to use appropriate barrier materials to filter out the transmitted electrons so that different directions 'see' different electrons, or design junctions for this purpose. I will describe in detail three such possible experiments. The second method is based upon the general discovery of a phase-diagram region of spin density wave / superconductivity coexistence, and upon some experiments suggesting fully gapped superconductivity. We show that such a coexistence state necessarily leads to Fermi surface nodal behavior, where the material is effectively 'normal' if the original pairing state is 's++' or fully symmetric s-wave. However, in the case of the sign-changing 's+/-', no nodes would be formed in the coexistence state so that thermodynamic and transport probes would see exponentially activated behavior.
Nov 20
Martin Varbanov (USC)
Quantum scattering theory on graphs with infinite tails
Quantum walks have proven extremely useful for developing quantum algorithms. The recent NAND tree algorithm and the proof of universality of quantum walks for quantum computations prompts us to develop the quantum scattering theory on graphs with infinite tails attached in detail. Propagating and bound states are defined, the equations that they need to satisfy derived and their properties explored. The orthogonality and completeness of the energy eigenbasis is proved. The S-matrix and its properties for such graphs is consequently examined. We derive formulas for the S-matrix under operations of cutting, attaching or connecting tails and prove unitarity preservation under such operations.
USC 
Caltech
UCLA ITP