USC
Physics Seminars 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. UPC location for remote viewing of HSC seminars: To be determined.
HSC seminar location: Herklotz seminar room, Zilkha Neurogenetic Institute HSC. Location for remote viewing of UPC seminars: Herklotz
January 15, 2pm SSL150
Yazhen Wang (University of Wisconsin-Madison)
Statistical Analysis of Annealing Experiment Data
We propose statistical methodologies to analyze computing experimental data from a D-Wave device and simulated data from the MCMC based annealing methods, and establish asymptotic theory and check finite sample performances for the proposed statistical methodologies. Our findings confirm bimodal histogram patterns displayed in input-output data from the D-Wave device and both U-shape and unimodal histogram patterns exhibited in input-output data from the MCMC based annealing methods. Further statistical explorations reveal possible sources for the U-shape patterns. On the other hand,our statistical analysis produces statistical evidences to indicate that input-output data from the D-Wave device are not consistent with the stochastic behaviors of any MCMC based annealing models under the study.
January 22, 2pm SSL150
Zoltan Zimboras (University College London)
Entanglement negativity of bosonic and fermionic Gaussian states
In pure states of many-body systems, entanglement is routinely studied via the von Neumann (or entanglement) entropy and various forms of Renyi entropies, which provides a complete characterization of bipartite correlations. The situation becomes more complicated for mixed states, e.g., if the system is composed of more than two parts, and one is interested in the entanglement between two non-complementary pieces. In such a scenario the entanglement can be characterized by a suitable measure called logarithmic negativity which has been the focus of recent interest. Similarly to pure-state entanglement, most of our analytical understanding of negativity in many-body lattice systems originates from studying Gaussian states. In this talk I will give an overview about the available methods to extract information about the entanglement negativity in quasifree lattice models. In particular, I will present some new results on tripartite entanglement in ground states of critical lattice models in one and two dimensions and, furthermore, even for systems driven far from equilibrium.
January 29, 11am SSC319
Mohd Hamdi Bin Abd Shukor (Kuala Lumpur)
Development of functional coatings
Several functional coatings (HAp, TiN, TiO2, Al2O3, CrAlN), were developed in our research group for various applications mainly biomedical, automotive and aerospace. The used of different techniques like magnetron sputtering, physical vapor deposition, thermal spray, anodization and combustion synthesis were utilized to deposit the coatings. The main objectives of applying such coatings were to improve the overall performance of the parts or devices in terms of mechanical properties and/or biological response in the case of biomedical implants. Finite element method (FEM) was carried out to ascertain the mechanical behavior of the coating.
Samples were subjected to relevant characterization analysis and physico-chemical tests. Analysis such as SEM/EDX, XRD, XPS, EBSD, TEM and FTIR were often employed for that purpose. Scratch test, adhesion test, micro or nano-indentation and tribological test were the usual tests conducted to determine the mechanical properties of the coatings. The subsequent in-vitro and in-vivo tests were carried out on bioceramic samples to investigate biocompatibility and corrosion behavior of the samples.
Magnetron sputtering system having RF and DC targets is one of main devices available in the lab. Bioceramic layers like Hydroxyapatite (HAp), TiO2, Al2O3, were produced onto different metallic alloy substrates like Ti6Al4V, Ti6Al7Nb and stainless steel. Anodization process was incorporated to produce TiO2 nanotube arrays to enhance the formation of HAp.
Recently, a newly developed powder based magnetron sputtering system was installed in the lab. It is undergoing rigorous assessment and calibration test. A lot of exciting experiments can be conducted if the system is able to successfully sputter powder.
February 5, 12am SSL150
Crystal Senko (JQI)
TBA
February 19, 2pm SSL150
Elizabeth Crosson (Caltech)
Simulated quantum annealing can be exponentially faster than classical simulated annealing
Simulated Quantum Annealing (SQA) is a Markov Chain Monte Carlo algorithm that samples the equilibrium thermal state of a Quantum Annealing (QA) Hamiltonian. In addition to simulating quantum systems, SQA can also be considered as another physics-inspired classical algorithm for combinatorial optimization, alongside classical simulated annealing. However, in many cases it remains an open challenge to compare the performance of QA and SQA. In this talk I will describe a recent proof which shows that SQA efficiently converges to the global minimum of a bit-symmetric cost function with a thin, high energy barrier. This cost function was designed so that classical simulated annealing would take exponential time to climb over the barrier with thermal fluctuations, while QA is able to tunnel through the barrier efficiently. Our work provides evidence for the growing consensus that SQA inherits at least some of the advantages of tunneling in QA, and so QA is unlikely to achieve exponential speedups over classical computing solely by the use of quantum tunneling.
February 25, 3pm SGM 414
Julea Butt (University of East Anglia)
Wired for Life: Survival Strategies and Technology Opportunities
Proteinaceous molecular wires are essential for life because they underpin the mechanisms by which living organisms gain energy from their environment. In essence they perform the electron transfer that is critical for energy conservation in respiration and photosynthesis. Viewed at the atomic level these proteins contain chains of electron-conducting transition metals within an insulating sheath of polypeptide - a design with similarities to the wires that move electrical current in our electronic devices. Unique insights into the electron transfer properties of these molecular wires are provided by their adsorption as electroactive films on conductive substrates. We apply this approach, complemented by (magneto-)optical spectroscopies and x-ray crystallography, to provide fresh perspectives on the cellular function of these proteins and to inform their deployment in biotechnologies. In this lecture proteins that contribute to the biogeochemical cycling of S and of Fe will illustrate the types of information revealed by our studies. These examples prompt re-evaluation of the merits of tetrathionate as a terminal respiratory electron acceptor and reveal the structural basis for electron exchange across the outer membrane of Gram negative bacteria. The latter provides inspiration for our most recent work that aims to deliver visible-light driven molecular synthesis by coupling intracellular enzyme catalysis to charge separation at extracellular dye-sensitised semiconductor particles.
March 4, 2pm SSL150
Mohammed Hassan (Caltech)
Catching Electrons in the Act: Tracing the Nonlinear Response of Bound Electrons
In the last decade, the real-time studies of electron dynamics in matter becomes increasingly important for the accurate clocking of microscopic phenomena as well as for the ultrafast switching of functionalities in nonlinear optical devices (1-4). These studies paved the way to exploring electron dynamics on its action time scale, but relevant techniques cannot provide direct access into the dynamics of the nonlinear response of bound electrons to optical fields. Here we demonstrate the world first optical attosecond pulses synthesized in the visible and nearby ranges, permit the access to the sub-fs probing and control of bound-electronic response (5). Vacuum ultraviolet spectra emanating from atoms of Krypton, exposed to intense waveform- controlled optical attosecond pulses, embody signatures and allow retrieval of a finite nonlinear response time of bound electrons up to ~115 as. Our study paves the way to novel electronic spectroscopies of bound electrons in atomic, molecular or lattice potentials of solids (6) as well as to light-based nonlinear photonics operating on sub-fs time scales and PHz rates.
References
1. Corkum PB & Krausz F (2007) Attosecond science. Nat Phys 3(6):381-387.
2. Schiffrin A, et al. (2013) Optical-field-induced current in dielectrics. Nature 493(7430):70.
3. Schultze M, et al. (2013) Controlling dielectrics with the electric field of light. Nature 493(7430):75-78.
4. Kruger M, Schenk M, & Hommelhoff P (2011) Attosecond control of electrons emitted from a nanoscale metal tip. Nature 475(7354):78-81.
5. Hassan MT, et al. (2016) Optical attosecond pulses and tracking the nonlinear response of bound electrons. Nature 530(7588):66-70.
6. Ivanov M & Smirnova O (2013) Opportunities for sub-laser-cycle spectroscopy in condensed phase. Chemical Physics 414(0):3-9.
March 9, 2pm SSC319
Ilia Zintchenko (ETH Zurich)
Randomised gap and amplitude estimation in quantum devices
We provide a new method for estimating spectral gaps in low-dimensional systems. Unlike traditional phase estimation, our approach does not require ancillary qubits, nor does it require well characterised gates. Instead, it only requires the ability to perform approximate Haar–random unitary operations, applying the unitary whose eigenspectrum is sought out and performing measurements in the computational basis. We discuss application of these ideas to in-place amplitude estimation and quantum device calibration.
March 25, 2pm SSL150
Micah McCauley (Northeastern)
The Active Role of Nucleic Acids Seen at the Single Molecule Level
While information storage is the main function of nucleic acids, interactions with ligands influence many cellular activities, often by directly altering nucleic acid stability and flexibility. Single molecule optical tweezers experiments examine these processes in detail, probing the flexibility of long strands of DNA, directly unzipping RNA hairpins and unraveling nucleosome core particles. These studies are combined with atomic force microscopy, thermodynamic models and recent advances in non-equilibrium statistics. Recent experiments have successfully characterized proteins that directly destabilize nucleic acid structures as well as other ligands that effectively prevent strand separation, and even proteins that bend and enhance DNA flexibility.
April 1, 2pm SSL150
Jess Riedel (Perimeter Institute)
Where are the branches in a many-body wavefunction?
When the wavefunction of a macroscopic system (such as the universe) unitarily evolves from a low-entropy initial state, we expect that it develops quasiclassical "branches", i.e., a decomposition into orthogonal components each taking well-defined, distinct values for macroscopic observables. Is this decomposition unique? Can the number of branches decrease in time? Answering these questions is hard because branches are defined only intuitively, much like early investigations of algorithms prior to the Church–Turing thesis. A rigorous definition would give an exponential speed up to certain (non-stationary) matrix-product state numerical simulations, as well as solve Kent's Set Selection problem in the consistent histories formalism, a 20-year-old open challenge in the foundations of quantum mechanics. I introduce some tentative definitions based on the idea of redundant information and the phenomenon of quantum Darwinism, while establishing several related uniqueness theorems. A key counterexample is provided by the Shor error-correction code, which demonstrates that branch structure with robust, redundant records on macroscopic scales can hide incompatible (noncommuting) structure on microscopic scales, with implications to holographic situations where locality breaks down.
April 22, 2pm SSL150
Götz Seibold (BTU Cottbus-Senftenberg)
Static and dynamical correlations of strongly disordered superconductors
In the last decades the failure of the BCS paradigm of superconductivity in several correlated materials led to a profound modification of the description of the superconducting phenomenon itself. A case in point is the occurrence of Cooper pairing and phase coherence at distinct temperatures, associated respectively with the appearance of a single-particle gap and a non-zero superfluid stiffness. This particular behavior is observed in several materials, which range from high-temperature cuprate superconductors to strongly-disordered films of conventional superconductors. For the latter system scanning tunneling microscopy measurements have revealed that the superconducting state becomes inhomogeneous, segregating into domains of large and suppressed superconducting order parameter. In this seminar I will discuss the static and dynamical response of such systems based on studies of the attractive Hubbard model with strong on-site disorder and by including fluctuations beyond the Bogoljubov-de Gennes approach. With increasing disorder charge and amplitude correlations decouple due to the formation of superconducting islands. This emergent granularity also induces an enhancement of the charge correlations on the SC islands whereas amplitude fluctuations are most pronounced in the 'insulating' regions. While charge and amplitude correlations are short-ranged at strong disorder we show that current correlations have a long-range tail due to the formation of percolative current paths. Moreover it turns out that for strongly disordered superconductors phase modes acquire a dipole moment and appear as a subgap spectral feature in the optical conductivity.
April 29, 2pm SSL150
Martin Gundersen (USC)
Medical Applications for Intense Nanosecond Pulsed Electric Fields
This talk will describe research that 1) initially investigated the response of biological cells to intense electric fields of short (nanosecond) duration, and 2) has recently resulted in a new company, Pulse Biosciences, which is developing this technology as a therapy for tumors and other conditions.
June 17, 2pm SSL150
Gabriel A. Durkin (UCLA)
Goldilocks Probes for Noisy Interferometry via Quantum Annealing to Criticality
Quantum annealing is explored as a resource for quantum information beyond solution of classical combinatorial problems. Envisaged as a generator of robust interferometric probes, we examine a Hamiltonian of N>>1 uniformly-coupled spins subject to a transverse magnetic field. The discrete many-body problem is mapped onto dynamics of a single one-dimensional particle in a continuous potential. This reveals all the qualitative features of the ground state beyond typical mean-field or large classical spin models. It illustrates explicitly a graceful warping from an entangled unimodal to bi-modal ground state in the phase transition region. The transitional `Goldilocks' probe has a component distribution of width N^{2/3} and exhibits characteristics for enhanced phase estimation in a decoherent environment. In the presence of realistic local noise and collective dephasing, we find this probe state asymptotically saturates ultimate precision bounds calculated previously. By reducing the transverse field adiabatically, the Goldilocks probe is prepared in advance of the minimum gap bottleneck, allowing the annealing schedule to be terminated `early'. Adiabatic time complexity of probe preparation is shown to be linear in N.
Archive
January-August 2015
September-December 2015
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