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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.
Amy Oldenburg (UIUC)
Magnetic and plasmon-resonant nanobiological imaging probes in optical coherence tomography
Optical coherence tomography (OCT) is used to interferometrically locate and sense nanoscale displacements of nanoparticles within biological tissues. Magnetic nanoparticles of 20nm size are modulated using a magnetic gradient to probe the viscoelastic response of the biological tissue microenvironment. In vivo imaging of the nanoparticles shows complementary information about the particle biodistribution in conjunction with whole-body MRI. Gold nanorods 15 x 45 in size are promising as orientational bioimaging probes, because they exhibit an alignment-specific surface plasmon optical absorption resonance sensed using spectroscopic OCT.
Jan 18
M. El-Naggar (USC)
Living Conductors: The Nature and Implications of Electrical Transport in Bacterial Nanowires
Bacterial nanowires are conductive pilus-like appendages produced by bacteria, most notably some .metal-reducers., in direct response to electron acceptor limitation. These recently discovered supramolecular assemblies represent a new paradigm in extracellular electron transfer, but the mechanism of electron transport remains unclear. This talk will feature quantitative measurements of transport across bacterial nanowires produced by the dissimilatory metal-reducing bacterium (DMRB) Shewanella oneidensis MR-1, whose electron transport system holds practical promise for renewable energy recovery and bioremediation. The Shewanella nanowires display a surprising non-linear electrical transport behavior, where the voltage dependence of the conductance reveals peaks indicating discrete energy levels with higher electronic density of states. These results indicate that the molecular constituents along the Shewanella nanowires possess an intricate electronic structure that plays a role in mediating the overall electron transport. We will highlight the vast implications of bacterial nanowires, especially for signal transduction at the biological-inorganic interface as well as devices that exploit this interface, such as microbial fuel cells. We will also discuss our recent efforts to develop experimental and image analysis tools that target the interactions between the living and non-living worlds at this interface.
Jan 23
B. Normand (ETH Zurich)
High-dimensional fractionalisation and spinon deconfinement in pyrochlore antiferromagnets
Spin S = 1/2 Klein models on the checkerboard and pyrochlore lattices contain in their ground-state manifold the subspace generated by the set of singlet dimer coverings, and thus possess an extensive ground-state degeneracy. Among the many exotic consequences is the presence of deconfined fractional excitations (spinons) which propagate through the entire system. While a realistic electronic model on the pyrochlore lattice is close to the Klein point, this point is in fact inherently unstable because any perturbation e restores spinon confinement at T = 0. Deconfinement is recovered in the finite-temperature region e << T << J, where the deconfined phase can be characterised as a dilute Coulomb gas of thermally excited spinons. The zero-temperature phase diagram away from the Klein point is analysed by means of a variational approach based on the singlet dimer coverings of the pyrochlore lattices and taking into account their nonorthogonality.
Jan 25
Juraj Topolancik (Harvard)
High-Quality Optical Resonators and Their Applications
High-quality (Q) optical resonators confine significant optical powers in small spaces for extended time periods. As a result, interactions of light with matter are greatly enhanced in these microstructures. They have attracted considerable scientific and technological interest due to their potential applications in optical signal processing, sensing and nonlinear optics. The first part of the talk will focus on whispering-gallery microresonators (silica microspheres) and will describe how we have employed them to monitor molecular transformations in complex biomembranes. The second part will consider nanofabricated cavities in photonic crystals (PhCs). Two-dimensional PhCs are periodic dielectric structures (usually air-holes in a high-refractive-index material such as silicon) that inhibit light propagation in bands of frequencies commonly referred to as photonic bandgap. Intentional breaking of the lattice periodicity introduces local defects in which light is trapped by the total internal and Bragg reflections. The exciting possibility of a highly-efficient photon confinement has established PhCs as a popular platform for designing optimized nanocavities. We have recently explored a conceptually different approach to photon localization in these structures. The design concept applies random structural perturbations uniformly throughout the artificial crystal by deliberately changing the shapes and orientations of its lattice elements. The disorder created this way represents random scatterers which impede propagation of Bloch-waves through the underlying periodic lattice. Optical modes guided along line-defects in disordered crystals experience strong backscattering which gives rise to localization. We have observed optical resonances with ultra-small modal volumes and the effective Qs of up to ~250,000. These Qs are comparable to the values measured on nanocavities engineered by meticulous parametric tuning of local defects. I will briefly discuss applications of these random nanocavities for ultra-sensitive biodetection.
Feb 1
Jeffrey Anker (Northwestern University)
Bright Shining Nanosensor Platforms with Magnetic Control and Other Features
Fluorescent and plasmonic nanoparticles are increasingly used in cellular imaging applications because they fluorescence and scatter light so brightly that single nanoparticles can be observed over extended periods without bleaching. In addition, nanoparticles can act as a platform onto which many components can be loaded. By loading new components onto nanoparticle platforms, new properties are created with diverse applications. For example, using vapor deposition to coat a fluorescent nanosphere with a hemispherical half-shell of opaque metal breaks the nanosphere's optical symmetry so that it reflects and fluoresces light in an orientation-dependent manner. As a result, the nanoparticle blinks when it rotates. If the nanoparticle is magnetic, it aligns with an external magnetic field and blinks when it rotates to follow a rotating magnetic field. The blinking signal from these magnetically modulated optical nanoprobes (MagMOONs) can be separated from unmodulated autofluorescence backgrounds. The blinking frequency provides a measure of the local viscosity, while the fluorescence spectrum provides a measure of the concentration of chemicals that interact with the fluorophores in the nanosensors. Magnetic nanosensors can also be guided and assembled into swarms using magnetic tweezers. Molecular plasmonic nanosensors are another example of multifunctional nanoplatforms. In addition to serving as brightly scattering spatial labels, the nanoprisms exhibit exquisite sensitivity to changes in local refractive index. When molecules adsorb to the nanoparticle surface, the resulting increase in local refractive index causes the nanoparticle extinction and scattering spectrum to redshift. Tracking this redshift in real-time facilitates the study of molecular binding kinetics and interactions. Overall, combinations of top-down and bottom-up nanofabrication processes provide control over nanoparticle optical, magnetic, and chemical properties allowing integration of features for a wide range of biomedical applications.
Feb 8
Research Orientation Meeting for First Year Graduate Students
TBA
TBA
Feb 15
Clifford Hicks (Stanford)
Scanning magnetic imaging of Sr2RuO4 and PrOs4Sb12-- searching for spontaneous time-reversal-symmetry breaking.
We present scanning squid magnetometer data on the superconducting materials strontium ruthenate (Sr2RuO4) and praseodymium-osmium-antimonide (PrOs4Sb12), both of which are believed to have spin-triplet pairing and to generate spontaneous time-reversal-symmetry-breaking fields below their superconducting transition temperatures. Our images do not show evidence for spontaneous TRSB fields, at the 0.1mG level, in contrast to muon spin rotation data which indicates gauss-scale fields in both materials. The fields indicated by uSR data must therefore have a length scale short compared with our spatial resolution (~3um) and/or a short time scale. Also, strontium ruthenate is believed to have a TRSB orbital order parameter, k_x + i k_y, which should result in spontaneous magnetic fields at sample edges and order parameter domain walls. Current experimental evidence suggests micron- to tens of micron-scale domains, comparable to or larger than our spatial resolution, however we do not see any edge fields. Our limits are roughly two orders of magnitude below current theoretical predictions.
Feb 22
Seth Lloyd (MIT)
Quantum Private Queries
Alice wants to ask Bob a question. Bob wants to give Alice the answer. But there's a catch: Alice doesn't want Bob to know what the question is. Classically, the only ways for Alice to make a provably private query are unwieldy. For example, Alice sends Bob a billion questions, of which her original question is one, and has him answer all of them. Quantum mechanically, however, Bob can answer Alice's question and provide her with a guarantee that he doesn't know what her question is. This talk shows how.
Feb 29
USC/CALTECH/UCLA Joint Condensed Matter Seminar
TBA
TBA
March 7
Stefan Vajda (ANL)
Clusters and Cluster-Based Nanostructures: New Materials with Distinct Physical and Chemical Properties
Small nanoparticles possess strongly size-dependent chemical and physical properties. In the smallest size regime, their properties can change by orders of magnitude by an addition or removal of a single atom, thus allowing a tuning of their properties with atomic precision. Small clusters consisting of only a handful of atoms can be also used as unique building blocks for larger 1D-3D structures. Samples of well defined sub-nanometer to few tens of nanometer large particles were prepared by size-selected cluster deposition from a molecular beam. The size and shape of produced nanoparticles, their thermal stability and temperature/reaction induced shape transformation was studied by using synchrotron-based X-ray techniques and scanning electron microscopies. The powerful combination of precise synthesis and characterization techniques will be demonstrated on the design of nanostructured materials with very specific physical (UV-VIS) and catalytic properties.
March 14
Mike Hsieh (USC)
The Quantum Control Landscape: A General Model of Quantum Optimal Control
The success of optimal control methods for quantum control simulations and experiments across a wide variety of physical systems is a desirable, but ill-understood phenomenon. Whether in computational simulations or laboratory experiments, quantum optimal control is generally a high-dimensional optimization problem with a very complex input-output mapping between the controls and the dynamical outcome. Why is it that relatively simple "optimization rules" such as local gradient search and global evolutionary algorithms are able to find good to excellent solutions to such problems? If the control process is abstracted as a directed search over a "quantum control landscape," a general model of the input-output mapping that assumes only (i) unitarity and (ii) controllability in the dynamics, it is found that the amenability of the control problem to solution by optimization may be found in the remarkably favorable topology of the landscape.
March 28
Chunrong Yin (USC)
Softlanding of mass-selected silver clusters on C60 film
The free clusters with electronic or geometric magic sizes are more stable than others. But what will happen for supported clusters due to the interaction between the clusters and surface? We softland mass selected silver clusters on Au(111) covered with C60 film at different temperatures and study thermally activated behaviors of clusters with LT-STM. On 1 ML C60/Au(111), when substrates are heated to high temperatures, clusters decay to a unique height which is also found in room temperature deposited clusters. Such unique clusters turn out to be quite stable in our studies system. While we didn't see such phenomena on cluster deposited on 2 ML C60/Au(111).
April 1, 11:00am, SSC 319
Barry Sanders (Calgary)
Quantum Walks on Circles in Phase Space via Cavity or Circuit Quantum Electrodynamics
ABSTRACT: We show how a quantum walk in phase space can be implemented via cavity or circuit quantum electrodynamics (CQED) where only the resonator field (i.e. the walker) needs to be driven and measured. The atom or Cooper pair box (i.e. the coin) is controlled indirectly via Jaynes-Cummings coupling. Decoherence can be tuned so that the transition from quantum to classical walk can be controlled, which confirms the quantum nature of the walk. In contrast to previous proposals for CQED realizations, the walker is not confined to one circle in phase space (fixed mean energy) but rather leaps to other circles in phase space. Despite this complication, the quantum enhanced diffusion of walker's phase can be cleanly observed and rigorously explained, thereby enabling the first experimental realization of a single-walker quantum walk without the need for direct control of the coin state.
April 4
Yoshifumi Morita
Optical Study of the Dirac Fermion: from unconventional superconductivity to carbon material
Making use of the Raman scattering as a probe, we shall reveal several properties of the Dirac fermion theoretically. Application to unconventional superconductivity etc. is also discussed.
April 11
Carlos Mochon (Perimeter Institute)
Quantum weak coin flipping with arbitrarily small bias
Abstract: Coin flipping by telephone (Blum '81) is one of the most basic cryptographic tasks of two-party secure computation. In a quantum setting, it is possible to realize (weak) coin flipping with information theoretic security. Quantum coin flipping has been a longstanding open problem, and its solution uses an innovative formalism developed by Alexei Kitaev for mapping quantum games into convex optimization problems. The optimizations are carried out over duals to the cone of operator monotone functions, though the mapped problem can also be described in a very simple language that involves moving points in the plane. Time permitting, I will discuss both Kitaev's formalism, and the solution that leads to quantum weak coin flipping with arbitrarily small bias.
April 16, SSC 319, 11am
Rosa DiFelice (National Center on nanoStructures and bioSystems at Surfaces, Italy)
Theoretical methods for the investigation of nano-materials
Understanding the electronic and structural properties of materials at the nano scale is a prominent goal of current research in materials science and condensed matter physics. The rationale underlying intense efforts in nano-science and nano-technology is the pursuing of ever smaller devices and ever denser circuits that assemble spontaneously without the need for costly lithographic processes. Experiments in this field call for theoretical interpretation and guidance. In my presentation I give an overview of the methods applied for the theoretical investigation at various possible levels, focusing on few examples from atomistic simulations by density functional theory and molecular dynamics.
April 18
Marcos Rigol (UCSC)
Thermalization and its mechanism for generic isolated quantum systems
Time dynamics of isolated many-body quantum systems has long been an elusive subject. Very recently, however, meaningful experimental studies of the problem have finally become possible stimulating theoretical interest as well. Progress in this field is perhaps most urgently needed in the foundations of quantum statistical mechanics. This is so because in generic isolated systems, one expects nonequilibrium dynamics on its own to result in thermalization: a relaxation to states where the values of macroscopic quantities are stationary, universal with respect to widely differing initial conditions, and predictable through the time-tested recipe of statistical mechanics. However, it is not obvious what feature of many-body quantum mechanics makes quantum thermalization possible, in a sense analogous to that in which dynamical chaos makes classical thermalization possible. Underscoring that new rules could apply in this case, some recent studies even suggested that statistical mechanics may give wrong predictions for the outcomes of relaxation in such systems. In this talk we demonstrate that an isolated generic quantum many-body system does in fact relax to a state well-described by the standard statistical mechanical prescription. Moreover, we show that time evolution itself plays a merely auxiliary role in relaxation and that thermalization happens instead at the level of individual eigenstates, as first proposed by J. M. Deutsch (1991) and M. Srednicki (1994). A striking consequence of this eigenstate thermalization scenario, confirmed below for our system, is that the knowledge of a single many-body eigenstate suffices to compute thermal averages---any eigenstate in the microcanonical energy window will do, as they all give the same result.
June 27
Cesar A. Rodriguez-Rosario (University of Texas / Harvard)
Non-Markovian Open Quantum Systems
We construct a non-Markovian canonical dynamical map that accounts for systems correlated with the environment. The physical meaning of not completely positive maps is studied to obtain a theory of non- Markovian quantum dynamics. The relationship between inverse maps and correlations with the environment is established. A generalized non- Markovian master equation is derived from the canonical dynamical map that goes beyond the Kossakowski-Lindblad Markovian master equation. Non-equilibrium quantum thermodynamics can be be studied within this theory.
USC 
Caltech
UCLA ITP