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 30, 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, 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 6, 2pm UPC
Ansgar Siemer (BIOC)
Solid-State NMR on Toxic and Functional Amyloid Fibrils
Solid-state NMR is a powerful tool to extract atomic resolution information from systems that are difficult to study otherwise. An important class of such systems is amyloid fibrils. Although amyloids were originally described as a key symptom and possible cause of several neurodegenerative diseases, they can also serve functional roles including structural support and cell signaling.
As an example of a functional amyloid, I will present solid-state NMR investigations on Orb2 a member of the CPEB protein family important for long-term memory. We were able confirm the location of the static amyloid core and characterize its secondary structure content. The amyloid core of these functional amyloids is formed by a glutamine rich domain similar to certain proteins responsible for neurodegenerative disease. We are therefore contrasting our results on Orb2 with solid-state NMR data of polyglutamine containing toxic amyloids to see what structurally distinguishes benign from toxic amyloids.
2015 BIOPHYSICS SEMINAR SE
February 13, 2pm SSL150
Gil Rafael (Caltech)
The Hilbert-glass transition: new universality of many- body dynamical quantum criticality
We study a new class of unconventional critical phenomena that is characterized by singularities only in dynamical quantities and has no thermodynamic signatures. I will develop a real-space renormalization group method for excited state (RSRG-X) that allows us to analyze such transitions, and will focus on the 1D disordered transverse field Ising
model with generic interactions. While thermodynamic phase transitions are generally forbidden in this model, using RSRG-X we find a finite-temperature dynamical transition between two localized phases. The transition is characterized by non-analyticities in the low frequency heat conductivity and in the long-time (dynamic) spin correlation function. The latter is a consequence of an up-down spin symmetry that results in the appearance of an Edwards-Anderson-like order parameter in one of the localized phases.
February 20, 2pm SSL150
Paul Brumer (Toronto)
Coherence, Decoherence and Incoherence in Natural Light Harvesting Systems
Establishing coherence, and resisting decoherence, are significant requirements in wide variety of quantum based processes. As a consequence, the experimental photon echo observations of unusually long lived coherences in complex light harvesting systems generated great enthusiasm for the idea that quantum coherence could persist in complex molecular systems at ambient temperatures. However, there are a number of issues, including the nature of the exciting light, which challenge the significance of such experimental results for natural light-induced dynamics.
We will review these results, and describe new developments in this area, such as the importance of various decoherence time scales and molecular properties for reaching mixed states, and the role of Fano resonances in generating long coherence time scales in the molecular response to natural incoherent light. Examples will be chosen from model systems, such as a generic v-level system, Rydberg atoms interacting with the cosmic microwave background, and PC645 irradiated by sunlight. The significance of these results for sustained quantum coherences in large molecular systems under natural incoherent excitation will be emphasized.
March 6, 2pm HSC
Tobias Ulmer (BIOC)
March 27, 2pm SSL150Mechanism of Integrin Transmembrane Signaling
When a blood vessel is injured, either by wounding or more chronically by atherosclerosis, the heterodimeric adhesion receptor integrin aIIbb3 is activated to crosslink blood platelets. This aggregation event can lead to occlusive thrombus formation, culminating in heart attack or stroke, the leading causes of death in the Western world. Receiving activating stimuli at the cytosolic integrin b tail leads to the dissociation of the complex formed between the integrin a and b transmembrane (TM) domains. This event causes a structural rearrangement of the large, non-covalently associated ab ectodomains that increases their ligand affinity. To understand the mechanism of integrin receptor activation, we elucidate the structural events that lead to ab TM dissociation and the structural and thermodynamic coupling between ecto- and TM domains.
March 12, 10am OHE 122 (refreshments will be served)
Atomistic Modeling of Solid-State Devices: From Qubits to TransistorsRajib Rahman (Purdue University)
Due to aggressive scaling, today’s transistors have reached sizes of tens of nanometers and are fast approaching the ultimate limits of scaling, as predicted by Moore’s Law. At the nanoscale, the atomic granularity of the devices and the associated quantum mechanical effects strongly influence device operation and need to be considered in theoretical models. To ensure continued progress in computing in the post Moore’s Law era, novel device concepts need to be developed utilizing quantum phenomena at the nanoscale. I will present an atomistic modeling technique for solid-state devices that combine material and device level description of electronic structure and transport from a full quantum mechanical treatment. This framework helps to model a variety of systems ranging from solid-state qubits to field-effect-transistors, and can help in designing the next generation of electronic devices.
In particular, I will show several applications of this method to model silicon qubits hosted in quantum dots and donors. 1) The method captures the precise electric field control of electronic and nuclear spins in donor qubits through the hyperfine and spin-orbit interactions , and helps in the first experimental realization of the Kane A-gate . 2) Spin-lattice relaxation times are computed from an atomistic electron-phonon Hamiltonian to interpret experimental measurements, and design guidelines are presented to enhance the relaxation times by an order of magnitude . 3) Electron-electron interaction is captured from a full configuration interaction technique in the tight-binding basis, and is used to obtain two-qubit exchange energy as a function of detuning electric field and qubit separation. Design considerations are presented to improve the electric-field tunability of exchange by several orders of magnitude in donor qubits. The computed single and multi-electron wavefunctions are also compared with tunneling probability measurements in scanning tunneling microscopy experiments to identify signatures of conduction band valley quantum interference in silicon .
I will also show atomistic quantum transport simulations of tunnel field-effect transistors (FET) in the emerging class of two-dimensional transition metal dichalcogenides. The simulations elucidate the material choice and design principles needed to achieve a steep sub-threshold slope transistor with large on-currents and high on/off ratio, which may help to scale down the power supply voltage and thus reduce the power consumption .
Biography: Rajib Rahman obtained his PhD degree in Electrical and Computer Engineering from Purdue University in 2009 in the area of computational nanoelectronics. Subsequently, Rajib spent three years in Sandia National Laboratories, New Mexico, as a postdoctoral fellow in the Silicon Quantum Information Science and Technology group. Both in his PhD and postdoc, Rajib developed large-scale computational techniques in the NEMO3D tool to investigate the properties of quantum bits in silicon based on quantum dots and impurities. In 2012, Rajib joined Purdue University as a Research Assistant Professor in the Network for Computational Nanotechnology (NCN). Rajib currently leads the device modeling effort of the Australian Centre for Quantum Computer and Communication Technology (CQC2T), and investigates silicon qubits and their interaction with a solid-state environment. At Purdue, Rajib also works on novel low energy field-effect transistors in emerging materials such as 2D transition metal dichalcogenides, graphene, and polarization engineered Nitride devices.
Ilya Krivorotov (UC Irvine)
Spin-orbitronics in metallic nanostructures
Manipulation of the spin of electron by an electric rather than magnetic field is at the core of spintronics. Spin orbit interaction (SOI) is a natural way of coupling the spin to an electric field. In this talk, I will review recent discoveries of surprisingly strong magneto-electric effects in metals, including SOI-induced pure spin currents and electric field control of magnetic anisotropy. I will show how these effects can be employed for generation and control of collective spin excitations in magnetic nanostructures. Applications of these effects in non-volatile computer memory and nanoscale microwave sources will be discussed.
April 3, 2pm UPC
Robert Farley (PHBI, USC)
Molecular Dynamics Simulations of Selective Ion Permeation in Biological Ion Channels
We have implemented a new application of Jarzynski’s Equality using non-equilibrium molecular dynamics simulations to investigate the movement of Na+ and K+ ions through K+- selective (KcsA) and Na+-selective (NavAb) ion channels. The simulations indicate that both channels may discriminate between the ions by preferential exclusion of the non-permeant ion rather than by selective binding of the permeant ion. The selectivity filter of KcsA adapts and responds to the presence of the ions with structural rearrangements that are different for Na+ and K+. Estimates of the K+/Na+ selectivity ratio and steady state ion conductance for KcsA from the simulations are in good quantitative agreement with experimental measurements. Non-equilibrium simulation data were combined with two-dimensional equilibrium free energy landscapes generated by umbrella sampling and weighted histogram analysis methods for multiple ions traversing the selectivity filter of the NavAb channel. The non-equilibrium simulations indicate that two or three extracellular K+ ions can block the entrance to the selectivity filter of NavAb. The block state occurs in an unstable local minimum of the free- energy landscape of two K+ ions that is ‘locked’ in place by modest applied forces. The work predicted by the non-equilibrium simulations that is required to break the K+ block is quantitatively equivalent to the large negative reversal potentials required to induce inward currents of K+ ions in two bacterial Nav channels, thus explaining the mechanism of outward rectification in these channels. In contrast to K+, three Na+ ions move favorably through the selectivity filter of NavAb together as a unit in a loose “knock-on” mechanism of permeation.
April 10, 2pm SSL150
Yaroslav Tserkovnyak (UCLA)
Bosonic condensation and spin superfluidity in solid state
The field of spintronics appears to approach a pivotal point in its development. Despite recent exciting discoveries of myriad spin-to-charge and spin-to-heat coupling effects, where the electron spin forms the cornerstone of diverse magnetoelectric and thermoelectric phenomena, there remains one serious impediment. Practically without exception, the spin transport is accomplished by diffusive carries, which suffer a rapid exponential decay. To compare with traditional electronics, it is as if integrated circuits operated with particles that decay or otherwise escape from the chip. There is a growing optimism, however, that efficient spin transport may be implemented by collective spin propagation through insulating magnetic media. One such route is via bosonic condensation of ferromagnetic magnons into a macroscopically coherent mode. Another approach strives to utilize macroscopic precession of the antiferromagnetic order, in order to realize a spin-superfluid state at room temperature. In this talk, I will discuss some recent theoretical advancements to these ends, while also touching upon the experimental developments that make us enthusiastic about their feasibility.
April 17, 2pm SSL150
Guanghou Wang (Nanjing University)
Clustering effect on topological transport of Cu-doped Bi2Te3 crystals
Great attention has been paid to enhance the electronic transport through topological surface states(TSS) in the study of three dimensional Bi2Se3/ Bi2Te3 crystals because the TSS transport is often hindered by the conductance of bulk electrons due to imperfect electron-hole hybridization, intrinsic Se/Te vacancies or antisite defects in the real topological samples. Such bulk dominance determines a non-TSS metallic transport in the topological devices. In this talk we will report the suppression of bulk conductance of the material by four orders of magnitude in the (Cu0.1Bi0.9)2Te3.06 crystals by intense aging. The 2D electron transport provided by the TSS is observed in such crystal as demonstrated by measurements of weak antilocalization effect, Shubnikov de Haas oscillations and scanning tunneling spectroscopy. Both STM/STS and the electrical measurements support a Fermi level inside the bandgap and reveal that Cu atoms are initially inside the quintuple layers and migrate to layer gaps to form Cu clusters during the aging. In combination with first-principles calculations, an atomic tunneling-clustering mechanism across diffusion barrier of 0.57eV at the interface between the quintuple layers is presented.
April 24, 2pm SSL150
Mehrtash Babadi (Caltech)
Far-from-equilibrium dynamics of isolated quantum many-body systems and non-thermal steady states
The equilibration of isolated quantum many-body systems is a fundamental and ubiquitous question in physics, and plays a central role in our understanding of a broad range of phenomena, from pump-probe experiments in condensed matter systems to the evolution of the early universe. The simplest perspective on the problem is the dichotomy between ergodic and non-ergodic systems. Recent theoretical and experimental investigations of strongly-correlated systems, however, suggest that certain systems can be trapped for long times in quasi-stationary “prethermalized” states with properties strikingly different from true thermal equilibrium. In the first part of this talk, I discuss deep interaction quenches in the fermionic Hubbard model and the emergence of such quasi-stationary states marked with short-range antiferromagnetic order. In the second part, I discuss the relaxation of far-from-equilibrium “spin spiral states” in the 3D quantum Heisenberg model. Of particular interest is the sudden appearance of steady states with diverging lifetimes as the spiral winding is tuned toward stable symmetry breaking ordered states. These prethermalized states are characterized by different bosonic modes being thermally populated at different effective temperatures, and by a hierarchical relaxation process reminiscent of aging in systems with quenched disorder.
May 1, 2pm HSC
Robert Chow (PHBI)
Perspectives on Molecular Mechanisms of Exocytosis
May 22, 2pm SSL150
Ilya Grigorenko (CityTech CUNY)
Can one induce superfluidity by thermal fluctuations?
We predict the existence of the superfluid state between two critical temperatures, which may be observed in an asymmetric Fermi system. The lower critical temperature corresponds to the superfluid transition induced by thermal fluctuations. The pairing in such a system takes place between two species of particles, with the asymmetry caused by the difference in the particle masses and their chemical potentials. The stability of the superfluid state is studied with respect to the changes of the asymmetry parameters. It is found that superfluidity is possible in a wide range of the asymmetry parameters when they satisfy a simple linear relation. We also predict that at zero temperature the system can undergo a quantum phase transition of the first order with respect to the change of the asymmetry parameters.
June 5, 2pm UPC
Radha Kalluri (OTOL)
From Mechanics to Ion Channels: Biophysics of Sensory Encoding in the Ear
2015 BIOPHYSICS SEMINAR SERIES
August 7, 2pm UPC
Moh El-Naggar (PHYS)
September 4, 2pm HSC
James Boedicker (PHYS)
2015 BIOPHYSICS SEMINAR SERIESPredicting and Controlling the Activity of Microbial Ecosystems
October 2, 2pm UPC
Emily Liman (BIOL)
November 6, 2pm HSC
Stephan Haas (PHYS)
2015 BIOPHYSICS SEMINAR SERIESJarzynski’s Equality, Ion Selectivity & Conduction
December 4, 2pm UPC
Ralf Langen (BIOC)
2015 BIOPHYSICS SEMINAR SERIES
USC Caltech UCLA ITP