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. Location for remote viewing of HSC seminars: ACB 238
HSC seminar location: Herklotz seminar room, Zilkha Neurogenetic Institute HSC.  Location for remote viewing of UPC seminars: Herklotz


Friday September 8, 2pm SSL 150
Tahir Yusufaly (USC)
TBA



Friday September 15, 2pm SSL 150
Peter Young (UC Santa Cruz)
Critical Phenomena and Griffiths-McCoy Singularities in Quantum Spin Glasses

Quantum phase transitions (i.e. phase transitions at zero temperature) in disordered systems can display unusual features, such as "infinite-randomness" critical phenomena, and Griffiths-McCoy (GM) power-law singularities even in the paramagnetic phase. In this lecture I will review these concepts in the context of quantum spin glasses, discuss the results of some (mainly old) Quantum Monte Carlo simulations, and conclude by describing some recent calculations with Rajiv Singh using series expansions to explore quantum spin glass critical behavior and GM singularities in a wide range of dimensions.



Friday September 22, 2pm SSL 150
Salvatore Mandrà (NASA)
Numerical and experimental results on problem optimization using classical and quantum heuristics

Quantum technologies have finally reached the astonishing level that commercialized quantum devices can compete with classical devices. Among the possible quantum paradigms, quantum annealing has the potential to be a disruptive technology and overcome classical heuristics in the optimization of energy landscapes. Unfortunately, excluding few theoretical and numerical results, suitable problems that show a quantum speed-up are still missing. In my talk, I will review some of my last results ([1-3]), including experimental and numerical results for a class of promising problems for which the D-Wave quantum device has the potential to show an advantage with respect to classical heuristics.

[1] S. Mandrà, Z. Zhu, W. Wang, A. Perdomo-Ortiz, H.G. Katzgraber, "Strengths and weaknesses of weak-strong cluster problems: A detailed overview of state-of-the-art classical heuristics versus quantum approaches", Physical Review A 94 (2), 022337
[2] S. Mandrà, Z. Zhu, H.G. Katzgraber, "Exponentially-Biased Ground-State Sampling of Quantum Annealing Machines with Transverse-Field Driving Hamiltonians", Physical Review Letters 118 (070502)
[3] S. Mandrà, H.G. Katzgraber, C. Thomas, "The pitfalls of planar spin-glass benchmarks: Raising the bar for quantum annealers (again)", Quantum Science and Technology 2 (3)




Friday October 13, 2pm SSC 319
David Hsieh (Caltech)
Hidden Odd-Parity Orders in Spin-Orbit Coupled Correlated Electron Systems

The 5d transition metal oxides are predicted to host a variety of exotic electronic phases emerging from the interplay of strong spin-orbit coupling and electron-electron correlations. In this talk, I will describe the application of our recently developed nonlinear optical anisotropy and microscopy techniques to uncover novel parity-breaking phases in such materials. In particular, I will present evidence of a magnetic multipolar ordered state in the doped spin-orbit assisted Mott insulator Sr2IrO4 and draw comparisons to the ordered state underlying the pseudogap region of high-Tc superconducting cuprates. I will also show evidence of a 3D electronic nematic phase in the correlated spin-orbit coupled metal Cd2Re2O7 and discuss its implications for the superconductivity state that emerges at lower temperature. 



Friday October 20, 2pm SSL 150
Guifre Vidal (Perimeter Institute)
Tensor networks, conformal field theory, and discrete geometry

At a quantum critical point, the universal properties of a quantum spin chain are captured by an emergent conformal field theory (CFT). First we will explain how to numerically characterize the emergent CFT starting only from a generic microscopic lattice Hamiltonian, using the Koo-Saleur formula augmented with the matrix product state formalism. Then we will explain in which sense a tensor network representation of critical systems can be interpreted as describing a discretized geometry -- as it is often suggested in the context of the AdS/CFT correspondence of high energy physics.



Friday October 27, 2pm SSL 150
Israel Felner (Hebrew University of Jerusalem)
Search for new high Tc superconductors and unusual irreversible magnetic behavior in three unrelated  substances

Following the phase diagram of the well-known 122 material Ba-Fe-As in which superconductivity (SC) emerges from magnetic states, we synthesized and measured dozens of 122 materials of the Y(Lu)Fe2-xMx(Si,Ge)2 type (M=3d element).  In all samples pronounced magnetic peaks appear at various temperatures.  Their nature will be discussed.  Unfortunately, no SC traces have been observed.  On the other hand, traces of two SC phases (with TC=32 K and 66 K) have been observed in inhomogeneous commercial and fabricated amorphous carbon doped with sulfur (a-CS).  
The non-superconducting a-CS samples  exhibit pronounced peaks in their virgin zero-field-cooled (ZFC) curves at TP ~50-80 K.  Around these peaks the field-cooled (FC) curves cross the ZFC plots, thus in a certain temperature range ZFC>FC.  This complex behavior disappears in the second ZFC run. 
The same peculiar observation (ZFC>FC) was observed in two other unrelated systems:
(i) In a chiral-based magnetic memory device where the main components are α-helix L-polyalanine adsorbed on gold, Al2O3, and Co or Ni layers.  The ZFC>FC phenomenon is observed only in the hard direction of the layers.
(ii) In pathological liver tissues taken from a patient with hematological malignancies.
The unusual ZFC>FC phenomenon cannot be ascribed to extra magnetic phases (oxygen or magnetite), and is believed to be an intrinsic property of the three unrelated systems.  We may assume that in the ground state the intrinsic local magnetic moments in each system are randomly distributed.  In the first ZFC runs, low dc magnetic field aligns these moments to flip along its direction in a FM manner up to TP.  Above TP, an antiparallel exchange (AFM) coupling is more favored and in the next ZFC and FC processes the net magnetic moments are lower and cross the ZFC branches.  Alternatively, we may speculate that all systems are in the so-called two-state system separated by a certain energy barrier.




November 3, 2pm HSC
Ralf Langen, Ph.D. (USC)
TBA



Friday November 10, 2pm SSL 150
Eliot Kapit (Tulane)

Improved quantum annealer performance from oscillating transverse fields

Quantum annealing is a promising application of quantum hardware for solving hard classical optimization problems. The runtime of the quantum annealing algorithm, in absence of noise or other effects such as the constructive interference of multiple diabatic crossings, and at constant adiabatic evolution rate, is proportional to the inverse minimum gap squared. In this article, we show that for a large class of problem Hamiltonians, one can improve in the runtime of a quantum annealer (relative to minimum gap squared scaling) by adding local oscillating fields, which are not amenable to efficient classical simulation. For many hard $N$-qubit problems these fields can act to reduce the difficulty exponent of the problem, providing a polynomial runtime improvement. We argue that the resulting speedup should be robust against qubit energy fluctuations ($1/f$ noise), in contrast to variable-rate annealing, which is not.  We demonstrate quantum speedups for two classes of hard first order transition (the Grover problem and $N$-spin transitions between polarized semiclassical states), and provide analytical arguments and numerical evidence to support our claims. The oscillating fields themselves can be added through current flux-qubit based hardware by simply incorporating oscillating electric and magnetic lines, and could thus be implemented immediately.



Friday November 17, 2pm SSL 150
Michael Fink (Harvard)
Water and Protein Surfaces: An Unfavorable Coexistence

Water is much smaller than most other biological molecules, and still, its peculiar property to form networks of hydrogen bonds amplifies the effects of aqueous solvation: they are large enough to strongly influence, or even dominate many interactions of apolar compounds in water. We argue that binding of hydrophobic molecules in water is largely determined by a mechanism that depends on the energetically unfavorable structure of water networks close to hydrophobic surfaces, and the rearrangement of these water molecules upon association of the surfaces. We test our hypotheses using a robust physical-organic model, the enzyme human carbonic anhydrase, rationalize thermodynamic and crystallographic results using molecular dynamics, and present experimental evidence for both enthalpic and entropic hydrophobic effects, and surprising instances of enthalpy-entropy compensation. These results help to improve computational methods in drug design, and to understand hydrophobic phenomena in physiological events.


111717_Fink
Friday January 19, 2pm SSL 150
Christina Steinke (Bremen)
Coulomb engineered two-dimensional materials: non-invasive control of band gaps and excitons


Heterojunctions are building blocks of various applications in modern optoelectronics. Common heterojunctions rely on interfaces of different materials in order to gain the desired spatial band-gap modulations.
Here we propose a new type of lateral heterojunction induced non-invasively within a single two-dimensional (2d) homogeneous monolayer. In 2d semiconductors the Coulomb interaction can modify band gaps on an eV scale and can be drastically manipulated by external screening. This allows to tune the local band gaps within a monolayer by laterally structured dielectric surroundings.
In addition, atomically thin materials have shown large potential for optical applications. We investigate the influence of the external screening on the excitonic properties of this new kind of heterojunction. We find that structured dielectrics imprint a peculiar potential energy landscape on excitons in these systems: While the ground-state exciton is least influenced, higher excitations are attracted towards regions with high dielectric constant of the environment.




Friday January 26, 2pm SSL 150
Jason Alicea (Caltech)
Majorana milestones

The search for condensed-matter analogues of Majorana fermions is now well underway, motivated by both the prospects of revealing new facets of quantum mechanics and longer-term quantum computing applications.  This talk will survey recent advances in this pursuit. In particular, I will describe strategies for "engineering" Majorana platforms from simple building blocks, preliminary experimental successes, and future milestones that reveal foundational aspects of Majorana physics directly relevant for quantum computation.  



Friday  February 2, 2pm SSL 150
Gürol M. Süel (UCSD)
The bacterial brain: Long-range electrical signaling among bacteria

I will share recent results building on our discovery of ion channel-mediated electrical signaling within bacterial biofilm communities. By modulating the membrane potential of bacteria, this electrical cell-to-cell signaling appears to play a critical role in the emergence of collective behavior and regulation of cellular activity as a function of space and time. I will discuss the intriguing physical properties associated with this new form of bacterial communication.




Friday February 9, 2pm SSL 150
Jonathan Habif (USC, ISI Waltham)
Quantum-Limited Optical Sensing and Communications   

Optical approaches to sensing and communications are commonplace technologies in military, intelligence and civilian information systems.  Often the sensitivity of passive sensing systems (such as cameras or laser warning receivers) and active sensing and communications systems (such as LADAR and FSO communications) is often assumed to be limited by classical or semi-classical sources of noise and uncertainty.  A full quantum mechanical treatment of optical states reveals the fundamental limits of sensitivity that can be achieved in sensing and communications systems.  Particularly for weak signals, this can result in order of magnitude improvement in sensitivity over traditional receiver implementations, revealing new capabilities for military and intelligence applications of interest.  This talk will discuss the approaches to determining the fundamental limitations for extracting information from optical states and provide several examples where current approaches fall far short from the quantum limit.  Additionally, I will connect this field of research to current and future research in quantum information.
Brief Bio
Dr. Jonathan L. Habif is an experimental physicist and research lead at the University of Southern California Information Sciences Institute (USC – ISI).  His research has focused on photon-starved, classical communication and imaging, quantum-secured optical communications in free-space and fiber, and integrated nano-photonics for both classical and non-classical applications.  Prior to joining ISI, Dr. Habif was with BBN technologies where he served as principal investigator for Government-sponsored research programs, partnering with university collaborators to demonstrate revolutionary optical technologies impacting traditional communications, sensing and computation systems.
Dr. Habif earned a Ph.D. from the University of Rochester in the field of superconducting quantum computing and continued this course of research as a postdoctoral associate at MIT.



Friday February 16, 2pm SSL 150
Eugene Terentjev (Cambridge)
Rotary molecular motors
 


ATP synthase (ATPase) either facilitates the synthesis of ATP in a process driven by the proton moving force (pmf), or uses the energy from ATP hydrolysis to pump protons against the concentration gradient across the membrane. ATPase is made of two rotary motors, F0 and F1, which compete for control of their shared shaft. We developed a physical model of F1 motor as a simplified two-state Brownian ratchet using the asymmetry of torsional elastic energy of the coiled-coil gamma-shaft. This stochastic model unifies the physical concepts of linear and rotary motors, and explains the stepped unidirectional rotary motion. Increasing the pmf torque exerted by the F0 motor can slow, stop and overcome the torque generated by F1, switching from ATP hydrolysis to synthesis. We discuss the motor efficiency, which is very low if calculated from the useful mechanical work it produces - but is quite high when the ‘useful outcome’ is measured in the number of H+ pushed against the chemical gradient.



Friday February 23, 2pm SSL 150
Moshe Goldstein (Tel Aviv)
Entanglement Entropy in Many-Body Systems: Disorder and Symmetry 

Entanglement has recently emerged as a central theme in the study of many-body systems. In this talk I will discuss two novel aspects of this subject. The first is the use of entanglement to characterize disordered topological phases, in particular the Kitaev chain. I will show that surprisingly, disorder may help induce entanglement and even topological behavior in these systems. 
The second has to do with systems with symmetries, which give rise to conservation laws. Similarly to the system Hamiltonian, a subsystem's reduced density matrix is composed of blocks characterized by symmetry quantum numbers, or charge sectors. I will present a geometric approach for extracting the contribution of individual charge sectors to a subsystem’s entanglement measures within the replica trick method, via threading of appropriate conjugate Aharonov-Bohm fluxes through a multi-sheeted Riemann surface.
Specializing to the case of 1+1D conformal field theory, I will describe a general exact result for the entanglement characteristics. I will then apply this result to a variety of systems, ranging from free and interacting fermions to spin and parafermion chains, and verify it numerically. For example, I will show that the total entanglement entropy, which scales as the logarithm of the subsystem size, is composed of square-root of log contributions of individual subsystem charge sectors for interacting fermion chains, or even subsystem-size-independent contributions when total spin conservation is also accounted for. I will also describe how measurements of the contribution to the entanglement from separate charge sectors can be performed with existing techniques. 




Friday March 2, 2pm SSL 150
Hai-Qing Lin (Beijing Computational Science Research Centre)
Studies on the Rabi Model

We report our recent studies on the quantum Rabi model (QRM). Firstly, by using a variational wave function, which facilitates to extract physics in entire parameter regime with high accuracy, we unveil a ground-state phase diagram of the QRM and argue that the main constituents are polaron and anti-polaron. Secondly, introducing an anisotropy into the QRM, in which the rotating- and counter-rotating terms are allowed to have different coupling strength, so that the model interpolates between two known limits with distinct universal properties. Through a combination of analytic and numerical approaches we compute phase diagram, scaling functions and critical exponents, and establish that the universality class at the finite anisotropy is the same as that of the isotropic limit. Our findings are relevant to a variety of systems that are able to realize strong coupling between matter and light.
 


Friday March 2, 3pm SSL 150
Gaurav Kumar Gupta (Bangalore)
Axion-Higgs interplay and anomalous magnetic phase diagram in TlCuCl_3   

TlCuCl3 has been widely studied due to its peculiar magnetic phase diagram, observation of magnetic Higgs mode, BE condensation of the magnons etc. We study its peculiar magnetic phase, starting from the DFT band structure. (1) We discover that there exists a Su-Schrieffer-Heeger (SSH) like Cu-chain along the z-direction. We construct a 3D version of the SSH model. (2) As an AFM order sets in, we show that the system naturally transforms into a topological axion insulator. (3) Finally, we present a Chern-Simon-Ginzburg-Landau theory for the competition and coexistence of axion, Higgs and Goldstone modes, and present an anomalous magnetic phase diagram described in terms of topological axion angle.



Friday March 23, 2pm SSL 150
Juan Atalaya-Chavez (UC Riverside)
Bacon-Shor code with continuous measurement of non-commuting observables


In this talk I will discuss the continuous operation of the four-qubit Bacon-Shor code ([[4,1,2]]), where the non-commuting gauge operators are measured weakly and at the same time. I will show how error syndromes in this case are defined in terms of time-averaged correlators of the measurement output signals. I will also present results for the logical error rate and the termination rate for this quantum error detecting code for several models of decoherence. Finally, I will answer the question whether the code operation with continuous measurements can be comparable in performance with the operation based on projective measurements.




Friday  March 30, 2pm ACB 238
Eugene Terentjev (Cambridge)
Nucleation and growth of amyloid filaments

The classical nucleation theory finds the rate of nucleation proportional to the monomer concentration raised to the power, which is the “critical nucleus size,” Nc. The implicit assumption, that amyloids nucleate in the same way, has been recently challenged by an alternative two-step mechanism, when the soluble monomers first form a metastable aggregate (micelle) and then undergo conversion into the conformation rich in beta-strands that are then able to form a stable growing nucleus for the protofilament. We look for theoretical expressions for the resulting rate of amyloid nucleation. At low monomer concentration in solution, the nucleation occurs via the classical route with Nc=3. At higher monomer concentration, the two-step “aggregation-conversion” mechanism of nucleation takes over. In this regime, the effective rate of the process is not directly related to the minimum size of the growing nucleus (which we find to be 7-8 for a typical Abeta peptide).



Friday April 13, noon SSC 319
Nicole Yunger Halpern (Caltech)
Quantum information in quantum cognition

Matthew Fisher recently postulated a mechanism by which quantum phenomena could influence cognition: Phosphorus nuclear spins may resist decoherence for long times. The spins would serve as biological qubits. The qubits may resist decoherence longer when in Posner molecules. We imagine that Fisher postulates correctly. How adroitly could biological systems process quantum information (QI)? We establish a framework for answering. Additionally, we construct applications of biological qubits to quantum error correction, quantum communication, and quantum computation. First, we posit how the QI encoded by the spins transforms as Posner molecules form. The transformation points to a natural computational basis for qubits in Posner molecules. From the basis, we construct a quantum code that detects arbitrary single-qubit errors. Each molecule encodes one qutrit. Shifting from information storage to computation, we define the model of Posner quantum computation. To illustrate the model's quantum-communication ability, we show how it can teleport information incoherently: A state's weights are teleported. Dephasing results from the entangling operation's simulation of a coarse-grained Bell measurement. Whether Posner quantum computation is universal remains an open question. However, the model's operations can efficiently prepare a Posner state usable as a resource in universal measurement-based quantum computation. The state results from deforming the Affleck-Lieb-Kennedy-Tasaki (AKLT) state and is a projected entangled-pair state (PEPS). Finally, we show that entanglement can affect molecular-binding rates, boosting a binding probability from 33.6% to 100% in an example. This work opens the door for the QI-theoretic analysis of biological qubits and Posner molecules.
Reference: arXiv:1711.04801 (with Elizabeth Crosson)



Friday April 20, 2pm SSL 150
Naihao Chiang (UCLA,Northwestern)
Nanoscale Vibrational Spectroscopy with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy

During the last few years, there has been an explosion of interest and activity in the field of nanoscale vibrational spectroscopy. The goal is to confine and manipulate light on the sub-nanometer length scale using the properties of the collective electronic excitations in noble metal nanostructures, known as surface plasmons. An improved understanding of the interactions between adsorbed molecules and local environment under various experimental conditions is having a significant impact in a number of research areas including electrochemistry, surface science, catalysis for energy conversion and storage, low dimensional materials, biomedical diagnostics, and art conservation science.
    In this presentation, I will focus on three recent advances in ultrahigh vacuum (UHV) tip-enhanced Raman spectroscopy (TERS) which illustrate the power of this emerging technique. The first example demonstrates that TERS reveals more detailed information on the adsorbate-substrate interaction which is not easily accessible by other techniques. In the second example, we interrogated the lifting of an accidental vibrational degeneracy of self-assembled molecular islands on Ag surfaces due to a strong lateral intermolecular interaction. This work demonstrates that UHV-TERS enables direct access of intermolecular interaction at the nanoscale. Last, I will show our recent results on the adsorption of molecular oxygen with cobalt phthalocyanine supported on a silver single crystal surface. This study establishes TERS as a chemically sensitive tool for probing catalytic systems at the molecular-scale.













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