USC Quantum Information, Condensed Matter and Biophysics Seminars

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).
Fall 2013

August 30, 2pm
Marian Breuer (UCL)

September 5, 2pm SSC 319
Carlos Pineda (Instituto de Fisica, Universidad Nacional Autonoma de Mexico)
Markovianity in quantum systems

We discuss the definition of quantum markovianity, and study two random matrix models, one with strong coupling (which presents non markovianity) and the other one with a tunable interaction (which serves to study the limits of the validity of the Markovian approximation). We also study several examples in which the degree of chaoticity can be tuned, to understand the relation between integrability and markovianity.

September 20, 2pm
Alex Neimark (Dept. of Chemical and Biochemical Engineering, Rutgers)
Self-Assembly and Transport in Soft Nanomaterials: Insight from DPD modeling

Soft nanomaterials, such as surfactant mesophases, phase-segregated block-copolymers, and ion-exchange membranes, often possess a complex hierarchical structure. They are built of nanoscale blocks arranged in self-assembled mesoscopic structures. Depending on the system and the environmental conditions, these self-assembled structures may have either regular symmetric or disordered fractal morphologies. Transport, mechanical, and rheological properties of a self-assembled system depend not only on its chemical composition but also on its morphology. Thus, the structure formation is the key problem to be considered toward a better understanding of engineering properties of self-assembled systems. The hierarchical structure of polymeric nanomaterials implies a hierarchical structure of a suite of modeling tools, which must span many orders of magnitude of spatial and temporal scales. I will present an overview of multiscale simulation methods employed in our group, which enable us to describe the macroscopic properties of complex nanostructured systems from ab-initio quantum mechanical calculations of electron density to atomistic molecular dynamics and Monte Carlo simulations to coarse-grained mesocsopic methods of dissipative particle dynamics. DPD). The main focus will be made on the DPD method, which will be illustrated drawing on the examples of micellization in surfactant solutions, conformation transitions in proteins, translocation through nanopores, structure formation and transport in nanostructured polyelectrolyte membranes, such as Nafion and sulphonated block-copolymers.

September 27, 2pm

Robin Kothari (Institute for Quantum Computing, University of Waterloo,
Nested quantum walks with quantum data structures

I'll talk about a new framework for designing quantum algorithms that extends the quantum walk framework of Magniez, Nayak, Roland, and Santha, by utilizing the idea of quantum data structures to construct an efficient method of nesting quantum walks. The new framework extends the known quantum walk framework while retaining its advantages: simplicity, ease of use, and a straightforward understanding of all resources used by the algorithm, such as queries, time or space.

The new framework is also powerful. In particular, the recently proposed learning graph framework of Belovs has yielded improved upper bounds for several problems, including the triangle finding problem and more general subgraph detection problems. I will exhibit the power of the new framework by giving simple explicit constructions that reproduce both the O(n^{35/27}) and O(n^{9/7}) learning graph upper bounds (up to logarithmic factors) for triangle finding.

October 25, 2pm

Sangeeta Kale, Professor in Physics and Dean (Academics) Defence Institute of Advanced Technology (DIAT), Girinagar, Pune 411 025, INDIA

Electrical-Interference-independent low field magnetic sensing using Optical Fiber based nanosensors

In spite of intense magnetic field sensing studies and evolution of low and high field magnetic sensors in commercial market, the search is still underway, especially in medical instrumentation. It has been observed that in environments where large electrical currents and circuits are in operation (MRI sections, for instance), sensors which work on potentiometric/amperometric approach cannot be deployed. Hence the search for electrical-interference-free magnetic field sensors is still underway and is quite a challenge. A combination of Optical Fiber Technology and Nanotechnology can provide an interesting and promising solution to such problems, especially due to the non-reactive environment given by the photonic devices towards electrical circuitry and wide property regime (and sensitivity) given by nanomaterials. Either a photonic crystal fiber (PCF) or a single/multimode fiber (SMF/MMF) has been used for such studies. Nanomaterials of magneto-optic properties (intrinsic or defect-induced) are used in conjunction with the fibers. Nanomaterials of iron oxide (Fe
3O4), lithium tantalate, lithium niobate and magnesium oxide (MgO) have been used as probable sensing materials. This talk focuses on exploring two systems, namely iron oxide in PCF and MgO in SMF for low-field magnetic sensing. Fe3O4 nanofluids have been deposited in the micro channels of PCF, which shows low-field sensing with the sensitivity of ~200 pm/mT. In an independent attempt, MgO has been deposited on the mirror tip of the SMF and the reflection property of the fiber is altered, which is a function of applied magnetic field. Evolution and manipulation of birefringence pattern, and the concept of refractive index variation of the nanomaterials, upon subjecting them to applied fields will be discussed in this seminar.
  1. Lithium Niobate Nanoparticulate Clad on the Core of Single Mode Optical Fiber for Temperature and Magnetic Field Sensing, Ch. N. Rao, Anoopam Bharadwaj, Suwarna Datar and S.N. Kale, Applied Physics Letters 101, 043102 (2012)
  2. Magnetic Nanoparticles for Biomedical Applications, Sangeeta Kale, Anup Kale, Sonia Kale, Satishchandra Ogale, Book Chapter No. 9, “Applications of Nanomaterials”, American Scientific Publishers, 2012. ISBN: 1-58883-181-7
  3. Sensitive weak magnetic field sensor based on Cobalt nanoparticles deposited in the microtunnels of PM-PCF optical fiber, Swati Gupta, Sandipan Nalawade, Shadie Hatamie, HV Thakur, S.N. Kale, AIP Conference Proceedings 1391, 437 (2011)
  4. Photonic crystal fiber injected with Fe3O4 nanofluid for magnetic field detection, Harneet V. Thakur, Sandipan M. Nalawade, Swati Gupta, Rohini Kitture, S. N. Kale Appl. Phys. Lett. 99, 161101 (2011)

November 1, 2pm
Thomas Seligman (Instituto de Ciencias Físicas, UNAM, Cuernavaca, México)
Intermediate environments: What do they represent and how can we manipulate them?

We consider a tripartite systems consisting of a central system of two qubits coupled only to a near environment of limited size which in turn is coupled to a far environment that may be infinite or as large as we can make it. We will show a model with a random matrix intermediate environment whose coupling to a far environment is represented by a master equation, as well as  a situation where all three subsystems are represented by Ising coupled kicked spin chais. Some ani-intutive results are obtained and interpreted.

November 22, 2pm
Justin Dressel (University of California Riverside)
An Action Principle for Continuous Quantum Measurements


Watching the gradual collapse of a quantum state has recently become possible in the laboratory. Unlike the conventional (textbook) wisdom about a measurement, which involves an instantaneous and discrete collapse of the state, such a gradual collapse is a continuous dynamical process. For example, the location of an electron in a double quantum dot can be gradually determined by the noisy current output by a nearby quantum point contact. Due to the intrinsic randomness of such a noisy signal, the gradual collapse is commonly modeled as a stochastic dynamical process, involving stochastic differential equations. As an alternative to this approach, we consider a complementary path integral formalism that uses a (canonical) doubling of the quantum state space itself. This path integral enables correlation functions to be computed perturbatively using standard diagrammatic techniques. Furthermore, the path integral supports an action principle that can be used to find the most likely paths through the canonical phase space that satisfy both initial and final boundary conditions.

Justin did his undergraduate work at the New Mexico Institute of Mining and Technology, where he received two B.S. degrees in Physics and Mathematics. After a brief detour as a Software Engineer working for the National Radio Astronomy Observatory on the Atacama Large Millimeter Array project, he received his Ph.D. at the University of Rochester in the field of Quantum Information and Foundations under the advisement of Professor Andrew Jordan. This past summer he was a Visiting Researcher in the group of Professor Franco Nori at RIKEN. Now he is working as a Postdoctoral Scholar in the group of Professor Alexander Korotkov at the University of California: Riverside.

December 6, 2pm
Matthew Mecklenburg, (Senior Staff Scientist, Center for Electron Microscopy and Microanalysis USC)
The Debye-Waller factor and the thermodynamic stability of graphene

Why are two dimensional materials stable and how can we use electron microscopy to understand their stability? 2D crystals are impossible, according to some interpretations of work by Landau and Peierls. Thus, suspended graphene's evident stability poses fundamental questions about long-range order in two-dimensional crystals. With information gathered from a variety of electron microscopy techniques that include aberration corrected imaging and diffraction we are able to understand the stability of graphene and other two dimensional materials. The good agreement between our ab initio theory and experiment indicates that ripples in the third dimension are not necessary to resolve graphene’s alleged stability paradox.

January 21 (2014), SSC 319, 11:30 am

Micah McCurdy, QRA Corp
Formulating Problems for a Quantum Computer

We discuss some aspects of the quotidian tasks of preparing problems to be solved using a device such as a DW-2. We discuss how to build up non-trivial networks from simple, well-known systems, using a technique we call "gluing", and we discuss some ideas for taking problems which are too large for existing hardware and break them up into smaller pieces, which we call "decomposition". We illustrate these techniques using the examples of integer factoring and subset-sum. Interested folk may choose to peruse arXiv:1312.5169 for a paper of ours concerning these ideas.

January 22 (2014), EEB 248, 10:30 – 12:00 pm

Austin J. Minnich, Assistant Professor of Mechanical Engineering and Applied Physics, California Institute of Technology
Heat under the microscope: Uncovering the microscopic processes that govern thermal transport

Thermal transport is a ubiquitous process that plays an essential role in nearly every technological application, ranging from space power generation to consumer electronics. In many of these applications, heat is carried by phonons, or quanta of lattice vibrations. Compared to other energy carriers such as electrons or photons, the microscopic properties of thermal phonons remain remarkably poorly understood, with much of our understanding still based on semi-empirical studies from over fifty years ago. In this talk, I will describe our efforts to uncover the microscopic processes that govern thermal transport by phonons using both experiment and computation. In particular, I will describe a new experimental technique that has enabled the first direct measurements of phonon mean free paths in a wide range of crystalline solids. I will demonstrate how these insights are advancing applications ranging from thermoelectric waste heat recovery to radio astronomy.

January 24 (2014), SSC 319, 10 am & 2 pm
Spyridon (Spiros) Michalakis (Caltech)
Lieb-Robison bounds: Methods and Applications (1 and 2)

The first talk will be an overview of Lieb-Robinson bounds and their application to Energy Filtering, a procedure that constructs projections onto spectrally isolated eigenstates. Both concepts are important ingredients for the construction of the quasi-adiabatic evolution unitary, the main tool in stability results (area law, correlation functions, spectral gap stability, quantization of Hall conductivity, etc).

The second talk will be on the construction of the quasi-adiabatic evolution unitary. This is a very powerful tool in the study of many-body physics, since it combines the power of adiabatic evolution with the elegance and simplicity of local quantum circuits.

February 21 (2014), 2 pm
Martin Linden (Stockholm University
Bayesian statistics, mean-field theory, and DNA looping at the single molecule level

Experimental techniques in single molecule biophysics and super-resolution microscopy have improved greatly in the last decade, and are delivering increasingly complex data on many cellular and molecular processes. However, data analysis methods are lagging behind both in terms of accuracy and ability to deal with large data sets, which means that a lot information in the data is lost.

In this talk, I will describe my ongoing work to improve this state of affairs with a combination of physical modelling and Bayesian statistics. In particular, I will discuss single molecule experiments using the tethered particle motion technique, and show how state-of-the art statistical methods have enabled us to extract new structural information about DNA looping by the bacterial transcription factor LacI, a classical model system for transcriptional regulation. Our analysis is the first demonstration of three coexisting looped states at the single molecule level, and hints at a subtle interplay between protein- and DNA deformation during loop formation, with possible implications for gene regulatory mechanisms.

April 4 (2014), 2 pm

Raul Garcia-Patron, ULB (Brussel)
Ultimate communication capacity of quantum optical channels

Optical channels, such as fibers or free-space links, are ubiquitous in today's telecommunication networks. A complete physical model of these channels must necessarily take quantum effects into account in order to determine their ultimate performances. Specifically, Gaussian bosonic quantum channels have been extensively studied over the past decades given their importance for practical purposes. In spite of this, a longstanding conjecture on the optimality of Gaussian encodings has yet prevented finding their communication capacity. In this talk we will present a recent result that solves this conjecture and establishes the ultimate achievable bit rate under an energy constraint. We will conclude discussing further implications of our result.

Joint work with V. Giovannetti, N. J. Cerf and A. S. Holevo

April 11 (2014), 12:00 - 1:30 pm (lunch served) SSL 150
Daniel Nagaj, University of Vienna
An Introduction to Quantum (Hamiltonian) Complexity

Finding (even describing) ground states of quantum systems is a challenging task. We will examine its complexity and look for a common language for computer science and physics to talk about the quantum analogue of constraint satisfaction problems: local Hamiltonians. First, we will focus on "destructive" results - hardness and completeness. Second, we will get creative and look into the future of quantum Hamiltonian complexity, with insights about correlations and entanglement motivating tensor network based algorithms.

Bio: Daniel Nagaj studied mathematical and theoretical physics at Comenius University in Bratislava, Slovakia, doing undergraduate research with Prof. Bužek at the Slovak Academy of Sciences. He finished his PhD in theoretical physics (quantum information) at MIT in 2008 under the guidance of Prof. Farhi, focusing on quantum complexity of local Hamiltonian problems as well as adiabatic quantum computing. After returning to work as a postdoc at the Research Center for Quantum Information of the Slovak Academy of Sciences for four years, he moved to Prof.Verstraete's group at the University of Vienna, where he investigates quantum complexity, quantum walks and tensor product state methods.

May 29 (2014), 2pm SSC 319
Angelo Lucia, Universidad Complutense de Madrid
Rapid Mixing of Quantum Local Dissipative Systems

Open quantum systems weakly coupled to the environment are modelled by
completely positive, trace preserving semigroups of linear maps. The
generators of such evolutions are called Liouvillians, and similarly to
the Hamiltonian in the case of coherent evolution, they encode the
physical properties of the system. In the setting of quantum many-body
systems on a lattice it is natural to consider local or exponentially
decaying interactions. We will focus on the case of maps with a unique
fixed point, and consider the scaling of the mixing time with respect to
the system size. In particular, if such scaling is sub-linear, a number
of good properties of the evolution can be obtained: local observables
are stable against perturbations, the fixed point has a finite
correlation length, and in the case of frustration-free systems,
satisfies an area law.

June 6 (2014), 2pm SSL 150
Leonardo Banchi, University College London
Optimal Quench for Distance-Independent Entanglement and Maximal Block Entropy

We optimize a quantum walk of multiple fermions following a quench in a spin chain to generate near ideal
resources for quantum networking. We first prove an useful theorem mapping the correlations evolved from
specific quenches to the apparently unrelated problem of quantum state transfer between distinct spins. This
mapping is then exploited to optimize the dynamics and produce large amounts of entanglement distributed in
very special ways. Two applications are considered: the simultaneous generation of many Bell states between
pairs of distant spins (maximal block entropy), or high entanglement between the ends of an arbitrarily long
chain (distance independent entanglement). Thanks to the generality of the result, we study its implementation
in different experimental setups using present technology: NMR, ion traps and ultracold atoms in optical lattices.

July 16 (2014), 2pm EEB539
Mark M. Wilde, LSU Baton Rouge
Renyi generalizations of the conditional quantum mutual information

The conditional quantum mutual information $I(A;B|C)$ of a tripartite state $\rho_{ABC}$ is an information quantity which lies at the center of many problems in quantum information theory. Three of its main properties are that it is non-negative for any tripartite state, that it decreases under local operations applied to systems $A$ and $B$, and that it obeys the duality relation $I(A;B|C)=I(A;B|D)$ for a four-party pure state on systems $ABCD$. It has been an open question to find Renyi generalizations of the conditional mutual information, that would allow for a deeper understanding of the original quantity and find applications beyond the traditional memoryless setting of quantum information theory. The present paper addresses this question, by defining different $\alpha$-Renyi generalizations $I_{\alpha}(A;B|C)$ of the conditional mutual information that all converge to the conditional mutual information in the limit $\alpha \to 1$. Furthermore, we prove that many of these generalizations satisfy the aforementioned properties. As such, the quantities defined here should find applications in quantum information theory and perhaps even in other areas of physics, but we leave this for future work. We also state a conjecture regarding the monotonicity of the Renyi conditional mutual informations defined here with respect to the Renyiparameter $\alpha$. We prove that this conjecture is true in some special cases and when $\alpha$ is in a neighborhood of one. Finally, we discuss how our approach for conditional mutual information can be extended to give Renyi generalizations of an arbitrary linear combination of von Neumann entropies, particular examples including the multipartite information and the topological entanglement entropy.

November 21 (2014), 2pm SSL 150
Nikhil Malvankar (UMass)

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