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 TBA
Marian Breuer (UCL) TBA
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, rkothari@iqc.ca) 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 email:
sangeetakale2004@gmail.com 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_{3}O_{4}), 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. Fe_{3}O_{4 }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.
References:
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)
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
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)
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
Abstract: 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.
Bio: 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 http://www.dbb.su.se/en/?p=researchgroup&id=167)
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.
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)
TBA