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Colloquia This Term
Colloquium
Friday, August 30, 2019
3:30 PM
Physics Building, Room 204

Jeffrey Teo
[Host: Bob Jones]
University of Virginia  Physics
"From interacting Majorana to universal fractional quasiparticles"

ABSTRACT:
Ising anyons, Majorana fermions (MF) and zero energy Majorana bound states have promising prospects in topological quantum computing (TQC) because of their ability to store quantum states nonlocally in space and insensitivity to local decoherence. Unfortunately, these objects are not powerful enough to assemble a TQC that can perform universal operations using topological braiding operations alone. On the other hand, there are anyonic quasiparticles, like the Fibonacci anyon in a ReadRezayi quantum Hall state, that are universal in the braidingbased TQC sense. However, these are quantum dynamical excitations, which can be challenging to spatially manipulate and susceptible to temperature fluctuations in a thermodynamic system. We propose and define a new notion of universal fractional quasiparticles, which are semiclassical static topological defects, supported by manybody interacting MFs in a superconducting spinorbit coupled topological electronic system.


Colloquium
Friday, September 6, 2019
2:30 PM
Physics Building, Room 204
Note special time.

Professor Or Hen
[Host: Nilanga Liyanage]
MIT  Massachusetts Institute of Technology
"Neutron stars droplets and the quarks within"

ABSTRACT:
Neutron stars are one of the densest stronglyinteracting manybody systems in our universe. A main challenge in describing the structure and dynamics of neutron stars steams from our limited understanding of the nuclear interaction at highdensities (i.e. shortdistances) and its relation to the underlaying quarkgluon substructure of nuclei.
In this talk I will present new results from highenergy electron scattering experiments that probe the shortranged part of the nuclear interaction via the hard breakup of ShortRange Correlations (SRC) nucleon pairs. As the latter reach densities comparable to those existing in the outer core of neutron stars, they represent ’neutron stars droplets’ who’s study can shed new light to the dynamical structure of neutron stars. Special emphasis will be given to the effect of SRCs to the behavior of protons in neutronrich nuclear systems and how it can impact the cooling rates and equation of state of neutron stars. Pursuing a more fundamental understanding of such interactions, I will present new measurements of the internal quarkgluon substructure of nucleons and show how its modification in the nuclear medium relates to SRC pairs and shortranged nuclear interactions.
Given time I will also discuss the development of new effective theories for describing shortranged correlations, the way in which they relate to experimental observables, and the emerging universality of shortdistance and highmomentum physics in nuclear systems.


Colloquium
Friday, September 13, 2019
3:30 PM
Physics Building, Room 204

Israel Klich
[Host: Bob Jones]
University of Virginia  Physics
"Quantum states, walks, tiles, and tensor networks"

ABSTRACT:
A major challenge of physics is the complexity of manybody systems. While true for classical systems, the difficulty is exasperated in quantum systems, due to entanglement between system components and thus the need to keep track of an exponentially large number of parameters. In particular, this complexity places a challenge to numerical methods such as quantum Monte Carlo and tensor networks. Here, exactly solvable models are of crucial importance: we use these to test numerical procedures, to develop intuition, and as a starting point for approximations.
In this talk, I will explain our current understanding of a new solvable "walk" model, the area deformed Motzkin model. The model shows that entanglement may be more acute than previously thought, in particular, it features a novel quantum phase transition between a nonentangled phase and extensively entangled “rainbow” phase. Most remarkably, the model motivated the construction of a new tensor network, providing, after many years, the first example for an exact tensor network description of a critical system. Finally, I will remark on open problems, and on exciting connections to other fields such as the notion of holography in field theory, and a famous problem in nonequilibrium statistical mechanics.


Colloquium
Friday, October 4, 2019
3:30 PM
Physics Building, Room 204

Professor Xiaoyang Zhu
[Host: Seunghun Lee]
Columbia University
"Ferroelectric Polarons, Belgian Waffles, and Principles for “Perfect” Semiconductors"

ABSTRACT:
Lead halide perovskites have been demonstrated as high performance materials in solar cells and lightemitting devices. These materials are characterized by coherent band transport expected from crystalline semiconductors, but dielectric responses and phonon dynamics typical of liquids. Here we explain the essential physics in this class of materials based on their dielectric functions and dynamic symmetry breaking on nano scales. We show that the dielectric function in the THz region may lead to dynamic and local ordering of polar nano domains by an extra electron or hole, resulting a quasiparticle which we call a ferroelectric large polaron, a concept similar to solvation in chemistry. Compared to a conventional large polaron, the collective nature of polarization in a ferroelectric large polaron may give rise to order(s)ofmagnitude larger reduction in the Coulomb potential. We show that the shape of a ferroelectric polaron resemble that of a Belgian waffle. Using twodimensional coherent phonon spectroscopy, we directly probe the energetics and local phonon responses of the ferroelectric large polarons. We find that that electric field from a nascent eh pair drives the local transition to a hidden ferroelectric order on picosecond time scale. The ferroelectric or Belgian waffle polarons may explain the defect tolerance and low recombination rates of charge carriers in lead halide perovskites, as well as providing a design principle of the “perfect” semiconductor for optoelectronics.


Colloquium
Friday, October 25, 2019
3:30 PM
Physics Building, Room 204

Rajveer Nehra
University of Virginia  Physics
"Phase space characterization of optical quantum states and quantum detectors"

ABSTRACT:
We are in the midst of a second quantum revolution fueled by “quantumness” of physical systems and the sophisticated measurement devices or detectors to produce and characterize these exotic systems. Thus, characterization of quantum states and the detectors is a key task in optical quantum science and technology. The Wigner quasiprobability distribution function provides such a characterization. In this talk, I present our recent results on quantum state tomography of a singlephoton Fock state using photonnumberresolving measurements using superconducting transitionedge sensor [1]. We directly probe the negativity of the Wigner function in our raw data without any inference or correction for decoherence, which is also an important indicator of the “quantumonly” nature of a physical system. For the second part of the talk, we discuss a method to characterize quantum detectors by experimentally identifying the Wigner functions of the detector positiveoperatorvaluemeasures (POVMs), a set of hermitian operators completely describing the detector [2]. The proposed scheme uses readily available thermal mixtures and probes the Wigner function pointbypoint over the entire phase space from the detector’s outcome statistics. In order to make the reconstruction robust to the experimental noise, we use techniques from convex quadratic optimizations.
References
1. R. Nehra, A. Win, M. Eaton, R. Shahrokhshahi, N. Sridhar, T. Gerrits,A. Lita, S. W. Nam, and O. Pﬁster, “Stateindependent quantum state tomography by photonnumberresolving measurements,” Optica 6,1356–1360 (2019). 2. R. Nehra and K. Valson Jacob (2019), “Characterizing quantum detectors by Wigner functions,” [arXiv:1909.10628].


Colloquium
Friday, November 1, 2019
3:30 PM
Physics Building, Room 204

Sherry Yennello
[Host: Simonetta Liuti]
Texas A&M University
"Studying the stars here on earth: Experimental investigations of the nuclear equationofstate "

ABSTRACT:
Heavyion collisions can produce nuclear material over a range of densities and proton fractions to study the nuclear equationofstate. These measurements are enabled by accelerating nuclei to – in some cases – GeV energies and detecting the fragments that are produced from the collisions. The detectors are multidetector arrays capable of measuring dozens of particles simultaneously from a single collision. Data rates can range up to many hundreds of collisions per second. One can either explore the characteristics of the individual fragments that are produced, often extracting particle ratios or double ratios, or correlations between the fragments – in particular transverse collective flow. From very low density to about three times normal nuclear density measurements have been made of the density dependence of the asymmetry energy. I will present an overview of how these measurements have been made and the constraints they have set on the nuclear equationofstate.


Colloquium
Friday, November 8, 2019
3:30 PM
Physics Building, Room 204

Professor Leo Stein
[Host: Kent Yagi]
University of Mississippi
"Testing Einstein with numerical relativity: theories beyond general relativity, and the precision frontier"

ABSTRACT:
Advanced LIGO and Virgo have already detected black holes crashing into each other at least ten times. With their upgrades we anticipate a rate of about 1 gravitationalwave detection per week. More signals and higher precision will take the dream of testing Einstein's theory of gravity, general relativity, and make it a reality. But would we know a correction to Einstein's theory if we saw it? How do we make predictions from theories beyond GR? And do current numerical relativity simulations have enough precision that we could be confident in any potential discrepancy between observations and predictions? I will discuss (i) how to perform simulations in beyondGR theories of gravity, and (ii) how numerical relativity simulations need to improve to be ready for the precision frontier of gravitational wave astrophysics.


Joint Colloquium with Physics and Astronomy/NRAO
Friday, November 15, 2019
3:30 PM
Physics Building, Room 203
Note special room.

Prof. Vasileios Paschalidis
[Host: Kent Yagi]
University of Arizona
"Multimessenger astronomy of compact binaries from the vantage point of computational gravity"

ABSTRACT:
We live in an exciting era where strongfield gravity has become a central pillar in the study of astrophysical sources. For the first time in history the detection of gravitational waves and simultaneous electromagnetic signals (multimessenger astronomy) from the same source have the potential to solve some of the most longstanding problems in fundamental physics and astrophysics. Computational gravity plays an important role in the success of the multimessenger astronomy program. Using the vantage point of computational gravity, in this talk we will we focus on how observations of colliding neutron stars can teach us about the state of matter at densities greater than the nuclear density, and with a critical eye assess what we have learnt so far from the first observation of a binary neutron star (event GW170817). We will also discuss how multimessenger detection of collisions of binary black holes may inform us about their environments andthe nature of black holes.


Colloquium
Friday, December 6, 2019
3:30 PM
Physics Building, Room 204

JenChieh Peng
[Host: Simonetta Liuti]
University of Illinois at UrbanaChampaign
"Exploring the Nucleon Sea"

ABSTRACT:
Direct experimental evidence for pointlike constituents in the nucleons
was first found in the electron deep inelastic scattering (DIS) experiment.
The discovery of the valence and sea quark structures in the nucleons
inspired the formulation of Quantum Chromodynamics (QCD) as the gauge field
theory governing the strong interaction. A surprisingly large asymmetry
between the up and down sea quark distributions in the nucleon was observed
in DIS and the socalled DrellYan experiments. In this talk, I discuss the
current status of our knowledge on the flavor structure of the nucleon sea.
I will also discuss the progress in identifying the "intrinsic" sea
components in the nucleons. Future prospect for detecting some novel
seaquark distributions will also be presented.




To add a speaker, send an email to bbc2x@Virginia.EDU Include the seminar type (e.g. Colloquia), date, name of the speaker, title of talk, and an abstract (if available). [Please send a copy of the email to physspeakers@Virginia.EDU.]
