Colloquia

Over the years my group has performed a number of experiments realizing relativistic physics using cold atoms — described by the 1D Dirac Hamiltonian in some degree of approximation.  I will begin by reviewing these results in conjunction with those from the whole of the cold-atom community.

With that backdrop, I describe a spin dependent bipartite Floquet lattice, in which the dispersion relation is linear for all points in the Brillouin zone. The (stroboscopic) Floquet spectrum of our periodically-driven Hamiltonian features perfect spin-momentum locking, and a linear Dirac dispersion.  These bands are protected by a Floquet topological invariant which we directly measure by using quantum state tomography.

In recent years there has been much interest in the field of "analogue gravity", in which cosmological or astrophysical phenomena like Hawking radiation are mimicked in a laboratory experiment.  At LSU, my research group in cold atom/condensed matter theory became interested in this field, motivated by the recent experiment of Eckel and collaborators, [Phys. Rev. X 8, 021021 (2018)] who used a rapidly expanding Bose-Einstein condensate (BEC) to reproduce the inflationary regime of the early universe.  I will present our work on the physics of inflation in expanding BEC's, and discuss other setups to detect analogue gravity phenomena like the Unruh effect.  

With the milestone discovery of the Higgs boson at the CERN Large Hadron Collider (LHC), high energy physics has entered a new era. The completion of the “Standard Model” (SM) implies, for the first time ever, that we have a relativistic, quantum-mechanical, self-consistent theoretical framework, conceivably valid up to exponentially high energies, even to the Planck scale. Yet, the SM leaves many unanswered questions both from the theoretical and observational perspectives, including the nature of the electroweak superconductivity and its phase transition, the hierarchy between the particle masses and between the observed scales, the nature of dark matter etc. There are thus compelling reasons to believe that new physics beyond the SM exists. We argue that the collective efforts of future high energy physics programs, in particular the future colliders, hold great promise to uncover the laws of nature to a deeper level. 

Atomtronics is the emerging technology of building circuits where the current is a flow of ultracold atoms propagating as coherent matter waves inside suitable waveguides.  In this talk I will describe our atomtronic technology in which the waveguides are created with laser light via the optical dipole potential, and then discuss two quantum sensors based on it.  First, we have demonstrated the atomtronic analogue of the dc SQUID and shown that it exhibits the quantum interference that gives the Superconducting Quantum Interference Device its name.   In the conventional SQUID this is seen as a periodic variation of critical current with magnetic flux.  In the atomtronic SQUID it causes a periodic variation of critical current with rotation, enabling the device to function as a gyro.  Second, we are developing an atomtronic version of the Fiber Optic Gyro, in which rotation is measured by the Sagnac effect.  In our device a Bose-Einstein condensate is split, reflected, and recombined inside a waveguide that is translated so that the wavepackets travel around a loop and realize a waveguide Sagnac atom interferometer. 

Atom interferometers exploit spatially delocalized quantum states to make a wide variety of highly precise measurements.  Recent technological advances have opened a path for atom interferometers to contribute to multiple areas at the forefront of modern physics, including searches for wave-like dark matter, gravitational wave detection, and fundamental quantum science.  In this colloquium, I will describe MAGIS-100, a 100-meter-tall atom interferometer being built at Fermilab to pursue these directions.  MAGIS-100 will serve as a prototype gravitational wave detector in a new frequency range, between the peak sensitivities of LIGO and LISA, that is promising for pursuing cosmological signals from the early universe and for studying a broad range of astrophysical sources.  In addition, MAGIS-100 will search for wave-like dark matter, probe quantum mechanics in a new regime in which massive particles are delocalized over macroscopic scales in distance and time, and act as a testbed for advanced quantum sensing techniques.  Finally, I will discuss the potential and motivation for follow-on atomic detectors with even longer baselines.

The ongoing development of superconducting qubits has brought some basic questions of many-body physics to the research forefront, and helped solve several of them. I will address two effects in quantum condensed matter highlighted by the development of a fluxonium qubit. The first one is the so-called cosine-phi problem stemming from the seminal paper of Brian Josephson. It predicted the phase dependence of the dissipative current across the Josephson junction. A fluxonium qubit enabled the observation of the effect, after nearly 50 years of unsuccessful attempts by other techniques. The second one is inelastic scattering ("splitting") of a microwave photon by quantum fluctuations of phase across a Josephson junction. This effect is the elementary mechanism driving the Schmid transition, which predicts a collapse of the Josephson current in a junction influenced by a dissipative environment.

Marking the semi-anniversary of the Fermilab Muon g−2 experiment (E989) Run 1 result, this colloquium will review the current status of the muon magnetic anomaly, the inferred evidence of possible particles outside the Standard Model (SM), and future prospects in this active research field.

The intrinsic magnetic field of a simple object, such as a compass needle, is expressed in terms of its magnetic moment.  The magnetic moment of a point particle, such as the electron, is predicted by relativistic quantum mechanics to be g = 2, in convenient dimensionless units.  For the electron, this prediction fails at the part-per-thousand level; the resulting magnetic anomaly, a_e = (g − 2)/2, is due to the electron’s couplings to virtual particles excited in the vacuum.

Muon, the electron’s 200 times heavier cousin, experiences far stronger couplings to massive virtual particles, including possible non-SM exotics. The SM provides a prediction for the muon magnetic anomaly a_µ with sub-ppm precision. Hence, a comparably precise measurement of a_µ offers a uniquely sensitive test for the presence of non-SM particles in nature. For almost 20 years a tantalizing discrepancy of ∼ 3 – 4σ has persisted between the measurements of a_µ, and the SM calculations. The Fermilab Muon g−2 Run 1 result brings a much awaited update to this test, with much more to come.

Precision rotation sensing is useful for navigation, geophysics, and tests of fundamental physics. Atom interferometers provide, by some measures, the most sensitive method for rotation sensing achieved to date. However, the best performance requires freely falling atoms in a large experimental apparatus. Many applications, such as navigating a vehicle, will benefit from a more compact geometry. One method to achieve this is by using trapped atoms that are suspended against gravity. We have implemented such an interferometer and used it to measure a rotation rate comparable to that of the Earth. The most recent iteration of the interferometer has demonstrated improvements by a factor of ten in rotation sensitivity and trap stability. A second new apparatus reduces the scale of the vacuum chamber and optical system to roughly the size of a microwave oven.

Whereas the structure of the atom has been  understood for many years, the internal structure of the proton (and neutron) is the subject of active research.  Understanding the nucleon is difficult because its structure is governed by quantum chromodynamics, or QCD, which has not been solved exactly in the non-perturbative or low-energy regime.  The proton's structure is intriguing, however, for many reasons.  For example, we think of the proton as being made of three quarks, but the mass of those quarks only accounts for about 1% of the proton's mass.  The remaining 99%, and hence 99% of the known mass in the universe, is due to exotic effects associated with the QCD vacuum.  While a great deal of work remains to be done, the way in which we visualize the proton has changed dramatically since the discovery of quarks.  Just as the structure of the atom was unveiled early in the 20th century, the structure of the proton is being unveiled in the first decades of the 21st century.  Another intriguing aspect of the proton arises from the fact that QCD is the only theory in nature that has essentially no free parameters. String theory, that attempts to unify our understanding of gravity and the quantum world, grew out of early efforts to understand the strong interaction.  Since string theory deals with the topology of space and time, it is tempting to believe that a deep understanding of the proton may one day provide a window into even more fundamental questions. The colloquium will cover some recent developments in our understanding of the nucleon, as well as providing a glimpse of where this rich area of research is heading in upcoming years.

In quantum mechanics, the role of an observer is fundamentally different from that of a classical observer.  The quantum mechanical observer necessarily plays an active role in the dynamics of the system that it is observing.  This apparent difficulty may be turned into a tool to drive an initially trivial system into a complicated quantum many-body state simply by observing it.  I will present two remarkable examples of states induced by measurement. In the first, we examine the role of a moving density measuring device interacting with a system of fermions, and in particular, show that it would leave behind a wake of purely quantum origin. In the second example, inspired by the recent invention of topological Floquet insulators, we will see how a suitably chosen set of density measurements, repeated periodically, will induce robust chiral edge motion on a lattice of free fermions. Our examples show how quantum mechanical observation can be added as a versatile tool to the arsenal of quantum engineering in condensed matter systems.

The origin of complexity remains one of the most important and, at the same time, the most controversial scientific problems. Earlier attempts were based on theory of dynamical systems but did not lead to a satisfactory solution of the problem. I believe that a deeper understanding is possible based on a recent development of statistical physics, combining it with relevant ideas from evolutionary biology and machine learning.

Using patterns in magnetic materials as the main example, I discuss some general problems such as (a) a formal definition of pattern complexity [1]; (b) self-induced spin glassiness due to competing interactions as a way to interpret chaotic patterns [2]; (c) multi-well states intermediate between glasses and ordinary ordered states and their relevance for the problem of long-term memory in complicated systems [3]; and (d) complexity of frustrated quantum spin systems [4]. I will also review a very recent experimental observation of self-induced spin-glass state in elemental neodymium [5].

Geometrically frustrated many-body systems show many interesting emerging phenomena, ranging from kinetic frustration to exotic spin ordering and chiral spin liquid phases. Ultracold atom systems offer great tunability and flexibility to realize such systems in a wide parameter range of interactions, densities, and spin-imbalance.

In this talk, I will present our recent results on site-resolved imaging of ultracold fermionic lithium atoms on a triangular optical lattice.

Degenerate Fermi gases with about one tenth of the Fermi temperature have been realized within a crossed dipole trap and successfully loaded into a two-dimensional triangular optical lattice. To characterize this lattice, we observed Kapitza-Dirac scattering using a molecular Bose-Einstein condensate. Collecting the emitted photons during Raman sideband cooling in the triangular lattice using a high-resolution microscope objective enabled the high-fidelity imaging of individual fermionic atoms in the lattice with single-site resolution.

The next step will be the realization of a triangular lattice Hubbard model by implementing an additional optical lattice to increase interactions.

This novel experimental platform will allow us to study spin and density correlations in the triangular Hubbard model to explore signatures of frustration and spin-hole bound states and may lead to a direct observation of non-vanishing chiral correlations.

Bosonic modes are widely used for quantum communication and information processing. Recent developments in superconducting circuits enable us to control bosonic microwave cavity modes and implement arbitrary operations allowed by quantum mechanics, such as quantum error correction against excitation loss errors. We investigate different bosonic encoding and error correction protocols, and provide a perspective on using bosonic quantum error correction for various applications.

The unique properties of superfluid ⁴He (low mechanical stiffness, zero viscosity, high structural and chemical purity and extremely low optical loss) may allow the direct pursuit of optomechanics with quantum mechanical oscillators. We have constructed an optomechanical system consisting entirely of a magnetically levitated drop of superfluid ⁴He in vacuum. Levitation removes a source of loss associated with physically clamped oscillators. and allows the drop to efficiently cool itself via evaporation. The drop’s optical whispering gallery modes (WGMs) and its surface vibrations couple to each other via the usual optomechanical interactions. We demonstrate the stable magnetic levitation of superfluid ⁴He drops in vacuum, and present measurements of the drops' evaporation rates, temperatures, optical modes and surface vibrations. We found optical modes with finesse ≈40 (limited by the drop's size). We found surface vibrations with decay rates ∼1 Hz (in rough agreement with theory). Lastly, we found that the drops reach a temperature T≈330 mK, and that a single drop can be trapped indefinitely.

The full potential of the Large Hadron Collider will ultimately be realized through the running period that will take collider operations into the late 2030s. The so-called High Luminosity era of the LHC (HL-LHC), planned to begin in 2027, will see up to a 5-fold increase in the nominal brightness of the LHC beams, which will consequently allow experiments to collect a data set of proton-proton collisions approximately 20 times larger than what has been collected so far in high-energy LHC running. This significant increase in data sample will be a boon for the physics program of the LHC experiments, furthering searches for ultra-rare phenomena and refining precision measurements of known processes. At the CMS experiment, a suite of novel upgrades will be deployed that will enable the experiment to cope with the onslaught of collisions in the high luminosity environment and fully capitalize on the opportunities presented in the HL-LHC era. In this talk I will focus on the MIP Timing Detector (MTD), a device capable of measuring the time-of-arrival for minimum-ionizing particles (MIPs) produced within CMS with a resolution of 30 picoseconds. This entirely new detector system will be built in part at UVA before being shipped to CERN and integrated with the CMS experiment. I will describe the principle of operation of the MTD, the current status of the project and UVA's role in the construction of the device. I will also discuss the expected impact the MTD will have on several high-profile physics signatures that are prime targets in the HL-LHC era and, in particular, the power of using timing information in the reconstruction of exotic long-lived particle decays. 

In this talk, I will present our recent efforts on dynamical simulations of correlated electron systems. I will first discuss new quantum molecular dynamics (QMD) methods based on advanced many-body techniques, such as Gutzwiller/slave-boson and dynamical mean-field theory, that are capable of modeling strong electron correlation phenomena. We apply our new QMD to simulate the correlation-induced Mott transition in a metallic liquid, and the nucleation-and-growth of Mott droplets in Hubbard-type models. I will also discuss the implementation of the ab initio Gutzwiller MD for simulating hydrogen liquids under high pressure. To overcome the obstacle of huge computational complexity in such large-scale simulations, I will discuss how simulation efficiency can be significantly improved with the aid of modern machine learning methods. In particular, deep-learning neural-network holds the potential of achieving large-scale quantum-accuracy simulation of correlated systems without the electrons.

‘Circuit QED’ is the quantum theory of superconducting qubits strongly interacting with microwave photons in electrical circuits. It is the leading solid-state architecture in the race to develop large-scale fault-tolerant quantum computers, and is the only technology that has demonstrated quantum error correction that actually extends the lifetime of quantum information. 

In this talk, I will present an elementary introduction to the basic concepts underlying superconducting quantum processors. Their ability to control and make quantum non-demolition (QND) measurements of individual microwave photons is a powerful resource for quantum computation, communication and simulation.   I will illustrate these capabilities with recent experiments on a programmable quantum simulator that uses efficient boson sampling of microwave photons to predict the Franck-Condon vibrational spectra of various small tri-atomic molecules.  Finally, I will briefly explore possible future directions for simulation of quantum many-body problems involving interacting bosons.

The ability to generate and manipulate photons with high efficiency and coherence is of critical importance for both fundamental quantum optics studies and practical device applications. However mainstream integrated photonic platforms such as those based on silicon and silicon nitride lack the preferred cubic c(2) nonlinearity, which limits active photon control functionalities. In this talk, I will present integrated photonics based on aluminum nitride (AlN) and lithium niobite (LN), whose non-centrosymmetric crystal structures give rise to the strong second-order optical nonlinearity. Together with their low optical loss, the integrated AlN and LN photonics can provide enhanced c(2) photon-photon interactions to achieve high fidelity photon control, including on-chip parametric down-conversion, coherent light conversion, spectral-temporal shaping, and microwave-to-optical frequency conversions.

The general lore, according to effective field theory, is that one ignores the high energy degrees of freedom when studying low energy phenomena. For example, we usually do not need to know quantum chromodynamics (QCD) (the strong nuclear force) in order to study fluid mechanics! However, the existence of 't Hooft anomalies (subtle phases in the partition function) may signal non-trivial intertwining between the high and low energy scales. This assertion can be seen in axion physics; axion is a hypothetical particle that may play important roles in solving a few puzzles in the Universe.  After a brief introduction to QCD and axions, I show how this intertwining takes place on axion domain walls (DW). To this end, I first discuss a new class of 't Hooft anomalies that was recently identified, and then use the anomalies to argue that quarks are deconfined (liberated) on axion DW. This newly discovered phenomenon implies that non-trivial interplay between different scales happens on the walls. Further, I confirm this picture by performing explicit calculations in a toy model, which is argued to be continuously connected to the full-fledged QCD.

Quantum field theory is a universal language in theoretical physics, which provides the foundation for the Standard Model of elementary particles, underlies the physics of the early universe, and describes a wealth of interesting phenomena in condensed matter. But despite its great successes, fully understanding this important framework is still very much a work in progress. Many insights, especially into theories with strong interactions, have been obtained using mathematical tools from string theory, and this has reshaped our understanding of what quantum field theory is. In this talk I will discuss new connections between quantum field theory and string theory that provide access to a remarkable class of theories that would not have been believed to exist based on conventional lore.

Neutrinos are the most elusive known elementary particles; they fly through all of us in vast numbers but are extremely difficult to detect. Immense progress has been made in analyzing their properties over the last decades, culminating in the surprising discovery of neutrino flavor oscillations. These neutrino oscillations imply that neutrinos have tiny but nonzero masses, which provides strong evidence for physics beyond the Standard Model of particle physics. With the increasing precision in neutrino measurements it has even become possible to use neutrinos as a tool to probe for further new physics, e.g. by studying how neutrinos scatter off electrons. In addition, neutrinos could prove uniquely helpful in the search for dark matter and provide complementary information to standard indirect detection signatures.

I will address two of the most fundamental questions about our Universe -- "What is the Universe made of?” and "How did complex structures come to exist?”. These translate into understanding the origins of dark matter and a mechanism to generate the asymmetry of matter over anti-matter in the early Universe. I will put these problems in context and then present a novel, testable, solution to both mysteries in the same framework.  The key idea is to rely on Standard Model particles called Beauty (B) Mesons which decay into dark matter particles. This mechanism has multiple testable signals. Some experimental searches are already under development, the results of which could yield the first hints of how nature chose to address these profound questions.

A hundred years after the prediction by Einstein, gravitational waves were directly detected for the first time in 2015 by LIGO, which marked the dawn of gravitational-wave astronomy. Gravitational waves are sourced by astrophysical compact objects, such as black holes and neutron stars. Due to their extremely large gravitational field and compactness, they offer us natural testbeds to probe strong-field gravity and dense matter physics. In this talk, I first give an overview of the current status of gravitational-wave observations. Next, I explain how well one can test General Relativity, constrain the equation of state of nuclear matter and measure nuclear parameters with gravitational waves. I also comment on how one can combine gravitational-wave information with the recent measurement of a neutron star radius by an X-ray payload NICER to further probe nuclear physics.

When learning about the properties of a quantum mechanical system, for instance, the energy levels of its bound states, it is useful to think of the system as closed and isolated from any environment, though we know in any laboratory setting, all systems eventually will interact with an environment. However, we can often engineer such interactions to be weak, short-ranged, and controllable, so that the isolated approximation is a good one.

I will argue that in many physically relevant field theories, the long-time observables or states of the theory can only be defined in the context of a quantum open system, where we take into account the interactions between the system and the environment continually in the evolution of the system. This is because excitations of the field theory will inevitably create their own environment, that is, states we must trace over. Resumming these interactions with the self-created environment is necessary to give a convergent expansion for observables over all of phase-space.

Microscopic quantum interactions between elementary particles control transport in macroscopic states of matter, such as in fluids and plasmas. In numerous states of interest, these microscopic interactions are strong, including in water, among electrons in graphene and in quark-gluon plasma — a state of nuclear matter that filled the early Universe and that is currently being recreated in particle colliders. While macroscopic theories describing the dynamics of such states, in particular, hydrodynamics (of fluids) and magnetohydrodynamics (of magnetized plasmas) have been partially understood, a full description of transport also requires a certain microscopic knowledge of its underlying quantum physics. After more than a century of striking advance in quantum theories, our theoretical understanding of these microscopic processes remains mostly limited to states with weak interactions. Recently, however, string theory also enabled explorations of strongly interacting states through the mathematical statement of holographic duality, which translates otherwise intractable problems into simpler analyses of black holes and gravitational waves.   

In my talk, I will first discuss new aspects of the macroscopic theory of hydrodynamics, focusing on the properties of the infinite series of higher-order corrections to the infamous Navier-Stokes equations. By using a novel concept of generalized global symmetries, which can encode the fact that the number of magnetic flux lines in Nature is conserved, I will then describe the construction of a new, comprehensive theory of magnetohydrodynamics. This reformulation has led to a number of general theoretical and experimental predictions for transport in magnetized plasmas. I will then move on to discuss the microscopic physics responsible for transport in strongly interacting states. Beginning with an introduction of holographic duality, this section will summarize holographic insights into the problem of the “unreasonable effectiveness of hydrodynamics” for the description of quark-gluon plasma. Then, I will discuss how the descriptions of microscopic physics and transport transition between strongly and weakly interacting pictures. Finally, by utilizing the mathematical structure behind our new theory of magnetohydrodynamics, a holographic dual of magnetized plasmas will be presented along with the first analyses of strongly interacting magnetized transport.

Direct experimental evidence for point-like 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 so-called Drell-Yan 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
sea-quark distributions will also be presented.

We live in an exciting era where strong-field 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 long-standing 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.

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 gravitational-wave 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 beyond-GR theories of gravity, and (ii) how numerical relativity simulations need to improve to be ready for the precision frontier of gravitational wave astrophysics.

Heavy-ion collisions can produce nuclear material over a range of densities and proton fractions to study the nuclear equation-of-state. 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 multi-detector 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 equation-of-state.

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 quasi-probability distribution function provides such a characterization. In this talk, I present our recent results on quantum state tomography of a single-photon Fock state using photon-number-resolving measurements using superconducting transition-edge 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 “quantum-only” 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 positive-operator-value-measures (POVMs), a set of hermitian operators completely describing the detector [2]. The proposed scheme uses readily available thermal mixtures and probes the Wigner function point-by-point 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, “State-independent quantum state tomography by photon-number-resolving measurements,” Optica 6,1356–1360 (2019). 2. R. Nehra and K. Valson Jacob (2019), “Characterizing quantum detectors by Wigner functions,” [arXiv:1909.10628].
 

Lead halide perovskites have been demonstrated as high performance materials in solar cells and light-emitting 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)-of-magnitude larger reduction in the Coulomb potential. We show that the shape of a ferroelectric polaron resemble that of a Belgian waffle. Using two-dimensional 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 e-h 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.

A major challenge of physics is the complexity of many-body 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 non-entangled 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 non-equilibrium statistical mechanics.

Neutron stars are one of the densest strongly-interacting many-body 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 high-densities (i.e. short-distances) and its relation to the underlaying quark-gluon substructure of nuclei.

In this talk I will present new results from high-energy electron scattering experiments that probe the short-ranged part of the nuclear interaction via the hard breakup of Short-Range 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 neutron-rich 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 quark-gluon sub-structure of nucleons and show how its modification in the nuclear medium relates to SRC pairs and short-ranged nuclear interactions. 

Given time I will also discuss the development of new effective theories for describing short-ranged correlations, the way in which they relate to experimental observables, and the emerging universality of short-distance and high-momentum physics in nuclear systems.

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 non-locally 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 Read-Rezayi quantum Hall state, that are universal in the braiding-based 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 semi-classical static topological defects, supported by many-body interacting MFs in a superconducting spin-orbit coupled topological electronic system.

Of course, multimessenger astronomy promises to revolutionize

astronomy and our understanding of nucleosynthesis. My

research shows that it goes further: astronomical observations

(via both photons and gravitational waves) provides a unique

laboratory to deepen our understanding of QCD and the

nucleon-nucleon interaction. Most current work is focused

on the equation of state. While the equation of state is

indeed important, in this talk, I show how we can

go beyond energy density and pressure. I present the first

large-scale Bayesian inference of neutron star observations

and nuclear structure data to obtain novel results on the

composition of dense matter and the nature of nucleonic

superfluidity.

Quantum Chromodynamics (QCD), the nuclear physics part of the Standard Model, is the first theory to demand that science fully resolve the conflicts generated by joining relativity and quantum mechanics.  Hence in attempting to match QCD with Nature, it is necessary to confront the innumerable complexities of strong, nonlinear dynamics in relativistic quantum field theory.  The peculiarities of QCD ensure that it is also the only known fundamental theory with the capacity to sustain massless elementary degrees-of-freedom, gluons (gauge bosons) and quarks (matter fields); and yet gluons and quarks are predicted to acquire mass dynamically so that the only massless systems in QCD are its composite Nambu-Goldstone bosons.  All other everyday bound states possess nuclear-size masses, far in excess of anything that can directly be tied to the Higgs boson.  These points highlight the most important unsolved questions within the Standard Model, namely: what is the source of the mass for the vast bulk of visible matter in the Universe and how is this mass distributed within hadrons?  This presentation will provide a contemporary sketch of the strong-QCD landscape and insights that may help in answering these questions.

In conventional imaging systems, the results are poor unless there is a physical mechanism for producing a sharp image with high signal-to-noise ratio.  In this talk, I will present two settings where computational methods enable imaging from very weak signals:  range imaging and non-line-of-sight (NLOS) imaging.

Lidar systems use single-photon detectors to enable long-range reflectivity and depth imaging.  By exploiting an inhomogeneous Poisson process observation model and the typical structure of natural scenes, first-photon imaging demonstrates the possibility of accurate lidar with only 1 detected photon per pixel, where half of the detections are due to (uninformative) ambient light.  I will explain the simple ideas behind first-photon imaging and lightly touch upon related subsequent works that mitigate the limitations of detector arrays, withstand 25-times more ambient light, allow for unknown ambient light levels, and capture multiple depths per pixel.

NLOS imaging has been an active research area for almost a decade, and remarkable results have been achieved with pulsed lasers and single-photon detectors.  Our work shows that NLOS imaging is possible using only an ordinary digital camera.  When light reaches a matte wall, it is scattered in all directions.  Thus, to use a matte wall as if it were a mirror requires some mechanism for regaining the one-to-one spatial correspondences lost from the scattering.  Our method is based on the separation of light paths created by occlusions and results in relatively simple computational algorithms.

Related paper DOIs:
10.1126/science.1246775
10.1109/TSP.2015.2453093
10.1109/LSP.2015.2475274
10.1364/OE.24.001873
10.1038/ncomms12046
10.1109/TSP.2017.2706028
10.1038/s41586-018-0868-6

Complex oxides are known to possess the full spectrum of fascinating properties, including magnetism, colossal magneto-resistance, superconductivity, ferroelectricity, pyroelectricity, piezoelectricity, multiferroicity, ionic conductivity, and more. This breadth of remarkable properties is the consequence of strong coupling between charge, spin, orbital, and lattice symmetry. Spurred by recent advances in the synthesis of such artificial materials at the atomic scale, the physics of oxide heterostructures containing atomically smooth layers of such correlated electron materials with abrupt interfaces is a rapidly growing area. Thus, we have established a growth technique to control complex oxides at the level of unit cell thickness by pulsed laser epitaxy. The atomic-scale growth control enables to assemble the building blocks to a functional system in a programmable manner, yielding many intriguing physical properties that cannot be found in bulk counterparts. In this talk, examples of artificially designed, functional oxide heterostructures will be presented, highlighting the importance of heterostructuring, interfacing, and straining. The main topics include (1) charge transfer induced interfacial magnetism and topologically non-trivial spin textures in SrIrO₃-based heterostructures and (2) lattice and chemical potential control of oxygen stability and associated electronic and magnetic properties in nickelate-and cobaltite-based heterostructures.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

This presentation describes an arc in mathematical/theoretical physics traversing concepts from equations, graphs, error-correction, and pointing toward an evolution-like process acting on the  mathematical laws that sustain reality.

Water wave patterns behind ships fuel human curiosity because they are both beautiful and easily observed.  These patterns called wakes were famously described in 1887 by Lord Kelvin.  According to Kelvin, the feather-like appearance of the wake is universal and the entire wake is confined within a 39 degree angle.  While such wakes have been observed, deviations from Kelvin’s predictions have also been reported.  In this talk summarizing my work with UVA alumnus Jonathan Colen I will present a quantitative reasoning based on classical surface water wave theory that explains why some wakes are similar to Kelvin’s prediction, and why others are less so.  The central result is a classification of wake patterns which all can be understood in terms of the problem originally treated by Kelvin.

Gravitational waves from the mergers of ten binary black holes and one binary neutron star were detected in the first two observing runs by the Advanced LIGO and Virgo detectors. In this talk, I will discuss the eleven gravitational-wave detections and the electromagnetic observations that accompanied the neutron-star merger. These detections confirmed many of the predictions of general relativity, and they initiated the observational study of strongly curved, dynamical spacetimes and their highly luminous gravitational waves. One aspect of these high gravitational-wave luminosities that LIGO and Virgo will be able to measure is the gravitational-wave memory effect: a lasting change in the gravitational-wave strain produced by energy radiated in gravitational waves. I will describe how this effect is related to symmetries and conserved quantities of spacetime, how the memory effect can be measured with LIGO and Virgo, and how new types of memory effects have been recently predicted. I will conclude by discussing the plans for the next generation of gravitational-wave detectors after LIGO and Virgo and the scientific capabilities of these new detectors. These facilities could detect millions of black-hole and neutron-star mergers per year, and they can provide insights on a range of topics from the population of short gamma-ray bursts to the presence of dark matter around black holes.

The next decade promises fundamental new scientific insights and discoveries from Multi-Messenger Astrophysics, enabled through the convergence of large scale astronomical surveys, gravitational wave astrophysics, deep learning and large scale computing. In this talk I describe a Multi-Messenger Astrophysics science program, and highlight recent accomplishments at the interface of gravitational wave astrophysics, numerical relativity and deep learning. I discuss the convergence of this program with large scale astronomical surveys in the context of gravitational wave cosmology. Future research and development activities are discussed, including a vision to leverage data science initiatives at the University of Virginia to spearhead, maximize and accelerate discovery in the nascent field of Multi-Messenger Astrophysics.

 

Black holes are bridges between astrophysics and fundamental physics.  I will describe three examples of this theme.  First, I will explain how contemporary theoretical ideas deriving from the holographic principle have proven useful for interpreting numerical simulations of electromagnetic outflows from spinning black holes.  These models are currently being tested against X-ray and radio observations of galactic black holes.  Second, I will describe a correspondence between black holes and lower dimensional fluids and discuss the possibility of probing this correspondence with gravitational wave memory experiments.  Finally, I will describe how gravitational wave observations of black hole tidal interactions might be used to find new symmetries acting on the event horizon.

We are entering a golden age of cosmology and astrophysics. In the coming decade we will have cosmological data for over a billion galaxies, a census of objects in the Milky Way, and a network of gravitational detectors spanning the globe that will detect thousands of events per year. This presents us with the unprecedented opportunity to learn how gravity behaves at the largest distances, and in the most extreme environments. In this talk I will describe how we can use current and upcoming data to understand the unexplained mysteries of the Universe, such as why the expansion of the Universe accelerating (dark energy). I will also discuss how to connect physics in these disparate regimes and how to test cosmology on small scales. To maximize the discovery potential of the data requires us to construct robust theoretical models, identify novel probes, and connect theory with observation, and I will describe projects where I have attempted to accomplish this. I will conclude the talk by discussing how this interdisciplinary effort will continue into the next decade and beyond.    

 

Observations have shown that nearly all galaxies harbor massive or supermassive black holes at their centers. Gravitational wave (GW) observations of these black holes will shed light on their growth and evolution, and the merger histories of galaxies. Massive and supermassive black holes are also ideal laboratories for studying strong-field gravity. Pulsar timing arrays (PTAs) are sensitive to GWs with frequencies ~1-100 nHz, and can detect GWs emitted by supermassive black hole binaries, which form when two galaxies merge. The Laser Interferometer Space Antenna (LISA) is a planned space-based GW detector that will be sensitive to GWs ~1-100 mHz, and it will see a variety of sources, including merging massive black hole binaries and extreme mass-ratio inspires (EMRIs), which consist of a small compact object falling into a massive black hole. I will discuss source modeling and detection techniques for LISA and PTAs, as well as present limits on nanohertz GWs from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration.

Quantum Chromodynamics (QCD), the theory of the strong interaction, is a cornerstone of the Standard Model of modern physics. It explains all nuclear matter as bound states of point-like fermions, known as quarks, and gauge bosons, known as gluons. The gluons bind not only quarks but also interact with themselves. Unlike with the more familiar atomic and molecular matter, the interactions and structures are inextricably mixed up, and the observed properties of nucleons and nuclei, such as mass and spin, emerge out of this complex system. To precisely image the quarks and gluons and their interactions and to explore the new QCD frontier of strong color fields in nuclei, the Nuclear Physics community proposes an US-based Electron Ion collider (EIC) with high-energy and high-luminosity, capable of a versatile range of beam energies, polarizations, and ion species. The community is convinced that the EIC is the right tool to understand how matter at its most fundamental level is made.

A global quantum revolution is currently underway based on the recognition that the subtler aspects of quantum physics known as superposition (wave-like aspect), measurement (particle-like aspect), and entanglement (inseparable link between the two aspects) are far from being merely intriguing curiosities, but can be transitioned into valuable, real-world technologies with performances that can far exceed those obtainable with classical technologies. The recent demonstration by the Chinese scientists of using a low-earth-orbit satellite to distribute entangled photons to two ground stations that are over a thousand kilometers apart is a stunning technological achievement—direct entanglement distribution over the best available fiber links is limited to a few hundred kilometers—and a harbinger of future possibilities for globally secure communications guaranteed by the power of quantum physics.

Harnessing the advantages enabled by superposition, measurement, and entanglement (SME)—the three pillars of quantum physics—for any given application is what is termed quantum engineering in general. In many instances, however, the details of the underlying science (high-temperature superconductivity, photosynthesis, avian navigation, are some examples) is still not fully understood, let alone how to turn the partially understood science into a potentially useful technology. Nevertheless, it has become clear in the last few decades that quantum engineering will require a truly concerted effort that will need to transcend the traditional disciplinary silos in order to create and sustain new breeds of science and technology communities that will be equally versed in quantum physics as they would be in their chosen area of technology. In this talk, I will present my vision for unleashing the potential of quantum engineering, taking some examples from ongoing and proposed research.

The bounty of gravitational-wave observations from LIGO and Virgo has opened up a new window onto the warped Universe, as well as a pathway to addressing many of the contemporary challenges of fundamental physics. I will discuss how catalogs of stellar-mass compact object mergers can probe the unknown physical processes of binary stellar evolution, and how these systems can be harnessed as standard distance markers (calibrated entirely by fundamental physics) to map the expansion history of the cosmos. The next gravitational-wave frontier will be opened within 3-6 years by pulsar-timing arrays, which have unique access to black-holes at the billion to ten-billion solar mass scale. The accretionary dynamics of supermassive black-hole binaries should yield several tell-tale signatures observable in upcoming synoptic time-domain surveys, as well as gravitational-wave signatures measurable by pulsar timing. Additionally, pulsar-timing arrays are currently placing compelling constraints on modified gravity theories, cosmic strings, and ultralight scalar-field dark matter. I will review my work on these challenges, as well as in the exciting broader arena of gravitational-wave astrophysics, and describe my vision for the next decade of discovery. 

I will begin by describing an interacting model in Quantum Mechanics where exact results about the ground state can be established by using tools from topology. I will then argue that such tools are also useful for tackling interesting problems in Quantum Field Theory. In particular, I will review Yang-Mills theory and argue that using topology one can make several predictions about its possible phases. We will then also extend the considerations to Quantum Chromodynamics and discuss possible connections with particle physics phenomenology and with condensed matter physics.

“An observation of neutron-antineutron oscillations (n-n ̅), which violate both B and B — L conservation, would constitute a scientific discovery of fundamental importance to physics and cosmology. A stringent upper bound on its transition rate would make an important contribution to our understanding of the baryon asymmetry of the universe by eliminating the post-sphaleron baryogenesis scenario in the light quark sector. We show that one can design an experiment using slow neutrons that in principle can reach the required sensitivity of 10¹⁰ s in the oscillation time, an improvement of 10⁴ in the oscillation probability relative to the existing limit for free neutrons. This can be achieved by allowing both the neutron and antineutron components of the developing superposition state to coherently reflect from mirrors. We present a quantitative analysis of this scenario and show that, for sufficiently small transverse momenta of n/n ̅ and for certain choices of nuclei for the n/n ̅ guide material, the relative phase shift of the n and n ̅ components upon reflection and the n ̅ annihilation rate can be small. While the reflection of n ̅ from surface looks exotic and counterintuitive and seems to contradict to the common sense, in fact it is fully analogous to the reflection of n from surface. The later phenomenon is well known and used in neutron research from its first years. We illustrate it with two selected example of gravitational and whispering-gallery quantum states of neutrons.”

[V.V. Nesvizhevsky, A.Yu. Voronin, Surprising Quantum Bounces, Imperial College Press, London, 2015]  

Human brains can solve many problems with orders of magnitude more energy efficiency than traditional computers.  As the importance of such problems, like image, voice, and video recognition, increases, so does the drive to develop computers that approach the energy efficiency of the brain.  Magnetic devices, especially tunnel junctions, have several properties that make them attractive for such applications.  Their conductance depends on the state of the ferromagnets making it easy to read information that is stored in their magnetic state.  In addition, current can manipulate the magnetic state.  Based on this electrical control of the magnetic state, magnetic tunnel junctions are actively being developed for integration into CMOS integrated circuits to provide non-volatile memory.  This development makes it feasible to consider other geometries that have different properties.  I describe two of the computing primitives that have been constructed based on the different functionalities of magnetic tunnel junctions.  The first of these uses tunnel junctions in their superparamagnetic state as the basis for a population coding scheme.  The second uses them as non-linear oscillators in the first nanoscale “reservoir” for reservoir computing.

Most of my talk will be about anomalous thermal relaxations, such as the Mpemba effect. Towards the end of my talk, I will also highlight a few topics in population dynamics that I have been working on. 

The Mpemba effect is a phenomenon when "hot can cool faster than cold" - a “shortcut” in relaxation to thermal equilibrium. It occurs when a physical system initially prepared at a hot temperature, cools down faster than an identical system prepared at a colder temperature. The effect was discovered as a peculiarity of water. Despite following observations in granular gasses, magnetic alloys, and spin glasses, the effect is still most often referred to as an “oddity” of water, although it is widespread and general.  I will describe how to define a Mpemba effect for an arbitrary physical system, and show how to quantify and estimate the probability of the Mpemba effect on a few examples. 

In the remaining time, I will briefly talk about the adaptation of bacterial populations and the immune system of bacteria with CRISPR.  Besides being the biology's newest buzzword and favorite gene editing tool, CRISPR is also a mechanism that allows bacteria to defend adaptively against phages and other invading genomic material. From the standpoint of physics and biology, the coevolution of bacteria and phages yields fascinating open questions. 

The steady increase in computational power of information processors over the past half-century has led to smart phones and the internet, changing commerce and our social lives.  Up to now, the primary way that computational power has increased is that the electronic components have been made smaller and smaller, but within the next decade feature sizes are expected to reach the fundamental limits imposed by the size of atoms.  However, it is possible that further huge increases in computational power could be achieved by building quantum computers, which exploit in new ways of the laws of quantum mechanics that govern the physical world.  This talk will discuss the challenges involved in building a large-scale quantum computer as well as progress that we have made in developing a quantum computer using silicon quantum dots.  Prospects for further development will also be discussed.

Many unusual properties of strongly correlated materials have been attributed to the proximity of quantum phase transitions (QPTs), where different types of orders compete and coexist, and may even give rise to novel phases.  In two-dimensional (2D) systems, the nature of the magnetic-field-tuned QPT from a superconducting to a normal state has been widely studied, but it remains an open question.  Underdoped copper-oxide high-temperature superconductors are effectively 2D materials and thus present a promising new platform for exploring this long-standing problem.  Although in cuprates the normal state is commonly probed by applying a perpendicular magnetic field (H) to suppress superconductivity, the identification and understanding of the H-induced normal state has been a challenge because of the complex interplay of disorder, temperature and quantum fluctuations, and the near-universal existence of charge-density-wave correlations. 

This talk will describe recent experimental advances in identifying and characterizing a full sequence of ground states as a function of H in underdoped cuprates.  In both the absence and the presence of charge order, the results demonstrate the key role of disorder in the H-tuned suppression of 2D superconductivity, giving rise to an intermediate regime with large quantum phase fluctuations, in contrast to the conventional scenario.  Most strikingly, the interplay of the “striped” charge order with high-temperature superconductivity leads to the emergence of an unanticipated, insulatinglike ground state with strong superconducting phase fluctuations, suggesting an unprecedented freezing (i.e. “the hidden order”) of Cooper pairs.  Possible scenarios will be discussed, including the implications of the results for understanding the physics of the cuprate pseudogap regime, as well as other 2D superconductors.

Layered transition-metal dichalcogenides (TMDs) are well known for their rich phase diagrams, which
encompass diverse quantum states including metals, semiconductors, Mott insulators, superconductors, and
charge density waves (CDWs). For instance, 2H-NbSe₂ and 2H-TaS₂ are canonical incommensurate CDW
systems, while 1T-TiSe₂ harbors a commensurate CDW order. There is a coexistence/competition of CDW
and superconductivity in 2H-NbSe₂ and 2H-TaS₂, though this is not the case for pristine 1T-TiSe₂. A subtle
interplay of CDW and superconducting orders, however, appears in each of these materials via chemical
intercalation or under pressure. Such a competition between or coexistence of proximate broken-symmetry
phases resembles many aspects of the phase diagram of cuprate high temperature superconductors
(HTSCs)—particularly, in the underdoped regime where the enigmatic pseudogap phase exists. The origin
of the CDW order in these compounds is an intriguing puzzle despite decades of research. We will present
our experimental data, which combine Angle Resolved Photoemission Spectroscopy, Scanning Tunneling
Spectroscopy, scattering and transport measurements, to provide new insights into the relative importance
of lattice and Coulomb effects in the CDW transitions of these compounds. These studies will also highlight
the distinctive impacts of disorder and doping in commensurate and incommensurate CDW systems.
Finally, comparing spectroscopic features of the CDW state of the TMDs with those of the normal state
underdoped HTSCs, we will discuss whether a CDW order can possibly be the origin of the pseudogap
phase in the cuprates.

2018 is the 100th Anniversary of the birth of Richard Feynman.
His discoveries and new formalisms, and the way he thought about
solving problems, transformed the way we think
about physics. I will talk about examples of how these impacted present results
in nuclear and particle physics.

I will also expand upon what might be called Feynmanʼs Scientific Method,
and how by following that method we can become better scientists ourselves
and nurture the next generation of scientists.

Is gravity a fundamental force? I will discuss a few scenarios in which gravity emerges from the dynamics of some underlying field theory. In holography (or AdS/CFT correspondence), Einstein's equations for the bulk gravity dual are linked to entanglement in the boundary field theory. In another example, the graviton emerges as a composite spin two massless particle in a scalar field theory.

"Searching for Supersymmetry with the ATLAS experiment"

"The Brave nu World"