# Colloquia / Seminars

The UNC Physics colloquium takes place on Mondays 3:30-4:30pm ET in Phillips 265 unless otherwise stated.

## Fall 2021

### M, Aug 30

##### Why Quantum Computing?

*Jon Engel, University of North Carolina at Chapel Hill*

I provide a basic introduction to the theory of quantum computing and its application in physics. After presenting a toy problem to illustrate general-purpose quantum algorithms, and then discussing a few of the best know algorithms of that type, I turn to the simulation of many-particle quantum systems, focusing on techniques such as the Variational Quantum Eigensolver that promise to be useful in the relatively near future. I end by presenting some work by my research group at UNC, which is dipping a toe into the field.

*4:00pm ET, Phillips 215*

### T, Sep 7

##### Topology and gauge theories

##### Emergent higher symmetry and topological order

*X.-G. Wen – 10am
Session chair: Drut
Introductory remarks: Ortiz*

##### Quantum information measure of space-time correlation

*X. Qi – 1pm*

*Session chair: Batista*

*Zoom link, p/w: 314159*

*YouTube live broadcast*

*More information*

### T, Sep 14

##### Machine learning and computational physics

##### Reconstructing quantum states with generative models

*R. Melko – 10am
Session chair: Nicholson
*

Generative models are a powerful tool in unsupervised machine learning, where the goal is to learn the unknown probability distribution that underlies a data set. Recently, it has been demonstrated that modern generative models adopted from industry are powerful enough to reconstruct quantum states, given projective measurement data on individual qubits. These virtual reconstructions can then be studied with probes that may be unavailable to the original experiment. In this talk I will outline the strategy for quantum state reconstruction using generative models, and show examples on experimental data from a Rydberg atom quantum simulator. I will discuss the continuing theoretical development of the field, including the exploration of powerful autoregressive models for the reconstruction of mixed and time-evolved quantum states.

##### Simulating strongly correlated systems with tensor networks

*F. Verstraete – 11am
Session chair: Ortiz
*

Tensor networks model the entanglement degrees of freedom of quantum many-body wavefunctions, and give rise to a powerful variational ansatz for simulating low-energy states of the corresponding quantum Hamiltonians. This talk will highlight recent advances in the field of tensor networks, including entanglement scaling methods, state of the art PEPS algorithms, and the description of symmetries in topological phases of matter.

*Zoom link, p/w: 314159*

*YouTube live broadcast*

*More information*

### T, Sept 21

##### Quantum simulation by quantum annealing

*H. Nishimori – 10am
Session chair: Papenbrock
*

After a brief introduction to quantum annealing and an overview of the status of quantum simulation by quantum annealing, I describe our recent quantum simulations using the D-Wave quantum annealers on the Kibble-Zurek mechanism for defect formation in a quantum chain and the Griffiths-McCoy singularity for low-dimensional diluted random magnets.

##### Quantum Computing for Nuclear Physics

*M. J. Savage – 11am
Session chair: Batista
*

Theoretical predictions of the properties and dynamics of quantum field theories and quantum many-body systems of importance to nuclear physics research, from dense and/or non-equilibrium matter, to systems of neutrinos, to jet production in heavy-ion collisions, require, in many instances, computational capabilities beyond the realm of classical computing. As highlighted by Feynman and others in the early 1980s, such systems may be amenable to future quantum simulations. I will discuss recent advances towards achieving these objectives and the connections to quantum information and other domain sciences.

*Zoom link, p/w: 314159*

*YouTube live broadcast*

*More information*

### T, Sep 28

##### Fermi surfaces large and small: unifying theories of the Kondo lattice and Hubbard models

** **S. Sachdev – 10am

Fermi surfaces which obey the Luttinger theorem are often referred to as “large”. However, it is possible to have “small” Fermi surfaces with electron-like quasiparticles, in certain metallic states (sometimes called FL*) which evade the Luttinger theorem using emergent gauge fields. It is relatively easy to construct FL* states in Kondo lattice models, but much harder in a single band Hubbard model. I will describe a new approach which yields a variational wavefunction for FL* in the Hubbard model, and also a theory for the transition between small and large Fermi surfaces. The key idea is to avoid fractionalizing the electron, and to instead fractionalize a paramagnon into a pair of ancilla qubits. I will note applications to the phase diagram of the cuprates.

##### Boundary criticality of the O(N) model in d = 3 critically revisited

*M. Metlitski – 11am
Session chair: Basar*

It is known that the classical O(N) model in dimension d > 3 at its bulk critical point admits three boundary universality classes: the ordinary, the extra-ordinary and the special. The extraordinary fixed point corresponds to the bulk transition occurring in the presence of an ordered boundary, while the special fixed point corresponds to a boundary phase transition between the ordinary and the extra-ordinary classes. While the ordinary fixed point survives in d = 3, it is less clear what happens to the extra-ordinary and special fixed points when d = 3 and N is greater or equal to 2. I’ll show that formally treating N as a continuous parameter, there exists a critical value Nc > 2 separating two distinct regimes. For N < Nc the extra-ordinary fixed point survives in d = 3, albeit in a modified form: the long-range boundary order is lost, instead, the order parameter correlation function decays as a power of log r. For N > Nc there is no fixed point with order parameter correlations decaying slower than power law. I’ll discuss how these findings compare to recent Monte-Carlo studies of classical and quantum spin models with SO(3) symmetry. Based on arXiv:2009.05119

*Zoom link, p/w: 314159*

*YouTube live broadcast*

*More information*

### M, Oct 4

##### A Flow and Aerosol Guide to the Orchestra

*Quentin Brosseau, University of Pennsylvania*

In the midst of the COVID 19 pandemic, live musical activities have come to a standstill to protect both musicians and the public. Among musical groups, orchestral ensembles both personnel heavy and instrumentally diversified face the challenge of contamination. In particular, assessing whether wind instruments are possible vectors of contamination through the dispersion of aerosol from human origin is a chief concern in mitigating the effect of the spread of the disease.

This study, made possible by the participation of members of the Philadelphia Orchestra, brings insight on the modes of production and early life of aerosol droplets emitted during the use of wind instruments. We show that aerosol size distribution shares similar characteristics as normal speech and vocalization, and is sensitive to the material of the instrument. Furthermore flow characterization shows that although small, the flow rate at instrument orifices is sufficient for the aerosol to be spread in the immediate environment of the musician and be picked up by the ambient for longer distance of travel.

*4:00pm ET*

*This lecture will be delivered via Zoom: https://unc.zoom.us/j/97576480231, p/w: 086511*

### F, Oct 8

##### Cosmology with Gravitational Waves and Gravitational Waves with Cosmology Data

*Kris Pardo, Jet Propulsion Laboratory, California Institute of Technology*

Gravitational waves offer us a whole new way of looking at our Universe. LIGO’s observations within the ~10-100 Hz range have taught us about fundamental properties of gravity, as well as populations of stellar mass black holes. Lower frequency gravitational waves are expected to be detected within the next two decades. This promises even more tests of fundamental physics and a better understanding of massive and supermassive black holes. In this talk, I will give an overview of the gravitational wave spectrum, discuss my contributions to using gravitational waves as a cosmological probe, and detail one way to bridge a gap in the gravitational wave frequency spectrum using upcoming photometric surveys. Finally, I will discuss plans to use space-based atom interferometers to detect gravitational waves in the last open part of the spectrum.

*3:30pm ET, Phillips 247*

*Lecture will also be available via Zoom: https://unc.zoom.us/j/94135079963?pwd=blJXa2hXV1hWeWJobmtoalBhMWEwdz09*

### M, Oct 11

##### The search for the QCD critical point

*Gökçe Başar, University of North Carolina at Chapel Hill
*

The strong force binds the building blocks of protons and neutrons, quarks and gluons, together and creates most of the observed mass in the universe. At a few trillion degrees, these bonds break, and matter transitions into a new phase called Quark Gluon Plasma (QGP). QGP once filled the universe when it was microseconds old, and today is recreated in the heavy ion collision experiments with the goal of understanding how matter behaves at these extreme temperatures and densities. What do we know about the phases of matter at these extreme environments? What is the nature of the phase transition between ordinary matter and QGP? Is there a critical point in the phase diagram and if there is how can we locate it? I will give an overview of the status of the theoretical and experimental efforts to answer these questions as well as the future challenges.

*3:30pm ET, Phillips 265*

*This lecture will be both in-person and available via Zoom: **https://unc.zoom.us/j/94217365737?pwd=WTBQNUJwaU9RblB4SDBISWZ5TGc1UT09*

*A recording of this lecture can be found here: *https://unc.zoom.us/rec/share/g394LmmpslPfz2pxYwDR4TfyfiLJRqG3tFMhwjwTD_A8ktHt7ATAkil7NPLxkDqQ.DG0OR7qwsX185MWd?startTime=1633980464000*, pwd: coolcosmos#1*

### M, Oct 18

##### The ties that bind: understanding nuclear forces from lattice QCD

*Amy Nicholson, University of North Carolina at Chapel Hill
*

There are many open questions in nuclear physics which only lattice QCD may be able to answer. One example is understanding the nature and origin of the fine-tuning of interactions between nucleons and nuclei observed in nature. The first step toward building a bridge between the underlying theory, QCD, and nuclear observables is full control over one- and two-nucleon systems. While enormous strides have been made in recent years in precision calculations of single-nucleon observables, the history of two-nucleon calculations has generated more questions than answers. In particular, there is a controversy in the literature between calculations performed using different theoretical techniques, even for calculations far from the physical point, chosen due to the exponentially simpler computational properties. In this talk, I will present the history and challenges behind one- and two-nucleon calculations in lattice QCD, as well as advances in understanding and controlling the associated systematics.

*3:30pm ET, Phillips 265
*

*This lecture will be both in-person and available via Zoom: https://unc.zoom.us/j/98744281549?pwd=Ti9TKzRoVStnNzIyR2pMZkc0WmR6dz09, p/w: 638211*

### M, Oct 25

**Galactic Archaeology and the Search for Dark Matter**

*Mariangela Lisanti, NIST, QuICS, University of Maryland
*

The Gaia mission is in the process of mapping nearly 1% of the Milky Way’s stars, nearly a billion in total. This data set is unprecedented and provides a unique view into the formation history of our Galaxy and its associated dark matter halo. I will review recent results demonstrating how the evolution of the Galaxy can be deciphered from the stellar remnants of massive satellite galaxies that merged with the Milky Way early on. This analysis is an inherently “big data” problem, and I will discuss how to leverage machine learning techniques to advance our understanding of the Galaxy’s evolution. Our results indicate that the local dark matter is not in equilibrium, as typically assumed, and instead exhibits distinctive dynamics tied to the disruption of satellite galaxies. The updated dark matter map built from the Gaia data has ramifications for direct detection experiments, which search for the interactions of these particles in terrestrial targets.

*4:00pm ET, via Zoom: https://unc.zoom.us/j/98391633039?pwd=bkY5SW5FSTVRWThIS0dOeGRncHpoZz09, p/w: 799373
*

*A recording of this lecture can be found here: https://unc.zoom.us/rec/share/nNgNZTL-EzLT7btt3aQTD3hNiWMazqiuiCI8geJsEWu28A75jMqOGDhC79MbRuYC.BnN0rTbDWwXkmPLV, p/w: coolcosmos#1*

### M, Nov 1

##### Quantum steampunk: Quantum information meets thermodynamics

*Nicole Yunger Halpern, NIST, QuICS, University of Maryland
*

Thermodynamics has shed light on engines, efficiency, and time’s arrow since the Industrial Revolution. But the steam engines that powered the Industrial Revolution were large and classical. Much of today’s technology and experiments are small-scale, quantum, far from equilibrium, and processing information. Nineteenth-century thermodynamics requires updating for the 21st century. Guidance has come from the mathematical toolkit of quantum information theory. Applying quantum information theory to thermodynamics sheds light on fundamental questions (e.g., how does entanglement spread during quantum thermalization? How can we distinguish quantum heat from quantum work?) and practicalities (e.g., nanoscale engines and the thermodynamic value of quantum coherences). I will overview how quantum information theory is being used to modernize thermodynamics for quantum-information-processing technologies. I call this combination quantum steampunk, after the steampunk genre of literature, art, and cinema that juxtaposes futuristic technologies with 19th-century settings.

*This lecture will be held remotely via Zoom: https://unc.zoom.us/j/93015929597?pwd=RTBXRUU3alNvRnlEek8yMjQ3WFp0dz09, p/w: 477402*

*A recording of this lecture can be found here: https://unc.zoom.us/rec/share/HRIA1MgRsq9kbZm6nSnRjNK1TZnW_YFWcwizVMn7o0pJ1wSL1s1dFxU95qzconME.KF2spqLSY9EQYT6q, p/w: coolcosmos#1*

### M, Nov 8

##### Graphene-based superconducting quantum Hall devices

*Gleb Finkelstein, Duke University
*

Superconductivity and the quantum Hall effect are some of the most studied phenomena in condensed matter physics. The more familiar of these phenomena – the superconductivity – results in vanishing electron resistance, and has its roots in the Cooper pairing of electrons. The quantum Hall effect is observed in quasi-two dimensional electron systems subject to high magnetic fields, and results in precise quantization of electrical resistance to better than one part in a billion. Combining superconductivity and the QH effect is predicted to be a promising route for creating novel “topological” quantum states of matter. In the past few years several groups made progress in inducing superconductivity in the QH regime. In particular, our group demonstrated the existence of supercurrent in a QH regime in graphene – a single atomic monolayer of graphite. I will review this result and our recent work on making a quantum Hall-based SQUID (superconducting quantum interference device)

*3:30pm ET, Phillips 265
*

*This lecture will be in-person and available remotely via Zoom: https://unc.zoom.us/j/92531103794?pwd=aDJmRHJCVjhveGYyMXRaeG1Pekw2dz09, p/w: 596014*

### M, Nov 15

##### Clock, quantum matter, and fundamental physics

*Jun Ye, JILA, National Institute of Standards and Technology, University of Colorado, Boulder*

Precise control of quantum states of matter and innovative laser technology are revolutionizing the performance of atomic clocks and metrology. Sr atoms cooled to a degenerate quantum gas and loaded into arrays of optical traps highlights new measurement capabilities building on the exploration of quantum and precision physics. This increasingly powerful physics frontier is promising greater opportunities for probing fundamental and emerging phenomena. I will highlight recent work where the gravitation red shift within a single atomic ensemble is measured at the 1 x 10^-20 level.

*4:00pm ET, via Zoom: https://unc.zoom.us/j/99144234796?pwd=QWZKbURYZkZWbkRuV3RqMkxVZmJnZz09, p/w: 856904
*

### M, Nov 22

##### Cavity Quantum Electrodynamics with Semiconductor Spin Qubits

*Jason Petta, Princeton University
*

Electron spins are excellent candidates for solid-state quantum computing due to their exceptionally long quantum coherence times, which is a result of weak coupling to environmental degrees of freedom. However, this isolation comes with a cost, as it is difficult to coherently couple two spins in the solid-state, especially when they are separated by a large distance. Here we combine a large electric-dipole interaction with spin-orbit coupling to achieve spin-photon coupling. Vacuum Rabi splitting is observed in the cavity transmission as the Zeeman splitting of a single spin is tuned into resonance with the cavity photon. We achieve a spin-photon coupling rate as large as gs/2π = 10 MHz, which exceeds both the cavity decay rate κ/2π = 1.8 MHz and spin dephasing rate γs/2π= 2.4 MHz, firmly anchoring our system in the strong-coupling regime [1]. We next utilize spin-photon coupling to achieve a resonant spin-spin interaction between two spins that are separated by more than 4 mm [2]. Our results demonstrate that microwave-frequency photons can be used as a resource to generate long-range two-qubit gates between spatially separated spins. I will also present extensions of this work to dot-donor systems, where theory suggests that high fidelity microwave readout of a single nuclear spin state is within reach [3].

*4:00pm ET, via Zoom: https://unc.zoom.us/j/96994594083?pwd=cUY1bnRHYmdGeEVoVm9reVVpbmZ2QT09, p/w 098125
*

### M, Nov 29

##### Planetesimals and Planets around White Dwarfs

*Siyi Xu, Gemini Observatory, Northern Operations Center, Hawaii
*

Recent studies from both observations and theories show that planetary systems can be present and active around white dwarfs. In the first part of the talk, I will present an optical transmission spectrum with Gemini/GMOS covering ten transits of WD 1856+534 b — a white dwarf with a transiting planet candidate. In the second part, I will discuss our on-going efforts to characterize planetesimals around bright white dwarfs identified in Gaia. I will also talk about new opportunities to characterize planetary systems around white dwarfs from current and future facilities.

*3:30pm ET, via Zoom: https://unc.zoom.us/j/97939441741?pwd=NEJxN1RsdWZ3UnpCMk45UVRiQmtyUT09, p/w: 477190
*

## Spring 2022

### M, Jan 24

##### Emergent hydrodynamics in a strongly interacting dipolar spin ensemble in diamond

*Chong Zu, Washington University
*

Conventional wisdom holds that macroscopic classical phenomena naturally emerge from microscopic quantum laws. However, building direct connections between these two descriptions has remained an enduring scientific challenge. In particular, it is difficult to quantitatively predict the emergent ‘classical’ properties of a system (for example, diffusivity, viscosity, and compressibility) from a generic microscopic quantum Hamiltonian. Here we introduce a hybrid solid-state spin platform in diamond, where the underlying disordered, dipolar quantum Hamiltonian gives rise to the emergence of unconventional spin diffusion at nanometre length scales [1]. In particular, the combination of positional disorder and on-site random fields leads to diffusive dynamics that are Fickian yet non-Gaussian. Finally, by tuning the underlying parameters within the spin Hamiltonian via a combination of static and driven fields, we demonstrate direct control over the emergent spin diffusion coefficient. If time permits, I will end by describing our recent efforts to realize a quantum simulation platform based upon spin defects in 2D [2].

[1] C. Zu, et al., Nature 597, 45-50 (2021)[2] E. Davis, et al., arXiv:2103.12742 (2021)

*3:30pm ET, via Zoom*: https://unc.zoom.us/j/95960889513?pwd=WEdiTS9kRi9zZVNoVkcwcyswdlkrUT09

Meeting ID: 959 6088 9513

Passcode: 205359

### M, Jan 31

##### Radium ions and radioactive molecules

*Andrew Jayich, University of California, Santa Barbara
*

The bottom row of the periodic table is famous for its radioactive elements, which compared to stable isotopes are little-explored. Many heavy radioisotopes have exotic nuclei which grant them enhanced discovery potential. Radioactive elements also hold promise for advancing technology. Modern atomic physics techniques, such as laser cooling and ion trapping, allow for efficient use of unstable elements and their study in highly-controlled environments. In this context we will discuss our recent work with laser-cooled radium ions. This heavy species is promising for controlling other radioactive atoms and molecules at the level of single quantum states and studying them with high precision spectroscopy. To date with trapped radium ions we have produced a number of radioactive molecules which are good candidates for studying time symmetry violation to address open questions in physics. We have also developed a new technique for rapidly identifying molecular ions. In addition to producing and controlling molecules, the radium ion holds promise in its own right, for example as a transportable optical clock candidate.

*3:30pm ET, Phillips 265*

### M, Feb 7

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*3:30pm ET, Phillips 265*

### M, Feb 14

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*3:30pm ET, Phillips 265*

### M, Feb 21

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*3:30pm ET, Phillips 265*

### M, Feb 28

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*3:30pm ET, Phillips 265*

### M, Mar 7

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*3:30pm ET, Phillips 265*

### M, Mar 21

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*3:30pm ET, Phillips 265*

### M, Mar 28

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*3:30pm ET, Phillips 265*

### M, Apr 4

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*3:30pm ET, Phillips 265*

### M, Apr 11

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*3:30pm ET, Phillips 265*

### M, Apr 18

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*3:30pm ET, Phillips 265*

### M, Apr 25

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*3:30pm ET, Phillips 265*