# 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

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

*Mariangela Lisanti, 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.

*4:00pm ET, via Zoom
*

### M, Nov 1

##### Quantum information meets thermodynamics

*Nicole Yunger Halpern, Harvard University, NIST
*

Abstract TBA.

*4:00pm ET, via Zoom*

### M, Nov 8

##### Title TBA

*Gleb Finkelstein, Duke University
*

Abstract TBA.

*4:00pm ET, Phillips 265
*

### M, Nov 15

##### Quantum measurements

*Jun Ye, JILA, University of Colorado, Boulder
*

Abstract TBA.

*4:00pm ET, via Zoom*

### M, Nov 22

##### Quantum control of nanometer scale systems

*Jason Petta, Princeton University
*

Abstract TBA.

*4:00pm ET, via Zoom*

### M, Nov 29

##### White Dwarfs

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

Abstract TBA.

*4:00pm ET, via Zoom*