# 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. *

*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: here, pwd: 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 here, pwd: 799373
*

*A recording of this lecture can be found here, pwd: 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, pwd: 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 here, pwd: 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 here, pwd: 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: here, pwd: 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 here, pwd: 477190
*

## Spring 2022

### F, Apr 8

##### The Missing Physicists

*Panel Discussion*

This special colloquium, “The Missing Physicists,” will feature a panel discussion of the March 2022 “Missing Physicists” series in *Science* about the barriers Black physicists face and potential models for change.

Panelists include:

- Mohammad Ahmed [he/him], NC Central University
- Roy Clarke [he/him], University of Michigan
- Zack Hall [he/him], UNC Chapel Hill
- Sheila Kannappan [she/her], UNC Chapel Hill
- Kent Wallace [he/him], Fisk University
- Jennifer Weinberg-Wolf [she/her], UNC Chapel Hill

The discussion will be moderated by Akaa Daniel Ayangeakaa [he/him] of UNC Chapel Hill.

*3:30pm ET, Chapman 125 or via Zoom*

### 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* here

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

##### Nanophotonic Interfaces to Control Plasmons and Spins

*Laura Kim, Massachusetts Institute of Technology
*

Light-matter interactions enabled by photonic quasiparticles play a crucial role in observing ultrafast phenomena as well as enabling next-generation nanophotonic devices and quantum technologies. In the first part of the presentation, I will present the first experimental demonstration of a mid-infrared light-emitting mechanism, originating from an ultrafast coupling of optically excited carriers into hot plasmon excitations in graphene. Such emission processes produce gate-tunable, non-Planckian emission behavior that is not dictated by the free-space photonic density of states. This work provides a platform for achieving ultrafast, ultrabright, on-chip mid-infrared light sources. In the second part of the presentation, I will present a diamond resonant metasurface that can mediate efficient spin-photon interactions and enable a new type of quantum imaging system. This quantum metasurface containing nitrogen-vacancy (NV) spin ensembles achieves local field concentration over a micron-scale NV layer, and it coherently encodes information about the local magnetic field on spin-dependent phase and amplitude changes of near-telecom light. The projected performance makes the studied quantum imaging metasurface appealing for the most demanding applications such as imaging through scattering tissues and spatially resolved chemical NMR detection.

*3:30pm ET, Phillips 265*

### W, Feb 9

##### Probing Quantum Materials with Scanning Probe Microscopy

*Yonglong Xie, Harvard University
*

Electrons inside matter can behave as complex particles that do not exist in the Standard Model. These seemingly impossible effects are examples of emergent phenomena—that is, unexpected collective behavior—of electrons in quantum materials. The discovery and characterization of new emergent phenomena in quantum materials not only expand the boundary of our knowledge, but also provide unique opportunities for future quantum technologies. However, these effects often manifest in subtle ways, and thus detecting them requires developing new, more sophisticated measurement tools.

In this talk, I will demonstrate how a class of experimental techniques called scanning probe microscopy can be a general tool for unlocking new phenomena in quantum materials. To illustrate the power of this approach, I will focus on our recent experimental observation of novel topological quantum states in magic-angle graphene, enabled by scanning single-electron-transistor microscopy. In addition, I will highlight other examples in which scanning probe microscopy permits the discovery of novel phases in other quantum materials. Finally, I will conclude by outlining how pushing the boundaries of existing scanning probe microscopy will enable the discovery and characterization of new emergent phenomena and functionalities in quantum materials, devices, and circuits.

*3:30pm ET, Phillips 215*

### M, Feb 14

##### The elusive lightness of neutrinos

*John F. Wilkerson, UNC/TUNL/ORNL
*

Neutrinos, enigmatic fundamental particles, were long assumed to be massless until a series of revolutionary experiments over the past two decades revealed that they actually exhibit complex behavior and must possess non-zero mass. From these and other recent measurements we know that neutrinos have minuscule masses, at least 500,000 times lighter than the electron. Yet we still do not know the neutrino’s actual mass nor why it is so light. Determining this absolute neutrino-mass scale is vital to our understanding of fundamental interactions, cosmology, astrophysics and ultimately to answering the underlying question on the origin of particle masses. This talk will review our current understanding of neutrinos and then address the question of how one “weighs” a neutrino? The techniques and latest results from cosmology, double beta decay and direct kinematical methods will be presented, with a focus on the Karlsruhe Tritium Neutrino experiment (KATRIN), which today (Feb. 14) published a new limit, reaching for the first time sub-eV sensitivity from a direct neutrino mass experiment.

*3:30pm ET, Phillips 265*

### W, Feb 16

##### Quantum point contacts in an oxide superconductor

*Evgeny Mikheev, Stanford University
*

Superconductors and semiconductors are typically thought of as distinct material categories. Each has fascinating and technologically useful electronic properties. What if one could combine them in one material? This is rare but possible: for example, the oxide SrTiO3 superconducts at carrier densities so low that its superconductivity can be tuned by applying electric fields. I will explain how this can be done locally by applying voltages to nanopatterned gate electrodes. This presents a unique opportunity to create superconductor/normal state junctions without the complexity of interfacing two dissimilar materials.

I will present my recent work [1, 2] on defining narrow constrictions between larger superconducting regions in SrTiO3. I will demonstrate how the combination of quantum confinement and local gate tuning results in quantized staircase shapes in normal state conductance. This is a hallmark of clean ballistic behavior that typically requires working with pristine high-mobility semiconductors. An even more distinctive quantization signature is observed in the superconducting state: each ballistic mode can only carry a finite quantum of supercurrent, and a staircase shape is seen in the critical current. This work inches us closer than ever to experimentally realizing superconducting junctions coupled by one or few perfectly transmitting ballistic modes. This a difficult but important technological goal: such junctions are key enablers for several approaches to protect quantum information from dephasing and bit-flip errors. My future target is to integrate oxide nanostructures into gate-tunable transmon qubits (gatemons), Andreev qubits, and nanowires with topological superconductivity.

[1] E. Mikheev, I. T. Rosen, D. Goldhaber-Gordon, Science Advance 7, eabi6520 (2021)[2] E. Mikheev, I. T. Rosen, M. A. Kastner, D. Goldhaber-Gordon, arXiv:2110.11535

*3:30pm ET, Phillips 215*

### M, Feb 21

##### For the new material’s era beyond Silicon age: discovering, understanding, and manipulating quantum materials.

*Na-Hyun Jo, Lawrence Berkeley National Laboratory
*

We are currently living on the edge of the silicon age, characterized by the rapid growth of the semiconductor industry. Yet, a new class of materials destined to become as familiar as silicon is underway. These materials are so-called quantum materials. In quantum materials, quantum effects manifest over a wide range of energy and length scales and give rise to exotic properties such as superconductivity, non-trivial topology, and many more. For the last decade, topological materials have been of great interest in the materials science community. With the steady rise of topological materials, the community’s attention is now shifting to strongly correlated topological materials that host a largely unexplored territory from both a theoretical and experimental perspective. In this colloquium, recent efforts to discover, understand, and manipulate the topological state of matter via strong correlation will be reviewed. First, I will describe how the spin degree of freedom can manifest a novel quantum state of matter. Second, the effect of lattice degree of freedom on manipulating quantum materials will be discussed. Finally, I will also briefly state the exciting future of the field.

*3:30pm ET, Phillips 265*

### W, Feb 23

##### Quantum information processing based on spins in semiconductor quantum dots

*Yinyu Liu, Harvard University
*

Abstract

The field of Quantum Information is of great excitement in both fundamental physics and industry. One promising platform for quantum computing is gate-defined quantum dots in semiconductors. The greatest limiting factor currently is that delicate quantum states can lose their quantum nature due to interactions with their environment. Other open challenges are to coherently control large-scale spin qubits and develop methods to entangle quantum bits that are separated by significant distances.

Silicon-based materials are promising due to the long lifetimes of electrons’ quantum states, but also challenging due to the difficulty in fabrication and valley degeneracy. I will report a singlet-triplet qubit with a qubit gate that is assisted by the valley states. This work would potentially relax the design and fabrication requirement for scaling. Moreover, strong coupling between electron spins and photons in hybrid circuit-QED architecture has been achieved in this research field. Quantum optics, long-distance quantum entanglement, and communication via photons are promised. To address that, I will present my project on indium arsenate (InAs) double quantum dots (DQD) that are embedded in circuit-QED architecture. We demonstrated the direct evidence of photon emission from a DQD in the microwave regime. By achieving stimulated emission from one DQD in these works, we invented a semiconductor single atom maser that can be tuned in situ. I will demonstrate that a semiconductor-based quantum dot is a promising platform for quantum information as well as for fundamental physics.

*3:30pm ET, Phillips 215*

### M, Feb 28

##### Quantum transduction and spin-orbitronics using hybrid magnonic systems

*Wei Zhang, Oakland University
*

Abstract

Hybrid magnonic systems have gained increased interest due to their potential impact in quantum transduction and coherent information processing. Magnons, as the fundamental excitation of magnetically ordered materials, exhibit promising features that can be tuned and made to coherently couple to other excitations, such as microwave photons, light, phonons, and other magnons. I will introduce our recent work in the coherent magnon-photon coupling in a permalloy-superconducting resonator device and the magnon-magnon coupling of a permalloy-yttrium iron garnet device. By controlling the coupling strength and the quantized spin waves, the hybrid magnon spectra can also exhibit magnetically induced transparency, which can be readily detected via our home-built magneto-optic spectroscopy. Lastly, I will show how spin-orbit effects can be used as an effective means to both tune and detect the dynamic magnetic properties in magnonic systems.

*3:30pm ET, Phillips 265*

### M, Mar 7

##### How do you Merge two Black Holes? The Present and Future of Gravitational-wave Astrophysics

*Carl Rodriguez, Carnegie Mellon University
*

Since 2015, The Laser Interferometer Gravitational-wave Observatory (LIGO) has detected ~100 gravitational waves from merging black holes and neutron stars, inaugurating a new era of observational astronomy. But how are these systems formed in the first place, and what can that tell us about the lives and deaths of massive stars and the star clusters and galaxies that make them? In this talk, I will attempt to answer these questions by describing how massive and old star clusters, such as the globular clusters in the Milky Way, are the ideal site for the production of binary black holes. I will show how the dynamical assembly of binaries in these dense stellar environments imprints detectable features in the gravitational waves themselves, and how multiple mergers in star clusters can produce black holes with masses that cannot be formed from single or binary stars. Finally, I will place these results within the broader context of galaxy formation and assembly, describing a new project to model star clusters self-consistently from collapsing giant molecular clouds in an MHD simulation of a Milky Way-mass galaxy. These results can provide a direct link between the study of globular cluster formation, the assembly of galaxies, massive black holes, and the future of gravitational-wave astronomy.

*3:30pm ET, Phillips 265*

### M, Mar 21

##### A Song of Ice and Fire: the Fate of Planetary Systems After Stellar Death

*Andrew Vanderburg, MIT
*

In the past 30 years, astronomers have discovered thousands of planets orbiting

stars outside the solar system. Most of the exoplanets we know of today orbit stars

that will eventually exhaust their nuclear fuel, expand into red giants, shed their

outer layers, and contract into dense remnants called white dwarfs. How does the

process of stellar death affect any orbiting planets in the system? I will review

our knowledge of planets beyond the main sequence and discuss new insights gleaned

from our discoveries of two very different systems: a disintegrating minor planet

around WD 1145+017 and an intact giant planet candidate around WD 1856+534. I will

conclude by discussing the prospects for habitability in white dwarf systems long

after the host star’s death and how with some luck, we may be able to test these

ideas in the next decade.

*3:30pm ET, Phillips 265*

### M, Mar 28

##### Changes in the physics higher ed landscape

*Andy Rundquist, Hamline University
*

As the provost for a small, private, liberal arts school in the midwest, where I’ve been on the physics faculty for 20 years, I am happy to share our approaches to the upcoming demographic changes we sometimes call the impending “demographic cliff.” I’ll also share some curricular and pedagogical approaches that we think will work well with those demographic changes, including ways to include student voices in assessment.

*3:30pm ET, Phillips 265*

### M, Apr 4

##### Departments with high-use of active learning in introductory STEM courses: How did they get there?

*Charles Henderson
*

There is now a convincing body of research showing that a wide variety of active learning instructional strategies consistently improve student learning and other desired outcomes when compared to traditional instruction. Like most fields, there is a substantial gap between the research-based knowledge about effective teaching and the actual practices of instructors. In 2019 we conducted a web-based survey of 3,769 instructors who taught introductory chemistry, mathematics, and physics courses. Survey responses suggest that the instructional practices are more strongly influenced by departmental factors than individual or institutional factors. To understand how these departments got there, we conducted interviews with 27 instructors in 16 departments that were in the top quartile in terms of the use of active learning in their introductory courses. We developed a model that highlights the relevant characteristics of departments that have high use of active learning instruction in their introductory courses. According to this model, there are four main characteristics of such departments (motivated people, knowledge about teaching, opportunities, and cultures and structures that support active learning) and two positive feedback loops. There are two main take-away messages for those interested in promoting the use of active learning. The first is that all four components are important. A weak or missing component will limit the desired outcome. The second is that desired outcomes are obtained and strengthened over time through the two positive feedback loops. It is not realistic to expect meaningful sustainable change to occur in less than three years.

*3:30pm ET, Phillips 265*

### M, Apr 11

##### Title TBA

*Speaker TBA
*

Abstract TBA

*3:30pm ET, Phillips 265*

### M, Apr 18

##### Increasing Accuracy in Measurements of the Hubble Constant: Is There Evidence for New Physics?

*Wendy Freedman, University of Chicago*

An important and unresolved question in cosmology today is whether there is new physics that is missing from our current standard Lambda Cold Dark Matter (LCDM) model. Recent measurements of the Hubble constant, Ho — based on Cepheids and Type Ia supernovae (SNe) — are discrepant at the 4-5-sigma level with values of Ho inferred from measurements of fluctuations in the cosmic microwave background (CMB). The latter assumes LCDM, and the former assumes that systematics have been fully accounted for. If real, the current discrepancy could be signaling a new physical property of the universe. I will present new results based on an independent calibration of SNe Ho based on measurements of the Tip of the Red Giant Branch (TRGB). The TRGB marks the luminosity at which the core helium flash in low-mass stars occurs, and provides an excellent standard candle. Moreover, the TRGB method is less susceptible to extinction by dust, to metallicity effects, and to crowding/blending effects than Cepheid variable stars. I will address the current uncertainties in both the TRGB and Cepheid distance scales, the promise of upcoming James Webb Space Telescope data, as well as discuss the current tension in Ho and whether there is a need for additional physics beyond the standard LCDM model.

Meeting ID: 960 5170 9190

Passcode: 347299

*3:30pm ET, Zoom only*

### M, Apr 25

##### The dark side of string theory

*Ulf Danielson, Uppsala University Sweden*

Cosmological horizons, as well as black hole horizons, pose great challenges to fundamental physics. This is true also in string theory, where the black hole information paradox is still not resolved, and it remains unclear if a cosmological constant can even exist. In this lecture I will give an introduction to the main problems, review the status of the field, and speculate about the future.

Meeting ID: 923 8228 8816

Passcode: 260778

*3:30pm ET, Zoom only*