## CQRT Seminars

The Atomic, Molecular and Optical (AMO) and Condensed Matter (CM) physics groups are hosting a joint seminar as part of the Center for Quantum Research and Technology (CQRT). This endowed seminar series brings in experts from across the country as well as across campus to discuss the latest in research advances in quantum science.

Seminars are scheduled for 2:30-3:30 pm on Tuesdays and/or Fridays, and are held in-person in Lin Hall 105, depending upon speaker availability and preference. Please check this web page or the email announcements for the current week's talks. By attending the seminar, either in person or on Zoom, you are agreeing to abide by our departmental code of conduct.

To get on the seminar mailing list, please contact one of the seminar orgainzers, Profs. Bihui Zhu and Kieran Mullen

### Spring 2024

**Title: Topological quantum synchronization: A case study of robust collective quantum dynamics**

Christopher W. Wächtler, University of California, Berkeley

*Tuesday, January 23rd, 2024*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Classical synchronization is ubiquitous across various systems, and it is just natural to ask how it is modified in the quantum world. Most of our current understanding in this emerging field arises from ad-hoc analysis of a few exemplary systems. However, these revealed a prominent obstacle: Synchronization in locally coupled networks is often only stable for fine-tuned parameters and initial conditions. This limits its applicability for future quantum devices. Moreover, genuine quantum effects of synchronization are currently quite elusive. The combination of synchronization with topological concepts offers the opportunity to advance in both of these directions: enhancing the robustness of synchronized dynamics and exploring novel quantum effects. The first half of my talk focuses on quantum van der Pol oscillators, which reduce to their classical analogues at the mean-field level and thus allow studying both the classical and quantum regime. I will discuss the emergence of topological synchronization in the classical scenario and demonstrated its persistence when quantum fluctuations are considered. In the second half, I will discuss an example of a purely quantum effect: Synchronization of fractionalized spins. In the gapped symmetric phase of the AKLT chain, the synchronized spin dynamics is significantly more robust than for previously investigated spin models. These results open the avenue to synchronization not only of microscopic but also emerging degrees of freedom.

**Title: Harnessing Quantum Fluctuations: Novel Approaches to Controlling Casimir Forces and Torques**

Jeremy Munday, University of California, Davis

*Tuesday, February 13th, 2024*

*2:30-3:30pm, 105 Lin Hall*

Abstract: While the classical vacuum of empty space is static, the reality of the quantum vacuum is anything but boring. Electromagnetic fields are constantly fluctuating and can give rise to measurable consequences. One striking effect is the Casimir force, where quantum fluctuations between two charge neutral plates give rise to an attractive interaction between them. In this seminar, we will trace the historical evolution of this phenomenon, highlighting advancements in measuring the Casimir force and related phenomena like the Casimir torque, and highlight our recent research leveraging anisotropic materials, epsilon-near-zero materials, and magnetic materials to manipulate and modify the Casimir force and torque. We will discuss broad applications spanning nanoscale physics, engineering, chemistry, and biology, while pointing towards promising future directions.

**Title: (Dis)entangling atoms for quantum simulations on quantum computers
**

*Susan Atlas, University of New Mexico*

*Tuesday, February 27th, 2024*

*2:30-3:30pm, 105 Lin Hall*

Abstract: As the NISQ (noisy intermediate-scale quantum) era evolves into the new era of fault-tolerant quantum computing, there is growing interest in developing methods for achieving practical quantum advantage in real applications. In the spirit of Feynmann’s original vision—simulating the quantumness of nature via a quantum computer—we are exploring novel approaches for mapping two key problems of molecular and materials physics onto quantum architectures, at scale, by exploiting the chemical concept of an atom-in-molecule: (1) improved functionals for describing electron correlation in density functional theory; and (2) ensemble charge-transfer force fields for describing bond formation and breaking in atomistic simulations of macromolecules and complex materials. In addition to improved simulation accuracy, the atom-in-molecule framework opens the prospect of exploiting natural fault tolerance effected by formal constraints on the electron density, and reinterpreting bond formation and breaking as an electronic phase transition.

**Title: Spintronic Phenomena for Reversible, Neuromorphic, Reservoir, and Secure Computing**

*Joseph Friedman, University of Texas, Dallas.*

*Tuesday, March 12th, 2024*

*2:30-3:30pm, 105 Lin Hall*

Abstract: The rich physics present in a wide range of spintronic materials and devices provide opportunities for a variety of computing applications. This presentation will describe six distinct proposals to leverage spintronic phenomena for reversible computing, neuromorphic computing, reservoir computing, and hardware security. The presentation will begin with a solution for reversible computing in which magnetic skyrmions propagate and interact in a scalable system with the potential for energy dissipation below the Landauer limit, followed by a paradigm for operating Boolean logic at terahertz clock frequencies utilizing the magnetoresistance of low-dimensional materials. Three neuromorphic systems for emulating neurobiological behavior with spintronic phenomena will then be presented: a purely-spintronic system that enables unsupervised learning with magnetic domain wall neurons and synapses, a reservoir computing system based on the dynamics of frustrated nanomagnets, and an approach for unsupervised learning that marks the first experimental demonstration of a neuromorphic network directly implemented with MTJ synapses. This presentation will conclude with a logic locking paradigm based on nanomagnet logic, the first logic locking system that is secure against both physical and algorithmic attacks.

**Title: Robust Classical Shadow Tomography in Shallow Circuits**

* Yizhuang You, UCSD*

*Friday, March 15th, 2024*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Extracting information efficiently from quantum systems is a major component of quantum information processing tasks. Randomized measurements, or classical shadows, enable predicting many properties of arbitrary quantum states using few measurements. While random single-qubit measurements are experimentally friendly and suitable for learning low-weight Pauli observables, they perform poorly for nonlocal observables. Prepending a shallow random quantum circuit before measurements maintains this experimental friendliness but also has favorable sample complexities for observables beyond low-weight Paulis, including high-weight Paulis and global low-rank properties such as fidelity. However, in realistic scenarios, quantum noise accumulated with each additional layer of the shallow circuit biases the results. To address these challenges, we propose the robust shallow shadows protocol. Our protocol uses Bayesian inference to learn the experimentally relevant noise model and mitigate it in postprocessing. This mitigation introduces a bias-variance trade-off: correcting for noise-induced bias comes at the cost of a larger estimator variance. Despite this increased variance, as we demonstrate on a superconducting quantum processor, our protocol correctly recovers state properties such as expectation values, fidelity, and entanglement entropy, while maintaining a lower sample complexity compared to the random single-qubit measurement scheme. We also theoretically analyze the effects of noise on sample complexity and show how the optimal choice of the shallow shadow depth varies with noise strength. This combined theoretical and experimental analysis positions the robust shallow shadow protocol as a scalable, robust, and sample-efficient protocol for characterizing quantum states on current quantum computing platforms.

**Title: Tunable Quantum Dissipation: Opportunities and Challenges**

*Archana Kamal, Univ. of Massachusetts., Lowell*

*Friday, March 29th, 2024*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Dissipation engineering is a powerful paradigm for correcting errors and realizing stable quantum coherences autonomously. The basic idea is to tailor the dissipation seen by a system, such that it relaxes and sustains the system in a desired target state. With growing capabilities in quantum information platforms this idea can now be realized in conjunction with parametric interactions that enable rapid tunability and in-situ reconfigurability for implementing strong dissipative couplings between spatially remote and off-resonant qubits. The resultant platform with tunable quantum dissipation offers novel functionalities for quantum state preparation and control, along with new opportunities for exploring fundamental physics of open systems. As an example, I will first discuss trade off-free protocols for robust and scalable entanglement generation using parametrically tunable dissipation. Next, motivated by the optimal regime of dissipation engineering, I will discuss our recent efforts on expanding the analytical and numerical framework of these platforms and some recent surprises for Lindblad renormalization and Zeno physics in the presence of strong dissipation.

**Title: Quantum Simulation with Non-Unitary Dynamics**

*Xiao Mi, Google AI*

*Friday, April 5th, 2024*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Many-body quantum simulation using experimental quantum processors is a highly promising avenue toward practical quantum advantage. Traditionally, such a task is accomplished via unitary dynamics which is limited by gate fidelities and oftentimes Trotter errors. Here we explore the non-unitary dynamics arising from the interplay between unitary evolution applied to a system of qubits and an engineered dissipative environment realized by auxiliary qubits that are frequently reset to their ground states. We find that by appropriate choice of interaction between the system qubits and auxiliaries, the quantum system may be steered to an entangled steady state resembling low-temperature states of 1D and 2D transverse Ising models. Furthermore, by coupling the system to auxiliaries stabilized to different states, we discover a new, subdiffusive form of quantum transport in the Heisenberg XXZ model. These results demonstrate the feasibility of utilizing engineered dissipation toward preparing complex quantum matter, significantly enriching the capability of near-term superconducting processors. As time allows, I will also briefly discuss another complementary approach toward practical quantum advantage, namely realizing high-fidelity analog evolution using tunable-coupling transmons.

**Title: Quantum state control via light-matter interactions in cold and ultracold atoms gases: quantum memory and holonomic quantum operations**

*Lindsay LeBlanc, University of Alberta*

*Friday, April 19th, 2024*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Neutral atomic gases provide fantastic opportunities for studying and controlling quantum phenomena, ranging from many-body physics to quantum computers. In our research, we use the well-known interactions between cold gases and electromagnetic radiation to harness various quantum degrees of freedom. Quantum memories, used for storing and manipulating photonic signals, will be a key component in quantum communications systems, especially in realizing critical quantum repeater infrastructure. In our work, we demonstrate two memory protocols in ultracold (sometimes Bose-condensed) atoms, which hold the potential for high-performance light storage: the Autler-Townes splitting (ATS) and superradiant approaches. These methods provide a path towards practical implementations in both ground- and satellite-based quantum communications systems, and we are working on both increasing performance and developing practical implementations. In a separate direction, our lab also uses ultracold ensembles to study unconventional quantum gates for quantum computing. In our work on holonomic operations, we engineer degeneracies into our system through Floquet driving, with the goal of realizing non-Abelian geometric phases. Our experiments reveal that we indeed rotate quantum states in this degenerate manifold, though we find that the naive expectation of geometric robustness to fluctuations is less resilient to real experimental issues than expected.

**Title: New Dimensions on the Interaction of Light and Matter: ****Quantum Materials, Quantum Light, and Quantum Control**

*Nathaniel Stern, Northwestern University,*

* April 30th, 2024*

*2:30-3:30pm, 105 Lin Hall*

Abstract: The quantum conception of light consisting of particles of discrete energy, or photons, underlies its interaction with matter. For solid materials, this understanding has led to transformational applications both as conventional as sensor and display technologies and as extraordinary as lasers. Despite this ubiquity, advances in materials continue to reveal nuances in the interaction of light with matter. The emergence of layered materials of atomic-scale thickness presents a new two-dimensional (2D) landscape in which to play with the interaction between photons and matter, revealing diverse opportunities for control based on morphology, surface chemistry, and electromagnetic environment. I will describe how the unique features of layered materials can be harnessed for generating and exploring optical phenomena. The properties of 2D materials give rise to spin-polarized half-light, half-matter superpositions that can be manipulated at picosecond timescales like a two-level spin. The polarization-sensitivity of these materials can be an ingredient of chiral interactions with light when integrated with photonic circuits. Shifting from light-matter superpositions to isolable quantum emitters, I will discuss recent insights into how the surface of 2D materials can be used to manipulate and to improve quantum light emission from defects through chemical functionalization. The confluence of spin, quantum emission, and quantum control available by combining low-dimensional materials with polarized light expands the toolbox for engineering quantum optical applications.

### Fall 2023

**Title: Exploring waveguide quantum electrodynamics with atomic matter waves**

*Dominik Schneble, Stonybrook University*

*Friday, September 29, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Understanding and harnessing light-matter interactions is central to the development of applications in quantum information science. One example is the emerging field of waveguide quantum electrodynamics that investigates the radiative coupling between quantum emitters and a low-dimensional photonic bath. Using ultracold atoms in an optical lattice, we have recently developed an analogue platform of artificial quantum emitters that decay by spontaneously emitting atomic matter-wave, rather than optical, radiation. Here, the emission of atoms into free space is analogous to that of photons near a photonic band gap. Our platform offers independent and full control of the quantum emitters' vacuum coupling and of their excitation energy, while spontaneous decay into modes other than the ones of interest is inherently suppressed. In my talk I will introduce the basic features and discuss some recent work on simulating atom-light interactions in novel regimes and at the boundary between quantum optics and condensed-matter physics.

**Title: Strange metallic transport of strongly interacting Hubbard model and hard core bosons**

*Assa Auerbach, Technion*

*Tuesday, October 24, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Using thermodynamic formulas for high temperature resistivities, we calculate the DC Hall coefficients and resistivities of hard core bosons and the t-J model on the square lattice, and obtain some insights into effects of strong interactions on non-fermi liquid transport in metals.

**Title: Resonant photonic lattices: Principles, design, fabrication, and applications**

**Robert Magnusson, UT Arlington**

*Friday, October 13, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: We discuss principles, properties, design, fabrication, and applications of resonant leaky-mode lattices. We review elementary facts related to diffractive optical elements as these constitute the original basis for periodic metamaterials. We show that perfect reflection is obtainable in 1D and 2D lattices and that its occurrence is immune to particle shape. We review the leaky-mode band structure and emphasize that its origin lies in the periodic assembly as opposed to particle resonance. The relation of lattice geometry to the leaky, resonant band edge and the nonleaky, bound-state band edge is discussed. The nonapproximated Rytov effective-medium formalism is shown to be effective in modeling the leaky band edges hosting the open radiant channel and the closed symmetry-blocked channel. Thus, all main resonance-lattice properties are embodied in Rytov-equivalent homogeneous waveguides overwhelmingly proving the waveguide character of all resonant lattice systems. Realizable 1D and 2D photonic lattices exhibit attractive features such as compactness, minimal interface count, high efficiency, and potential monolithic fabrication with attendant robustness under harsh conditions. Applications include various optical components and bio- and chemical sensors. The governing resonance effects hold across the spectrum, from visible wavelengths to the microwave domain, by simple scaling of wavelength to period and proper materials choice.

**Title: Quantum Computing and Simulations with Long Chains of Trapped Ions**

**Marko Cetina, Duke University**

*Friday, October 20, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: To apply today's quantum hardware to challenging problems, we need to efficiently use native interactions while minimizing the effects of noise.

In quantum computers and simulators based on long chains of trapped ions, the dominant errors arise from fluctuating trap-induced electric fields. I will present four ways to counter this noise: error correction, direct laser cooling, engineering of new expressive gates, and quantum simulation using parallel individual addressing.

I will conclude with an application of these techniques to the preperation of near-ground states of the long-range 1D XY model, which break the U(1) symmetry of the underlying Hamiltonian, resulting in long-range off-diagonal order.

**Title: Synthetic matter with small interatomic spacing**

*Qiyu Liang, Purdue University*

*Tuesday, October 24, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Synthetic matter assembled atom-by-atom has demonstrated an unparalleled level of control in ultracold atomic experiments, unveiling a multitude of new possibilities in quantum simulation, computing, sensing, metrology, and chemistry. Yet, achieving the same proximity between atoms as in top-down approaches remains challenging. We aim for sub-micron interatomic spacing, employing blue-detuned, short-wavelength tweezer traps, projected through a digital micromirror device. This will enable us to embark on two separate research avenues: (1) A monolayer of atomic arrays positioned at a distance shorter than the dipole transition wavelength serves as an ideal mirror for normally incident resonant light. Quantum nonlinear optics becomes attainable through the introduction of Rydberg-Rydberg interactions. This approach closely resembles experiments utilizing disordered ensembles, albeit without the deleterious dissipative component in the emergent photon interactions. Our work will deepen our understanding of light-atom interactions, test the basic ingredients for high-fidelity all-optical devices and open up possibilities to investigate optical many-body phenomena. (2) At small spacing, an admixture of many nearby Rydberg states leads to a large “spaghetti” of energy levels. Microscopic understanding in this regime is lacking. Gaining insight would facilitate applications of Rydberg technologies in various contexts, including the aforementioned atomic mirror. In particular, we envision a reciprocal synergy between our expanding knowledge of short-distance interactions and improvement of collectively enhanced imaging. We plan to apply collectively enhanced imaging capability with single Rydberg atom resolution to study spin dynamics in a driven, dissipative many-body system.

**Title: Quantum Interferometry in Imaging and Entanglement Measurement**

**Mayukh Lahiri, Oklahoma State University**

*Friday, October 27, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: I will discuss two interferometry-based imaging techniques that rely on two types of optical interference. One of the techniques – based on “interference by path identity” – acquires image without detecting the photons that interact with the object. This technique, therefore, allows to obtain object-information at a wavelength for which no detectors are available. The other technique – based on interference of light from independent sources – enables phase imaging through intensity-intensity correlation. It allows image acquisition when random phase fluctuations wash out intensity interference patterns, that is, when standard interferometric phase imaging schemes fail. I will also briefly mention how quantum interferometry is currently being applied to measure entanglement in mixed photonic states without detecting all entangled photons.

**Title: Searching for Symmetry Violation with Molecular Ions**

**Matthew Grau, Old Dominion University**

*Friday, November 3, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Molecules have emerged as powerful instruments for conducting precise tests of fundamental symmetries, such as the search for the electron electric dipole moment (eEDM). However, molecules are generally difficult to trap and cool, making it challenging to perform narrow linewidth measurements with long interrogation times without a complicated laser-cooling setup capable of repumping multiple rovibrational levels. Our approach harnesses molecular ions, which offer distinct advantages. They can be readily stored in ion traps for prolonged durations and sympathetically cooled by laser-cooled atomic ions. Notably, atomic lutetium ions are amenable to direct laser cooling, making them ideal candidates to serve as sympathetic coolants and to form pre-cooled molecular ions. Furthermore, Lu-176 boasts one of the largest nuclear electric quadrupole moments of any long-lived isotope, rendering it exceptionally sensitive to the CP-violating nuclear magnetic quadrupole moment (nMQM). I will describe how we can harness these properties in molecular ions containing lutetium, such as LuOH⁺, to probe new physics through the simultaneous investigation of nMQM and eEDM.

**Title: Coherent and magneto-optical Kerr spectroscopy at extremely high magnetic fields**

*Denis Karaiskaj, University of South Florida*

*Tuesday, November 7, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Magnetic field- and polarization-dependent measurements on bright and dark excitons in monolayer MoSe_{2} and WSe_{2} combined with time-dependent density functional theory calculations reveal intriguing phenomena. Magnetic fields up to 25 T parallel to the WSe_{2} plane lead to a partial brightening of the energetically lower lying exciton, leading to an increase of the dephasing time. Using a broadband femtosecond pulse excitation, the bright and partially allowed excitonic state can be excited simultaneously, resulting in coherent quantum beating between these states. The magnetic fields perpendicular to the WSe_{2} plane energetically shift the bright and dark excitons relative to each other, resulting in the hybridization of the states at the K and K′ valleys. Our experimental results are well captured by time-dependent density functional theory calculations. These observations show that magnetic fields can be used to control the coherent dephasing and coupling of the optical excitations in atomically thin semiconductors.

In the presence of magnetic fields perpendicular to the MoSe_{2} plane the coherence at negative and positive time delays is dominated by intervalley biexcitons. We demonstrate that magnetic fields can serve as a control to enhance the biexciton formation and help search for more exotic states of matter, including the creation of multiple exciton complexes and excitonic condensates.

Finally, static and magneto-optical Kerr effect measurements are used to investigate a host of kagome lattice helimagnets and topological magnetic materials. These measurements are performed at extremely high magnetic fields and as a function of temperature enabling us to investigate their full phase diagram.

*N. P. Pradhan et al., Nanoscale 15, 2667 (2023)**Liu et al., Physical Review B 106, 035103 (2022)**V. Mapara et al., Nano Letters 22, 1680 (2022)**J. Paul et al., Rev. Sci. Instrum. 90, 063901 (2019)**P. Das et al., Nanoscale 12, 22904 (2020).**C. E. Stevens et al., Nature Communications 9, 3720 (2018)**C. E. Stevens et al., Optica 5, 749 (2018)**J. Paul, et al., Phys. Rev. B 95, 245314 (2017)**C. E. Stevens, et al., Solid State Commun. 266, 30 (2017)**P. Dey, et al., Phys. Rev. Lett. 116, 127402 (2016)*

**Title: Adiabatic oracle for Grover Algorithm**

**Nikolai Sinitsyn, Los Alamos National Lab**

*Tuesday, November 14, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Conventional quantum algorithms use certain resources that are assumed to be given at almost no cost but are hard to provide in practice. An alternative is to work with “heuristic” approaches, such as DWave-like quantum annealing but there is no mathematical evidence that they work without similar “oracle”-like assumptions at some stage [1].

In this talk I will argue that a practically useful and un-classically fast quantum computing is possible. Namely, I will describe a novel “hybrid” approach to hardware that realizes Grover’s oracle for numerous computational problems by means of quantum annealing in polynomial time and realized the search for the solution with the desired quantum speedup [2-3]. The proposed scheme should achieve quantum supremacy on these computational problems using only ~60 qubits, without the access to a universal quantum gate set, and without the need for “all-to-all” qubit connectivity. No error correction is needed as the outcome of the time-dependent control is topologically protected.

References:

[1] Bin Yan and NA Sinitsyn. Analytical solution for nonadiabatic quantum annealing to arbitrary Ising spin Hamiltonian. Nature Communications volume 13,

Article number: 2212 (2022).

[2] Bin Yan and NA Sinitsyn. An adiabatic oracle for Grover's algorithm. arXiv/2207.05665.

[3] N. A. Sinitsyn and Bin Yan. “Topologically protected Grover's oracle for the Partition Problem”, Phys. Rev. A 108, 022412

**Title: From Dipolar to Rydberg Photonics**

**Hadiseh Alaeian, Purdue University**

*Friday, November 17, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Strong light-induced interactions between atoms are known to cause nonlinearities at a few-photon level which are crucial for applications in quantum information processing. At densities higher than 1 atom per cubic wavelength, such interactions give rise to density shifts and broadenings, and when confined to less than a wavelength size, such dipolar interaction leads to collective blockade phenomena, which mostly have been studied in the context of strongly interacting Rydberg states.

Here we study these phenomena for low-lying excited atomic states confined in thin atomic clouds that are generated via the pulsed Light-Induced Atomic Desorption (LIAD) technique. For the first few nanoseconds, the transient light-induced dipolar interaction of the low-lying lines of Rubidium leads to shifts and broadenings well beyond the well-known Lorentz-Lorenz limit. In the second experiment, we combine the high densities achievable in thermal atomic vapors with an efficient coupling to a slot waveguide. In contrast to free-space interactions, atoms aligned within the slot exhibit repulsive interactions that are further enhanced by a factor of 8 due to the Purcell effect. The corresponding blueshift of the transition frequency of atoms arranged in the essentially one- dimensional geometry vanishes above the saturation, providing a controllable nonlinearity at the few-photon level.

Towards the end of my talk, I will introduce our novel platform in thin-film cuprous oxide, which allows us to realize strongly interacting Rydberg excitons in a solid-state system that is inherently suitable for scalability and integration. The results of these studies pave the way towards a robust scalable platform for quantum nonlinear chiral optics and all-optical quantum information processing in an integrable and scalable platform, and potentially at elevated temperatures.

**Title: Current(less) Trends in Spintronics: Magnetoionics & Magnonics for Energy Efficient Computing**

**Julius de Rojas, Oklahoma State University**

*Friday, December 1, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Quickly approaching fundamental hurdles, including power constraints and manufacturing limitations, have left conventional computing hardware struggling to maintain energy efficiency as dimensions continue to shrink, leading to several approaches to low-energy data storage and processing. In this talk, I will overview magneto-ionics and magnonics as potential candidates for data storage and processing. I will then present our study of magneto-ionics in oxygen-based and nitrogen-based systems as a low-energy means to toggle magnetization states. I will focus on the “structural ion” approach, in which the mobile ions are already present in the target material and discuss its potential advantages and challenges. I will then conclude with our investigation into the static and dynamic behavior of “pseudo-3D” trilayer square artificial spin ice structures for magnonic applications, in which a nonmagnetic copper layer of varying thick-ness is inserted between Permalloy (Ni81Fe19) layers. We show that the copper thickness enables interlayer coupling between layers to be finely controlled, leading to bespoke magnetization states and resonance spectra tuning, a potentially programmable degree of freedom for magnonic and microwave devices.

### Spring 2023

**Title: Strange Vortices in Spinor Superfluids**

David Hall, Amherst University

*Tuesday, January 24th, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Vortices play fundamental roles in science, mathematics, and technology. Many aspects of vortices in superfluids, such as the quantization of circulation, have been extensively studied. With the addition of a spin degree of freedom, however, less-familiar phenomena are encountered: spinor vortices can have no core with circulation that is not quantized, or they can be singular with fractional units of circulation, depending on the underlying magnetic phase of the superfluid they inhabit. Singular spinor vortices can also have unusual vortex cores that consist of superfluid in magnetic phases distinct from the vortex itself. In this talk I will describe our recent experiments with spinor vortices in dilute-gas Bose-Einstein condensates, as well as experimental progress towards realizing "noncommuting" vortices in magnetic phases with only discrete symmetries. These strange vortices can leave behind traces of their collisions in the form of "rung vortices," with possible implications for interferometry and quantum information.

**Title: Metal-dielectric photonic crystal organic light emitting diodes.**

Matthew White, University of Vermont

*Tuesday, January 31st, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: A microcavity OLED, consisting of a conventional OLED stack with two metallic mirror electrodes, shows narrow-band emission centered around specific peak resonant wavelengths. These cavity modes are analogous to the energy states found in any resonator system, including musical instrument strings and 1-dimensional quantum square wells. We demonstrate how the microcavity OLED can be used as a unit cell in a planar, 1-dimensional photonic crystal and explore how the dimensional and material properties of the device impact the spectral electroluminescence dispersion. Stacking N microcavities splits the resonant modes into N discrete states; a photonic band centered at the single microcavity state. Devices are fabricated by thermal evaporation with in-vacuo shadow mask transfers to enable parallel-connected unit cells. The photonic density of states is controlled by varying the thickness of the semitransparent metal mirror electrodes. A Peierls bandgap is tuned by varying only alternate mirror thicknesses. Device parameters, including N, the thickness of the semi-transparent metal electrodes, and dipole emission position are correlated to emission properties such as peak wavelength, FWHM, and Q-factor for each of the photonic states. The band structure is optimized by employing optically similar silver alloys as anode and cathode. The experimental results are guided by a predictive computationalmodeling tool, which is critically important for the complex-architecture devices.

**Title: Dynamics of Bose-Einstein condensates in a moving optical lattice**

Qingze Guan, Washington State University

*Friday, February 10th, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Ultracold quantum gases are powerful platforms for building quantum simulators due to the long coherence time and the high tunability. By manipulating laser-atom interactions, various quantum systems can be simulated in cold atoms, such as Bloch oscillation of electrons in solids, Josephson junction oscillations in superconductors, matter with synthetic gauge fields, and topological materials. This presentation will focus on the dynamics of Bose-Einstein condensates in a moving optical lattice with particular emphasis placed on the role of interaction in the system. It will cover the Ramsey interferometers, self-trapping, Landau-Zener tunneling, spin dynamics, and scattering phenomena. The good agreement between theory and experiment validates such a system as a testbed for theories and a powerful platform for other quantum simulations.

**Title: Quantum Back-action Limits in Dispersively Measured Bose-Einstein Condensates**

Emine Altuntas, JQI, University of Maryland

*Friday, February 17th,*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Most quantum technologies simultaneously require quantum limited measurements and feedback control to establish and maintain quantum coherence and entanglement, with applications ranging from quantum state preparation to quantum error correction. Large-scale applications of these capabilities hinge on understanding system-reservoir dynamics of many-body quantum systems, whose Hilbert space grows exponentially with system size. Ultracold atoms are an ideal platform for understanding the system-reservoir dynamics of many-body systems. In this talk, I will present the characterization of measurement back-action in atomic Bose-Einstein condensates, weakly interacting with a far-from resonant, i.e., dispersively interacting, laser beam. We theoretically describe this process using a quantum trajectories approach where the environment measures the scattered light and I will present our measurement model based on an ideal photodetection mechanism. Next, I will discuss our experimental quantification of the resulting wavefunction change with two observations: the contrast of a Ramsey interferometer [1] and the deposited energy [2]. These results are necessary precursors for achieving true quantum back-action limited measurements of quantum gases and open the door to quantum feedback control of ultracold atoms.

[1] E. Altuntaş and I. B. Spielman, arXiv preprint arXiv:2209.04400 (2022).

[2] E. Altuntaş and I. B. Spielman, arXiv preprint arXiv:2212.03431 (2022).

**Title: Quantum Computing with Neutral Atoms.**

Ivan Deutsch, University of New Mexio

*Friday, February 24th, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: One of the earliest proposals for scalable quantum computers was to encode qubits in individual optically- trapped, ultracold neutral atoms. Like their more famous cousins, atomic ions, qubits encoded in the energy levels of neutral atoms are all identical, can have long coherence times, and can be controlled with a variety of magneto-optical fields, with tools that build on decades of development for atomic clocks and precision metrology. Unlike with ions, quantum computing architectures have proceeded more slowly, as neutral atoms are harder to trap and they only weakly interact in their ground state. New developments in trapping and laser technology has now opened the door to high-fidelity operation with potentially hundreds to thousands of qubits - neutral atoms are back in the game! In this seminar I will discuss how high-fidelity quantum logic can be implemented through coherent control of superpositions of atoms in ground and highly excited Rydberg states. I will also describe how optimal control can be used to implement a variety of protocols for quantum information processing with neutral atoms, including the performing quantum logic with “qudecimals" (d=10 dimensional systems) encoded in nuclear spins.

**Title: Coherent photoexcited dynamics in molecular systems**

Sergei Tretiak, Los Alamos National Lab

*Friday, March 3rd, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: I will overview some applications of Non-adiabatic EXcited-state Molecular Dynamics (NEXMD) framework developed at several institutions and is released to public. The NEXMD code is able to simulate tens of picoseconds photoinduced dynamics in large molecular systems and dense manifold of interacting and crossing excited states. As an application, I will exemplify ultrafast coherent excitonic dynamics guided by *intermolecular* conical intersections (CoIns). Both simulations and time-resolved two-dimensional electronic spectroscopy track the coherent motion of a vibronic wave packet passing through CoIns within 40 fs, a process that governs the ultrafast energy transfer dynamics in molecular aggregates. Our results suggest that intermolecular CoIns may effectively steer energy pathways in functional nanostructures. In the second example, we use dynamical simulations to compute X-ray Raman signals, which are able to monitor the coherence evolution. The observed coherences have vibronic nature that may survives multiple conical intersection passages for several hundred femtoseconds at room temperature. These spectroscopic signals are possible to measure at XFEL facilities, paving the way for detailed coherence measurements in functional organic materials. Observed relationships between spatial extent/properties of electronic wavefunctions and resulting electronic functionalities allow us to understand and potentially manipulate excited state dynamics and energy transfer pathways toward optoelectronic applications.

Relevant references

- A. De Sio, E. Sommer, X. T. Nguyen, L. Gross, D. Popović, B. Nebgen, S. Fernandez-Alberti, S. Pittalis, C. A. Rozzi, E. Molinari, E. Mena-Osteritz, P. Bäuerle, T. Frauenheim, S. Tretiak, C. Lienau,
*“Intermolecular conical intersections in molecular aggregates”*__Nature Nanotech. 16, 63 – 68 (2021).__ - D. Keefer, V. M. Freixas, H. Song, S. Tretiak, S. Fernandez-Alberti, and S. Mukamel,
*“Monitoring Molecular Vibronic Coherences in a Bichromophoric Molecule by Ultrafast X–Ray Spectroscopy”*__Chem. Sci.__**12**, 5286 – 5294 (2021). - V. M. Freixas, D. Keefer, S. Tretiak, S. Fernandez-Alberti, and S. Mukamel,
*“Ultrafast coherent photoexcited dynamics in a trimeric dendrimer probed by X-ray stimulated-Raman signals,”*__Chem. Sci.,__**13**, 6373 – 6384 (2022).

**Title: Atomic clocks and quantum sensors for advancing science in microgravity and space environment**

Nan Yu, NASA Jet Propulsion Laboratory

*Tuesday, March 7th, 2023
2:30-3:30pm, Lin Hall *

Abstract: With the advent of laser cooling, trapping, and manipulation of atomic particles, advanced atomic clocks, atom interferometer sensors, and atomic quantum systems have been demonstrated in research laboratories. These new technologies and resulting capabilities will have a significant and diverse impact on space science and exploration, including gravity measurements for earth and planetary sciences, gravitational wave detection, direct detection of dark energy and matter in astrophysics and tests of fundamental physics. This talk will provide an overview of clock and quantum sensor technologies and their applications in space with a focused discussion on the direct detection of dark energy scalar field experiment on Einstein Elevator.

**Title: Improving the Efficiency and Safety of Battery and Fuel Cell Materials through Computational Analysis**

Michelle Johannes, NRL

*Tuesday, March 21st, 2023
2:30-3:30pm, Lin Hall *

Abstract: Although batteries and fuel cells work as complete and complex electrochemical systems, but a surprising number of their performance metrics can be traced back to the individual materials components that comprise the anode, cathode and electrode. Optimizing these materials in terms of cost, efficiency and safety is a concern for the commercial and government sectors.

In this talk, I will discuss how costly laboratory searches can often be replaced by “cheap” computational simulations and how the underlying materials physics, including subtle quantum mechanical effects, can determine battery/fuel cell performance. I will briefly introduce density functional theory (DFT), a fully quantum mechanical description of how electrons organize, reorganize and move in relevant materials and then describe some successes (and failures) of the method in designing and improving common battery materials. I will highlight the property of “transport” - both ionic and electronic which is often the determining factor in whether a material is useful or not for electrochemical systems.

**Title: Quantum state control of single molecular ions for quantum information processing and precision spectroscopy**

Chin-Wen Chou, NIST-Boulder

*Tuesday, March 28th, 2023
2:30-3:30pm, Lin Hall *

Abstract: Over the past decades, atoms are trapped and laser cooled to near zero temperature, minimizing the motional effects in spectroscopy. Internal states of atoms can be coherently manipulated and prepared in pure quantum states, including entangled states that are impactful in quantum information processing, sensing, and metrology. This talk will describe the effort of the Ion Storage group at NIST in bringing molecular ions to equal footings with atoms in terms of state control and spectroscopic precision. The project builds on laser cooling and trapping techniques, frequency comb technology, and quantum-logic spectroscopy protocols nowadays routinely employed in cold-atom research and trapped ion optical clocks. That enables demonstrations, on single molecular ions, of coherent quantum state manipulation, nondestructive state detection, spectroscopy with better than part-per-trillion resolution, and quantum entanglement. The group is exploring new opportunities in physics and chemistry offered by the richer structure and broader species selections in molecules.

**Title: Controlling spin and light at room temperature in Lead-Halide Inspired Hybrid Semiconductors**

Matthew Beard, NREL

*Friday, April 7th, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: In this presentation I will discuss our studies of controlling the charge carrier dynamics, light/matter interactions, and spin populations in metal-halide organic/inorganic hybrid systems.

Lower dimensional perovskites are of particular interest since the lower degree of symmetry of the metal-halide connected octahedra and the large spin-orbit coupling can potentially lift the spin degeneracy. To achieve their full application potential, an understanding of spin-relaxation in these systems are needed. Here, we report an intriguing spin-selective excitation of excitons in a series of 2D lead iodide perovskite (n = 1) single crystals by using time- and polarization-resolved transient reflection spectroscopy. Exciton spin relaxation times as long as ~26 ps at low excitation densities and at room temperature were achieved for a system with small binding energy, 2D EOA_{2}PbI_{4}(EOA=ethanolamine).

We have recently studied and developed a novel class of chiral hybrid semiconductors based upon layered metal-halide perovskite 2D Ruddlesden-Popper type structures. These systems exhibit chiral induced spin selectivity (CISS) whereby only one spin sense can transport across the chiral layer and the other spin sense is blocked for one handedness of the chiral perovskite layer. We show that chiral perovskite layers are able to achieve > 80% spin-current polarization. We have also studied spin-injection from the chiral-layer to a non-chiral perovskite layer and find high spin-injection efficiency. We developed novel spin-based LEDs using non-chiral perovskite NCs as the light emitting layer that promotes light emission at a highly spin-polarized interface. The LED spin-polarization is limited by spin-depolarization within the MHP NCs.

Finally, if time permits I will discuss our efforts for developing novel organic inorganic magnetic systems. Low dimensional copper halide perovskite anitferromagnets display interesting a tunable magnonic behavior. The organic cations offer interesting opporutnies to control the weak intralayer AFM interaction as well as symmetry breaking in the inorganic layer.

**Title: Integrated Optical Control of Atomic Quantum Systems**

Amit Agrawal, NIST

*Friday, April 7th, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Over the last decade, flat optical elements composed of an array of deep-subwavelength dielectric or metallic nanostructures of nanoscale thicknesses – referred to as metasurfaces – have revolutionized the field of optics. Because of their ability to impart an arbitrary phase, polarization or amplitude modulation to an optical wavefront as well as perform multiple optical transformations simultaneously on the incoming light, they promise to replace traditional bulk optics in applications requiring compactness, integration and/or multiplexing. Recent demonstrations including imaging, polarimetry, quantum-light generation and LIDAR demonstrate the range of technologies where metasurfaces have already had a significant impact. In this talk, we demonstrate the versatility of wavefront shaping metasurfaces as a compact, efficient and multifunctional interface to trap neutral atoms or address trapped ions for applications in quantum information science and atomic clocks. In another integration step, combining metasurfaces with photonic integrated circuits, replacing bulk optical elements, promises increased complexity and functionality in a batch-fabricated optical microsystem ultimately fully replacing the laboratory optical table to enable cold atom clocks and quantum computers.

**Title: Exciton fine structure in perovskite nanocrystal quantum dots**

Peter Sercel, California Institute of Technology

*Friday, April 28th, 2023*

*2:30-3:30pm, 105 Lin Hall*

Abstract: Colloidal lead halide perovskite nanocrystal quantum dots are promising as classical light sources for devices such as light emitting diodes and displays owing to the fast, efficient radiative decay in these materials. They are currently being considered for potential applications in quantum information processing as well since, for example, they can emit single photons or correlated photon pairs, and have been shown to have long optical coherence times. Such applications hinge to an extent on our understanding and control of the exciton fine structure.

In this talk we will discuss the exciton fine structure in perovskite nanocrystals and its determinants: The electron-hole exchange interaction, which splits the exciton into a dark (optically inactive) singlet level and a bright (optically active) triplet, the effect of the crystal lattice symmetry in splitting the bright triplet, and the possible role of the Rashba effect. We will focus then on the particular role of the nanocrystal shape in conjunction with the “long range” exchange interaction in controlling the fine structure.

A unique aspect of the structure of perovskite nanocrystals is a roughly 45 degree misalignment between the orthorhombic crystal lattice vectors from the nanocrystal bounding facets. This misalignment breaks the orthorhombic symmetry, creating coupling between bright exciton states. The result is a previously unrecognized avoided-crossing “fine-structure gap” within the bright triplet which can be observed via quantum beating in ensemble-level transient absorption measurements [1]. We predict as a further consequence the existence of chiro-optical effects, which are a manifestation of “extrinsic chirality”.

[1] Y. Han, W. Liang, Y. Li, X. Lin, F. Sun, F. Zhang, P.C. Sercel, K. Wu,** **Lattice distortion inducing exciton splitting and coherent quantum beating in CsPbI_{3} perovskite quantum dots, *Nature Materials*, **21**, 1282–1289, (2022.)

**Title: Liquid and solid like behavior of ultra-dilute Bose-Einstein condensates of magnetic atoms**

P. Blair Blakie, Department of Physics, University of Otago, Dunedin, New Zealand

*Monday, May 15th, 2023
2:30-3:30pm, 105 Lin Hall *

Abstract: Over the last decade, ultra-cold gases of highly magnetic lanthanide atoms have emerged as an intriguing platform for studying superfluid gases with long-range interactions.

One surprising discovery in these systems was the observation that beyond meanfield effects, arising from quantum fluctuations, could become important in the behavior of the gas. This has led to liquid like behavior, such as the production of quantum droplets, and more recently, a crystallization transition to a supersolid state of matter. These curious phenomena occur in an ultra-dilute gas about a billion times less dense than ordinary water!

I will overview some of the basic theory of this system and discuss some of the wonderful progress made in experiments studying Bose-Einstein condensates of highly magnetic atoms.

**Title: High-entropy oxides as candidates for artificial magnetoelectrics**

Robert Kruk, Karlsruhe Institute of Technology

*Tuesday, May 23rd, 2023
2:30-3:30pm, Lin Hall *

Abstract: The quest for artificial, heterostructured magnetoelectrics is driving the search for new magnetic materials that exhibit enhanced sensitivity to voltage stimulation. In principle, the system of choice for this purpose can be the new class of materials known as High Entropy Oxides (HEOs). HEOs are single phase solid solutions consisting of five or more elements in equiatomic or near-equiatomic proportions incorporated into the cationic sub-lattice(s). What sets HEOs apart is their remarkable chemical complexity, encapsulated within a single crystallographic structure, often resulting in unique functionalities. From a local structure standpoint, HEOs exhibit an exceptionally large number of distinct metal-oxygen-metal pairings. Consequently, the magnetic correlations in HEOs, influenced by the coordination geometry, valence, spin state, and type of hybridized metal cations, are naturally influenced by an extensive variety of neighboring ionic configurations. These conditions give rise to a complex magneto-electronic free-energy landscape within HEOs, potentially leading to the stabilization of unconventional spin-electronic states. This form of inherently imbalanced magnetism has the potential to be influenced by external stimuli, such as voltage, presenting opportunities for the development of artificial magnetoelectrics. Examples of these systems, including perovskites and spinels, will be discussed in the context of their magneto-electronic properties, which are a consequence of the extreme local chemical disorder.