Seminars

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.

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Robert Magnusson, UT Arlington

Friday, October 13, 2023
2:30-3:30pm,  105 Lin Hall

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Marko Cetina, Duke University

Friday, October 20, 2023
2:30-3:30pm,  105 Lin Hall

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Qiyu Liang, Purdue University

Tuesday, October 24, 2023

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

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Mayukh Lahiri, Oklahoma State University

Friday, October 27, 2023
2:30-3:30pm,  105 Lin Hall

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Matthew Grau, Old Dominion University

Friday, November 3, 2023
2:30-3:30pm,  105 Lin Hall

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Denis Karaiskaj, University of South Florida

Tuesday, November 7,  2023
2:30-3:30pm,  105 Lin Hall

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Nikolai Sinitsyn, Los Alamos National Lab

Tuesday, November 14, 2023
2:30-3:30pm,  105 Lin Hall

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Hadiseh Alaeian, Purdue University

Friday, November 17, 2023
2:30-3:30pm,  105 Lin Hall

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Julius de Rojas, Oklahoma State University

Friday, December 1, 2023
2:30-3:30pm,  105 Lin Hall

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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

1. 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). 
2. 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).  
3. 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 EOA₂PbI₄(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 CsPbI₃ 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.

Title:  New Materials Discovery by Molecular-beam Epitaxy 

Hanjong Paik, OU  

Tuesday, August 23,  2022
2:30-3:30pm (105 Lin Hall/Zoom Link will be announced).

Abstract:   The synthesis of complex oxide thin films by the molecular-beam epitaxy (MBE) technique provides a great potential for unleashing novel and hidden properties of the material from the dull ground states. Especially, when it utilizes the lattice coherency from the substrates, i.e., strain, the properties of thin films can be dramatically altered in comparison to their bulk form. Occasionally, this approach results in metastable phase and pseudomorphic polymorphism, thus, introducing the unexpected emergent properties owing to the power of epitaxial-strain-symmetry-stabilization at the interface. Therefore, the thin-film approach for new quantum material discovery will be the ultimate platform for the fundamental study of new quantum phenomena at the surface and interface. 

  In this talk, I would like to present how ozone-assist oxide MBE can be useful to discover new material properties, especially, relevant to the strongly correlated electronic system. I will talk about (1) room temperature high-electron-mobility and its 2DEG behavior of the perovskite stannate interface for the transparent power electronics (2) how simple metal ruthenium dioxide becomes a superconductor via. strain-stabilization, and (3) realization of epitaxial topological crystalline insulator Sr₃SnO anti-perovskite system with in-situ spectroscopy. In addition to describing the above material synthesis and characterization, I also would like to discuss several challenging materials systems, for example, some cubic-to-hexagonal perovskite polymorphic systems, noble pyrochlore oxides system, and materials growth challenges containing alkaline metal elements (i.e., Li-, K-, Na- containing material system) for the fundamental study of quantum materials. 

Title:    Developing Quantum Information Devices and Systems using New Materials, Experiments, Nanostructures and Data-driven Models

Safura Sharifi, OU Dept. of Electrical and Computer Engineering

Tuesday, September 6, 2022
2:30-3:30pm (105 Lin Hall).

Abstract:   Quantum devices and systems are the heart of revolutionary technologies that open up new ways to collect and process information, perfectly secure communications, optimize computation problems, accurately measure physical phenomena, and many more. My research includes the applications of new materials, experiments, nanostructures, and data-driven models to impact the field of quantum science and technology. My presentation will explain integrating computational and experimental techniques to introduce new development, design, and implementation tools for quantum devices and quantum optical systems with more diverse and enhanced functionalities. My research presentation will discuss combining the knowledge of designing the optical structures and the dominant performance of rare-earth material to generate, manipulate, and store quantum light as well as control and use coherent atom-atom interactions for quantum information processing. In addition, I will discuss how to engineer the functionality of multilayer nanostructures to control optical properties for various applications ranging from spacecraft thermal control systems to advanced gravitational wave detectors that can minimize thermal noise below the standard quantum limit. Furthermore, I will provide my perspective on developing predictive performance models using computational and machine learning techniques to combat noise in optical and quantum optical systems and predict the performance of future experiments.

Title:  Colloidal Assembly at Fluid Interfaces

Sepideh Razavi, OU Dept. of Chemical, Biological and Medical Engineering.

Friday, September 16, 2021
2:30-3:30pm,  105 Lin Hall

Abstract:  The ubiquity of self-assembly - the process of creating organizational order in systems of components - in nature has inspired technological developments towards synthetic building blocks that assemble into desirable structures with a unique set of properties. Isotropic spherical colloids are a simple example of such building blocks where their spatial arrangement yields photonic crystals that exhibit structural color. The key step towards engineering the assembly process is the ability to tune the interparticle interactions. There is a concerted effort in the field to identify the factors that impact the interparticle interactions and control the assembly process. How is the assembly in bulk different from the 2D assembly in the presence of a fluid interface? What happens when shape or surface anisotropic (i.e., Janus) particles are used as building blocks for assembly? In this talk, I will present experiments on the application of fluid interfaces as a template for assembly; specifically, I will discuss the role of particle surface properties in tuning the mechanical stability and flow behavior of the assembled monolayer, important for applications in which the interface undergoes large deformations producing compression and shear stresses at the interface.

Title:  Molecular transistors as substitutes for quantum information applications

Mario Borunda, Oklahoma State University

Friday, September 23, 2022
2:30-3:30pm,  105 Lin Hall

Abstract:  Applications of quantum information generally rely on the generation and manipulation of qubits. Still, there are ways to envision a device with a continuous readout but without the entangled states. In this talk, I will discuss an alternative to the qubit, namely the solid-state version of the Mach–Zehnder interferometer, in which the local moments and spin polarization replace light polarization. Transistors based on such systems lead to the possibility of fabricating logic gates that do not require entangled states.

Title:    Strong coupling theory of twisted multilayer graphene systems: correlatedinsulators, collective excitations and superconductivity   

Eslam Khalaf, Harvard University

Tuesday, October 4th, 2022

2:30-3:30pm (105 Lin Hall/Zoom Link will be announced).

Abstract:    I will discuss a recently developed strong coupling theory of magic-angle twisted bilayer graphene. I will begin by showing how the electronic structure of twisted bilayer graphene makes it possible to relate its flat bands to the lowest Landau levels. This leads to a model of twisted bilayer graphene consisting of two sets of U(4) symmetric Landau-level-like Chern bands with opposite Chern numbers that are tunnel-coupled. I will show how this model broadly captures most of the observed features of twisted bilayer graphene: correlated trivial and topological Chern insulators at integer fillings, fractional Chern insulators and superconductivity. The correlated insulators are understood as generalized quantum Hall ferromagnets. I will then discuss the nature of charged excitations on top of such correlated insulators showing that they admit non-trivial charged excitations in addition to single-particle excitations. These excitations are intimately tied to band topology and can take the form of skyrmions -- real space spin textues -- or spin polarons -- bound states of an electron and a spin waves. I will show that there is a very natural mechanism for superconductivity which leads to pairing of these charged excitations. This mechanism is purely repulsive in origin and relies on band topology in a fundamental way. At the end, I will discuss how these insights generalize to a class of multilayer graphene systems with alternating twist angle and contrast the properties of these systems compared to their bilayer counterpart.

Title:    CANCELLED - Will be rescheduled

Alexander Efros, Naval Research Lab

Tuesday, October 18th, 2022
2:00-3:00pm (105 Lin Hall/Zoom Link will be announced).

Abstract:   

Title:    Entanglement and learnability transitions

Sarang Gopalakrishnan, Princeton University

Tuesday, October 25th, 2022
2:30-3:30pm (105 Lin Hall/Zoom Link will be announced).

Abstract:    Continuously monitored quantum systems can undergo "entanglement transitions" in their steady states (conditioned on the measurement outcomes). I will briefly review our understanding of entanglement transitions and why it is challenging to observe them in realistic experiments. I will introduce a variant on the entanglement transition, called the "charge sharpening" transition, and explain how it can be related to a transition in the ability of an eavesdropper to "learn" the state of a system using local measurements.

Title:  Quantum Simulation and Quantum State Engineering: Prospects and Challenges

 Vito Scarola, Virginia Tech.

Friday, November 4th, 2022
2:30-3:30pm,  105 Lin Hall

Abstract:  Analogue and digital quantum simulation might offer solutions to important but otherwise intractable models.  Yet challenges, such as heating in analogue simulators and noise in digital quantum devices, remain as obstacles.  My theory group seeks to guide experimental setups and construct methods for implementing quantum simulation.  I will review our theory work and experimental progress in using atoms in optical lattices as analogue simulators to probe the phase diagram of the Fermi-Hubbard model.  I will also discuss digital quantum simulation in near term devices.  Here noise limits coherence and therefore gate depth.  I will discuss new ideas for constructing ultracompact quantum algorithms for unbiased quantum simulation.   

Our research also explores directions for new and interesting experiments that can probe fundamentals of quantum many-body states on near-term quantum devices.  I will focus on graph states which have applications in measurement-based quantum computing, quantum networking, and quantum metrology.  We introduce a formalism for their construction as robust quantum states hosting symmetry protected topological order.  We find that certain graph states can be engineered to self-correct unwanted errors.  More generally, I will discuss graph states as important quantum resource states for atomic, molecular, and optical setups. 

Title:    Ultra-Low-Voltage, Beyond CMOS Microelectronics

R. Ramesh, Rice University

Monday, November 7th, 2022
3:00-4:00pm (105 Lin Hall/Zoom Link will be announced).

Abstract:   Despite ever-improving computing efficiency, information technology (IT) represents the fastest growing energy consumer and will have significant implications for U.S. energy consumption. This impending cliff threatens the nation’s ability to solve important problems across science, technology, national security, and energy. Without improvements in computing efficiency, the explosion of the Internet of Things (IoT) and artificial intelligence (AI) applications will exponentially increase energy consumption. A complete rethinking of how computing is performed today is needed to develop the next generation of beyond-CMOS microelectronics. Our scientific mission is built on a core guiding principle that a significant opportunity exists for use-inspired basic science to enable highly energy-efficient computing by exploiting correlated phenomena and consequently lowering the operating voltage. Orders of magnitude improvement in energy efficiency are possible by exploiting correlations (electronic charge/spin and dipolar). We aim to design and manipulate this energy barrier to specifically reduce the operating voltage substantially below what is achievable by today's CMOS technology. This fundamental physics approach to solving systems-level techno-economic problems can lead to dramatically lower energy consumption, in addition to a completely new hierarchy of logic-in-memory information technology building blocks.

Title:    Semiconductor Nanocrystals: from discovery to modern development

Sasha Efros

Tuesday, November 15th, 2022
2:30-3:30pm (105 Lin Hall/Zoom Link will be announced).

Abstract:   Semiconductor nanocrystals (NCs) are the most heavily studied of the nanoscale semiconductors. The size dependence of NC optical properties was discovered independently more than 30 years ago in two different materials: in semiconductor-doped glasses by Ekimov et al (1981), and in aqueous solutions by Brus et al (1983).  I will briefly discuss the history of this discovery, the main obstacles in the development of this field, and the critical breakthroughs along the way.¹

Today, semiconductor NCs have become much more than objects of scientific curiosity. The demonstration of tunable, room-temperature lasing using NC quantum dot solids, the development of NC-based light-emitting diodes and photovoltaic cells, quantum dots TV produced by Samsung, and the first commercial products in the area of NC bio-labeling are just a few illustrations of the broad technological potential of these materials. 

Nonradiative Auger recombination is the central non-radiative relaxation process, which negatively affects the performance of all these devices.  I will discuss why nonradiative Auger recombination is enhanced in NCs, and how we can suppress it.²  

Finally, I am going to discuss the unusual optical properties of the recently discovered CPbX₃ (X=Cl, Br. I) NCs, which are connected with a ground bright exciton state.³ We calculate the lowest quantum confined levels of electrons and holes and the spectra of the allowed optical transitions. The calculations take into account the cubic shape of the perovskite NCs, which results in an inhomogeneous electric field of emitted and absorbed photons.   The symmetry of the ground exciton state has been analyzed, and the radiative decay time has been calculated.   The results of our theoretical calculations have explained the 200 ps radiative decay time and polarization properties measured in experiments on single CsPb(BrCl₂)  quantum dots.

1.     Al. L. Efros and L. E. Brus, ACS Nano 15, 6192–6210 (2021)

2.     Al. L. Efros and D. J. Nesbitt, Nature Nanotech. 11,  661 (2016)

3.      M. A. Becker, R. Vaxenburg, G. Nedelcu, P. C. Sercel, A. Shabaev, M. J. Mehl, J. G. Michopoulos, S. G. Lambrakos, N. Bernstein, J. L. Lyons, T. Stöferle, R. F. Mahrt, M. V. Kovalenko, D. J. Norris, G. Rainò, and Al. L. Efros, Nature, 553, 189 (2018)

Title:   Optical Materials for Far-UV Chiral Sensing and Ultrafast Near-Infrared Optoelectronics

Kevin McPeak, Louisiana State University

Tuesday, December 6th, 2022
2:30-3:30pm (105 Lin Hall).

Abstract:   I will discuss two distinct research thrusts my laboratory has pursued over the last several years, i.e., investigating resonant plasmonic-biomolecular interactions and the emergent optical and electronic properties of noble-transition metal alloys. 

Resonant plasmonic-molecular chiral interactions are a promising route to enhanced biosensing. However, biomolecular optical activity primarily exists in the far-ultraviolet regime, posing significant challenges for spectral overlap with current nano-optical platforms. I will show experimentally and computationally the enhanced chiral sensing of a resonant plasmonic-biomolecular system operating in the far-UV. Our calculations show that detectable enhancements in the chiroptical signals from small amounts of biomolecules are possible only when tight spectral overlap exists between the plasmonic and biomolecular chiral responses. We support this conclusion experimentally by using Al gammadion arrays to enantiomerically discriminate ultrathin (< 10 nm thick) films of Tyrosine. Our results demonstrate the importance of using far-UV active metasurfaces for enhancing natural optical activity.

Noble-transition metal alloys are a new class of materials for interband-driven hot hole generation with NIR light. NIR hot-carrier generation in pure, noble metals suffers from insufficient energy to exceed the interband energy threshold (IET) (e.g.,> 2 eV), and in pure transition metals, rapid hot-carrier thermalization. Band hybridization in select noble-transition metal alloys can overcome these issues, yielding emergent properties that facilitate tuning the hot-carrier distribution and lifetime.

References

1.    Tiago Ramos Leite, Lin Zschiedrich, Orhan Kizilkaya, and Kevin M. McPeak “Resonant Plasmonic–Biomolecular Chiral Interactions in the Far-Ultraviolet: Enantiomeric Discrimination of sub-10 nm Amino Acid Films” Nano Lett. 22, 18, 7343–7350, (2022)

2.    Sara KF Stofela, Orhan Kizilkaya, Benjamin T Diroll, Tiago R Leite, Mohammad M Taheri, Daniel E Willis, Jason B Baxter, William A Shelton, Phillip T Sprunger, Kevin M McPeak, “A Noble-Transition Alloy Excels at Hot-Carrier Generation in the Near Infrared,” Advanced Materials 32 (23), 1906478 (2020)