Interfacial Engineering of Quantum Materials
Quantum materials are fascinating platforms where macroscopic quantum phenomena occur. As a prominent example, superconductors conduct electricity with no dissipation at low temperatures, holding great promise to address global energy challenges. It is of tremendous scientific and technological interest to enhance superconducting transition temperatures. In this talk I will elucidate the principles of interfacial engineering and co-operative interactions through studies of bulk superconductors, and demonstrate how I implement these concepts in fabricating an iron selenide/strontium titanate interfacial superconductor. In the first example, I map out the electronic band structure and quantify electron-phonon interactions of calcium-intercalated graphite. I discover a key phonon mode, enabled by the interface between calcium superlattice and graphene sheets, which facilitates superconductivity in this compound. In the second example, I combine two time-domain experiments to study an iron selenide superconductor. By precisely determining the atomic displacements and electronic energy shifts, I discover a co-operative interplay between electron-phonon and electron-electron interactions, which is crucial for unconventional superconductivity. Following the acquired insight, I fabricate ultrathin iron selenide on strontium titanate substrates and investigate the interfacial superconductivity. Characterized by in situ transport and in situ photoemission spectroscopy, the transition temperature of the interfacial layer is one order of magnitude higher than the counterpart for bulk iron selenide. This high transition temperature persists even when additional iron selenide layers are deposited. This study sets the foundation for manipulating superconductivity and other quantum phenomena via interfacial engineering.
Thomas Purdy (NIST)
Thomas Purdy (NIST)
Can Evolutionary Dynamics Be Understood Quantitatively?
The basic laws of evolution have been known for more than a century and there is overwhelming evidence for the facts of evolution. Yet little is understood quantitatively about the dynamical processes that drive evolution: by physicists' standards the theory of evolution is far from fully-fledged. Huge advances in DNA sequencing technology and laboratory experiments have enabled direct observations of evolution in action and, together with theoretical developments, opened up great opportunities for dramatically advancing our understanding. This talk will focus on framing questions and on recent progress addressing some of these.
Quantum sensing in a new single-molecule regime
Quantum optics has had a profound impact on precision measurements, and recently enabled probing various physical quantities, such as magnetic fields and temperature, with nanoscale spatial resolution. Such advancements in ‘quantum sensing’ have brought the elusive dream of performing nuclear magnetic resonance spectroscopy (NMR) on individual biomolecules closer to reality. In my talk, I will discuss the development and application of novel quantum metrological technologies to study biological systems at a single-molecule level. I will start with a general introduction to quantum sensing, with a focus on the measurement of magnetic fields at a nanoscale. I will then show how we utilize such sensing techniques to control the temperature profile in living systems with subcellular resolution. Finally, I will provide an outlook on how quantum sensing and single-molecule biophysics can be utilized to perform NMR spectroscopy with unprecedented sensitivity, possibly down to the level of individual biomolecules.
Prototyping a Quantum Computer with Trapped Ions
Trapped atomic ions are an ideal system for quantum computation, with optically-accessible qubit states with long coherence times and fidelities exceeding 99% . This combination allows us to take advantage of coherence and entanglement--two distinctly quantum phenomena--to realize one- and two- qubit gates and to explore quantum algorithms that promise to scale better than their classical counterparts on particular problems like factoring large numbers . In this talk, I will describe the trapped ion quantum computing architecture [3,4], share first signals from a new room-temperature surface-electrode-trap universal quantum computing system we are building at the University of Maryland, and present related experiments that use motion on the ion chain instead of the ion's electronic state for quantum simulation.
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 A Montanaro. NPJ Quant. Inf. 2, 15023 (2016).
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 S Debnath, et al. Nature 536, 63 (2016).
Atomic Layer Semiconductor 2D Nanoelectromechanical Systems (NEMS)
Atomically thin semiconducting crystals derived from new classes of layered materials have rapidly emerged to enable two-dimensional (2D) nanostructures with unusual electronic, optical, mechanical, and thermal properties. While graphene has been the forerunner and hallmark of 2D crystals, newly emerged 2D semiconductors offer intriguing, beyond-graphene, attributes. The sizable and tunable bandgaps of compound and single-element 2D semiconductors offer attractive perspectives for strong multiphysics coupling and efficient transduction across various signal domains. In this presentation, I will describe my research group’s latest efforts on investigating how mechanically active atomic layer semiconductors and their heterostructures interact with optical and electronic interrogations, and on engineering such structures into new ultrasensitive transducers and signal processing building blocks. Using single- and few-layer transition metal di-chalcogenide (TMDC) crystals, we demonstrate multimode resonant 2D nanoelectromechanical systems (NEMS) with extraordinary electrical tunability. We have also found remarkably broad dynamic range (DR~70 to 100dB) in these 2D NEMS, via deterministic measurement of device intrinsic noise floor and onset of nonlinearity. I will describe spatial mapping and visualization of mode shapes and Brownian motion in these 2D multimode resonators, along with their applications in resolving intrinsic anisotropy and structural asymmetry. I shall then discuss emerging device applications, from classical information processing technologies to 2D NEMS operating in their quantum regime.
Recent progress in the understanding of non-equilibrium quantum dynamics of many-body systems will be discussed. Applications to electron systems and ultracold atoms will be considered. Examples include photo-induced Cooper pairing in superconductors, prethermalization in 1d condensates, mesonic resonances in the Fermi Hubbard model. Possible connections of these phenomena to other areas of physics will be pointed out.
Emergent Phenomena at Oxide Interfaces
The plethora of fascinating properties observed in oxide heterostructures has attracted a lot of interest. Most noticeably, the confined electron gas formed at the interface between the two insulators LaAlO3 and SrTiO3 features e.g. gate-tunable superconductivity, ferromagnetism and non-volatile memory effects. Numerous studies have been devoted to understanding the origin of the conductivity along with enhancing its properties. Recently, we found that substituting LaAlO3 with γ-Al2O3 can produce a confined electron gas with an electron mobility exceeding 100,000 cm2/Vs. Here, I show that the γ-Al2O3/SrTiO3 interface conductivity originates from oxygen vacancies and use defect engineering to control various interface properties. In addition, I find that the high mobility coexists with a strain-tunable magnetic order below 40 K and a positive, non-saturating magnetoresistance of up to 80,000% at 15 T. The study evidences that the γ-Al2O3/SrTiO3 heterostructure is an exciting alternative to LaAlO3/SrTiO3 and paves the way for combining lattice, spin and electronic degrees of freedom.
Quantum Acoustics with Superconducting Qubits
The ability to engineer and manipulate different types of quantum mechanical objects allows us to take advantage of their unique properties and create useful hybrid technologies. Thus far, complex quantum states and exquisite quantum control have been demonstrated in systems ranging from trapped ions to superconducting resonators. Recently, there have been many efforts to extend these demonstrations to the motion of complex, macroscopic objects. These mechanical objects have important applications as quantum memories or transducers for measuring and connecting different types of quantum systems. In particular, there have been a few experiments that couple motion to nonlinear quantum objects such as superconducting qubits. This opens up the possibility of creating, storing, and manipulating non-Gaussian quantum states in mechanical degrees of freedom. However, before sophisticated quantum control of mechanical motion can be achieved, we must realize systems with long coherence times while maintaining a sufficient interaction strength. These systems should be implemented in a simple and robust manner that allows for increasing complexity and scalability in the future.
In this talk, I will describe our recent experiments demonstrating a high frequency bulk acoustic wave resonator that is strongly coupled to a superconducting qubit using piezoelectric transduction. In contrast to previous experiments with qubit-mechanical systems, our device requires only simple fabrication methods, extends coherence times to many microseconds, and provides controllable access to a multitude of phonon modes. We use this system to demonstrate basic quantum operations on the coupled qubit-phonon system. Straightforward improvements to the current device will allow for advanced protocols analogous to what has been shown in optical and microwave resonators, resulting in a novel resource for implementing hybrid quantum technologies.
Spectral signatures of many-body localization with interacting photons
Statistical mechanics is founded on the assumption that a system can reach thermal equilibrium, regardless of the starting state. Interactions between particles facilitate thermalization, but, can interacting systems always equilibrate regardless of parameter values? The energy spectrum of a system can answer this question and reveal the nature of the underlying phases. However, most experimental techniques only indirectly probe the many-body energy spectrum. Using a chain of nine superconducting qubits, we implement a novel technique for directly resolving the energy levels of interacting photons. We benchmark this method by capturing the intricate energy spectrum predicted for 2D electrons in a magnetic field, the Hofstadter butterfly. By increasing disorder, the spatial extent of energy eigenstates at the edge of the energy band shrink, suggesting the formation of a mobility edge. At strong disorder, the energy levels cease to repel one another and their statistics approaches a Poisson distribution - the hallmark of transition from the thermalized to the many-body localized phase. Our work introduces a new many-body spectroscopy technique to study quantum phases of matter.
On systems with and without excess energy in environment
How does a microscopic system like an atom or a small molecule get rid of the excess electronic energy it has acquired, for instance, by absorbing a photon? If this microscopic system is isolated, the issue has been much investigated and the answer to this question is more or less well known. But what happens if our system has neighbors as is usually the case in nature or in the laboratory? In a human society, if our stress is large, we would like to pass it over to our neighbors. Indeed, this is in brief what happens also to the sufficiently excited microscopic system. A new mechanism of energy transfer has been theoretically predicted and verified in several exciting experiments. This mechanism seems to prevail “everywhere” from the extreme quantum system of
the He dimer to water and even to quantum dots. The transfer is ultrafast and typically dominates other relaxation pathways.
Can there be interatomic/intermolecular processes in environment when the system itself (again, an atom or small molecule) does not possess excess energy? The answer to this intriguing question is yes. The possible processes are introduced and discussed. Examples and arguments are presented which make clear that the processes in question play a substantial role in nature and laboratory.
Topological and 2D materials: new playground for physics and devices
Topological materials and two-dimensional (2D) materials (including graphene) have become among the most actively studied systems in condensed matter physics emerged in the last decade, as also reflected in two recent physics Nobel prizes (2016 and 2010). These classes of materials, with significant overlap and connection between them, have also received attention beyond condensed matter physics, ranging from high-energy physics to electronics industries. These materials bring unprecedented freedom to realize and engineer novel electronic bandstructures (kinetic energy-momentum dispersion, used to be thought as a well-settled subject), create condensed matter analogs of Dirac/Weyl/Majorana fermions and other exotic particles, and are also considered promising in potential device applications for new ways of energy-efficient information processing, such as spintronics or perhaps even quantum computing. In this talk, I will discuss some of their unusual properties, potential applications and future prospects, drawing examples from a personal journey of experimental exploration of these materials. I will describe, for example, how they can defy our usual intuition about conductors, allowing a conductor cut in half to maintain the same conductance (as in a “topological insulator”), and two pieces of conductors (such as graphene) stacked together to behave like an insulator. Conduction by spin-helical Dirac fermions on the surface of topological insulators, can give rise to characteristic topological transport with “half-integer” quantum number measured in quantum Hall effect and Aharonov-Bohm effect, and may provide efficient ways to generate and store spin based information. Furthermore, these materials can serve as building blocks or inspirations to create more complex material structures and new states of matter so far difficult to realize.
Supercurrent in the quantum Hall regime
One of the promising routes towards creating novel topological states and excitations is to combine superconductivity and quantum Hall (QH) effect. However, signatures of superconductivity in the QH regime remain scarce, and a superconducting current through a QH weak link has so far eluded experimental observation. By utilizing high mobility graphene/boron nitride heterostructures we demonstrate the existence of a novel type of supercurrent-carrying states in a QH regime at magnetic fields as high as 2 Tesla. At low magnetic fields, devices demonstrate the Fraunhoffer pattern and Fabri-Perot oscillations, confirming their uniformity and ballisticity. In the QH regime, when Landau quantization is fully developed, regions of superconductivity can be observed on top of the conventional QH fan diagram. The measured supercurrent is very small, on a few nA scale, and periodic in magnetic field. We discuss possible mechanisms that could mediate supercurrent along the QH edge states.
Strong THz fields and nonlinear THz interactions with charges, dipoles, and spins
Dramatic advances in tabletop generation of THz pulses with high field levels have enabled nonlinear THz spectroscopy and THz control over molecules and materials. The most widely used approach for generation of strong fields in the 0.2-2 THz range involves excitation of phonon-polaritons in ferroelectric host crystals, and a THz polaritonics platform has been developed that includes capabilities for THz field control and visualization as well as direct and strong coupling to collective spin waves (magnons). Recent developments in THz generation, nonlinear spectroscopy, and control over charges, dipoles, and spins will be discussed. THz-driven liberation and acceleration of carriers in solids can induce highly nonlinear responses such as cascaded impact ionization and billion-fold changes in conductivity, insulator-metal electronic/structural phase transitions, electroluminescence, and chemical reactions. Nonlinear manipulation of molecular dipoles and collective magnetic spins has been demonstrated in recent 2D THz spectroscopy measurements that include the extension of 2D magnetic resonance to the THz frequency range. Nonlinear THz spectroscopy offers new fundamental insights and potential applications in security, THz sensing and imaging, and high-bandwidth signal processing.