Andreev and Majorana Weyl crossings in multi-terminal Josephson junctions
We analyze the Andreev spectrum in a four-terminal Josephson junction between topological superconductors. We find that a topologically protected crossing in the space of three superconducting phase differences can occur between the two Andreev bound states with lower energy. We discuss the possible detection of this crossing through the transconductance quantization, in units of 2e^2/h, between two voltage-biased terminals. Our prediction provides another example of topology in multi-terminal Josephson junctions.
Exploiting Orbital Angular Momenta and Nonlinear Frequency Conversion in Plasmonic Devices
Surface plasmons (SPs) are evanescent waves generated through the collective oscillations of electrons at a metal/dielectric interface under optical excitation. Due to the strong light-electron coupling and near-field nature, surface plasmons offer: (1) the opportunity for sub-wavelength spatial confinement of optical waves is enabled; and (2) giant local field enhancement of optical waves is permitted. These unique attributes lead to the long-envisaged optical circuits, and allowed breakthroughs in the generally termed “plasmonics” . For example, in the realizations of optical nano-antennas , surface plasmon vortices [3-5], and sub-wavelength guiding in plasmonic two-wire transmission-line (TWTL) [6-8] that have all enriched the scientific world. Optical vortices are waves carrying orbital angular momentum and exhibit helical phase fronts. Helical phase front leads to discontinuous azimuthal phase jumps and the number of phase discontinuities within a 2 range is referred to as the topological charge of an optical vortex. Generation of optical beams carrying orbital angular momentum has received increasing attentions recently, both in the far-field and in the near-field. Near-field vortices are typically generated through the excitation of SPs. In this talk, I will first introduce our recent progress on applying surface plasmon vortex for selectable particle trapping and rotation. The ability to spatially shape the near-field spatial patterns of surface plasmon vortices will be addressed as well. Moreover, in all past studies, SP vortices were excited by far-field circularly polarized light. This means the functionality of the SP devices were merely converting the far-field spin angular momentum to orbital angular momentum in the near-field. Next, I will focus on the creation of surface plasmon vortex using non-angular momentum excitation. Nonlinear optical frequency conversion in plasmonics has attracted immense research attention recently. Through various device geometries, harmonic generations of various orders have been reported. However, in these demonstrations, the near-field effect utilized has been limited to localized surface plasmons. It was not until last year that nonlinear frequency conversion utilizing propagating surface plasmon polaritons being reported in a single plasmonic nanowire. A plasmonic TWTL is comprised of two metallic nanowires with a nano-gap between these two nanowires. Depending on the laser excitation polarization, TE or TM mode could be selectively excited within a plasmonic TWTL. It has been demonstrated the two modes could be freely converted through the geometrical design to the TWTL [6,7]. Here I will present our most recent experimental data on ultrafast second-harmonic generations (SHG) in a plasmonic TWTL. We also show that regardless of the fundamental harmonic mode excited by the laser beam, the SHG signals are always the TM mode if no special treatment to the laser beam or the TWTL is provided. In the second part of this talk, the functionality of a plasmonic two-wire transmission-line will be introduced. I will also extend the studies into the nonlinear optical regime, where interesting modal behaviors are observed.
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. P. Biagioni, J.-S. Huang, and B. Hecht, “Nanoantennas for visible and infrared radiation,” Rep. Prog. Phys. 75, 024402 (2012).
. Y. Gorodetski, A. Niv, V. Kleiner, and E. Hasman, “Observation of the spin-based plasmonic effect in nanoscale structures,” Phys. Rev. Lett. 101, 043903 (2008).
. W.-Y. Tsai, J.-S. Huang, and C.-B. Huang, “Selective trapping or rotation of isotropic dielectric micro-particles by optical near field in a plasmonic Archimedes spiral,” Nano Lett. 14, 547 (2014).
. C.-F. Chen, C.-T. Ku, Y.-H. Tai, P.-K. Wei, H.-N. Lin, and C.-B. Huang, “Creating optical near-field orbital angular momentum in a gold metasurface,” Nano Lett. 15, 2746 (2015).
. W.-H. Dai, F.-C. Lin, C.-B. Huang, and J.-S. Huang, “Mode conversion in high-definition plasmonic optical nanocircuits,” Nano Lett. 14, 3881 (2014).
. Y.-T. Hung, C.-B. Huang, and J.-S. Huang, “Plasmonic mode converter for controlling optical impedance and nanoscale light-matter interaction,” Opt. Express 20, 20342 (2012).
. P. Geisler, G. Razinskas, E. Krauss, X.-F. Wu, C. Rewitz, P. Tuchscherer, S. Goetz, C.-B. Huang, T. Brixner, and B. Hecht, “Multimode plasmon excitation and in situ analysis in top-down fabricated nanocircuits,” Phys. Rev. Lett. 111, 183901 (2013).
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.
Quantum LEGOs: Building large quantum systems atom-by-atom
The realization of large-scale controlled quantum systems is an exciting frontier in modern physical science. In this talk, I will introduce a new approach based on cold atoms in arrays of optical tweezers. We use atom-by-atom assembly to deterministically prepare arrays of individually controlled cold atoms. A measurement and feedback procedure eliminates the entropy associated with the probabilistic trap loading and results in defect-free arrays of over 60 atoms . Strong, coherent interactions are enabled by coupling to atomic Rydberg states. We realize a programmable Ising-type quantum spin model with tunable interactions and system sizes of up to 51 qubits. Within this model we observe transitions into ordered states (Rydberg crystals) that break various discrete symmetries, verify high-fidelity preparation of ordered states, and investigate dynamics across the phase transition in large arrays of atoms .
An alternative, hybrid approach for engineering interactions is the coupling of atoms to nanophotonic structures in which guided photons mediate interactions between atoms. I will discuss our progress towards entangling two atoms that are coupled to a photonic crystal cavity and I will outline the exciting prospects of this approach for scaling the system to large distances in a quantum network.
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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 Measurements in Cavity Optomechanics
Over the last several years, research in the field of cavity optomechanics has developed extraordinarily sensitive and low loss devices as well as clever measurement techniques to probe macroscopic mechanical systems in the quantum regime. If one observes carefully, the noise in optically detected mechanical resonators can reveal a remarkable tale of the fundamental quantum mechanics of measurement embodied by Heisenberg’s microscope type physics. In this talk, I will review the basic consequences of quantum measurement backaction in the context of recent cavity optomechanics experiments ranging from nanoscale integrated photonic devices, to millimeter scale vibrating membranes, to extremely large interferometric gravitational wave observatories. I will then discuss how these effects are being harnessed for useful purposes. One set of applications consists of manipulating the quantum state of light with a mechanically mediated nonlinearity to generate squeezed light for quantum metrology or to coherently convert quantum information between vastly different frequency domains. Another type of application involves carefully measuring the size of quantum noise for use as a fundamental scale for temperature metrology.
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.
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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Fractional Statistics from Topological Magnetism
There is considerable excitement about in realizing non-Abelian Anyons, particles whose phase upon exchange depends on the path taken. Their observation signals the emergence of new topological phases and offers a route to quantum computation. I will outline the reasons to expect such particles in a particular quantum spin-liquid, where there is long-range entanglement but no magnetic order. I will then describe a particularly promising van-der-Walls material for realizing such particles, RuCl3. Via Raman spectroscopy, we have uncovered the Fermi statistics emerging from the Majorana particles in this insulating system. Time permitting, I will discuss the possibility of creating novel heterostructures in our new cleanroom in a glovebox with RuCl3.
Machine learning approaches to entangled quantum states
Artificial neural networks play a prominent role in the rapidly growing field of machine learning and are recently introduced to quantum many-body systems. This talk will focus on using a machine-learning model, the restricted Boltzmann machine (RBM) to describe entangled quantum states. Both short- and long-range coupled RBM will be discussed. For a short-range RBM, the associated quantum state satisfies an entanglement area law, regardless of spatial dimensions. I will present our recently constructed exact RBM models for nontrivial topological phases, including a 1d cluster state and a 2d toric code. For a long-range RBM, the captured entanglement entropy scales linearly with the number of variational parameters in the RBM model, in sharp contrast to the log-scaling in matrix product state representation.