Colloquium Abstracts (Fall 2009)
Superconducting-based Artificial atoms for Quantum Computing
Roberto Ramos
Superconducting micro-fabricated solid-state devices such as the Josephson junction have been shown to be viable candidates for building qubits in quantum computation (1). When cooled to ultra-low temperatures, individual devices demonstrate quantized energy level states, much like an artificial atom. When these giant “atoms” are coupled, these states combine to form quantum states which have no classical analog – these states are said to exhibit “quantum entanglement”- which is believed to be the criteria for quantum computing to work (2). We are currently studying the dynamics of Josephson junctions coupled to other quantum devices (junctions and harmonic oscillators), in different configurations. By approximating the coupled devices as harmonic oscillators with an anharmonic perturbation, we have calculated the energies of entangled quantum states and predicted multiple avoided crossings in their energy spectra – which is the signature of entanglement in couple d systems (3). In this presentation, I will discuss these energy spectroscopy predictions for coupled qubit systems containing more than two qubits. I will discuss the progress of our low-temperature experiments designed to measure these macroscopic quantum properties. If there is time, I will also discuss plans to measure macroscopic quantum states in graphene-based junctions.
Nonlinear Optics at the Nanoscale
Eric Mazur
We explore nonlinear optical phenomena at the nanoscale by launching femtosecond laser pulses into long silica nanowires. Using evanescent coupling between wires we demonstrate a number of nanophotonic devices. At high intensity the nanowires produce a strong supercontinuum over short interaction lengths (less than 20 mm) and at a very low energy threshold (about 1 nJ), making them ideal sources of coherent white-light for nanophotonic applications. The spectral broadening reveals an optimal fiber diameter to enhance nonlinear effects with minimal dispersion. We also present a device that permits a number of all-optical logic operations with femtosecond laser pulses in the nanojoule range.
Anisotropic electric properties – a new field in solid state physics
Peter Weinberger
"Anisotropic electric properties" sounds awfully "academic", actually it is not, since they provide the physical phenomena on which
• spin-polarized Scanning Tunnelling Microscopy (sp-STM)
• domain wall & domain wall motions
• and by the way also GMR & TMR
are based on.
Sp-STM enables the discovery of possibilities for switching small magnetic islands, even of individual atoms, on suitable substrates (miniaturization of memory devices on the Angstrom scale).
Domain wall motions most likely will serve as the principle behind completely new types of memories („solid state devices“, so-called „race track memories“) with no moving parts at all.
(GMR & TMR don‘t need any further p.r., simply switch on a desk top computer or a laptop. These effects were already honored with a Nobel Prize!)
How does a quantum mechanical system thermalize?
David Weiss
I will describe experiments with atoms in optical lattices that create effectively 1D Bose gases. These are the first experimentally observed integrable many-body systems, which means among other things that they can be exactly solved and that to a first approximation they do not thermalize, as we have observed by making quantum Newton's cradles. The question we are now addressing is what happens when integrability is lifted. Does the gas thermalize, or, as in some classical systems, is there some regime of non-integrability in which the system still does not thermalize? We think we are close to a definitive experimental answer, but since it's still preliminary you'll have to come to my talk to hear it.
Resonance phenomena: a tool for mixing, a tool for control
Dmitri Vainchtein
In my talk I discuss several aspects of transport phenomena in near-integrable multiscale dynamical systems. I will start with an introduction into what makes a system chaotic and how these properties can be quantified. In the next part of the talk I consider mixing via resonances-induced chaotic advection in volume-preserving flows. I show that proper characterization of the mixing quality requires introduction of two different metrics. The first metric determines the relative volumes of the domain of chaotic streamlines and the domain of regular streamlines. The second metric describes the time for homogenization inside the chaotic domain. In the last part of the talk I illustrate how the capture into resonance, that by itself is random in nature and, consequently, is rather inefficient as a mechanism of regular transport, can be structured with little additional cost. As a model problem I consider the Hamiltonian dynamics of a charged particle in an electromagnetic field.
Resonance phenomena: a tool for mixing, a tool for control
Michele Parrinello
We is a generalized form of Langevin equation in which the noise is correlated (colored) rather then being white to devise a number of very powerful sampling methods. After revising the theory that is behind our approach we show how one can model the noise to achieve optimal sampling in ordinary and in ab-initio (Car-Parrinello) molecular dynamics. Most remarkably our sampling method can be used to introduce quantum effect at zero additional cost with respect to a standard simulation.
G. Bussi and M. Parrinello, J. Chem. Phys., vol. 126 (1), pp. 014101, 2007
M. Ceriotti, G. Bussi and M. Parrinello, Phys. Rev. Lett., vol 102 (2), pp. 020601, 2009
M. Ceriotti, G. Bussi and M. Parrinello, Phys. Rev. Lett., vol 103 (3), pp. 030603,
Topological Insulators and Topological Band Theory
Charles Kane
A topological insulator is a material with a bulk excitation gap generated by the spin orbit interaction, which is topologically Distinct from an ordinary insulator. This distinction —characterized by a topological invariant - necessitates the existence of conducting states on the sample boundary which have a massless Dirac dispersion relation. These materials have attracted considerable interest as a fundamentally new class of insulators with applications from quantum transport to quantum computing. In this talk we will outline our theoretical discovery of this electronic phase and describe recent experiments in which its signatures have been observed in both two and three dimensional systems. We will close by arguing that the proximity effect between an ordinary superconductor and a 3D topological insulator leads to a novel two dimensional interface state which may provide a new venue for realizing proposals for topological quantum computation.
What is a Photon – A Mind-Boggling Concept?
B.-S Skagerstam
Recent developments in electronic and optical technology have made it possible to experimentally realize well-localized ONE-photon states. In this talk, directed to a general audience, I will first remind ourselves about the basic rules of quantum mechanics and then discuss in what sense quantum-mechanical interference of ONE-photon states has been experimentally verified. Then I outline a relativistic and quantum-mechanical description of SINGLE photons and show how the experimentally verified Berry phase of linearly polarized light naturally emerges in such a framework. Geometry, including non-commutative aspects, and quantization are connected in this approach. The use of t'Hooft-Polyakov non-Abelian magnetic monopoles finds here an amazing application. If time permits some aspects of the space-time evolution of a localized ONE-photon wave-packet will also briefly be touched upon.
Topological Insulators and Topological Band Theory
Jiping Huang
I shall briefly review the exciting development of econophysics as an interdisciplinary research, and then present a specific topic: the “invisible hand” in resource allocation. It is believed that the generalized "invisible hand" exists not only in economic markets, but also in many biological and social systems. However, the microscopic mechanism under the “invisible hand” is unclear, so far. In our recent study [PNAS 106, 8423 (2009)], we focused on the biasedly distributed resource allocation problem. First, we designed and conducted a series of behavioral economic experiments. We found that the allocation of a virtual resource could reach the efficient state in these experiments, even if direct negotiations among the participants, as well as the external instructions which might create certain collaboration among them, were completely forbidden. In other words, we demonstrated the existence of the “invisible hand”. Next, based on the minority game (MG), we constructed a model called market-directed resource allocation game (MDRAG). The new model can explain what we found in our experiments. In the mean time, through a large number of the MDRAG simulations, we are able to establish a possible mechanism for the “invisible hand”. A more interesting discovery is related to a number of phase transitions found in the MDRAG simulations. Around the critical region of these phase transitions, the directing power of the “invisible hand” can be released completely, which could lead to an efficient, stable and unpredictable market. Also, our analytic theory agrees with the simulations.