Simulating a system of two driven coupled qubits, we show that the time-averaged probability to find one driven qubit in its ground or excited state can be controlled by an ac drive in the second qubit. Moreover, off-diagonal elements of the density
matrix responsible for quantum coherence can also be controlled via driving the second qubit, i.e., quantum coherence can be enhanced by appropriate choice of the bi-harmonic signal. Such a dynamic synchronization of two differently driven qubits has an analogy with harmonic mixing of Brownian particles forced by two signals through a substrate. Nevertheless, the quantum synchronization in two qubits occurs due to multiplicative coupling of signals in the qubits rather than via a nonlinear harmonic mixing for a classical nano-particle.
In this paper we consider a two-dimensional metamaterial comprising an array of qubits (two level quantum objects). Here we show that a two-dimensional quantum metamaterial may be controlled, e.g. via the application of a magnetic flux, so as to prov
ide controllable refraction of an input signal. Our results are consistent with a material that could be quantum birefringent (beam splitter) or not dependent on the application of this control parameter. We note that quantum metamaterials as proposed here may be fabricated from a variety of current candidate technologies from superconducting qubits to quantum dots. Thus the ideas proposed in this work would be readily testable in existing state of the art laboratories.
We present molecular-dynamic simulations of memory resistors (memristors) including the crystal field effects on mobile ionic species such as oxygen vacancies appearing during operation of the device. Vacancy distributions show different patterns dep
ending on the ratio of a spatial period of the crystal field to a characteristic radius of the vacancy-vacancy interaction. There are signatures of the orientational order and of spatial voids in the vacancy distributions for some crystal field potentials. The crystal field stabilizes the patterns after they are formed, resulting in a non-volatile switching of the simulated devices.
We have extended our recent molecular-dynamic simulations of memristors to include the effect of thermal inhomogeneities on mobile ionic species appearing during operation of the device. Simulations show a competition between an attractive short-rang
ed interaction between oxygen vacancies and an enhanced local temperature in creating/destroying the conducting oxygen channels. Such a competition would strongly affect the performance of the memristive devices.
Reversible bipolar nano-switches that can be set and read electronically in a solid-state two-terminal device are very promising for applications. We have performed molecular-dynamics simulations that mimic systems with oxygen vacancies interacting v
ia realistic potentials and driven by an external bias voltage. The competing short- and long-range interactions among charged mobile vacancies lead to density fluctuations and short-range ordering, while illustrating some aspects of observed experimental behavior, such as memristor polarity inversion.
Following the discovery of Bose-Einstein condensation (BEC) in ultra cold atoms [E. Gosta, Nobel Lectures in Physics (2001-2005), World Scientific (2008)], there has been a huge experimental and theoretical push to try and illuminate a superfluid sta
te of Wannier-Mott excitons. Excitons in quantum wells, generated by a laser pulse, typically diffuse only a few micrometers from the spot they are created. However, Butov et al. and Snoke et al. reported luminescence from indirect and direct excitons hundreds of micrometers away from the laser excitation spot in double and single quantum well (QW) structures at low temperatures. This luminescence appears as a ring around the laser spot with the dark region between the spot and the ring. Developing the theory of a free superflow of Bose-liquids we show that the macroscopic luminesce rings and the dark state are signatures of the coherent superflow of condensed excitons at temperatures below their Berezinskii-Kosterlitz-Thouless (BKT) transition temperature. To further verify the dark excitonic superflow we propose several keystone experiments, including interference of superflows from two laser spots, vortex formation, scanning of moving dipole moments, and a giant increase of the luminescence distance by applying one-dimensional confinement potential. These experiments combined with our theory will open a new avenue for creating and controlling superflow of coherent excitons on nanoscale.
We report the results of molecular dynamics simulation of a spatiotemporal evolution of the locally photoexcited electrons and holes localized in two separate layers. It is shown that the ring-shaped spatial pattern of luminescence forms due to the s
trong in-layer Coulomb interaction at high photoexcitation power. In addition, the results predict (i) stationary spatial oscillations of the electron density in quasi one-dimensional case and (ii) dynamical phase transition in the expansion of two-dimensional electron cloud when threshold electron concentration is reached. A possible reason of the oscillations and a theoretical interpretation of the transition are suggested.
