Controlled generation of coherent matter currents using a periodic driving field

We study the effect of a strong, oscillating driving field on the dynamics of ultracold bosons held in an optical lattice. Modeling the system as a Bose-Hubbard model, we show how the driving field can be used to produce and maintain a coherent atomic current by controlling the phase of the intersite tunneling processes. We investigate both the stroboscopic and time-averaged behavior using Floquet theory, and demonstrate that this procedure provides a stable and precise method of controlling coherent quantum systems.     

Phase Time for the Tunneling of Ultracold V-Type Atoms Through a Mazer Cavity

We study the tunneling time of ultracold V-type atoms interacting a high quality microwave cavity. Here atomic coherence is introduced in the system by a strong driving field which couples the two lower states of the three-level atom. It is found that in the presence of coherence, mazer action or the scattering like nature of the interaction may be examined for extended energies of the incident cold atoms. Our results show that position and amplitudes of the peak values of the phase time(traversal time) may be very effectively controlled by the coherent driving field. Further, here we obtained superclassical values of the phase time corresponding to much higher values of the transmission amplitudes of the tunneling atoms which may be advantageous in the possible experimental realization of the superclassical tunneling time of the traversing cold atoms. In addition, we examine a mirror reflection type symmetry in the phase time curve for a judicious choice of the external driving field.     

High-efficiency asymmetric diffraction based on PT-antisymmetry in quantum dot molecules

We present preparation of asymmetric grating with higher diffraction efficiency in quantum dot molecules by combining the tunneling effect and parity-time antisymmetry.In the presence of tunneling between two quantum dots,the system exhibits the striking PT antisymmetry via spatially modulating the driving field and the detuning with respect to the driven transition.For this reason,the asymmetric grating could be achieved.The results show that the diffraction efficiency can be adjustable via changing the driving intensity,detuning,tunneling strength,and interaction length,and then the high-order diffraction can be reached.The scheme provides a feasible way to obtain the direction-controlled diffraction grating,which can be helpful for optical information processing and realization of controllable optical self-image.     

Quantum control and entanglement using periodic driving fields

We propose a scheme for producing directed motion in a lattice system by applying a periodic driving potential. By controlling the dynamics by means of the effect known as coherent destruction of tunneling, we demonstrate a novel ratchetlike effect that enables particles to be coherently manipulated and steered without requiring local control. Entanglement between particles can also be controllably generated, which points to the attractive possibility of using this technique for quantum information processing.     

Coherent Destruction of Tunneling of Bosons with Effective Three-Body Interactions

The tunneling dynamics of dilute boson gases with three-body interactions in a periodically driven double wells are investigated both theoretically and numerically.In our findings,when the system is with only repulsive twobody interactions or only three-body interactions,the tunneling will be suppressed;while in the case of the coupling between two- and three-body interactions,the tunneling can be either suppressed or enhanced.Particularly,when attractive three-body interactions are twice large as repulsive two-body interactions,CDT occurs at isolated points of driving force,which is similar to the linear case.Considering different interaction,the system can experience different transformation from coherent tunneling to coherent destruction of tunneling(CDT).The quasi-energy of the system as the function of the periodically driving force shows a triangular structure,which provides a deep insight into the tunneling dynamics of the system.     

Two Level Systems Driven by a Stochastic Perturbation

Here we consider a two level system driven by an external harmonic field whose amplitude is perturbed by a white noise term. In the limit of small splitting, dynamical localization, i.e. coherent destruction of tunneling, is proved for times of the order of 1/ε, where ε is the two-level splitting. The same type of localization is proved if the driving field is simply the white noise.     

Fractional quantum Hall states of atoms in optical lattices

We describe a method to create fractional quantum Hall states of atoms confined in optical lattices. We show that the dynamics of the atoms in the lattice is analogous to the motion of a charged particle in a magnetic field if an oscillating quadrupole potential is applied together with a periodic modulation of the tunneling between lattice sites. In a suitable parameter regime the ground state in the lattice is of the fractional quantum Hall type, and we show how these states can be reached by melting a Mott-insulator state in a superlattice potential. Finally, we discuss techniques to observe these strongly correlated states.     

Superresolution via enhanced evanescent tunneling

We here propose the concept of enhanced evanescent tunneling (EET). Our analysis indicates that by means of a suitable control field, the transmission of evanescent waves across a forbidden gap can be enhanced by several orders of magnitude-well beyond the ordinary frustrated total internal reflection case. We show how such a phenomenon can be used to probe both the amplitude and phase of the evanescent portion of the angular spectrum, thereby allowing target superresolution. In principle EET can be manifested in other areas of physics where wave tunneling is involved.     

Analysis of the performance of the quantum wire resonant tunneling field-effect transistor

We develop a theory of a resonant tunneling through a quantum wire placed in transverse electric field and show that transistor action can be caused via the electric-field-induced changes of the symmetry of electron wavefunctions. We evaluate the current–voltage characteristics, transconductance and speed of the proposed device and show how they can be further improved by means of bandgap engineering.     

Effect of time reversal symmetry breaking on the density of states in small superconducting grains

We show that in ultrasmall superconducting grains any concentration of magnetic impurities or infinitely small orbital effect of magnetic field leads to destruction of the hard gap in the tunneling density of states, and find analytically the exponential tail at low energies. Thus, the tunneling density of states exhibits the "soft gap" behavior. As the energy approaches zero, it vanishes linearly with excitation energy.     

Synthetic gauge fields for vibrational excitations of trapped ions

The vibrations of a collection of ions in a microtrap array can be described in terms of tunneling phonons. We show that the vibrational couplings may be tailored by using a gradient of the trap frequencies together with a periodic driving of the trapping potentials. These ingredients allow us to induce effective gauge fields on the vibrational excitations, such that phonons mimic the behavior of charged particles in a magnetic field. In particular, microtrap arrays are well suited to realize a quantum simulator of the famous Aharonov-Bohm effect and observe the paradigmatic edge states typical from quantum-Hall samples and topological insulators.     

Tunneling measurement of a single quantum spin

Measurement of the tunneling current of spin-polarized electrons via a molecule with a localized spin provides information on the orientation of that spin. We show that a strong tunneling current due to the shot noise suppresses the spin dynamics, such as the spin precession in an external magnetic field, and the relaxation due to the environment (quantum Zeno effect). A weak tunneling current preserves the spin precession with the oscillatory component of the current of the same order as the noise. We propose an experiment to observe the Zeno effect in a tunneling system and describe how the tunneling current may be used to read a qubit represented by a single spin 1/2.     

Nonlinear Landau-Zener tunneling under biharmonic driving

We address the response of a two-mode Bose-Einstein condensate to a biharmonic high-frequency driving field that directly couples the two mode. By use of a high-frequency approximation we find that asymmetric biharmonic high-frequency driving yields new effects on Nonlinear Landau-Zener tunneling as compared to a symmetric off-diagonal modulation. In particular, we detect an unclosed loop structure and the disappearance of the energy level below the loop structure.     

Theoretical aspects of electron transport in modulated structures

In the theory of transport in modulated structures we have studied both transport perpendicular and parallel to the heterojunction interfaces. In perpendicular transport we have investigated models for tunneling through double barriers and find that resonant tunneling and sequential tunneling lead to the same expression for the current as long as the width of the energy distribution of the injected electrons is larger than the width of the resonant level in the diode. We present results for phonon assisted tunneling between two wells in a model which remains valid even when the barrier shrinks and the tunneling probability becomes very high. In parallel transport we show that very satisfactory agreement with extensive measurements of the mobility in modulation doped structures in the whole temperature range from 4 K to 300 K can be obtained if one takes into account the complete quasi-two-dimensional subband structure and all the relevant scattering mechanisms. Having established this we apply this program to systems with more complicated double channel structures, and show how one can tailor the conductivity of a channel in which perpendicular resonant tunneling affects parallel transport.     

Visualization of coherent destruction of tunneling in an optical double well system

We report on a direct visualization of coherent destruction of tunneling (CDT) of light waves in a double well system which provides an optical analog of quantum CDT as originally proposed by Grossmann, Dittrich, Jung, and H?nggi [Phys. Rev. Lett. 67, 516 (1991)10.1103/PhysRevLett.67.516]. The driven double well, realized by two periodically curved waveguides in an Er:Yb-doped glass, is designed so that spatial light propagation exactly mimics the coherent space-time dynamics of matter waves in a driven double well potential governed by the Schr?dinger equation. The fluorescence of Er ions is exploited to image the spatial evolution of light in the two wells, clearly demonstrating suppression of light tunneling for special ratios between the frequency and amplitude of the driving field.     

Superradiance and Collective Gain in the Atom-Assisted Multimode Optomechanical System

We investigate that the muti-mode optomechanical system coupled with the two-level atoms. If the driving pump field is resonance with the anti-Stokes sideband, the system is at the superradiative state. For the driving filed in the Stokes sideband, the collective gain can be observed. We study a scheme that how the atomic medium affect these superradiance and collective gain. Our results show that the presence of the atom can enhance the superradiant behavior. In the mode splitting regime, the mode splits into thirds with the presence of the atoms with the anti-Stokes sideband. In addition, we also show that the use of atoms in this system could provide us a way to switch the system form superradiative state to collective gain.

     

Controlling quantum discord dynamics in cavity QED systems by applying a classical driving field with phase decoherence

We investigate a two-level atom interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence and find that a stationary quantum discord can arise in the interaction of the atom and cavity field as the time turns to infinity. We also find that the stationary quantum discord can be increased by applying a classical driving field. Furthermore, we explore the quantum discord dynamics of two identical non-interacting two-level atoms independently interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence. Results show that the quantum discord between two atoms is more robust than entanglement under phase decoherence and the classical driving field can help to improve the amount of quantum discord of the two atoms.     

Single-particle tunneling in strongly driven double-well potentials

We report on the first direct observation of coherent control of single-particle tunneling in a strongly driven double-well potential. In our setup atoms propagate in a periodic arrangement of double wells allowing the full control of the driving parameters such as frequency, amplitude, and even the space-time symmetry. Our experimental findings are in quantitative agreement with the predictions of the corresponding Floquet theory and are also compared to the predictions of a simple two mode model. Our experiments reveal directly the critical dependence of coherent destruction of tunneling on the generalized parity symmetry.     

Control of localization and suppression of tunneling by adiabatic passage

We show that a field of frequency omega combined with its second harmonic 2omega driving a double-well potential allows us to localize the wave packet by adiabatic passage, starting from the delocalized ground state. The relative phase of the fields allows us to choose the well of localization. We can suppress (and restore) the tunneling subsequently by switching on (and off) abruptly the fields at well-defined times. The mechanism relies on the fact that the dynamics is driven to an eigenstate of the Floquet Hamiltonian which is a localized state.