Guo-Qin Ge

Protecting quantum coherence and discord from decoherence of depolarizing noise via weak measurement and measurement reversal

Protection of the quantum coherence and discord in realistic quantum systems interacting with the environment of depolarizing noise is an important subject in quantum information processing. Weak measurement and measurement reversal can effectively suppress the amplitude damping-class decoherence. In this paper, we examine the effect of this protocol in the protection of quantum coherence and discord subjected to depolarizing noise environments. Our scheme consists of a prior weak measurement on each qubit before interacting with noisy channels followed by post measurement reversal. It is found that quantum coherence and discord can be enhanced to an optimal value by performing weak measurements and adjusting measurement parameters on each qubit. In addition, the maximal value of the quantum coherence and discord is found to be independent of the initial-state parameters.

Enhancement of Quantum Correlations in Qubit-Qutrit Systems under the non-Markovian Environment

We investigate the time evolution of quantum correlations of a hybrid qubit-qutrit system under the classical Ornstein-Uhlenbeck (OU) noise. Here we consider two different one-parameter families of qubit-qutrit states which independently interact with the non-Markovian reservoirs. A comparison with the Markovian dynamics reveals that for the same set of initial condition parameters, the non-Markovian behavior of the environment plays an important role in the enhancement of the survival time of quantum correlations. In addition, it is observed that the non-Markovian strength has a positive impact on the correlations time. For the initial separable states it is found that there is a finite time interval in which the geometric quantum discord is frozen despite the presence of a noisy environment and that interval can be further prolonged by using the non-Markovian property. Moreover, its decay can be significantly delayed.

Coexistence of three-wave, four-wave, and five-wave mixing processes in a superconducting artificial atom.

We present a theoretical study of multiwave mixing in a driven superconducting quantum qubit (artificial atom) with a cyclic Ξ-type three-level structure. We first show that three-wave mixing (3WM), four-wave mixing (4WM), and five-wave mixing (5WM) processes can coexist in the microwave regime in such an artificial system due to the absence of selection rules. Because of electromagnetically induced transparency suppression of linear absorption in a standard Ξ-type configuration, the generated 4WM is enhanced greatly and its efficiency can be as high as 0.1% for only a single artificial atom. We also show that Autler-Townes splitting occurs in the 3WM and 5WM spectra and quantum interference has a significant impact on the total signal intensity being a coherent superposition of these two signals.

On the tunneling time of ultracold atoms through a system of two mazer cavities

We study the resonant tunneling of ultraslow atoms through a system of high quality microwave cavities. We find that the phase tunneling time across the two coupled cavities exhibits more frequent resonances as compared to the single cavity interaction. The increased resonances are instrumental in the display of an alternate sub and superclassical character of the tunneling time along the momentum axis with increasing energies of the incident slow atoms. Here, the intercavity separation appears as an additional controlling parameter of the system that provides an efficient control of the superclassical behavior of the phase tunneling time. Further, we find that the phase time characteristics through two cavity system has the combined features of the tunneling through a double barrier and a double well arrangements.

Tunable surface plasmon polaritons at the surfaces of nanocomposite media

Surface plasmon polaritons (SPPs) at the interfaces of composite media have some important properties not found in conventional SPPs, i.e. , SPPs at metal-dielectric interfaces. We present a useful scheme to control basic features of SPPs at the interface between a tripod-type atomic system and a silver-silica (AgSiO2 ) composite film. There is some interaction between the SPPs at the interface and localized surface plasmons (LSPs) due to Ag-nanoparticles in the composite. We show that the SPPs dispersive properties are strongly dependent on the coherent driving fields applied in the atomic system. Similarly, the filling ratio of the nanoparticles in the composite and the incident wavelength also affect different features of the SPPs. Our model provides more degrees of freedom for tuning the fundamental properties of SPPs at the interfaces associated with composite media.

Nonlinear manipulation of tunable microwave amplification and attenuation in superconducting circuits

We demonstrate the controllable nonlinear microwave modulation in a cyclically driven three-level superconducting Josephson system. By designing two subtle matched conditions in the -type atom-field configuration, a new physical mechanism – combined action of nonlinear wave mixing and wave interference – is developed and leads to not only amplification but also attenuation for two microwave signals. Our results show that such a nonlinear manipulation of the signal transition from enhancement to damping can be tuned in a large scope by controlling the relative phase and the driving-field frequency and thus the solid-state Josephson system can act as a phase- and frequency-controlled amplitude modulator. Our study opens up a fascinating perspective for its widespread applications in nonlinear optics and quantum information science.