Li Jia-Hua

Optical bistability via quantum interference in a four-level atomic medium

We theoretically investigate optical bistability (OB) behaviour in a four-level atomic system confined in a unidirectional ring cavity. It is shown that OB can be controlled by adjusting the intensity of the coupling field and the value of the cooperativity parameter. More importantly, our numerical results show that the threshold intensity of OB can be reduced without decreasing the range of the hysteresis loop via adjusting the effect of spontaneously generated coherence (SGC) or the relative phase between two applied fields. The influence of the frequency detuning of the coupling field on the OB behaviour is also discussed.

Laser-polarization-dependent spontaneous emission of the zero phonon line from single nitrogen–vacancy center in diamond

We investigate spontaneous emission properties and control of the zero phonon line (ZPL) from a diamond nitrogen–vacancy (NV) center coherently driven by a single elliptically polarized control field. We use the Schrodinger equation to calculate the probability amplitudes of the wave function of the coupled system and derive analytical expressions of the spontaneous emission spectra. The numerical results show that a few interesting phenomena such as enhancement, narrowing, suppression, and quenching of the ZPL spontaneous emission can be realized by modulating the polarization-dependent phase, the Zeeman shift, and the intensity of the control field in our system. In the dressed-state picture of the control field, we find that multiple spontaneously generated coherence arises due to three close-lying states decaying to the same state. These results are useful in real experiments.

Remote Preparation of an Entangled State in Nonideal Conditions

In this paper, the theoretical investigation of remote preparation of an entangled state is studied in nonideal conditions. Our studies include two parts. In the first part, we consider the remote state preparation (RSP) of an entangled state through two equally noisy quantum channel states, namely, a mixture of Bell states. Studies show there is a particular mixed-state channel for which all pure entangled states remain entangled after this inexact RSP. In the second part, we suppose that noises which quantum channels suffer from can be expressed as the Lindblad operators. The master equation of the system can be expressed in the Lindblad form. Through solving the master equation, we calculate the fidelity as a function of decoherence rates and parameters of the state to be prepared. For a given entangled state, we investigate the influence of different types of noises on the fidelity.

Trace distance of remote state preparation through noisy channels

In this paper, we study remote state preparation (RSP) by w state through noisy channels. The trace distance is used to describe how close the original state is to the output state. Studies show the trace distance is a function of decohenrence rates and angles of the state to be prepared. At the same time, we investigate the influence of different types of noises on the trace distance and find the various types of noises have different degrees of influence on the trace distance for a definite qubit. We also study changes of the trace distance against polar and azimuthal angles.

Controllable Optical Bistability and Multistability in a Four-Level Atomic System with Closed-Loop Configuration

We theoretically investigate optical bistability (OB) and multistability (OM) behaviour of a closed-loop configuration atomic system driven by a degenerate coupling field and a degenerate probe field inside a unidirectional ring cavity. It is found that the OB and OM behaviour can be controlled by adjusting the intensity and the frequency detuning of the coupling field, respectively. Interestingly, our numerical results show that it is easy to realize the transition from OB to OM or vice versa by adjusting the intensity of the coupling field under a appropriate frequency detuning. The effect of the atomic cooperation parameter on the OB behaviour is also discussed.

Cluster States from Quantum Logic Gates with Trapped Ions in Thermal Motion

Following the recent proposal by Briegel et al. [Phys. Rev. Lett. 86 (2001) 910], a procedure is proposed for one-step realizing quantum control phase gates with two trapped ions in thermal motion. It is shown that the scheme can also be used to create a new special type of entangled states, i.e., cluster states of many trapped ions. In the scheme the two-trapped ions are simultaneously excited by a single laser beam and the frequency of the laser beam is slightly off resonance with the first lower vibration sideband of the trapped ions. The distinct advantage of the scheme is that it does not use the vibrational mode as the data bus. Furthermore, our scheme is insensitive to both the initial motional state and heating (or decay) as long as the system remains in the Lamb–Dicke regime.

Efficient Scheme for Greenberger–Horne–Zeilinger State and Cluster State with Trapped Ions

We propose a scheme to generate the Greenberger?Horne?Zeilinger (GHZ) states and the cluster states of many trapped ions. In the scheme, the ion is illuminated by a single laser tuned to the first lower vibrational sideband. The scheme only requires resonant interactions. Thus the scheme is very simple and the quantum dynamics operation can be realized at a high speed, which is important in view of decoherence.

Proposal for quantum entanglement of six photons

We propose a different scheme to achieve six-photon entangled states based entirely on the concept of quantum erasure. To begin with, a scheme for making use of a group of four entangled photons to generate six-photon entangled states is presented. Then, with the same technique, the preparation of the six-photon entanglement from five-particle entanglement which is then combined with Bell states is considered. Our experimental methods can be used for the investigations of measurement-based quantum computation and multi-party quantum communication. We find that the success probability is determined by the small coefficients of the entangled states.

Control of Spontaneous Emission via a Single Elliptically Polarized Light in a Five-Level Atomic System

We investigate the features of the spontaneous emission spectra in a cold five-level atomic system coupled by a single elliptically polarized control field. We use wave function approach to derive the explicit and analytical expressions of atomic spontaneous emission spectra. It is shown that some interesting phenomena such as spectral-line enhancement, spectral-line suppression, spectral-line narrowing, spectral-line splitting and dark fluorescence can be observed in the spectra by appropriately modulating the phase difference between the right-hand circularly (LHC) and left-hand circularly (RHC) polarized components of the elliptically polarized control field and the intensity of external magnetic field. The number of emission peaks, the positions of fluorescence-quenching points can be also controlled. Furthermore, we propose an ultracold 87Rb atomic system for experimental observation. These investigations may find applications in high-precision spectroscopy.

The effect of intersubsystem coupling on the geometric phase in a composite system driven by decohering quantized fields

We investigate the behaviour of the geometric phase of a bipartite system with intersubsystem coupling via a quantum jump approach, where one of the subsystems is driven by a one- or two-mode quantized field which is also subjected to decoherence. The results show that the first correction due to the decohering field is only quadratic in decaying rate λ when the evolution of the system is adiabatic, and the lowest order correction is related to the first order in λ when the evolution of the system is non-adiabatic. Moreover, an interesting interpretation of this phenomenon is given.