Abstract:
It is shown that, through a super-radiant Rayleigh scattering, a strong far off-resonant pump laser applied to a Bose-Einstein condensates(BEC) can induce a non-demolition coupling of the many-mode quantized vacuum field to the BEC. This effective interaction will force the total system of the BEC plus the light field to evolve from a factorized initial state to an ideal entangled state and thus result in the quantum decoherence in the BEC. Since the effective coupling coefficients are mainly determined by the Rabi frequency of the pump laser, the quantum decoherence process can be controlled by adjusting the intensity of the pump laser. To study the physical influence of decoherence on the BEC, we investigate how the coherent tunneling of BEC in a well-separated tight double wall is suppressed by the effectively-entangled vacuum modes.

Abstract:
The two photon interference phenomenon is theoretically investigated for the general situations with an arbitrary input two photon state with and without photon polarization. For the case without polarization, the necessary-sufficient condition for the destructive interference of coincidence counting is given as the symmetric pairing of photons in the light pulses. For both case it is shown that the "dip" in coincidence curve can be understood in terms of the free induction decay mechanism. This observation predicts the destructive interference phenomenon to occur even for certain cases with separable input two photon state, but it can only be explained in terms of "the two photon (not two photons)interference ".

Abstract:
We study the dynamical process of disentanglement of two qubits and two qutrits coupled to an Ising spin chain in a transverse field, which exhibits a quantum phase transition. We use the concurrence and negativity to quantify entanglement of two qubits and two qutrits, respectively. Explicit connections between the concurrence (negativity) and the decoherence factors are given for two initial states, the pure maximally entangled state and the mixed Werner state. We find that the concurrence and negativity decay exponentially with fourth power of time in the vicinity of critical point of the environmental system.

Abstract:
We study how to recover the unitarity of Lee model with the help of bi-orthogonal basis approach, when the physical coupling constant in renormalization exceeds its critical value, so that the Lee's Hamiltonian is non-Hermitian with respect to the conventional inner product. In a very natural fashion, our systematic approach based on bi-orthogonal basis leads to an elegant definition of inner product with a non-trivial metric, which can overcome all the previous problems in Lee model, such as non-Hermiticity of the Hamiltonian, the negative norm, the negative probability and the non-unitarity of the scattering matrix.

Abstract:
We study two-photon scattering in a one-dimensional coupled resonator arrays (CRA) by a two-level system (TLS), which is localized as a quantum controller. The $S$-matrix is analytically calculated for various two-photon scattering processes by TLS, e.g., one photon is confined by TLS to form a bound state while the other is in the scattering state. It is discovered from the poles of the $S$-matrix that there exist two kinds of three-body bound states for describing two bound photons localized around TLS.

Abstract:
We present a quantum field theoretical approach based on the Lehmann-Symanzik-Zimmermann reduction for the multi-photon scattering process in a nano-architecture consisting of the coupled resonator arrays (CRA), which are also coupled to some artificial atoms as the controlling quantum node. By making use of this approach, we find the bound states of single photon for an elementary unit, the T-type CRA, and explicitly obtain its multi-photon scattering S-matrix in various situations. We also use this method to calculate the multi-photon S-matrices for the more complex quantum network constructed with main T-type CRA's, such as a H-type CRA waveguide.

Abstract:
We study the $\Lambda$-atoms ensemble based quantum memory for the storage of the quantum information carried by a probe light field. Two atomic Rabi transitions of the ensemble are coupled to the quantum probe field and classical control field respectively with a same detuning. Making use of the hidden symmetry analysis developed recently for the on-resonance EIT case (Sun, Li, and Liu, Phys. Rev. Lett. 91, 147903 (2003)), we show that the dark states and dark-state polaritons can still exist for the case of two-photon resonance EIT. Starting from these dark states we construct a complete class of eigen-states of the total system. A explicit form of the adiabatic condition is also given in order to achieve the memory and retrieve of quantum information.

Abstract:
We study the propagation of a probe light in an ensemble of $\Lambda$-type atoms, utilizing the dynamic symmetry as recently discovered when the atoms are coupled to a classical control field and a quantum probe field {[Sun {\it et al.,} Phys. Rev. Lett. {\bf 91}, 147903 (2003)]}. Under two-photon resonance, we calculate the group velocity of the probe light with collective atomic excitations. Our result gives the dependence of the group velocity on the common one-photon detuning, and can be compared with the recent experiment (E. E. Mikhailov, Y. V. Rostovtsev, and G. R. Welch, quant-ph/0309173).

Abstract:
By using a two-mode description, we show that there exist the multistability, phase transition and associated critical fluctuations in the macroscopic tunneling process between the halves of a double-well trap containing a Bose-Einstein condenstate. The phase transition that two of the triple stable states and a unstable state merge into one stable state or a reverse process takes place whenever the ratio of the mean field energy per particle to the tunneling energy goes across a critical value of order one. The critical fluctuation phenomenon corresponds to squeezed states for the phase difference between the two wells accompanying with large fluctuations of atom numbers.

Abstract:
The idea of quantum state storage is generalized to describe the coherent transfer of quantum information through a coherent data bus. In this universal framework, we comprehensively review our recent systematical investigations to explore the possibility of implementing the physical processes of quantum information storage and state transfer by using quantum spin systems, which may be an isotropic antiferromagnetic spin ladder system or a ferromagnetic Heisenberg spin chain. Our studies emphasize the physical mechanisms and the fundamental problems behind the various protocols for the storage and transfer of quantum information in solid state systems.