Production of entanglement in Raman three-level systems using feedback

We examine the theoretical limits of the generation of entanglement in a damped coupled ion-cavity system using jump-based feedback. Using Raman transitions to produce entanglement between ground states reduces the necessary feedback bandwidth, but does not improve the overall effect of the spontaneous emission on the final entanglement. We find that the fidelity of the resulting entanglement will be limited by the asymmetries produced by vibrations in the trap, but that the concurrence remains above 0.88 for realistic ion trap sizes.Comment: 8 pages, 8 figure

Screening of qubit from zero-temperature reservoir

We suggest an application of dynamical Zeno effect to isolate a qubit in the quantum memory unit against decoherence caused by coupling with the reservoir having zero temperature. The method is based on using an auxiliary casing system that mediate the qubit-reservoir interaction and is simultaneously frequently erased to ground state. This screening procedure can be implemented in the cavity QED experiments to store the atomic and photonic qubit states.Comment: 4 pages, 5 figure

State transfer in dissipative and dephasing environments

By diagonalization of a generalized superoperator for solving the master equation, we investigated effects of dissipative and dephasing environments on quantum state transfer, as well as entanglement distribution and creation in spin networks. Our results revealed that under the condition of the same decoherence rate $\gamma$, the detrimental effects of the dissipative environment are more severe than that of the dephasing environment. Beside this, the critical time $t_c$ at which the transfer fidelity and the concurrence attain their maxima arrives at the asymptotic value $t_0=\pi/2\lambda$ quickly as the spin chain length $N$ increases. The transfer fidelity of an excitation at time $t_0$ is independent of $N$ when the system subjects to dissipative environment, while it decreases as $N$ increases when the system subjects to dephasing environment. The average fidelity displays three different patterns corresponding to $N=4r+1$, $N=4r-1$ and $N=2r$. For each pattern, the average fidelity at time $t_0$ is independent of $r$ when the system subjects to dissipative environment, and decreases as $r$ increases when the system subjects to dephasing environment. The maximum concurrence also decreases as $N$ increases, and when $N\rightarrow\infty$, it arrives at an asymptotic value determined by the decoherence rate $\gamma$ and the structure of the spin network.Comment: 12 pages, 6 figure

Teleportation of the one-qubit state with environment-disturbed recovery operations

We study standard protocol $\mathcal{P}_0$ for teleporting the one-qubit state with both the transmission process of the two qubits constitute the quantum channel and the recovery operations performed by Bob disturbed by the decohering environment. The results revealed that Bob's imperfect operations do not eliminate the possibility of nonclassical teleportation fidelity provided he shares an ideal channel state with Alice, while the transmission process is constrained by a critical time $t_{0,c}$ longer than which will result in failure of $\mathcal{P}_0$ if the two qubits are corrupted by the decohering environment. Moreover, we found that under the condition of the same decoherence rate $\gamma$, the teleportation protocol is significantly more fragile when it is executed under the influence of the noisy environment than those under the influence of the dissipative and dephasing environments.Comment: 8 pages, 4 figure

Decoherence control in microwave cavities

We present a scheme able to protect the quantum states of a cavity mode against the decohering effects of photon loss. The scheme preserves quantum states with a definite parity, and improves previous proposals for decoherence control in cavities. It is implemented by sending single atoms, one by one, through the cavity. The atomic state gets first correlated to the photon number parity. The wrong parity results in an atom in the upper state. The atom in this state is then used to inject a photon in the mode via adiabatic transfer, correcting the field parity. By solving numerically the exact master equation of the system, we show that the protection of simple quantum states could be experimentally demonstrated using presently available experimental apparatus.Comment: 13 pages, RevTeX, 8 figure

State transfer in intrinsic decoherence spin channels

By analytically solving the master equation, we investigate quantum state transfer, creation and distribution of entanglement in the model of Milburn's intrinsic decoherence. Our results reveal that the ideal spin channels will be destroyed by the intrinsic decoherence environment, and the detrimental effects become severe as the decoherence rate $\gamma$ and the spin chain length $N$ increase. For infinite evolution time, both the state transfer fidelity and the concurrence of the created and distributed entanglement approach steady state values, which are independent of the decoherence rate $\gamma$ and decrease as the spin chain length $N$ increases. Finally, we present two modified spin chains which may serve as near perfect spin channels for long distance state transfer even in the presence of intrinsic decoherence environments $\mathcal {F}{[\rho(t)]}$.Comment: 11 pages, 11 figure

Finite-time destruction of entanglement and non-locality by environmental influences

Entanglement and non-locality are non-classical global characteristics of quantum states important to the foundations of quantum mechanics. Recent investigations have shown that environmental noise, even when it is entirely local in influence, can destroy both of these properties in finite time despite giving rise to full quantum state decoherence only in the infinite time limit. These investigations, which have been carried out in a range of theoretical and experimental situations, are reviewed here.Comment: 27 pages, 6 figures, review article to appear in Foundations of Physic