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We discuss the reversible storage of an entangled two-mode wavepacket in separated two atomic ensembles based on electromagnetically induced transparency. By introducing wavepacket dark-states, we show the dynamic transfer of quantum information of the wavepacket between the optical field and the atomic ensembles. We examine in detail the performance of the scheme focusing on the reversible storage of an entangled two-photon wavepacket in the case where the effects of the environment and the thermal motions of atoms are taken into consideration.

We discuss the generation and stabilization of entanglement between two distant atoms in a driven cascaded two-cavity system with homodyne-based feedback control. As the scheme works freely, the two atoms can fall into a steady state close to the Bell state $$\left| \varPhi _{+}\right\rangle $$?+ with an amount of entanglement inverse to its decay rate in a narrow parameter region. The homodyne feedback enlarges this region and makes the system robust against the unexpected imperfections.

This study proposes the first high-capacity quantum secure direct communication (QSDC) with two-photon six-qubit hyper-entangled Bell states in two longitudinal momentum and polarization degrees of freedom (DOFs) of photon pairs, which can be generated using two 0.5 mm-thick type-I β barium borate crystal slabs aligned one behind the other and an eight-hole screen. The secret message can be independently encoded on the photon pairs with 64 unitary operations in all three DOFs. This protocol has a higher capacity than previous QSDC protocols because each photon pair can carry 6 bits of information, not just 2 or 4 bits. Our QSDC protocol decreases the influence of decoherence from environment noise by exploiting the decoy photons to check the security of the transmission of the first photon sequence. Compared with two-way QSDC protocols, our QSDC protocol is immune to an attack by an eavesdropper using Trojan horse attack strategies because it is a one-way quantum communication. The QSDC protocol has good applications in the future quantum communication because of all these features.

We present some compact circuits for a deterministic quantum computing on the hybrid photon---atom systems, including the Fredkin gate and SWAP gate. These gates are constructed by exploiting the optical Faraday rotation induced by an atom trapped in a single-sided optical microcavity. The control qubit of our gates is encoded on the polarization states of the single photon, and the target qubit is encoded on the ground states of an atom confined in an optical microcavity. Since the decoherence of the flying qubit with atmosphere for a long distance is negligible and the stationary qubits are trapped inside single-sided microcavities, our gates are robust. Moreover, ancillary single photon is not needed and only some linear-optical devices are adopted, which makes our protocols efficient and practical. Our schemes need not meet the condition that the transmission for the uncoupled cavity is balanceable with the reflectance for the coupled cavity, which is different from the quantum computation with a double-sided optical microcavity. Our calculations show that the fidelities of the two hybrid quantum gates are high with the available experimental technology.

We propose a deterministic scheme for preparing any number of distant atoms in a W state. The atoms are respectively trapped in spatially separated optical cavities connected by the fibers. Our scheme utilizes the adiabatic passage along dark state, and is robust against the atomic spontaneous emission and the decays of cavities. The numerical simulation of entanglement fidelity shows that the losses of fibers can be negligible under certain conditions. All of the advantages make the protocol more practical than previous ones.

The light transmission properties through two-dimensional photonic crystal waveguides based on coupling of square cavities are studied by the finite-difference time-domain technique. Through interlacing the adjacent cavities along the direction vertical to the waveguide, the coupling distance between the adjacent cavities is extended, and the group velocity of the guiding modes can be slowed by five-fold compared with that in vacuum. Because of the different spatial field distributions of various resonant modes, the corresponding group velocities are also different for the same CROW structure.

We investigate the formation of discrete breathers (DBs) and the dynamics of the mixture of two-species Bose-Einstein condensates (BECs) in open boundary optical lattices using the discrete nonlinear Schr\"{o}dinger equations. The results show that the coupling of intra- and interspecies interaction can lead to the existence of pure single-species DBs and symbiotic DBs (i.e., two-species DBs). Furthermore, we find that there is a selective distillation phenomenon in the dynamics of the mixture of two-species BECs. One can selectively distil one species from the mixture of two-species BECs and can even control dominant species fraction by adjusting the intra- and interspecies interaction in optical lattices. Our selective distillation mechanism may find potential application in quantum information storage and quantum information processing based on multi-species atoms., Comment: 11 pages, 5 figures, one column

Simple schemes are proposed for implementing deterministic teleportation, superdense coding, and quantum information splitting with W-type entangled states. The physical realization of these schemes should be much simpler than previous ones due to the assistance of an auxiliary particle. We illustrate the ideas in cavity quantum electrodynamics. The important features of our schemes can also be demonstrated in other systems.

We study the teleportation of an unknown quantum state from a sender (Alice) to a receiver (Bob) via the control of many supervisors (Charlie 1, Charlie 2, . . .) in a network. It has been shown that such a task can be achieved by distributing a GHZ-type entangled state among the participants in advance. In the protocols with GHZ-type entanglement channel, the achievement of teleportation between Alice and Bob is conditioned on the cooperation of all the supervisors. In other words, if anyone of the supervisors does not cooperate, the teleportation will fails. In this paper, we introduce another kind of controlled teleportaton protocol with other types of entangled states acting as the quantum channel, in which the teleportation between Alice and Bob can be realized with high degree of endurance against uncooperation of part of supervisors.