Cheng-Zu Li

Quantum repeaters based on Rydberg-blockade-coupled atomic ensembles

We propose a scheme for realizing quantum repeaters with Rydberg-blockade-coupled atomic ensembles, based on a recently proposed collective encoding strategy. Rydberg-blockade-mediated two-qubit gates and efficient cooperative photon emission are employed to create ensemble-photon entanglement. Thanks to deterministic entanglement swapping operations via Rydberg-based two-qubit gates, and to the suppression of multiexcitation errors by the blockade effect, the entanglement distribution rate of the present scheme is higher by orders of magnitude than the rates achieved by other ensemble-based repeaters. We also show how to realize temporal multiplexing with this system, which offers an additional speedup in entanglement distribution.

Generating polarization-entangled W states using weak nonlinearity

We present a scheme for preparing three-photon polarization-entangled W states. The signal photons and a strong probe field interact via weak cross-Kerr nonlinearity that conditionally causes a phase shift on the probe coherent state. A W state is then prepared with a high success probability by the homodyne measurement and postselection. The generating method is simple and efficient.

Quantum key distribution based on phase encoding in long-distance communication fiber

A robust two-way quantum key distribution system based on phase encoding is demonstrated in 50 km and 100 km commercial communication fiber. The system can automatically compensate for birefringence effects and remain stable over 23 h. A low quantum bit error rate and high visibility are obtained. Furthermore, the storage fiber is unnecessary and train of pulses is only needed in the test with 100 km fiber.

Quantum coherence effects in a four-level diamond-shape atomic system

Abstract A symmetric four-level closed-loop ♢ type (the diamond-shape) atomic system driven by four coherent optical fields is investigated. The system shows rich quantum interference and coherence features. When symmetry of the system is broken, interesting phenomena such as single and double-dark resonances appear. As a result, the controllable double electromagnetically induced transparency (EIT) effect is generated, which will facilitate the implementation of quantum phase gate (QPG) operation.

A lower bound on entanglement of formation of 2⊗n system

Abstract A lower bound on entanglement of formation of 2⊗ n systems is given by decomposing an 2× n dimension Hilbert space into many 2×2 dimension subspace. We also compared this lower bound with the relative entropy entanglement. The possible generalization was discussed simply.

Scalable one-way quantum computer using on-chip resonator qubits

College of Science, National University of Defense Technology, Changsha 410073, People’s Republic of China(Dated: November 14, 2011)We propose a scalable and robust architecture for one-way quantum computation using couplednetworks of superconducting transmission line resonators. In our protocol, quantum information isencoded into the long-lived photon states of the resonators, which have a much longer coherencetime than the usual superconducting qubits. Each resonator contains a charge qubit used for thestate initialization and local projective measurement of the photonic qubit. Any pair of neighboringphotonic qubits are coupled via a mediator charge qubit, and large photonic cluster states can becreated by applying Stark-shifted Rabi pulses to these mediator qubits. The distinct advantage ofour architecture is that it combines both the excellent scalability of the solid-state systems and thelong coherence time of the photonic qubits. Furthermore, this architecture is very robust againstthe parameter variations.

Coherence induced by incoherent pumping field and decay process in three-level Λ type atomic system

Abstract Following the method proposed by Kozlov et al. [Victor V. Kozlov, Yuri Rostovtsev, Marlan O. Scully, Phys. Rev. A 74 (2006) 063829], we have investigated the atomic coherence induced by incoherent pump and vacuum spontaneous decay process in a Λ type three-level atomic system. The system can be in a coherent population trapping state and multi-steady states in different conditions. Interestingly, two kinds of new states are derived from the system with different pumping rate and decaying rate. They are the “robust” steady state and the “weak” steady state. Under the action of pump field and vacuum reservoir, these two kinds of states exhibit stable or unstable characteristics, respectively. Moreover, by investigating the difference between these states, we reveal the mechanism of coherence excitation and level-population transition. The special feature of the Λ atomic system will promise fruitful applications in quantum optics.

Robust quantum repeater with atomic ensembles against phase and polarization instability

We propose an alternative scheme for quantum repeater without phase stabilization and polarization calibration of photons transmitted over long-distance channel. We introduce time-bin photonic states and use a two-photon interference configuration to robustly generate entanglement between distant atomic-ensemble-based memory qubits. Our scheme can be performed with current experimental setups through making some simple adjustments.

Generation of Bell states of two cavities based on interactions with a single atom

This paper presents a scheme to generate some Bell states of two orthogonal polarized modes in two separated cavities deterministically. The fidelity of the generated state is less than 1, which is mainly caused by fluctuations of interaction time between atoms and cavities, but the value of the fidelity is within our practical interest.

Necessary and sufficient condition of separability of any system

The necessary and sufficient condition of separability of a mixed state of any systems is presented, which is practical in judging the separability of a mixed state. This paper also presents a method of finding the disentangled decomposition of a separable mixed state.