ShengLi Zhang

Linear optical implementation of the two-qubit controlled phase gate with conventional photon detectors

Thermal entanglement of a two-qubit Heisenberg chain in presence of the Dzyaloshinski-Moriya (DM) anisotropic antisymmetric interaction and entanglement teleportation when using two independent Heisenberg chains as quantum channel are investigated. It is found that the DM interaction can excite the entanglement and teleportation fidelity. The output entanglement increases linearly with increasing value of input one, its dependences on the temperature, DM interaction and spin coupling constant are given in detail. Entanglement teleportation will be better realized via antiferromagnetic spin chain when the DM interaction is turned off and the temperature is low. However, the introduction of DM interaction can cause the ferromagnetic spin chain to be a better quantum channel for teleportation. A minimal entanglement of the thermal state in the model is needed to realize the entanglement teleportation regardless of antiferromagnetic or ferromagnetic spin chains.

Local Gaussian operations can enhance continuous-variable entanglement distillation

Entanglement distillation is a fundamental building block in long-distance quantum communication. Though known to be useless on their own for distilling Gaussian entangled states, local Gaussian operations may still help to improve non-Gaussian entanglement distillation schemes. Here we show that by applying local squeezing operations both the performance and the efficiency of existing distillation protocols can be enhanced. We find that such an enhancement through local Gaussian unitaries can be obtained even when the initially shared Gaussian entangled states are mixed, as, for instance, after their distribution through a lossy-fiber communication channel.

Limitation of decoy-state Scarani-Acin-Ribordy-Gisin quantum-key-distribution protocols with a heralded single-photon source

For the Bennett-Brassard 1984 (BB84) quantum key distribution, longer distance and higher key generating rate is shown with a heralded single-photon source (HSPS) [Phys. Rev. A. 73, 032331 (2006)]. In this paper, the performance of the Scarani-Acin-Ribordy-Gisim (SARG) protocol utilizing the HSPS sources is considered and the numerical simulation turns out that still a significant improvement in secret key generating rate can also be observed. It is shown that the security distance for HSPS+SARG is 120 km. However, compared with the HSPS+BB84 protocols, the HSPS+SARG protocol has a lower secret key rate and a shorter distance. Thus we show the HSPS+BB84 implementation is a preferable protocol for long distance transmittance.

Cavity-QED-based phase gate for photonic qubits

We present a two-qubit phase gate for photonic qubits using a three-level atom trapped in an optical cavity. In the scheme, the two ground states are coupled to the excited state by two cavity fields, respectively, and one of them is driven by a classical laser field. It is shown that, based on the large-detuning condition, the decay of atomic spontaneous emission can be effectively suppressed. The influence of cavity decay on our scheme is also investigated.

Roles of thermal noise and detector efficiency in distillation of continuous variable entanglement state

We address the degradation of continuous variable (CV) entanglement distillation in the presence of thermal environment and nonunity detecting efficiency. We explore the logarithmic negativity as a criteria and compare the output entanglement between the schemes of single-photon subtraction (PS) and two-PS. The probability of success is also investigated. We show that compared with detector efficiency, thermal noise plays a more dominant role in Gaussian entanglement distillation. Our results shed more light on the noise mechanism that influences realistic CV entanglement distillation and is of experimental relevance in developing a more efficient entanglement distillation scheme.

Distillation of phase-damped continuous-variable entanglement with photon subtraction

Photon subtraction (PS) is a widely used technique in the distillation of noise-damped continuous-variable Gaussian entangled states. In the literature, all distillation schemes with the PS technique are restricted to pure Gaussian entanglement or amplitude-damped entanglement, and it is not clear whether PS is applicable in overcoming the phase noise or even more general noise. In this paper, we show that the PS technique is versatile and can still be used to distill entanglement that is subject to phase noise and simultaneous phase and amplitude noise.

Characterization of photon statistics in a single-photon source via variable attenuation

We propose a simple but effective scheme for characterizing photon-number statistics of a practical single-photon source. In this scheme, the variable attenuation method which frequently appears in decoy state quantum cryptography is utilized here to enhance the estimation of photon-number statistics. A much stricter bound for vacuum and single-photon proportions is obtained and this result, in turn, is shown to be applicable to the unconditional secure quantum cryptographic communication with single-photon devices.

Local squeezing-assisted two-copy and single-copy continuous variable entanglement distillation

Entanglement distillation is a physically feasible operation which helps to partially or completely eliminate noise-induced decoherence in the distribution of entangled states. In this paper, we investigate the two-copy and single-copy distillation of an amplitude-damped continuous variable two-mode squeezed vacuum state. We consider local squeezing as one of the important tools [Phys. Rev. A84, 062309 (2011)] to enhance distillation. Our result is twofold. First, we show that local squeezing is useful to improve the distillation of two-copy interference-based distillation. Second, we show single-copy photon subtraction-based distillation is more efficient in providing more entangled outputs. However, such an advantage would diminish and two-copy distillation could succeed with much higher probability of success for a fixed amount of distilled entanglement.

Information–disturbance tradeoff in sending direction information via antiparallel quantum spin

When sending unknown direction information, antiparallel spins contain more direction information than parallel spins (Gisin and Popescu 1999 Phys. Rev. Lett. 83 432). In this paper, the optimal information?disturbance tradeoff bound for antiparallel spins is derived. The quantum measurements which attain the optimal tradeoff bound are obtained. This result can be of practical relevance for posing some general limits on Eves eavesdropping process. Finally, we also present a comparison between the bound for antiparallel spins and the bound for parallel spins.

Quantum circuit implementation of the optimal information-disturbance tradeoff of maximally entangled states

We give a direct derivation for the information–disturbance tradeoff in estimating a maximally entangled state, which was first obtained by Sacchi (2006 Phys. Rev. Lett. 96 220502) in terms of the covariant positive operator valued measurement (POVM) and Jamiolkowskis isomorphism. We find that, the Cauchy–Schwarz inequality, which is one of the most powerful tools in deriving the tradeoff for a single-particle pure state still plays a key role in the case of the maximal entanglement estimation. Our result shows that the inequality becomes equality when the optimal tradeoff is achieved. Moreover, we demonstrate that such a tradeoff is physically achievable with a quantum circuit that only involves single- and two-particle logic gates and single-particle measurements.