ChenHui Jin

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.

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.

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.