Yi-Bo Zhao

Multiple stochastic paths scheme on partially-trusted relay quantum key distribution network

Quantum key distribution (QKD) technology provides proven unconditional point-to-point security based on fundamental quantum physics. A QKD network also holds promise for secure multi-user communications over long distances at high-speed transmission rates. Although many schemes have been proposed so far, the trusted relay QKD network is still the most practical and flexible scenario. In reality, the insecurity of certain relay sections cannot be ignored, so to solve the fatal security problems of partially-trusted relay networks we suggest a multiple stochastic paths scheme. Its features are: (i) a safe probability model that may be more practical for real applications; (ii) a multi-path scheme with an upper bound for the overall safe probability; (iii) an adaptive stochastic routing algorithm to generate sufficient different paths and hidden routes. Simulation results for a typical partially-trusted relay QKD network show that this generalized scheme is effective.

Decoy states for quantum key distribution based on decoherence-free subspaces

Quantum key distribution with decoherence-free subspaces has been proposed to overcome the collective noise to the polarization modes of photons flying in quantum channel. Prototype of this scheme have also been achieved with parametric-down conversion source. However, a novel type of photon-number-splitting attack we proposed in this paper will make the practical implementations of this scheme insecure since the parametric-down conversion source may emit multi-photon pairs occasionally. We propose decoy states method to make these implementations immune to this attack. And with this decoy states method, both the security distance and key bit rate will be increased.

Quantum repeaters free of polarization disturbance and phase noise

Original quantum repeater protocols based on the single-photon interference suffer from the phase noise of the channel, which makes the long-distance quantum communication infeasible. Fortunately, two-photon interferencelike quantum repeaters can be immune to the phase noise of the channel. However, this type of quantum repeaters may still suffer from polarization disturbance of the channel. Here we propose a quantum repeaters protocol, which is free of the polarization disturbance of the channel based on the invariance of the antisymmetric Bell state

Apply current exponential de Finetti theorem to realistic quantum key distribution

|{\ensuremath{\psi}}^{\ensuremath{-}}⟩=(|H⟩|V⟩\ensuremath{-}|V⟩|H⟩)/\sqrt{2}

Computational Complexity of Continuous Variable Quantum Key Distribution

under collective noise. Our protocol is also immune to the phase noise with the Sagnac interferometer configuration. Through single-atom cavity-QED technology and linear optics, this scheme can be implemented easily.

Security proof of differential phase shift quantum key distribution in the noiseless case

In the realistic quantum key distribution (QKD), Alice and Bob respectively get a quantum state from an unknown channel, whose dimension may be unknown. However, while discussing the security, sometime we need to know exact dimension, since current exponential de Finetti theorem, crucial to the information-theoretical security proof, is deeply related with the dimension and can only be applied to finite dimensional case. Here we address this problem in detail. We show that if POVM elements corresponding to Alice and Bobs measured results can be well described in a finite dimensional subspace with sufficiently small error, then dimensions of Alice and Bobs states can be almost regarded as finite. Since the security is well defined by the smooth entropy, which is continuous with the density matrix, the small error of state actually means small change of security. Then the security of unknown-dimensional system can be solved. Finally we prove that for heterodyne detection continuous variable QKD and differential phase shift QKD, the collective attack is optimal under the infinite key size case.

Field experiment on a robust hierarchical metropolitan quantum cryptography network

The continuous variable quantum key distribution has been considered to have the potential to provide high secret key rate. However, in present experimental demonstrations, the secret key can be distilled only under very small loss rates. Here, by calculating explicitly the computational complexity with the channel transmission, we show that under high loss rate it is hard to distill the secret key in present continuous variable scheme and one of its advantages, the potential of providing high secret key rate, may therefore be limited.

Gigahertz sine-wave gate-control low-pass filtering ultrared single-photon detector

Differential phase shift quantum key distribution systems have a high potential for achieving high speed key generation. However, its unconditional security proof is still missing, even though it has been proposed for many years. Here, we prove its security against collective attacks with a weak coherent light source in the noiseless case (i.e. no bit error). The only assumptions are that quantum theory is correct, the devices are perfect and trusted and the key size is infinite. Our proof works on threshold detectors. We compute the lower bound of the secret key generation rate using the information‐theoretical security proof method. Our final result shows that the lower bound of the secret key generation rate per pulse is linearly proportional to the channel transmission probability if Bob’s detection counts obey the binomial distribution.