Zheng-Fu Han

Faraday–Michelson system for quantum cryptography

Quantum key distribution provides unconditional security for communication. Unfortunately, current experiment schemes are not suitable for long-distance fiber transmission because of instability or backscattering. We present a uni-directional intrinsic-stabilization scheme that is based on Michelson-Faraday interferometers, in which reflectors are replaced with 90 degree Faraday mirrors. With the scheme, key exchange from Beijing to Tianjin over 125 kilometers with an average error rate is below 6% has been achieved and its limited distance exceeds 150 kilometers. Experimental result shows the system is insensitive to environment and can run over day and night without any break even in the noise workshop.Quantum key distribution provides unconditional security for communication. Unfortunately, current experimental schemes are not suitable for long-distance fiber transmission because of phase drift or Rayleigh backscattering. In this Letter we present a unidirectional intrinsically stable scheme that is based on Michelson-Faraday interferometers, in which ordinary mirrors are replaced with 90 degree Faraday mirrors. With the scheme, a demonstration setup was built and excellent stability of interference fringe visibility was achieved over a fiber length of 175 km. Through a 125 km long commercial communication fiber cable between Beijing and Tianjin, the key exchange was performed with a quantum bit-error rate of less than 6%, which is to our knowledge the longest reported quantum key distribution experiment under field conditions.

Realizing quantum controlled phase flip through cavity QED

We propose a scheme to realize quantum controlled phase flip (CPF) between two rare-earth ions embedded in the respective microsphere cavity via interacting with a single-photon pulse in sequence. The numerical simulations illuminate that the CPF gate between ions is robust and scalable with extremely high fidelity and low error rate. Our scheme is more applicable than other schemes presented before based on current laboratory cavity-QED technology, and it is possible to be used as an applied unit gate in future quantum computation and quantum communication.

Field Experiment on a “Star Type” Metropolitan Quantum Key Distribution Network

Quantum key distribution (QKD) networks have recently attracted growing attention. The topology of the local QKD network is the basis of the next-generation global secure communication network. In this letter, we report a realization of a wavelength-routing star type QKD network which can span a metropolis using a commercial backbone optical fiber network without trusted relays. The longest and the shortest fiber lengths between two geographically separated nodes are 42.6 and 32 km, respectively, and the maximum average quantum bit-error rate is below 8%. A novel analysis model with experimental validation is also proposed to evaluate the users performance in this network under the condition of maximum multiuser crosstalk.

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

Computational Complexity of Continuous Variable Quantum Key Distribution

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

Generation of multi-atom Dicke states through the detection of cavity decay

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.

Fiber-taper-coupled zeolite cylindrical microcavity with hexagonal cross section.

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.

Quantum computation without strict strong coupling on a silicon chip

We propose a scheme to create W states, and further generalize universal Dicke states of n largely detuned atoms, through detecting the leaky polarized photons from an optical cavity. The generation of entangled states in our scheme has a very high success probability since the atomic spontaneous emission is strongly suppressed. The scheme has potential practicability based on near coming laboratory cavity QED technology.

A frequency-coded quantum key distribution scheme

Whispering-gallery modes (WGMs) in a zeolite cylinder have been effectively coupled with a low-loss fiber taper. The fiber transmission spectrum directly shows the WGM distribution, which agrees well with the theoretical prediction based on geometric optics. Due to other scattering and absorbing mechanisms, the measured quality factors of the WGMs are limited to approximately 800. This result shows that the fiber taper provides a powerful tool for coupling WGMs of a zeolite cylinder, and this taper-coupled zeolite can be a potential microcavity system for the cavity quantum electrodynamics and the microlaser.