General theory for decoy-state quantum key distribution with arbitrary number of intensities

We develop a general theory for quantum key distribution (QKD) in both the forward error correction and the reverse error correction cases when the QKD system is equipped with phase-randomized coherent light with arbitrary number of decoy intensities. For this purpose, generalizing Wang's expansion, we derive a convex expansion of the phase-randomized coherent state. We also numerically check that the asymptotic key generation rates are almost saturated when the number of decoy intensities is three.Comment: This manuscript has been revised extensivel

Controlling passively-quenched single photon detectors by bright light

Single photon detectors based on passively-quenched avalanche photodiodes can be temporarily blinded by relatively bright light, of intensity less than a nanowatt. I describe a bright-light regime suitable for attacking a quantum key distribution system containing such detectors. In this regime, all single photon detectors in the receiver Bob are uniformly blinded by continuous illumination coming from the eavesdropper Eve. When Eve needs a certain detector in Bob to produce a click, she modifies polarization (or other parameter used to encode quantum states) of the light she sends to Bob such that the target detector stops receiving light while the other detector(s) continue to be illuminated. The target detector regains single photon sensitivity and, when Eve modifies the polarization again, produces a single click. Thus, Eve has full control of Bob and can do a successful intercept-resend attack. To check the feasibility of the attack, 3 different models of passively-quenched detectors have been tested. In the experiment, I have simulated the intensity diagrams the detectors would receive in a real quantum key distribution system under attack. Control parameters and side effects are considered. It appears that the attack could be practically possible.Comment: Experimental results from a third detector model added. Minor corrections and edits made. 11 pages, 10 figure

Feasibility of 300 km quantum key distribution with entangled states

A significant limitation of practical quantum key distribution (QKD) setups is currently their limited operational range. It has recently been emphasized (Ma et al 2007 Phys. Rev. A 76 012307) that entanglementbased QKD systems can tolerate higher channel losses than systems based on weak coherent laser pulses (WCP), in particular, when the source is located symmetrically between the two communicating parties, Alice and Bob. In the work presented here, we experimentally study this important advantage by implementing different entanglement-based QKD setups on a 144 km free-space link between the two Canary Islands of La Palma and Tenerife. We established three different configurations where the entangled photon source was placed at Alice's location, asymmetrically between Alice and Bob and symmetrically in the middle between Alice and Bob, respectively. The resulting quantum channel attenuations of 35, 58 and 71 dB, respectively, significantly exceed the limit for WCP systems (Ma et al 2007 Phys. Rev. A 76 012307). This confirms that QKD over distances of 300 km and even more is feasible with entangled state sources placed in the middle between Alice and Bob. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft