Long-distance Quantum Key Distribution System

Quantum cryptography provides a new method for absolutely secure communication, which is guaranteed by the inviolability of a Law of Nature, and any advanced technologies cannot decipher it. For the purpose to establish a practical long-distance quantum key distribution system, I tried in my thesis to find soluteions to some main obstacles of technologies and principles, such as generation of entangled photon source, single-photon detection, and secure & stable quantum key distribution protocols.We achieved concurrent parametric up-conversion and down-conversion, which were induced by non-collinear optical parametric amplification in a type-I BBO crystal. The observed phenomena are actually spontaneous photonic cascades due to four-wave mixing in the nonlinear optical crystal, leading to the generation of entangled rainbow pairs consisted of ultraviolet and visible photons which exhibited angularly-resolved spectra of continuously tunable wavelengths. We could distill multiple entangled photon pairs from the entangled rainbows with different wavelength. Based on the multi-wavelength entangled photon pairs, we put forward a scenario for an efficient quantum network.We devised a novel kind of spike-cancellation gated-mode single-photon detector, which produced a fake spike by a variable capacitance and cancelled the spikes in a differential network. With the spike cancellation, we achieved a high-performance near-infrared single-photon detector based on InGaAs/InP avalanche photodiode. Its ratio of dark-count probability to quantum efficiency reached, to the best of our knowledge, the lowest value as low as 1.7×10^-6 per detection pulse. On the basis of the experimental test on spike-cancellation gated-mode technique, the new design was further developed as stable and robust near-infrared single-photon detector as a commercial product of high performance and convenience in operation.We put forward and achieved quantum key distribution based on Sagnac interferometer, which was embodied in "Quantum cryptography roadmap" of Los Alamos. And a Plug&Play quantum key distribution system was realized in a 50 km fiber with the mean photon number of 0.1/pulse, exhibiting a long-term stability up to 12 h and a qubit error rate less than 4%. In the Plug&Play system, we achieved 155 km single-photon routing with the fringe contrast of 87% by using our high-performance single-photon detector.The environmental stress and temperature change may induce random changes of fiber birefringence, which make it impossible to keep a polarization state stable in long-distance fiber. We developed a feedback control at single-photon level to compensate for unpredictable polarization scrambling in long-distance fiber. Polarizations of single-photon pulses have been controlled with long-term stability of more than 10 hours in long-distance fibers. Experimental tests of long-term operations in 50, 75 and 100 km fibers demonstrated that such a single-photon polarization control supported stable polarization encoding in long-distance fibers to facilitate stable "one-way" fiber system for polarization-encoded quantum key distribution up to 100 km.We succeeded in simulating intercept-resend attack experimentally, and put forward a scenario to beat the photon-number-splitting attack by monitoring bright reference pulses. The scenario can keep a secure distance up to 146 km for quantum key distribution with a coherent light source.