In order to implement fault-tolerant quantum computation, entanglement generation with low error probability and high success probability is required. We have proposed the use of squeezed coherent light as a probe to generate entanglement between two
atoms by communication, and shown that the error probability is reduced well below the threshold of fault-tolerant quantum computation [Phys. Rev. A. {bf 88}, 022313 (2013)]. Here, we investigate the effect of photon loss mainly due to finite coupling efficiency to the cavity. The error probability with the photon loss is calculated by the beam-splitter model for homodyne measurement on probe light. Optimum condition on the amplitude of probe light to minimize the error probability is examined. It is shown that the phase-squeezed probe light yields lower error probability than coherent-light probe. A fault-tolerant quantum computation algorithm can be implemented under 0.59 dB loss by concatenating five-qubit error correction code.
The phase randomized light is one of the key assumptions in the security proof of Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol implemented with an attenuated laser. Though the assumption has been believed to be satisfied for c
onventional systems, it should be reexamined for current high speed QKD systems. The phase correlation may be induced by the overlap of the optical pulses, the interval of which decreases as the clock frequency. The phase randomness was investigated experimentally by measuring the visibility of interference. An asymmetric Mach-Zehnder interferometer was used to observe the interference between adjacent pulses from a gain-switched distributed feedback laser diode driven at 10 GHz. Low visibility was observed when the minimum drive current was set far below the threshold, while the interference emerged when the minimum drive current was close to the threshold. Theoretical evaluation on the impact of the imperfect phase randomization provides target values for the visibility to guarantee the phase randomness. The experimental and theoretical results show that secure implementation of decoy BB84 protocol is achievable even for the 10-GHz clock frequency, by using the laser diode under proper operating conditions.
