| Budget Amount *help |
¥3,200,000 (Direct Cost: ¥3,200,000)
Fiscal Year 2005: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 2004: ¥2,400,000 (Direct Cost: ¥2,400,000)
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| Research Abstract |
Recently there have been growing interests in continuous-variable (CV) quantum information processing. Experimental demonstration of CV quantum teleportation, quantum dense coding, quantum telecloning, and quantum key distribution, (QKD) have been reported. One important advantage of CV quantum information is that efficient implementation of the essential steps in quantum information processing is achievable with currently available technologies. For example, a homodyne detector operating at room temperature provides a tool for measuring with near-unit efficiency the quadrature amplitude of the electromagnetic field. For the case of CV QKD using coherent states, the preparation of an ideal light source is easily actualized by just attenuating a laser light. In addition, it is expected that CV QKD will allow one to achieve higher secret key rates than single-photon QKD schemes. In this research, we have achieved "plug & play" and free-space implementations of continuous-variable (CV) quantum key distribution (QKD). In a CV QKD system, a homodyne detector is used to detect a weak signal light by superposing the signal with a local oscillator (LO) whose intensity is much stronger than the signal. In conventional "plug & play" (or auto compensating) systems, two pulses traverse an identical optical path and the intensities of them become equal. In our experiment we use an acousto-optic modulator and have successfully controlled the intensities of the signal and the LO. For free-space implementation of CV QKD the stability of the double interferometer is a crucial problem. We have separated the signal and LO in time longer than the coherence time of the pulse by exploiting the birefringence of EOM crystals. In this setup the signal and LO traverse along the same ray path, so the stability of the interferometer is greatly improved.
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