Controlling excess noise using acousto-optic modulator for quantum cryptography with continuous variables

In a two-way type plug and play system of quantum cryptography (QC), there are no troublesome adjustments that need to be made over long distances, and also it is stable for many hours. However, it is widely known that Rayleigh backscattering has an adverse effect on the performance of a two-way QC system using single photon detection. The backscattered (BS) light has the same wavelength as signal light and is generated anywhere in a fiber, hence it cannot be decoupled by controlling the detection time or using optical filtering. The BS light increases the probability of false detection, thus it reduces the safety of QC. In continuous variables QC using homodyne detection, the BS light induces excess noise, which is surplus noise to a shot noise limit and also reduces the safety. A QC experiment to control the excess noise using acousto-optic modulator was performed and the results were presented.