  We experimentally demonstrate a bias-free optical quantum random number
generator with real-time randomness extraction to directly output uniform
distributed random numbers by measuring the vacuum fluctuation of quantum
state. A phase modulator is utilized in the scheme to effectively reduce the
influence of deviations between two arms of the generator caused by the
imperfect practical devices, which is an innovative solution in the field of
quantum random number generator. In the case where the feedback modulation
frequency is much faster than the phase jitter, an unbiased result can be
obtained by an additional subtraction between the compensation signal and its
average value to eliminate residual deviation. A following randomness extractor
is applied to eliminate the influence of residual side information introduced
by the imperfect devices in practical system.
