  We demonstrate a high bit-rate quantum random number generator by
interferometric detection of phase diffusion in a gain-switched DFB laser
diode. Gain switching at few-GHz frequencies produces a train of bright pulses
with nearly equal amplitudes and random phases. An unbalanced Mach-Zehnder
interferometer is used to interfere subsequent pulses and thereby generate
strong random-amplitude pulses, which are detected and digitized to produce a
high-rate random bit string. Using established models of semiconductor laser
field dynamics, we predict a regime of high visibility interference and nearly
complete vacuum-fluctuation-induced phase diffusion between pulses. These are
confirmed by measurement of pulse power statistics at the output of the
interferometer. Using a 5.825 GHz excitation rate and 14-bit digitization, we
observe 43 Gbps quantum randomness generation.
