  We consider the problem of quantum noise extraction from polarization
swapping in a gain-switched VCSEL. The principle of operation of a quantum
random number generator is based on the generation of laser pulses with one of
two orthogonal polarization states, followed by digitization of
polarization-resolved pulses with a comparator. At intensity values of laser
pulses close to the threshold value of the comparator, the contribution of the
classical noise of the photodetector will have a crucial role in making a
decision on the choice of a logical zero or one. We show how to evaluate the
contribution of classical noise and how to calculate the quantum reduction
factor required for post-processing.
