  Pseudo-random number generators are widely used in many branches of science,
mainly in applications related to Monte Carlo methods, although they are
deterministic in design and, therefore, unsuitable for tackling fundamental
problems in security and cryptography. The natural laws of the microscopic
realm provide a fairly simple method to generate non-deterministic sequences of
random numbers, based on measurements of quantum states. In practice, however,
the experimental devices on which quantum random number generators are based
are often unable to pass some tests of randomness. In this review, we briefly
discuss two such tests, point out the challenges that we have encountered and
finally present a fairly simple method that successfully generates
non-deterministic maximally random sequences.
