  We address joint photodetection as a method to discriminate between the
classical correlations of a thermal beam divided by a beam splitter and the
quantum entanglement of a twin-beam obtained by parametric downconversion. We
show that for intense beams of light the detection of the difference
photocurrent may be used, in principle, in order to reveal entanglement, while
the simple measurement of the correlation coefficient is not sufficient. We
have experimentally measured the correlation coefficient and the variance of
the difference photocurrent on several classical and quantum states. Results
are in good agreement with theoretical predictions taking into account the
extra noise in the generated fields that is due to the pump-laser fluctuations.
