  Blind Source Separation (BSS) is an active domain of Classical Information
Processing. The development of Quantum Information Processing has made possible
the appearance of Blind Quantum Source Separation (BQSS). This article
discusses some consequences of the existence of the entanglement phenomenon,
and of the probabilistic aspect of quantum measurements, upon BQSS solutions.
It focuses on a pair of spins initially separately prepared in a pure state,
and then with an undesired coupling between these spins. An unentanglement
criterion is established for the state of an arbitrary qubit pair, expressed
first with probability amplitudes and secondly with probabilities. It is
stressed that the concept of statistical independence of the sources, widely
used in classical BSS, should be used with care in BQSS, and possibly replaced
by some disentanglement principle. It is shown that the coefficients of the
development of any qubit pair pure state, over the states of the standard
basis, can be expressed with the probabilities of results in the measurements
of well-chosen spin components.
