Speckle patterns and purity of quantum states

Almost any quantum information processing experiment starts, ideally, with the preparation of a quantum system in a nice pure state (and that pure state has to be recreated many times). But how could one measure directly the purity of that state? I show how laser speckle patterns lead to one solution of this problem. I will also discuss the other obvious question: how can one verify that one repeatedly produces the same state in an experiment?

References:
S.J. van Enk and C.W.J. Beenakker, Phys. Rev. Lett. 108, 110503 (2012).
Lucia Schwarz and S.J. van Enk, Phys. Rev. Lett. 106, 180501 (2011).