Measuring statistical property using a single click of a detector

In 1993 Yakir Aharonov and Lev Vaidman introduced the concept of "protective measurement" which allowed to measure quantum states given a single (protected) quantum system. 

Their work created a major controversy since according to the prevailing interpretations of quantum mechanics the quantum state has only a statistical meaning as a property of an ensemble of identically prepared systems. There were numerous theoretical and philosophical papers about the interpretation of this result, but only now the first experimental implementation of this proposal has been performed, employing an advanced quantum optics setup. An experimental group in Turin, under the guidance of  the former student of Aharonov and Vaidman, Eliahu Cohen, and Lev Vaidman measured the expectation value of polarization using detection of a single photon. As its name suggests, expectation value is a statistical concept, so measuring it using a single photon is a striking paradoxical result, challenging the foundations of quantum mechanics. 

The paper appeared in Nature Physics on August 14th.