Determining the quantum expectation value by measuring a single photon

Fabrizio Piacentini*, Alessio Avella, Enrico Rebufello, Rudi Lussana, Federica Villa, Alberto Tosi, Marco Gramegna, Giorgio Brida, Eliahu Cohen, Lev Vaidman, Ivo P. Degiovanni, Marco Genovese

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


One description provides only probabilities for obtaining various eigenvalues of a quantum variable. The eigenvalues and the corresponding probabilities specify the expectation value of a physical observable, which is known to be a statistical property of an ensemble of quantum systems. In contrast to this paradigm, here we demonstrate a method for measuring the expectation value of a physical variable on a single particle, namely, the polarization of a single protected photon. This realization of quantum protective measurements could find applications in the foundations of quantum mechanics and quantum-enhanced measurements.

Original languageEnglish
Pages (from-to)1191-1194
Number of pages4
JournalNature Physics
Issue number12
StatePublished - 1 Dec 2017


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