Determining the quantum expectation value by measuring a single photon

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. A photonic experiment demonstrates protective measurements, a type of weak measurements. These make it possible to determine the expectation value of the polarization of a photon from a single measurement.