Anomalous weak values via a single photon detection
Is it possible that a measurement of a spin component of a spin-1/2 particle yields the value 100? In 1988 Aharonov, Albert and Vaidman argued that upon pre- and postselection of particular spin states, weakening the coupling of a standard measurement procedure ensures this paradoxical result. This theoretical prediction, called weak value, was realized in numerous experiments, but its meaning remains very controversial, since its "anomalous" nature, i.e. the possibility to exceed the eigenvalues range, as well as its "quantumness" are debated. We address these questions by presenting the first experiment measuring anomalous weak values with just a single click, without any statistics. The measurement uncertainty is significantly smaller than the gap between the measured weak value and the nearest eigenvalue. Beyond clarifying the meaning of weak values, this result represents a breakthrough in understanding quantum measurement foundations, paving the way to further applications of weak values to quantum photonics.