Search

Recent controversy regarding the meaning and usefulness of weak values is reviewed. It is argued that in spite of recent statistical arguments by Ferrie and Combes, experiments with anomalous weak values provide useful amplification techniques for precision measurements of small effects in many realistic situations. The statistical nature of weak values is questioned. Although measuring weak values requires an ensemble, it is argued that the weak value, similarly to an eigenvalue, is a property of a single pre- and post-selected quantum system. This article is part of the themed issue 'Second quantum revolution: foundational questions'. [ABSTRACT FROM AUTHOR]

In 1993, Elitzur and Vaidman introduced the concept of interaction-free measurements, which allowed finding objects without "touching" them. In the proposed method, since the objects were not touched, even by photons, thus, the interaction-free measurements can be called "seeing in the dark." Since then, several experiments have been successfully performed and various modifications were suggested. Recently, however, the validity of the term "interaction-free" has been questioned. The criticism of the name is briefly reviewed and the meaning of the interaction-free measurements is clarified. [ABSTRACT FROM AUTHOR]

We present surprising experimental evidence regarding the past of photons passing through an interferometer. The information about the positions through which the photons pass in the interferometer is retrieved from modulations of the detected signal at the vibration frequencies of mirrors the photons bounce off. From the analysis we conclude that the past of the photons is not represented by continuous trajectories, although a "common sense" analysis adopted in various welcher weg measurements, delayed-choice which-path experiments, and counterfactual communication demonstrations yields a single trajectory. The experimental results have a simple explanation in the framework of the two-state vector formalism of quantum theory. [ABSTRACT FROM AUTHOR]