Jan Dzwiewior et al.
Universality of Weak Values
The concept of weak values, which was introduced by Aharonov, Albert and Vaidman in 1988, continues to inspire a range of successful experimental applications, yet, it also has retained a highly controversial status. The foundational debates revolve around questions such as the ontological status of the weak value, its physical meaning and consequently also its role in solving paradoxes and giving novel answers to fundamental questions in quantum mechanics. This experimental study touches on these debates by developing the already known concept of the universality of weak values further and consolidating their meaning as measurable properties in an operational sense. Weak values are defined based on weak interactions of pre- and postselected quantum systems with external systems. They provide a simple and universal description of the seemingly complicated modification of interactions due to the pre- and postselection. Here we demonstrate this universality experimentally by weakly coupling several distinct external systems to a pre- and postselected system. After postselection all three external systems are changed in the same manner. This modification is fully encompassed by the complex weak value, which depends on the pre- and postselected system only and is thus independent of the specific properties of the external systems. The experiment focuses in particular on the special case of a local interaction of a photon inside an interferometer, for which the weak value of a spatial projector is relevant. As external systems other degrees of freedom are employed, which enables a clear high fidelity demonstration. In turn, as any misalignment of the interferometer can be seen as coupling to other degrees of freedom, we can use the universal character of weak values to provide a simple but precise alignment procedure for interferometric setups.
