Yuval Gefen

Title

Weak Measurements and Weak Values, a Peephole into Quantum Mechanics

Abstract

The measurement of any observable in quantum mechanics is a probabilistic process described by the projection postulate. According to von Neumann this involves a collapse of the system’s wavefunction onto an eigenstate of the measured quantity. A strikingly different concept is that of weak measurement. Weakly measuring an observable (i.e., measuring it while weakly disturbing the system), provides only partial information on the state of the system. More than 2 decades ago, Yakir Aharonov and collaborators have invoked the notion of weak value: a two-step procedure – weak measurement followed by a strong one, where the outcome of the first measurement is kept provided a second post-selected outcome occurs. Weak values may give rise to exotic and non-intuitive expectation values of the measured observables, and can be exploited to devise efficient amplification protocols. At the same time weak value protocols provide us with non-destructive peepholes into quantum phenomena, avoiding the collapse of the corresponding wavefunctions. I will review these concepts with special emphasis on applications to quantum solid state systems.