Abstract:

The state of a quantum system is described by a wave function evolving in time according to the Schroedinger equation. If a measurement is carried out on the system, its wave function collapses, i.e., it changes according to the random outcome of the measurement. During a sequence of measurements on a single system, its quantum state thus follows a stochastic trajectory composed of the normal quantum mechanical time evolution interrupted by collapses at each measurement. The resulting state of the system, at any time, can be used to predict the probabilities and mean values for the measurement of physical observables.

In this talk we ask whether, like in archaeology, information obtained from measurements in the present also improves our knowledge about the state of the system in the past. The answer is yes, and in the talk I shall show how such “hindsight” knowledge can be formally defined in quantum mechanics and how we can represent it via a time evolving (past) state, which at any time depends on both earlier and later measurement outcomes. I will show applications of the theory to recent experiments on atoms and superconducting qubits, and I will discuss how the new concept and formalism of past quantum states relate to some questions of more foundational character.

Date: 9/10/2015

Time:15:00 (coffee & cookies will be served at 00:00)

Place:FORTH Seminar Room 1