## Events at Physics |

**Physics Department Colloquium**

******NOTE SPECIAL START TIME******

**Quantum trajectories and the quantum state of the past**

**Date:**

**Friday, January 29th**

**Time:**3:00 pm

**Place:**2241 Chamberlin Hall

**Speaker:**Klaus Molmer, Aarhus University

**Abstract:**The state of a quantum system is described by a wavefunction 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 in which the normal quantum mechanical time evolution is interrupted by collapses at each measurement.

The resulting state of the system, at any time, successfully predicts probabilities and mean values for the measurement of physical observables. In this talk we ask whether the sequence of measurements also adds to our knowledge about the state of the system at earlier times during the experiment.

The answer is yes, and 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 experiments on atoms and superconducting qubits, and I will discuss how the concept and formalism of past quantum states touch upon both practical and foundational aspects of quantum physics.

**Host:**Mark Saffman