# Quantum correlations in space and time

### Friday, 09 Aug. 2013, 12:00 - 13:00

**Presenter:** Prof. Joseph Fitzsimons

**Host:** P. Walther

**Where:** Ernst Mach Hörsaal, 2nd floor, Boltzmanngasse 5 - 1090 Vienna

In ordinary, non-relativistic, quantum physics, time enters only as a parameter and not as an observable : a state of a physical system is specified at a given time and then evolved according to the prescribed dynamics. While the state can, and usually does, extend across all space, it is only defined at one instant of time, in conflict with special relativity where space and time are treated on an equal footing. In this talk, I will examine the consequences of extending the notion of the quantum density matrix to multiple spatial and temporal measurements. To this end, I will introduce the concept of a pseudo-density matrix which treats space and time indiscriminately. This matrix in general fails to be positive for timelike separated measurements, motivating the definition of a measure of causality that discriminates between spacelike and timelike correlations. I will present the results of recent NMR experiments to measure causal correlations and their decay under the effects of noise.

In the second half of the talk, I will present an application on the pseudo-density framework to bounding the capacity of quantum channels, and show how it can be used to obtain new bounds for the capacity of the depolarizing channel. I will conclude with a discussion of possible future research directions.