Luetkenhaus, Norbert

Tuesday March 29, 2011
Time: 17:15 - 18:00
Place: ETH Science City, HIT J 51
Host: Renato Renner

Testing Quantum Devices for Quantum Communication

Norbert Luetkenhaus
Institute for Quantum Computing, University of Waterloo, Canada

Quantum Communication requires a number of different devices: quantum channels, quantum memories, quantum repeaters. As experimental effort is under way to realize these tools, we will necessarily start with imperfect devices. Channel loss is one example, decoherence in quantum memory another.

In my group we study simple test procedure to make sure that quantum devices can be of use for quantum communication. The criteria being that the performance of the devices cannot be explained by a process that measures out the input state in order to re-prepare the output from classical information (measure/reprepare strategy). This approach links the performance of devices as quantum devices to the ability to maintain entanglement. The technical key point will be methods to verify and quantify entanglement on quantum systems which consist of a combination of a discrete variable and a continuous variable system with just a handful of observed expectation values.

We concentrate on optical devices which operate in infinite dimensional Hilbert spaces and utilize simple test state, such as laser pulses, and quadrature measurements. We are able to show that a three different test states can be as powerful as an infinite set of coherent states with a Gaussian distribution of amplitudes to demonstrate quantum behavior.

In a quantitative approach, we give bounds on the amount of entanglement that can be distributed and/or stored with the devices.