# Theoretical Physics

**21 October 2015**

**Time:**15:00 to 16:00

**Location:**EC Stoner 8.60

Theoretical Physics Seminar: Christian Arenz (Aberystwyth)

Wednesday, Oct 21 at 3pm @ EC Stoner 8.60

Control of open quantum systems

The interaction of a quantum system with its environment is usually considered to be detrimental for quantum information processing. Quantum features one wants to use for quantum information tasks are washed out quickly so that the implementation of quantum gates becomes noisy. Quantum control theory of open quantum systems offers here valuable way to go. In the first part of my talk I show that decoherence of a single spin resulting from the interaction with a spin environment can be overcome using methods from quantum control theory. In presence of the spin environment the implementation of universal gates in a noiseless manner becomes possible by acting with one control field on the system spin alone.

In the second part of my talk I am going to argue that rather than fighting against the environment a dissipative process can also have a beneficial effect on the dynamics that is induced by some controls. I show that every dissipative process exhibiting some set of states that are invariant under the noise process can enlarge the set of unitary operations that can be implemented by means of classical fields. In conclusion a dissipative process can turn a quantum system into a system which is universal for quantum information tasks.

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