|Location: Hörsaal A (1.3.14)
Time: Tuesday, 25.05.2010, 14 h c.t.
It is of both fundamental and practical interest to understand the quantum dynamics of a central spin coupled to an ensemble of bath spins and to mitigate the associated decoherence. Electron spin qubits in GaAs quantum dots interacting with a mesoscopic nuclear spin bath are an ideal model system for this problem and also promising candidates for solid state quantum computing. I will present two sets of experiments on this system demonstrating significant progress towards the above goals. By operating the qubit as a feedback loop controlling its own environment, the fluctuations of the nuclear bath can be reduced. The qubit first performs a partial measurement of the state of the bath and then polarizes it depending on the outcome. This feedback operation results in a nearly tenfold enhancement of the inhomogeneous dephasing time, T2*. Measuring the Hahn echo coherence of our qubit gives detailed insight into the rich dynamics of the system. The echo response shows collapses and revivals that originate from the Larmor precession of the nuclei and are accurately described by a semiclassical model. More advanced pulse sequences that dynamically decouple the qubit from its environment result in coherence times of up to 200 ms, two orders of magnitude longer than previously reported for these qubits. Our results imply good prospects for GaAs based spin qubits to reach the quantum error correction threshold in spite of nuclear decoherence. They also level the way for further studies focusing on the quantum aspects of the nuclear bath.