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Phys Rev Lett. 2010 Nov 19;105(21):216803. Epub 2010 Nov 19.

Enhancing the coherence of a spin qubit by operating it as a feedback loop that controls its nuclear spin bath.

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Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.


In many realizations of electron spin qubits the dominant source of decoherence is the fluctuating nuclear spin bath of the host material. The slowness of this bath lends itself to a promising mitigation strategy where the nuclear spin bath is prepared in a narrowed state with suppressed fluctuations. Here, this approach is realized for a two-electron spin qubit in a GaAs double quantum dot and a nearly tenfold increase in the inhomogeneous dephasing time Tâ‚‚* is demonstrated. Between subsequent measurements, the bath is prepared by using the qubit as a feedback loop that first measures its nuclear environment by coherent precession, and then polarizes it depending on the final state. This procedure results in a stable fixed point at a nonzero polarization gradient between the two dots, which enables fast universal qubit control.

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