Hyperfine dephasing of electron spin qubits in GaAs quantum dots

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Electron spins in GaAs quantum dots have proven their viability as semiconductor qubits, but one important source of decoherence in these devices is the hyperfine interaction of the electrons with the nuclear spins of the host lattice. While much progress has been made in understanding and reducing its effect, several questions that will be addressed experimentally in this project remain. One major task is a detailed study of the effectiveness of dynamical decoupling. We have shown earlier that this technique can extend the dephasing time to at least 200 µs by repeatedly inverting the qubit state, but the limiting factors are not understood, and much longer times are expected theoretically. Furthermore, we will investigate the conjecture that dephasing of nuclear spins due to quadrupole coupling to electric fields also limits electron coherence. If it is correct, a significant enhancement of the coherence time by choosing an optimal orientation of the external field is expected.The second half of the project will focus on more fundamental questions concerning the properties of the nuclear spin bath. So far, all experiments can be explained with a classical model of this bath consisting of about 10^6 nuclei, but given its mesocopic nature it is interesting if quantum effects can be observed. We will explore to what extent a description via a classical spectral density is adequate, and what this spectrum would look like. Furthermore, we will directly probe whether a back action of the qubit on the nuclear spins can be detected. Such a back action is required by the laws of quantum mechanics and lies at the heart of the uncertainty relation. It would thus be a qualitative hallmark of quantum mechanical behavior. To this end, we will measure the correlations between successive single shot measurements of the qubit and how they depend on the manipulation of the qubits between the two measurements. Our earlier theoretical and experimental work demonstrates the viability of this novel measurement concept.