For the realisation of scalable solid-state quantum-bit systems, spins in semiconductor quantum dots are promising candidates. A key requirement for quantum logic operations is a sufficiently long coherence time of the spin system. Recently, hole spins in III-V-based quantum dots were discussed as alternatives to electron spins, since the hole spin, in contrast to the electron spin, is not affected by contact hyperfine interaction with the nuclear spins. Here, we report a breakthrough in the spin coherence times of hole ensembles, confined in so called natural quantum dots, in narrow GaAs/AlGaAs quantum wells at temperatures below 500 mK. Consistently, time-resolved Faraday rotation and resonant spin amplification techniques deliver holespin coherence times, which approach in the low magnetic field limit values above 70 ns. The optical initialisation of the hole spin polarisation, as well as the interconnected electron and hole spin dynamics in our samples are well reproduced using a rate equation model.

We have investigated spin and carrier dynamics of resident holes in high-mobility two-dimensional hole systems in GaAs/ Al 0.3 Ga 0.7 As single-quantum wells at temperatures down to 400 mK. Time-resolved Faraday and Kerr rotation as well as time-resolved photoluminescence spectroscopy are utilized in our study. We observe long-lived hole-spin dynamics that are strongly temperature dependent, indicating that in-plane localization is crucial for hole-spin coherence. By applying a gate voltage, we are able to tune the observed hole g factor by more than 50%. Calculations of the hole g tensor as a function of the applied bias show excellent agreement with our experimental findings.

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