Engineering ultralong spin coherence in two-dimensional hole systems at low temperatures

For the realization of scalable solid-state quantum-bit systems, spins in semiconductor quantum dots (QDs) 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 QDs 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 the so-called natural QDs, in narrow GaAs/AlGaAs quantum wells at temperatures below 500 mK. Consistently, time-resolved Faraday rotation and resonant spin amplification techniques deliver hole-spin coherence times, which approach in the low magnetic field limit values above 70 ns. The optical initialization of the hole spin polarization, as well as the interconnected electron and hole spin dynamics in our samples, are well reproduced using a rate equation model.