1. School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
   
2. Center for NanoScience and Department für Physik, Ludwig-Maximilians-Universität, 90539 Munich, Germany
   
3. Materials Department, University of California, Santa Barbara, California 93106, USA

Correspondence to: Brian D. Gerardot¹Richard J. Warburton¹ Correspondence and requests for materials should be addressed to B.D.G. (Email: b.d.gerardot@hw.ac.uk) or R.J.W. (Email: r.j.warburton@hw.ac.uk).

The spin of an electron is a natural two-level system for realizing a quantum bit in the solid state^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}. For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon-related spin relaxation^{1, 2, 3, 4, 5, 6, 7}. However, this advantage is offset by the hyperfine interaction between the electron spin and the 10⁴ to 10⁶ spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds^{8, 9, 10, 11, 12, 13, 14}. Spin-echo techniques have been used to mitigate the hyperfine interaction^{14, 15}, but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this^{16, 17} but is very difficult to realize in practice^{12, 18, 19}. Exploring materials with zero-spin nuclei is another option, and carbon nanotubes²⁰, graphene quantum dots²¹ and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin-3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak^{22, 23}. We demonstrate here high fidelity (about 99 per cent) initialization of a single hole spin confined to a self-assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid-state quantum networks²⁴ that can intra-convert the spin state with the polarization of a photon.