Optical pumping of a single hole spin in a quantum dot

A quantum dot that can be optically initialized to contain a well-defined and very stable hole spin has been designed, with a relaxation time long enough to allow potential applications in solid-state quantum networks. 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 10 4 to 10 6 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 nanotubes 20 , graphene quantum dots 21 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 networks 24 that can intra-convert the spin state with the polarization of a photon.