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Arguably, the only really useful single photon source in quantum technology is a source of close-to-perfect single photons. The demands are stringent in terms of purity (the level of anti-bunching), coherence (the level of indistinguishability) and brightness (the efficiency of the entire device). Such a source would find applications in device-independent quantum cryptography. The quantum repeater requires in addition a stationary bit at each node, for instance a coherent spin. Ideally therefore, a system is required with a coherent spin and efficient spin-photon interface.
Implementing these ideas in the solid-state is appealing as nano-technology can potentially add a lot of device functionality. However, a solid-state environment is a source of noise, phonons in the lattice, charge noise from fluctuating charges in the solid and spin noise from the fluctuating nuclear spins of the host atoms. Also, a solid-state emitter may emit not just a photon (the zero-phonon-line) but also a photon together with a phonon.
Reported here is progress on two prominent solid-state emitters, a semiconductor quantum dot [1] and the NV colour centre in diamond. The two emitters have complementary strengths and weaknesses. A semiconductor quantum dot is a source of high quality single photons but single spins dephase rather rapidly. Conversely, the NV centre hosts a highly coherent spin but the photons are of low quality.
In the case of quantum dots, the noise in high quality material is very low [2] and in the best case (resonant excitation at low temperature), transform-limited linewidths have been achieved [3]. A micro-cavity is under development with the goal of increasing the brightness [4]. A quantum dot electron spin dephases rapidly on account of the spin noise in the nuclei. A hole spin is a viable alternative and dephases much less rapidly than an electron spin in the presence of unprepared nuclear spins [5].
In principle, a resonant micro-cavity confers multiple benefits to the NV centre photons: faster recombination, preferential emission into the micro-cavity mode, and, crucially, an increase in the zero-phonon-line fraction. In practice, it has been difficult to secure these advantages, largely because nano-fabrication of diamond is very challenging. However, an increase in the zero-phonon-line fraction from 3% to close to 50% has now been achieved by embedding ultra-pure diamond membranes in a micro-cavity [6].
[1] R. J. Warburton, Nature Materials 12, 483 (2013)
[2] A. V. Kuhlmann et al., Nature Physics 9, 570 (2013)
[3] A. V. Kuhlmann et al., Nature Communications 6, 824 (2015)
[4] L. Greuter et al., Physical Review B 92, 045302 (2015)
[5] J. Prechtel et al., Nature Materials 15, 981 (2016)
[6] D. Riedel et al., arXiv:1703.00815 (2017)
