Realisation of a spin–photon interface using singlet–triplet qubits in self-assembled quantum dot molecules

Abstract

We explore the prospects of using doubly charged, optically active, self-assembled quantum dot molecules in the (1,1) regime as a spin–photon interface. Using optical means, we investigate the coherence properties of the singlet–triplet subspace in the vicinity of the sweet spot, where that subspace becomes atomic-clock like. We compare frequency domain measurements, making use of coherent population trapping, with time domain measurements based on coherent spin control. We find T2∗ coherence times in the range of 10 ns to 200 ns, determined by charge fluctuations that vary from sample to sample. In conjunction with measurements of the Hahn spin echo, which we observe for up to 300 ns, we conclude that additional sources of decoherence are playing a role. We report a spin-measurement protocol making use of ancillary optical cycling transitions present in the system. We find an improvement of detection efficiency by two orders of magnitude compared to conventional direct optical spin detection, suggesting single-shot readout to be achievable. The principal result of this thesis is the demonstration of the coherence of the optical interaction by quantifying the amount of entanglement between a spontaneously emitted single photon and the resident pseudo-spin. A visibility of the quantum coherence approaching 50 %, together with strong classical correlations unambiguously prove quantum entanglement between the two systems.