The tin-vacancy centre in diamond: a coherent spin-photon interface for quantum network nodes

Abstract

Quantum networks require spin-photon interfaces capable of generating entanglement between a stationary qubit and flying photons. So far, such an interface with a large emission rate of indistinguishable photons and excellent spin properties has been missing. Group IV colour centres in diamond offer strong emission into the zero-phonon line, an inversion symmetric structure and integration into nanophotonics. Especially the tin-vacancy centre in diamond has both excellent optical properties and long spin coherence times at elevated cryogenic temperatures. High-fidelity quantum control, however, has remained elusive. In this thesis, we introduce a new platform: deterministically strained tin-vacancy centres in a thin diamond membrane. The crystal strain allows microwave control of the spin state with a gate fidelity of 99.36(9) % at aligned magnetic fields with highly spin-selective optical transitions. Dynamical decoupling protocols are used to protect the spin coherence for up to 5.7(11) ms. We identify the properties of the spin bath, allowing us to understand material engineering challenges for next-generation devices. The crystal strain suppresses phonon-induced dephasing processes, enabling coherence times of up to 223(10) μs at 4 K, a record-value for group IV colour centres. For a low-strain tin-vacancy centre in a diamond nanopillar device, we show polarisation of more than 65 % of a strongly coupled 13C nuclear spin. We find that optically induced laser-detuning independent dephasing limits the gate fidelities of the all-optical stimulated Raman drive, confirming microwave control as the most viable route for quantum control of the spin-photon interface. Lastly, we report on the development of a versatile open optical microcavity platform for quantum materials. The combination of high-fidelity quantum control shown in this thesis and achievements in related works, like fibre-packaged diamond waveguides and an intrinsic strongly coupled nuclear spin, renders the tin-vacancy centre as a prime candidate for quantum network nodes.