Photonic integration of electron-nuclear spin systems in semiconductor quantum dots

Abstract

Semiconductor quantum dots (QDs) are bright sources of single photons suitable for photonic integration. This makes them a promising platform for quantum information processing. One approach to quantum computing is based on the measurement of multidimensional photonic cluster states. These are states of highly entangled photons, which can be generated using the light-matter interface of an electron and the nuclear spins within an optically active QD. Here we integrate the electron-nuclear spin system of a semiconductor QD into an open microcavity. We report progress on three fronts towards the generation of multidimensional photonic cluster states: First, we improve the photon collection efficiency through an actively stabilised open microcavity. Our design has a small footprint (30 × 30 mm), is cryogenic compatible, and adaptable to various quantum-emitter platforms. We demonstrate a cavity collection efficiency of 28(10)%, a single-photon end-to-end efficiency of 5.4(3)%, and cavity length stability of 153 pm root-mean-square, corresponding to 16% of the full width at half maximum of the cavity mode. Second, we examine a negatively charged QD under a magnetic field along the growth direction (Faraday geometry). We measure optical transitions enhanced by a Purcell factor of 5.9, demonstrate light hole-enabled spin initialisation, and study the viability of coherent electron spin control in cavity-coupled QDs in Faraday geometry. Third, we demonstrate nuclear spin control on the level of single-spin fluctuations. By purifying an ensemble of 50,000 nuclear spins, we prolong the electron spin inhomogeneous dephasing time by a factor of 89, reaching T₂* = 296(5) ns. Further, we engineer classically mixed nuclear macrostates on a lattice and present a coherent extension to Schrödinger cat states. Together, these are key steps towards the generation of large-scale, multidimensional photonic cluster states with semiconductor QDs.