A quantum dot in a microcavity as a coherent spin-photon interface

Single photons can encode and transmit information at the speed of light and, hence, are the most promising candidates for flying qubits in quantum networks and measurement-based quantum computation. Although photons do not naturally interact with each another, spin-photon interfaces can mediate the necessary interaction or entanglement between them. An ideal platform combines high-rate photon emission with a coherent spin in an efficient interface.
Semiconductor quantum dots coupled to optical cavities are excellent single-photon sources. Due to the ability to charge a single electron or hole onto a quantum dot, they can further be used to engineer strong spin-photon interfaces. The key challenge is achieving high quantum dot-cavity coupling and a coherent spin while being able to coherently manipulate the spin
state in the cavity.
In this thesis, we address these demands by employing a single InAs quantum dot in a tunable open microcavity – a system that has already reported a record-high single-photon source efficiency.
First, we implement a highly-efficient light-matter interface – a one-dimensional atom. We observe an extinction in photon transmission of 99.2% resulting in a photon bunching of close to 600. This showcases the nonlinearity of the system at level of single photons: a near-perfect transmission of the two-photon component and reflection of single photons. The open nature of the cavity allows for in-situ tuning of the quantum dots’s coupling efficiency to the cavity, resulting in full control over the photon statistics from bunching to anti-bunching. Furthermore, we implement a chiral interface and measure directional transmission with an isolation of 10.7 dB, the highest non-reciprocal response recorded with a single quantum emitter. Lastly, we operate in the back-reflection regime and directly observe a photon-number dependent time-delay upon scattering of a laser pulse: single-, two-, and three-photon components incur a different time delay of 144.02 ps, 66.45 ps and 45.51 ps, respectively. This is a fingerprint of stimulated emission at the level of a few photons.
In the second part, we establish a spin-photon interface. We achieve fast single-shot readout of an electron spin within 3 ns with a fidelity of 95.2%. For the first time, this allows readout faster than the quantum dot relaxation and dephasing times. Further, we observe time-resolved quantum jumps in the spin state using many readout repetitions and predict that single-shot readout should be achievable with 89,9% fidelity in Voigt geometry. Finally, we implement coherent control of a hole spin in the microcavity. We observe hole-spin Rabi oscillations with frequencies up to 1 GHz. The spin mediates interaction with the host nuclear spins, narrowing the nuclear spin distribution – an effect which has not yet been exploited on the hole. The nuclear cooling results in an enhancement of the spin coherence time from 30 ns to 500 ns.
In summary, a quantum dot in a microcavity can combine the best of all worlds in the same system: coherent single photons, a coherent spin, an interface for high efficiency spin-photon interaction, and fast manipulation and readout of a spin state. The results pave the way for the implementation of two-qubit gates between photons, high rate spin-photon entanglement and ultimately the generation of photonic cluster states.