Hybrid integration of different quantum material platforms is probably the right path leading to eventual realizations of practical photonic quantum computing and communication. Amongst these platforms, quantum dots stand out as not only near-perfect sources emitting the purest, indistinguishable photons, but also viable spin qubits for inducing non-local correlation, i.e., entanglement, between flying photons. Entanglement generation between multiple photons is an indispensable ingredient for realizing measurement-based quantum photonic computing and loss-tolerant quantum repeaters, yet it remains to be experimentally demanding.

Self-assembled InAs quantum dots integrated on photonic-crystal waveguides are especially promising in this endeaver, since the entanglement between two guided photons can be mediated by a quantum-dot spin owing to a deterministic spin-photon interface.

The same interface could also serve as a medium to swap information between flying photons and a stationary spin qubit, fulfilling DiVincenzo’s sixth criteria for long-distance quantum communication.

In this thesis, we show that such a waveguide-integrated spin-photon interface enables a plethora of quantum protocols suitable for quantum information processing.

Notably, we report the on-chip generation of high-fidelity entanglement between a guided photon and the embedded quantum-dot hole spin, which lays experimental grounds for, say, two-photon entangling gates, deterministic Bell-state analyzers and spin-photon controlled-phase gate necessary for one-way quantum repeaters. The spin-photon entanglement is generated with 74% fidelity, on par with existing solid-state platforms while the generation speed is two-orders of magnitude faster.

Our second work proposes that the information carried by a flying photon can be stored in the stationary quantum dot with fidelity exceeding 95% in a realistic experimental setting. The proposal is strikingly robust to losses with performance comparable to that of the atomic platform. We therefore expect it to be useful for performing fast deterministic SWAP gates, as well as memory-assisted satellite quantum key distribution.

Finally, we demonstrate entanglement between two photons and the quantum-dot electron spin, endowed by significant improvements across most aspects of the platform. Using nuclear spin narrowing, the electron spin dephasing time is extended by fifteen-fold, with single-qubit gate fidelity of Fπ ≈ 98%. Fidelities of spin-photon and biphoton-spin entanglement are 77% and 56%, respectively, predominantly limited by cross-excitation errors which can be mitigated in the next-generation sample. The path towards higher photons is thus straightforward, and 4-qubit entanglement should be within reach.