Quantum interface engineering with solid-state spins and photons

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This project falls within the EPSRC "quantum technologies" research area. The project focuses on experimental quantum communication technologies, using in particular III-V quantum dots (QDs), which are widely considered to be amongst the leading quantum node candidates for use in quantum optical networks. It will investigate a new generation of lattice-matched GaAs QDs, which promise a major improvement in the coherence properties of spin quantum bits (qubits) relative to the state of the art in self-assembled InGaAs QDs. The overall objective is to demonstrate that they can serve as a quantum networking node, which requires showing simultaneously: high-efficiency photon collection, qubit control, and a nuclear quantum memory. This will allow the generation of photons from the QD which are entangled with its solid-state spin qubits for a sufficient time that they can be used to network with other quantum nodes to realize a quantum network.Initially the focus will be on improving the optical interface between the spin qubits in the QDs and the outgoing photons. Enhancing photon collection efficiency is crucial, as while GaAs QDs have shown great promise in terms of their long spin coherence times, an important metric for preserving the quantum state, they lag other QD candidates (e.g. InGaAs) in terms of photon collection efficiency. The vision is to place the quantum emitter (QD) into a photonic microcavity, which would enhance emission and coupling into a fibre mode for long distance transmission. Next, we will demonstrate qubit control of the QD electron spin, which will be performed all-optically. This combination of spin control within an efficient optical interface will be unique, and will allow proof-of-concept demonstrations such as a deterministic photon-photon quantum gate. Finally, a quantum memory will be made possible by capitalizing on the hyperfine interaction between the single electron and the proximal nuclear spins to facilitate an optically addressable memory using the electron, enabling this QD-based quantum node to have dedicated quantum memories. The project aims to deliver benchmarking results on a multitude of quantum protocols forming the backbone of a quantum communication and computing network, such as quantum gates between photon qubits, generation of quantum repeater states, and distribution and storage of entanglement. Realizing a viable quantum optical network would enable a truly quantum internet between quantum nodes, paving the way for fully private communications (ensured by the principles of quantum mechanics), quantum cryptography, and distributed quantum computing. The work is performed in collaboration with the Quantum Optical Materials and Systems group at the University of Cambridge, the Semiconductor Physics group at Johannes Kepler University Linz, and the Photonic Nanomaterials Group at the University of Oxford.