TY - BOOK

T1 - A Quantum Dot Source of Time-Bin Multi-Photon Entanglement

AU - Appel, Martin Hayhurst

PY - 2021

Y1 - 2021

N2 - Quantum states of multiple entangled photons constitute an important resourcefor measurement-based quantum computing and all-photonic quantum repeaters.However, the generation of such states is challenging, and the probabilistic schemespursued until now are difficult to scale. Here, we investigate deterministic entanglementgeneration using a spin-photon interface which, through repeated opticalmanipulation, can emit longs strings of entangled photons. Specifically, we employa solid-state InAs quantum dot charged with a single hole spin. Additionally, weembed the quantum dot in a photonic crystal waveguide, thereby strongly couplingthe emitter to a single optical mode and modifying the light-matter interaction.A common limitation encountered with quantum dots is the incompatibility ofcoherent spin control and optical cycling transitions. By applying an in-plane magneticfield and by selectively coupling the linear optical dipoles to the waveguidemode, we measure a broadband increase in optical cyclicity up to X14:7 while retainingthe ability to drive optical Raman transitions. The waveguide geometry alsoallows selective pumping of the optical transitions leading to 98% spin initialisationfidelity. We demonstrate a T2 = 23:2 ns spin dephasing time, which exceeds mostexperiments employing comparable nanostructures.These capabilities allow the realisation of a time-bin entanglement protocol,which we analyse in great detail. By combining resonant optical pulses and Ramanpulses, the protocol can generate GHZ states and linear cluster states containingthe QD spin and N photons, where each photon is emitted in a superposition of twotemporal modes. This protocol is insensitive to T2 , thanks to a built-in spin-echoprocess, and is compatible with high magnetic fields and waveguides. We calculateerror rates of 2:1% pr. photon while considering realistic parameters and optimaluse of the waveguide. The protocol is implemented experimentally, and we realize aspin-photon Bell state with a 66.6% fidelity and 124 Hz detection rate. By using aself-stabilising double-pass interferometer, we are able to construct exact GHZ andBell state delity estimates. Extending to three qubits, we observe clear signaturesof coherence which, however, lack the amplitude for certiffiable entanglement. Byconstructing an exhaustive Monte Carlo simulation, we are able to include nearly allrelevant errors and identify our modest 88.5% spin rotation fidelity as the leadingerror mechanism. Other experiments have demonstrated better spin control, andwe discuss several possible paths towards achieving higher fidelity and scaling upto more qubits.

AB - Quantum states of multiple entangled photons constitute an important resourcefor measurement-based quantum computing and all-photonic quantum repeaters.However, the generation of such states is challenging, and the probabilistic schemespursued until now are difficult to scale. Here, we investigate deterministic entanglementgeneration using a spin-photon interface which, through repeated opticalmanipulation, can emit longs strings of entangled photons. Specifically, we employa solid-state InAs quantum dot charged with a single hole spin. Additionally, weembed the quantum dot in a photonic crystal waveguide, thereby strongly couplingthe emitter to a single optical mode and modifying the light-matter interaction.A common limitation encountered with quantum dots is the incompatibility ofcoherent spin control and optical cycling transitions. By applying an in-plane magneticfield and by selectively coupling the linear optical dipoles to the waveguidemode, we measure a broadband increase in optical cyclicity up to X14:7 while retainingthe ability to drive optical Raman transitions. The waveguide geometry alsoallows selective pumping of the optical transitions leading to 98% spin initialisationfidelity. We demonstrate a T2 = 23:2 ns spin dephasing time, which exceeds mostexperiments employing comparable nanostructures.These capabilities allow the realisation of a time-bin entanglement protocol,which we analyse in great detail. By combining resonant optical pulses and Ramanpulses, the protocol can generate GHZ states and linear cluster states containingthe QD spin and N photons, where each photon is emitted in a superposition of twotemporal modes. This protocol is insensitive to T2 , thanks to a built-in spin-echoprocess, and is compatible with high magnetic fields and waveguides. We calculateerror rates of 2:1% pr. photon while considering realistic parameters and optimaluse of the waveguide. The protocol is implemented experimentally, and we realize aspin-photon Bell state with a 66.6% fidelity and 124 Hz detection rate. By using aself-stabilising double-pass interferometer, we are able to construct exact GHZ andBell state delity estimates. Extending to three qubits, we observe clear signaturesof coherence which, however, lack the amplitude for certiffiable entanglement. Byconstructing an exhaustive Monte Carlo simulation, we are able to include nearly allrelevant errors and identify our modest 88.5% spin rotation fidelity as the leadingerror mechanism. Other experiments have demonstrated better spin control, andwe discuss several possible paths towards achieving higher fidelity and scaling upto more qubits.

M3 - Ph.D. thesis

BT - A Quantum Dot Source of Time-Bin Multi-Photon Entanglement

PB - Niels Bohr Institute, Faculty of Science, University of Copenhagen

ER -