PhD Defense: Martin Hayhurst Appel

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

Quantum states of multiple entangled photons constitute an important resource for quantum information processing. Here, we investigate deterministic entanglement generation using a solid-state quantum dot embedded in a photonic crystal waveguide.  A common limitation encountered with quantum dots is the incompatibility of coherent spin control and optical cycling transitions. By applying an in-plane magnetic field and by selectively coupling the linear optical dipoles to the waveguide mode, we measure a broadband increase in optical cyclicity up to 14.7 while retaining the ability to drive optical Raman transitions and perform high fidelity optical spin pumping.These capabilities allow the realisation of a time-bin entanglement protocol, which we analyse in detail. By combining resonant optical pulses and Raman pulses, the protocol can generate GHZ states and linear cluster states containing the QD spin and N photons, where each photon is emitted  in a superposition of two temporal modes. This protocol is insensitive to inhomogeneous dephasing, thanks to a built-in spin-echo process, and is compatible with high magnetic fields and waveguides. We calculate error rates of 2.1% pr. photon for realistic parameters.The protocol is implemented experimentally. By using a novel self-stabilising double pass interferometer, we measure a spin-photon Bell state with a 66.6% fidelity and 124 Hz detection rate. This fidelity is well reproduced by a Monte Carlo simulation, and we discuss strategies for improved entanglement generation, which mainly focus on improved spin rotations.