Photon Sources for Linear Optical Quantum Computing

Quantum photonic technologies have many exciting applications including secure commu- nication, quantum enhanced measurement and quantum computing. Linear optical quantum computing (LOQC) is a technology of particular interest - especially given that recent progress in on-chip waveguide technology removes the requirement for complex and costly bulk optics setups and state-of-the-art detectors have detection efficiencies of over 90%. Arguably the largest remaining technological hurdle for LOQC is the development of a suitable photon source. A suitable source would produce single, indistinguishable photons deterministically. Additionally, it would be beneficial if the generated photons were entangled, as this can significantly reduce the degree of multiplexing needed to implement LOQC. Quantum dots are a suitable candidate system for these photon sources as they exhibit bright single photon emission and can act as the interface between light and a trapped spin qubit. These properties have resulted in proposals to generate multi-photon entangled states suitable for use in LOQC. This thesis presents some of the progress we have made towards the creation of a suitable photon source. After introducing the background material, we demonstrate pulsed resonant excitation using a single-electron-charged quantum dot. Deterministic excitation is demonstrated by performing Rabi oscillations and Ramsey interference in the excitonic population. We also investigate Ramsey interference in a Faraday geometry magnetic field and observe a variety of beats and oscillations in the interferograms. We develop a model to explain our results and conclude that controlling the phase between the two Ramsey interference pulses allows a degree of control over the state of the trapped spin. We then also demonstrate the coherent optical manipulation of a trapped spin in a Voigt geometry magnetic field. Once we have presented the manipulation of the excitonic state and the state of the trapped spin, we proceed to investigate the properties of the light produced by the resonant excitation of a quantum dot. Hong Ou Mandel interference experiments allow us to probe the indistinguishability of the photons resulting from the resonant excitation of the negative trion transition. Repeating the measurement using light generated from a similar system (this time with a trapped hole rather than a trapped electron) that is embedded in a micropillar cavity, we find that the cavity enhancement of the transition results is higher indistinguishabilities. We make use of this bright source of indistinguishable photons to perform an on-chip quantum enhanced measurement and observe the phase superresolution associated with N00N states. In the final experimental chapter, we propose and implement a scheme to generate multi- qubit single photon states. We show that by repeatedly driving a micropillar-cavity-enhanced Raman transition of a single-hole-charged quantum dot in a Voigt geometry magnetic field it is possible to coherently superpose a photon across multiple time bins. The scheme is conceptually similar to proposed schemes for producing multi-photon entangled states. Lastly, we propose a scheme that makes use of the capabilities shown in the three experimental chapters to overcome several of the experimental difficulties associated with generating multi-photon entangled states.Funded by the EPSRC Centre for Doctoral Training in Photonic Systems Development and Toshiba Research Europe Ltd