Time-bin entanglement with a single quantum dot

This thesis presents the development of an on-demand source of time-bin entangled photons, created from the biexciton-exciton radiative cascade in a single quantum dot. To achieve a source of time-bin entangled photons, we determined that the quantum dot must have two key properties: a dephasing time that is longer than the lifetime and high count rates, which are necessary to perform the correlation measurements that prove the entanglement. Theoretical calculations are presented, which demonstrate the violation of Bell’s inequality after post-selection of emitted photons that originate from the same time-bin and travel the same path of a Mach-Zehnder interferometer. Two different types of quantum dots are studied by optical spectroscopy to determine which quantum dot suits the requirements for time-bin entanglement best: InAs/GaAs self-assembled quantum dots embedded in a planar microcavity and InAsP quantum dots embedded in tapered nanowire waveguides. The planar microcavity and nanowire increase the measured count rates of the quantum dot since they make the photon emission more directional. As a result, count rates of up to 300,000 counts per second have been measured. We conclude that the InAs/GaAs self-assembled quantum dots satisfy the requirements for a source of time-bin entangled photons since they have a long exciton dephasing time, a short exciton lifetime, and high count rates. Finally, we conclude that resonant excitation of the quantum dot is essential for time-bin entangled photons; otherwise, the phonons reveal the time of emission to the environment and thus degrade the degree of entanglement. Initial experiments towards resonant two-photon excitation of the biexciton in a quantum dot are designed and tested. The measured laser extinction ratio is 105, achieved by crossed polarizations only. This extinction ratio is only two orders of magnitude away from achieving resonant excitation necessary for the time-bin entanglement measurements.