Thomas Hummel

A thesis for the degree of Doctor of Philosophy defended August 2019.

The PhD School of Science, Faculty of Science, Quantum Optics, Niels Bohr Institute, University of Copenhagen

Supervisor:
Ravitej Uppu
Peter Lodahl

Multiphoton generation from a single quantum dot in a photonic nanostructure

Single photons are very actively pursued for quantum optical applications due to their robust nature against environmental influences. These applications benefit from deterministic generation of indistinguishable single photons. Quantum dot sources fabricated in a heterostructure with electrical contacts allows the control of coupling between a photonic crystal waveguide and the quantum dot. This control over the Purcell enhancement allows a tuneable efficiency of the source. The electrodes also allow the study of the Stark tuning of quantum dots and to analyse the effect of charge noise.

The control over quantum dot sources allows the deterministic generation of indistinguishable single photons. Generating multi-photon states using multiple quantum dot single photon sources induces problems since the generated photons are distinguishable from each other. This occurs due to the random growth nature of the quantum dots. Generating a multi photon state from a single quantum dot source requires a temporal-to-spatial mode converter (i.e. demultiplexer). An efficient 4 mode demultiplexer is demonstrated in this thesis with an output four fold coincidence rate of 1:05  0:05 Hz. Thorough analysis of all efficiencies from the source till detection shows that this is the most efficient demultiplexer yet. The generated four photon state can be used as a resource to generate heralded polarisation entanglement between two photons. With an indistinguishability between the photons of 95%, a maximum heralding efficiency of 81% can be achieved.

The entanglement gate is built with a port-to-port efficiency of 88%, averaged over all combinations. Incorporating end-to-end losses by coupling the entanglement gate to the demultiplexer results in a maximum achievable heralding efficiency of 31%. The demultiplexer is built such that it can be upgraded to 8 spatial modes. Adding a second heralded entanglement gate allows proof of principle experiments of device independent quantum key distribution.