Academic Publications

This thesis describes the first demonstration of telecom optical fibre-based quantum key distribution using single photons from an InAs/GaAs quantum dot. The generation of single photons with wavelengths around 1.3 μm has been achieved by growing a low density of large InAs/GaAs quantum dots through molecular beam epitaxy techniques. In order to efficiently select and preferentially couple long-wavelength emission into single-mode optical fibre, the InAs quantum dots were incorporated into pillar microcavity structures.

Micro-photoluminescence measurements reveal a number of excitonic complexes in the studied quantum dots, including a biexciton cascade and charged excitonic emission. Strong enhancement of the collected intensity of the emission from excitonic states was achieved when temperature tuned onto resonance with the fundamental mode of the pillar microcavity, as compared to the off- resonance intensity. Purcell enhancement of the spontaneous emission rate is also demonstrated.

Strongly correlated photon emission is observed, not only within a single emission line but also between different emission lines such as emission from a biexciton-exciton cascade. Polarization correlations between the biexciton and exciton states show that the quantum dots measured have good potential to generate pairs of entangled photons, provided that the fine structure splitting can be reduced. High resolution spectroscopy is achieved by introducing a Fabry-Perot etalon with a linewidth significantly less than that of the quantum dot. The enhanced resolution allows deeper investigation into the fine structure splitting of the exciton state, and provides a useful tool for measuring samples where pairs of entangled photons are sought.

The lifetime of the emission and the suppression of multi-photon emission is compared by optical excitation energies tuned above and below the bandgap energy of GaAs. Exciting the quantum dots with below band excitation yield significant improvements in the performance of the source

in terms of jitter on the emission time and suppression of g measured and the corresponding correlation curve is presented.

The combination of the high quality emission and the enhanced photon extraction efficiency due to the presence of a cavity, enabled the incorporation of the single photon source into a BB84 phase encoded quantum key distribution system. Key transfer with the quantum dot source is shown to have a quantitative advantage at long distances compared with that which could be securely achieved using uniform intensity laser pulses at 1310 nm in the same system.