Advanced quantum light sources for quantum networking

Abstract

All of quantum photonics relies on being able to reliably generate quantum states encoded in a particular degree of freedom of light. A key piece of technology is therefore the photon source. The choice of which degree of freedom to encode information in is an interesting one, there is no universal best option. The historically common option of polarisation is straightforward to manipulate and detect, but is restricted to a two dimensional Hilbert space. More modern choices such as orbital angular momentum and path encodings have become popular as they are high dimensional and offer greater information capacity per photon. The downside of these options is they are difficult to integrate into existing communication networks. Time and frequency are in a unique position of being naturally high dimensional and compatible with single-mode fibre which makes them compatible with standard telecommunication equipment. The first half of this thesis is about generating time-frequency encoded quantum states in a scalable, lossless and arguably simpler way than other techniques commonly used to generate time-frequency encoded states. This is done through the process of domain-engineering in parametric downconversion. This thesis will walk through the basics of nonlinear optics and particular three-wave mixing before going on to discuss the principles of domain-engineering and how it can be used to manipulate the time-frequency structure of photon pairs produced in parametric downconversion. The experimental characterisation of a high dimensional frequency-bin entangled source is discussed along with other potential time-frequency states which could be generated using the same domain-engineering techniques. The second half of the thesis is centred around building a bright and low noise single-photon source at telecommunication wavelengths using a frequency converted quantum dot. The performance of the source before and after conversion is compared with the end result that the frequency conversion process does not significantly alter the single-photon nature of the source. Using the mathematical machinery developed to describe three-wave mixing, we show how the time-frequency properties of quantum dots can be improved using frequency conversion. With a bright and low noise source realised, a demonstration of quantum key distribution is carried out. The range and key rate of this demonstration compare favourably to to other single-photon sources in the literature. Finally, theoretical predictions of a decoy state quantum key distribution protocol using this source are carried out which extends the range by around 100 km compared to the protocol without decoy states.