Quantum networking with optimised parametric down-conversion sources

Abstract

Quantum information processing exploits superposition and entanglement to enable tasks in computation, communication and sensing that are classically inconceivable. Photonics is a leading platform for quantum information processing owing to the relative ease in which the encoding and manipulation of quantum information can be achieved, but there are a set of characteristics that photons themselves must exhibit in order to be useful. The ideal photon source for building up multi-qubit states needs to produce indistinguishable photons with high efficiency. Indistinguishability is crucial for minimising errors in two-photon interference, central to building larger states, while high heralding rates will be needed to overcome unfavourable loss scaling. Domain engineering in parametric down-conversion sources negates the need for lossy spectral filtering allowing one to satisfy these conditions inherently within the source design. Contained in this Thesis are two experimental investigations. Within the first investigation, we present a telecom-wavelength parametric down-conversion photon source that operates on the achievable limit of domain engineering. The source is capable of generating photons from independent sources which achieve two-photon interference visibilities of up to 98.6 ± 1.1% without narrow-band filtering. As a consequence, we can reach net heralding efficiencies of 67.5%, corresponding to collection efficiencies exceeding 90%. These sources enable us to efficiently generate multi-photon graph states, constituting the second experimental investigation. Graph states, and their underlying formalism, have been shown to be a valuable resource in quantum information processing. The generation and distribution of a 6-photon graph state—defining the topology of a quantum network—allows us to explore prospective issues with networks that invoke protocols beyond end-to-end primitives, where users only require local operations and projective measurements. In the case where multiple users wish to establish a common key for conference communication, our proof-of-principle experiment concludes that employing N-user key distribution methods over 2-user methods, results in a 2.13 ± 0.06 key rate advantage.