New devices and techniques for quantum information protocols

Abstract

Quantum technologies aim to make direct use of the physics of the very small to revolutionise how we sense, compute, and communicate information. Quantum communications promises theoretically unbreakable future security and fully operational quantum key distribution (QKD) systems are already available commercially. As the information carrier of choice for quantum communications is the photon, uptake will be facilitated by the increased reproducibility and scalability offered by integrated photonic devices. The first experimental section of the thesis makes direct use of fibre-coupled femtosecond laser written (FLW) devices in two experimental studies. The first presents an implementation of the phase-sensitive state comparison amplifier, or SCAMP, and represents the first example of probabilistic amplification making use of an integrated component. This demonstrates the ability of fibre-coupled FLW devices to integrate seamlessly into modern telecommunications infrastructure. The second study proposes a novel technique using the photonic lantern as a low-loss temporal multiplexer for the output of a quantum communications receiver. The technique trades optimal key generation rate for cost effectiveness by facilitating single-detector QKD. Femtosecond laser writing is unique so far among integrated photonic platforms in its ability to implement the photonic lantern; the technique is proved in principle via characterisation of a polarisation Bennett-Brassard 1984 QKD receiver temporally multiplexed by an FLW photonic lantern combined with staggered optical delays. This experiment represents the first use case of the photonic lantern in a quantum communications protocol and is an important step towards mass consumer uptake. The second experimental section introduces a creative new technique for manipulation of single-photon level thermal light, displaced photon subtraction, which presents as an optical loss but allows manipulation of the relative amplification and suppression of the conditioned output mean photon number via the displacement amplitude. The operation is translated to a realistic experimental implementation and proved in principle using commercially available fibre optical components, showing its immediate compatibility with current infrastructure and potential compatibility with integrated platforms. This technique represents a new tool for protocols in quantum information that make use of thermal light which have enjoyed a recent resurgence of interest in the literature as an easy to produce source of two-mode classically correlated single-photon level light.