Spatially Structured Light for High-Dimensional Quantum Information

Abstract

Encoding information on single photons of light is an important part of many quantum information applications. There are many photonic degrees of freedom which can be used for encoding quantum information, and we refer to these diverse states as structured light. Structured light is used for many quantum information such as quantum key distribution, entanglement distribution, sensing, and quantum walks. One advantage of structured light is that we can encode more than 1 bit of information on a single photon. This has obvious benefits for quantum communication in allowing one to increase the information density of the communication channel. However another benefit which comes out of the security proofs of quantum communication protocols is an increased tolerance to errors. This can be useful if one want to communicate in a noisy environment. In this thesis we focus on the transverse spatial modes of light, in particular the Laguerre-Gaussian modes and how these can be used for high-dimensional quantum protocols. This culminates in three works detailing new quantum protocols and the implementation of an adaptive optics system to improve key rates in free-space channels. We present a new protocol for quantum key distribution which is a high-dimensional extension of the round-robin differential-phase-shift protocol and experimentally demonstrate the protocol using Laguerre-Gaussian modes with an azimuthal index up to ℓ = 15. We also describe a high-dimensional extension to quantum certified deletion. First however, we discuss the liquid crystal devices that are used to generate these states and show the kinds of interesting light that can be produced. We detail an experimental demonstration of magic windows using liquid crystal devices, and show a new and optimal method for diffractive focussing which is confirmed experimentally with liquid crystal devices. We also present 2 works on photon pair sources which are a fundamental piece of quantum communication systems. We describe a method for tailoring the symmetry of momentum entangled states and show the full Laguerre-Gaussian mode correlations from an SPDC source. Many different technological steps are required for the implementation of quantum information protocols, and here we demonstrate advances across many of these steps from states preparation to source characterization to protocol implementation.