title: Quantum optics in the infrared: single-cycle THz field electro-optical sampling with single-photon detectors
Creators: name: Shields, Taylor
Publisher: name: University of Glasgow
Year: 2023

description: This thesis represents a summary of the activities I performed throughout my PhD, mainly on quantum optics in the infrared. The field of quantum optics was revolutionised by the experimental demonstration of correlated photon-pair generation via the non linear interaction known as spontaneous parametric down-conversion (SPDC). Quantum-enhanced optical systems utilising the unique properties of these quantum sources have blossomed in recent years particularly in the fields of quantum communications and quantum metrology. Currently, most measurements taking advantage of these non-classical sources have narrowed their focus on the near-infrared to telecommunications spectral region (0.7 − 1.6 µm) with good reason. Going beyond this spectral range, at longer wavelengths, presents significant technological problems in both generation, manipulation and detection of mid-infrared radiation. Fundamental advantages such as reduced scattering and propagation losses in well-established fields presents a compelling case to investigate non-classical states within the mid-infrared window. This thesis details the challenges associated with delving into the 2 µm regime and demonstrates the realisation of a compact and robust quantum source of entangled photon-pairs using custom designed non-linear crystals and superconducting nanowire single-photon detectors (SNSPD). The demonstration of two-photon interference and polarisation entanglement at 2.1 µm provides a solution which could prove valuable in the implementation of future freespace daylight quantum communications and high sensitivity metrology. This thesis follows up on this work by addressing a promising application of 2 µm sources in the field of integrated photonics. The testing of the performance of a type of novel hollow-core nested antiresonant nodeless fibre (HC-NANF) with mid-infrared radiation indicates high polarisation purity and low attenuation properties. This technology may provide the radical solution to ensure the future capacities of modern communication networks are met. The proposal to employ the quantum properties of entangled photon states generated via SPDC to enhance detection in wavelengths far beyond the mid-infrared is a challenging concept. This thesis moves in the direction of the application of non-classical states for the metrological detection of terahertz (THz) radiation of which has garnered significant interest in countless areas of scientific endeavour in recent times. It tackles the enhanced phase estimation granted by using NOON states in metrology applications and takes the first step in utilising alternative detection strategies for measuring THz radiation. Traditional techniques for measuring or reconstructing THz electric fields rely on balanced detection using photodiodes with inherent shot-noise limitations on the system. Terahertz time-domain spectroscopy (THz-TDS) using electro-optic sampling (EOS) and ultrashort pulsed probes is typically employed to measure directly the electric field of THz radiation. This thesis reports on the first step in the direction of using non-classical states for THz detection demonstrating that electric fields can be measured with single-photon detectors using a squeezed vacuum as the optical probe. The approach achieves THz electro-optical sampling using phase-locked single-photon detectors at the shot-noise limit and thus paves the way toward quantum-enhanced THz sensing.