Resources for Integrated Quantum Sensing in the Mid-infrared

Quantum states of light, with sub-classical noise statistics, are heraldedas a potential route towards enhanced absorption spectroscopy. In thisthesis we develop key infrastructure in the pursuit of quantum-enhancedabsorption spectroscopy in the 2 μm-band via the adoption of integrated silicon photonics as a deployable and scalable solution.Characterising quantum states of light in the 2 μm-band requires shot-noise limited homodyne detectors and so we start by presenting the design and characterisation of a homodyne detector that we use to make the first observation of megahertz speed vacuum shot-noise in this band. The device, designed primarily for pulsed illumination, has a 3-dB bandwidth of 13.2 MHz, total conversion efficiency of 57% at 2.07 μm, and a common-mode rejection ratio of 48 dB at 39.2 MHz.We then utilise a silicon chip to implement an all-optical noise suppressionscheme aimed at reducing the intensity noise of state-of-the-art pulsed lasers in this band via nonlinear interferometry. We find initial designs capable of noise suppression but with the addition of unwanted noise amplification from modulation instability.In the final results chapter we look to expand the applicability of quantumstates in absorption spectroscopy by analysing the effect of sample saturationon estimate precision in absorption measurements. We compare both classicaland quantum probes. A limit is derived on the maximum precision gained fromusing a nonclassical probe and a measurement strategy for saturating this bound is presented. Finally, we evaluate amplitude-squeezed light as a viable route to gaining a quantum advantage under saturation