Quantum sensing and imaging in the infrared
Creators: name: Pearce, Emma Louise
Publisher: name: Imperial College London
Year: 2023

description: Infrared sensing and imaging is invaluable to many areas of research for its ability to probe molecular responses. However, these investigations are often limited by poor detection technology compared to that available at visible wavelengths, as well as the cost of infrared lasers. This thesis presents methods to enhance sensing and imaging in the infrared by using non-degenerate photon pairs generated in nonlinear optical processes. The number correlations of photon pairs produced via spontaneous four-wave mixing in a polarisation-maintaining fibre are first exploited for noise rejection. Using a tuneable visible pump laser, infrared absorption spectroscopy is performed by heralding the infrared photons with their corresponding visible partner. This allows rejection of uncorrelated noise at the infrared wavelength which otherwise obscures the absorption feature of a gallium arsenide sample under investigation. A compact, portable device for infrared imaging with visible detection is presented. The system is comprised of a spontaneous parametric down-conversion crystal in an SU(1,1) nonlinear interferometer. Introducing an object in the infrared light can be observed as a change in the visible interference, without the need to detect the infrared photons. This is known as "imaging with undetected photons". A rapid analysis technique is also presented to allow real-time display of transmission and phase information. The system is the first of its kind to be demonstrated outside of a laboratory environment and shows important advances towards practical deployment of the technique. A detailed theory of the SU(1,1) interferometer is outlined to model the "undetected photon" approach, accounting for the effects of internal losses, detection losses, seeding, and gain on the interference visibility and contrast. The stimulated parametric down-conversion regime is then demonstrated experimentally to verify this, by introducing an infrared seed beam alongside the pump. Interference visibility is shown to reach above 90% with just microwatts of infrared power and no phase locking, hence compensating for losses in the system. The coherence length of the down-conversion is also drastically enhanced, with potential for measurements at range. Preliminary results in the high-gain regime are presented, also demonstrating a significant enhancement to interference visibility and contrast. The effect of high gain on image resolution and field-of-view is considered. The increased brightness of both seeding and high-gain promise to reduce camera exposure times, further to the goal of rapid acquisition and analysis of IR images as well as potential for imaging weakly reflective or highly absorbing targets. A gain-unbalanced interferometer is also explored, showing the ability to regain high visibility in the presence of signal loss but not idler loss. The unbalancing is also used to demonstrate operation in a regime where very few IR photons reach the sample, while retaining visibility and a detectable number of signal photons.