Imaging at the quantum noise limit and beyond: propagation effects in the generation of multi-spatial-mode squeezed light and optimised detection of a laser beam position

Fradgley, Ellie (2022). Imaging at the quantum noise limit and beyond: propagation effects in the generation of multi-spatial-mode squeezed light and optimised detection of a laser beam position. University of Birmingham. Ph.D.

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Abstract

Light is commonly used as a measurement tool in both scientific and everyday applications. It is increasingly true that such measurements are limited by the quantum nature of light, which means there is fundamental minimum level of noise present on any measurement made using a classical beam. Typically, this limitation can be overcome by increasing the optical power, however this is not always possible: too much power can damage a system, or introduce other problems which dominate over the quantum noise. This thesis explores two ways of improving the signal to noise ratio (SNR) of a system limited by quantum intensity fluctuations, specifically where the spatial distribution of the light is also significant. The first method, explored in the first part of this thesis, concerns the use of the nonlinear optical process of 4 wave mixing (4WM) to generate twin beams which exhibit localised intensity correlations. A high quantum efficiency camera is used to capture images of the beams, allowing the correlations to be studied. Correlations at low spatial frequencies are observed, however a distortion effect caused by the matched propagation of the beams as they pass through the nonlinear media prevents correlations from being observed at higher spatial frequencies. The impact of this matched propagation on the ability to use the produced squeezed light to make sub-shot noise measurements, is explored. A simulation of the propagation of the twin beams through the cell is produced and verified experimentally. There is evidence to suggest a nontrivial transverse displacement of the spatially correlated modes in the twin beams occurs which impedes the ability to see squeezing in higher spatial modes. With the aid of the simulation and experimental data it should be possible to account for the displacements in future measurements and produce a mapping of the correlations between the two beams. The second method, explored in the second part of this thesis, investigates an optimised scheme for detecting small displacements of a Gaussian beam. This is particularly desirable in applications such as atomic force microscopy (AFM), which currently can operate at the shot noise limit. By using a detection mode which is maximally sensitive to the displacement, improvements can be seen in the SNR of a measurement limited by shot noise without the need for advanced optics setups. Rather, the conventional split photodetector used to detect small displacements is replaced with a photodetector with a detection mode which saturates the Cramer-Rao bound of the measurement. A theoretical improvement to the SNR of π/2 compared to the conventional method of using a split photodetector is calculated. This is verified experimentally by building the split and optimised detectors and measuring the displacement of an optical beam. An improvement in the SNR of π/2 is observed.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):