School of Physics - Theses
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ItemRelative intensity squeezing: by four-wave mixing in rubidium( 2010)This thesis is a theoretical and experimental study of the production of relative intensity squeezed light through four-wave mixing in a rubidium vapour. Relative intensity squeezing enhances measurement precision by using quantum-correlated “twin beams” to eliminate photon shot-noise. The “double-^” four-wave mixing process produces twin beams by stimulating a four-stage cyclical transition resulting in the emission of time-correlated “probe” and “conjugate” photons. Measuring and subtracting the corresponding beam intensities cancels the photon shot-noise, enabling measurements beyond the shot-noise limit. An ab initio analysis of the double-^ scheme determined the experimental phase-matching conditions required to generate efficient mixing. Expressions for the expected level of squeezing were then derived. Deviations from perfect matching were considered and a spatial bandwidth for the mixing process was derived. This bandwidth was used to explain recent experiments obtaining multi-mode squeezed light from this system. Optical losses are an experimental inevitability that destroy quantum correlations by randomly ejecting photons. Expressions were derived to quantify the degradation of squeezing caused by losses. Sensitivity to unbalanced losses was exhibited and an optimum level of relative beam loss was observed. Near-resonant absorption within the vapour causes losses to compete with squeezing, so an interleaved gain/loss model was formulated to analyse the interplay of the two processes. A novel theoretical framework was developed and used to derive expressions for the level of squeezing produced in the presence of absorption. Four-wave mixing resonances were observed experimentally and the intensity noise spectra of the resulting beams were characterised. Gain dependence on beam power, cell temperature and laser detuning was determined. Relative intensity squeezing of 3 dB was demonstrated and physical insight into the experimental results was gained through analysis with the theoretical model. Factors limiting the measured level of squeezing are discussed and design improvements proposed.