Toward implementing conditional quantum logic on-chip using the Kerr nonlinearity
Creators: name: Begley, Stephen Patrick
Publisher: name: Imperial College London
Year: 2012

description: In this thesis, I investigate and develop techniques applicable to photonic quantum computing on a microscale chip, using dye molecules as both single-photon sources and as nonlinear media to mediate photon-photon interactions. An overview of linear optical quantum computing is presented to introduce the physical phenomena and measurement techniques relevant to this field of study, while highlighting the problems of the significant resource overhead required by schemes which do not utilise a nonlinearity. Potential challenges to using the Kerr nonlinearity to implement quantum logic are then discussed. Details of the experimental methods most relevant to developing a quantum logic gate harnessing the Kerr nonlinearity are introduced, including techniques to detect single photons using the second-order correlation function g2(t), the necessary light-matter interactions for capturing photons into a waveguide and coupling them to an embedded dye molecule, Mach-Zehnder interferometry and waveguide design. This lays the groundwork for the following chapters detailing the experimental setups and results obtained. The first task to be undertaken was to develop a microscope system capable of locating and focusing on single dye molecules in a condensed matter matrix. A confocal scanning microscope was built and control software developed. While the final setup would involve a cryostat the design phase did not require the cryostat to be in place. The combined system was tested using calibration grids and micron-sized scattering targets. Output could be collected by charge-coupled device (CCD) camera, or by raster-scanning a laser spot and reading detecting the scattered light on a photodiode or avalance photodiode (APD). The microscope proved capable of focusing incident 785nm excitation light to a diffraction-limited spot of sub-micron size. Clear images of calibration grids and microspheres embedded in crystalline polymethyl methacrylate (PMMA) were obtained with single-photon sensitivity. A sample of Dibenzoterrylene (DBT) in a solution of anthracene was spin-coated onto a glass slide and imaged by the microscope at room temperature, which yielded good images of anthracene crystals, but fluorescence from DBT could not be observed, despite an analysis of the system showing it to have a high enough collection efficiency to image any flouorescing molecules clearly. A spin-coated sample of DBT in PMMA was prepared to test the activity of the DBT stock solution at room temperature, and to provide insight into photobleaching rates and fluorescence stability within a solid state matrix. Bright fluorescence (&gt; 10 8 photons per second) was observed from this sample, and its photobleaching rate was modelled and fit to data in the saturated and unsaturated intensity regimes. A refined approach to producing DBT / Anthracene crystals is therefore required. It was determined that the number of fluorescing DBT molecules in spin-coated PMMA decayed by a factor of 1/e in a time of 1/(0:154 x I), where I is the incident intensity. Following this, Finite-Difference Time-Domain (FDTD) simulations of directional couplers were carried out to investigate the effect of varying the guide separation on the inter-guide coupling, and to quantify this for guides made from Silicon Nitride. Traditional directional coupler structures were modelled, and the rate of inter-guide coupling was found to vary exponentially with the guide separation with a decay constant of 0.006nm-1, and the approximations of calculating the expected coupling from the mode distribution and overlap were justified by. Novel 3-guide directional couplers were also modelled, and it was found that they also performed well as microscopic beamsplitters, and offered scope for further investigation.