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 (> 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.