Diploma thesis

2 Introduction
The reason why truth is so much
stranger than fiction, is that there is no
need for it to be consistent.
Mark Twain
Quantum mechanics has been one of the most important paradigm change in physics
to this day. Initiated by Max Planck with his hypothesis of quantised black body
radiation in the year 1900 and Einsteins explanation of the photoelectric effect 1905,
the photon theory was one of the first achievements of quantum theory. Yet it took
over 50 years for the first undeniable proof of its existence by Kimble et. al. 1977
[1].
It is safe to assume that neither Einstein nor Planck, at the beginning of the
20th century, ever considered the generation of single photons. However, the advent
of quantum information and the rapid progress in quantum information processing
with linear optical gates [2] and cluster states [3] has placed high demands on
the generation of single photons. Required are pure deterministic or at least pure
heralded single photon states.
At the dawn of the 21st century, single photon sources of all kind have emerged.
They came a long way from the first experiments with strongly attenuated lasers
beams some decades ago. They range from quantum dots in pillar microcavities [4],
falling neutral atoms [5], trapped ions in cavities [6], defects in diamond nanocrystals
[7], a single molecule in solid [8], four-wave mixing [9], to parametric downconversion
[10, 11, 12]. They all display completely different approaches to the same goal,
explored by scientists around the world.
Until recently, it has been very difficult to generate the desired pure single photon
states. In October 2007, a new type of pure heralded single photon sources has
been presented [10]. The authors rely on parametric downconversion in bulk KDP,
pumped by a ultrashort laser source.
In this thesis, we take a deep dive into this thrilling field of nonlinear optics
and theoretically investigate different methods to implement similar single photon
sources. We push this approach forward, with ideas for a higher brightness and
a more straightforward implementation in KTP. Waveguiding structures embedded
in nonlinear crystals are a perfect candidate. They exhibit a much higher
brightness than bulk crystals. We explore the possibility to shift the wavelength of
the generated photons up to the telecommunication wavelength of 1550 nm, where
propagation loss through optical fibers is minimal. On the experimental side, we
characterised different sample waveguides in the laboratory with second harmonic
generation. The results enable us to compare the developed theory with obtained
3