Effiziente Erzeugung verschränkter Photonenpaare

Efficient Generation of Entangled Photon-Pairs The first experiments
with correlated photons have been performed in the context of
EPR-Bell experiments on the realistic and local properties of
quantum mechanics. The source used there produced pairs of
polarization entangled photons from a 2-photon decay of Calcium
atoms. The technical requirements of these experiments were high
(vacuum systems, stronge dye-lasers, etc.) whereas the efficiency of
the source was quite low. An important step forward was the
introduction of spontaneous parametric down conversion (SPDC),
which has become the most common source in quantum optics for
generating correlated or entangled photon pairs. In this process
photons of an intense pump laser convert to photon pairs in an
optical nonlinear crystal. Conservation of energy and momentum
leads to strong correlations between the generated photons. With
this kind of two-photon source it was possible to realize or
improve many experiments on the foundations of quantum mechanics
addressing the EPR-Paradoxon and in the new field of quantum
information. But again, more advanced experiments and applications
suffer from the limited ef- ficiency of the fluorescence process. Many
photon pairs are lost by spatial and spectral filtering, which is
necessary to achieve polarization entanglement and long coherence
times. Different techniques have been implemented to increase the
number of photon pairs using two-crystal arrangements, focusing
techniques or periodically poled crystals. Most of these methods
have the disadvantage that no entangled photons have been observed.
It is the aim of this work to increase the yield and to improve the
mode definition of entangled photon pairs generated by resonant
enhancement of the pump mode and the fluorescence modes. As a first
step a linear cavity for the pump mode was realized. Since the
conversion probability is proportional to the pump power it was
possible to increase the photon pair count rate by factor of 7 over
the previous source. Besides the possibility of further improvement
on already established pair correlation experiments, such an
enhancement allows to build a compact source for photon pairs, in
which an expensive argon-ion laser is replaced by a cheap diode
laser. Among other applications such sources are of strong interest
for quantum cryptography. 3 In many quantum information experiments
optical fibers are use to carry the photons over long distance.
Therefore, light from the parametric down-conversion source has to
be efficiently coupled into fibers. In the second part we report on a
new method to optimize collection efficiency by matching the angular
distribution of the parametric fluorescence to the spatial mode of
an optical fiber. By using this technique, we detected 366500
polarization-entangled photon pairs per second in the near-infrared
region in single-mode optical fibers for 465 mW pump power (at 351.1
nm) with a 2 mm BBOcrystal. The entanglement of the photon pairs
was verified by measuring polarization correlations of more than 96%
in a HV-basis and in a ±45-basis. To our knowledge, such enormous
count rates of highly entangled photon pairs have not been reached
yet with any other technique. In the third part of this thesis we
investigated the process of parametric downconversion in a cavity
which is resonant to certain longitudinal down-conversion modes
only. The idea of placing the parametric down-conversion source
inside a cavity is not new. Such a device is usually referred to as
a single or double resonant optical parametric oscillator (OPO) and
is mainly used to generate squeezed quantum states. In that kind of
application the system is operating close to but still under the
threshold of oscillation. In our application the situation is quite
different. The system is operating far below threshold so that
mainly spontaneous emission occurs. In that mode correlations
between single photons can still be observed. But bouncing the
light back and forth inside the cavity increases the interaction
length and hence enhances the signal levels of the down-conversion
fields. Further, by resonating two certain modes only, the bandwidth
is reduced by orders of magnitude and the coherence time is found
to be inverse proportional to the bandwidth. A similar experiment
has already been realized with a type-I parametric down-converter
in the resonator. We have tried to realize a compact double
resonant OPO far below threshold with a type-II parametric
down-converter in a high-finesse cavity to realize a bright source
of entangled photon pairs with extremely narrow bandwidth.