Abstract: 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.
Publication Year: 2002
Publication Date: 2002-07-10
Language: en
Type: article
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Cited By Count: 1
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