www.qutools.com quED Manual 7
Therefore, an additional birefringent crystal has to be put behind the down-conversion crystals
to counteract this second effect, too. The described double-crystal compensation technique
ensures a complete temporal indistinguishability of the emission processes even though a free-
running laser diode as a pump source is used and no spectral filtering of generated photons is
applied in the quED.
Description
Referring to Fig. 3, there is shown the source for the generation of polarization-entangled
photon pairs. For convenience, the whole optical set-up mounted on an aluminum breadboard
can be divided into two basic blocks: the pump-beam block and the down-conversion block. All
the components of the pump assembly are installed on a rectangular pedestal. This block
comprises a laser diode head, two mirrors, beam-shaping optics, two half-wave retarder, a pre-
compensation birefringent crystal, and nonlinear down-conversion crystals. The blue laser
diode is built in the laser diode mount. The mount features a protective circuit to maximally
eliminate laser diode failures due to electrostatic discharge and a thermoelectric module to
precisely regulate the operating temperature of the diode. The strongly divergent light from the
laser diode is focused using a telescope consisting of a collimating aspheric lens and a negative
spherical lens.
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