Figure 2
Experimental setup: The state source is composed by a 2-mm-thick
(BBO) nonlinear crystal
pumped by a cw krypton laser operating at 413 nm, generating photon pairs at 826 nm by type-II spontaneous parametric down-conversion. Crystal
is cut and oriented to generate either one of the polarization-entangled Bell states
or
. Walk-off and phase compensation is provided by the half-wave plate
followed by a 1-mm-thick BBO crystal
[
23], together with two 1-mm-thick crystalline quartz plates
inserted in one of the down-converted photon paths. The unconverted laser beam transmitted by crystal
is discarded by means of a dichroic mirror
. The detection stages are composed by photon counting diode modules
and
, preceded by 8 nm full width at half maximum interference filters
and
centered at 825 nm, and by circular apertures
of
and
of
. Single and coincidence counts with 5 ns resolving time are registered by a computer-controlled electronic module (CC). Polarization analyzers are composed of quarter-wave plates
and
, half-wave plates
and
, followed by polarizing cubes
and
. The state source produces state
. For each pair, the photon emerging in the upper path goes straight to the polarization analyzer and to the detection stage 1. The lower-path photon is directed by mirror
through the circular aperture
into the state mixer (an unbalanced Michelson interferometer), composed by the beam splitter BS, mirrors
and
, quarter-wave plates
and
, variable circular apertures
and
, and by the half-wave plate
, whose purpose is to compensate for an unwanted slight polarization rotation caused by the beam splitter. The quarter-wave plate
is switched off, which means that if the lower photon follows the path labeled 4, there is no change to its polarization and the half-wave plate
changes the state to
. On the other hand, if the lower photon follows the path labeled 5,
is oriented with the fast axis at 45° in order to flip its polarization. The path length difference, 130 mm, is much larger than the coherence length of the down-converted fields, ensuring an incoherent recombination at BS. The pair detected by CC is in state
, where
is defined by the relative sizes of apertures
and
.
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