26 quED Manual www.qutools.com
Bell state discrimination
The two Bell states:
can be directly prepared in the quED by rotating the pre-compensation crystal, please see
sections 1.1 for more details. They both show correlations in the horizontal/vertical basis, i.e.
maximum coincidence count rates are observed for the polarizer combinations
and minimum coincidence count rates for
. However, in the diagonal basis the two states:
can be easily distinguished by the polarization measurement; here the symbols
and
represent the diagonal polarization states of photons:
Whereas
shows correlations in the diagonal basis and the maximum is observed for the
polarizer combinations , the state
shows
anticorrelations and the maximum is observed for the polarizer combinations
.
By inserting the half-wave plate (optionally delivered with the quED) rotated at into one of
the down-conversion arms one can prepare the two other Bell states
and
showing
anti-correlations in the horizontal/vertical basis. Analogously to -class Bell states -states can
be distinguished in the diagonal basis.
Bell experiment
The most widespread version of Bell’s inequality used in experimental tests is the one from
Clauser
,
Horne
,
Shimony
and
Holt
(hereafter referred to as CHSH). The main idea behind the
CHSH-Bell inequality is that, in local realistic theories, the absolute value of a particular
combination of correlations between two particles is bounded by 2:
where
and
denote the local measurement settings of two observers, each receiving
one of the particles. In our particular experimental configuration with entangled photons
and
are the orientations of polarizing filters positioned in the two arms of the source.
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