20 quED Manual www.qutools.com
2. Verify the existence of correlations in the horizontal-vertical polarization basis
(maximum coincidence count rates for the filter combinations “horizontal/horizontal”
(HH) and “vertical/vertical” (VV) and minimum coincidence count rates for the filter
combinations “horizontal/vertical” (HV) and “vertical/horizontal” (VH)).
3. Rotate the polarizing filters to positions corresponding to polarization analysis in the
diagonal basis. Choose the combination of positions, which corresponds to the
expected minimum in coincidence count rate for the Bell state
(i.e. “+45° /-45°” (PM)
or “-45° /+45°” (MP)) and register the coincidence rate.
4. Rotate the polarizing filters to positions corresponding to maximum in coincidence
count rate in diagonal polarization basis and register the rate.
5. Calculate visibility of the correlations in the diagonal polarization basis according to
, where
is the maximum/minimum
coincidence count rate. If the visibility is lower than expected, we refer to section 4.1.
Troubleshooting
Improving the Entanglement Quality
In the following you can find a few issues to be addressed in order to reach a high quality of
polarization entanglement.
• In principle the amount of horizontally-polarized photon pairs detected during a time
unit should equal approximately the amount of vertically-polarized photon pairs. This
situation corresponds to the theoretical maximally-entangled state having equal
amplitudes of its two constituting terms, please see also section 1.1. Unfortunately, the
polarizers also have a small spatial effect on the modes when rotated due to a very
small yet finite wedge angle. Thus, one cannot reliably measure the ratio of
horizontally- and vertically-polarized photon pairs. If the ratio, however, is very
asymmetric (worse than ≈ 60/40) it is advisable to modify the coupling very carefully for
more symmetric rates. Geometrically, this means to move the overlap region of pump
and collection modes slightly back or forward in the BBO crystal (see Figure 9).
• In case the entanglement quality is still not satisfactory, this indicates that the
collection of the down-conversion photons is more or less asymmetric. The emission
cones of the two BBO crystals are preadjusted to overlap maximally. Due to the spectral
width, however, one can get reasonable coincidence rates from asymmetric collection
modes (see Figure 10) The procedure in Figure 11 aims to correct this error.
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