The Bridge - February 2018 - 26

Feature
events in the 4-Channel APD (the originating APD
of the backflash), we rotate a linear polarizer in
front of the fiber capture to scan through the linear
polarization Hilbert space. Each of the five output
signals from the five APDs used is routed
to a timetagger device that records the exact times
of all individual photodetection events.
5.2 Experimental Results
Since we are interested in observing correlations
between backflash polarization and individual APDs
in the 4-Channel APD, during the data-processing
step we isolate backflash events associated with
a particular APD. We achieve this by identifying all
photodetection times from the backflash detecting
APD that occur within a specific time window after
the firing of one of the four APDs in the BB84
receiver since that backflash photon must have
originated from that BB84 APD.
Note that this approach does
not handle the case where
additional backflash events
from other APDs happen to
occur whether because of a
pulse with multiple photons or
spontaneous emission within the
same time window and therefore
are incorrectly attributed to this
APD. However, we make the
reasonable assumption that such
peculiar events are rare and
their effects will be masked by
legitimate backflashes that occur
much more frequently.
Rotating the linear polarizer, we
observe that the intensities of
backflash events associated with
each APD change as a function of the polarizer angle
due to the hypothesized polarization dependence of
the backflashes. Therefore, we are able to measure
the maximum and minimum intensities of the
backflashes from each APD.

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We calculate the fringe visibility
(3)

where Imax is the maximum observed intensity and
Imin is the minimum observed intensity. We apply
this concept to our backflash measurements to give
a quantitative measure of our ability to measure a
specific polarization in the backflash data.
Figure 4 presents the visibility values for the five
primary slices from each APD with ν = 0 indicating
minimum visibility and ν = 1 indicating maximum
visibility. With visibility values generally above 0.5,
it is clear that we do have an ability to identify the
polarization angles associated with maximum and
minimum counts for each APD.

Figure 4: Visibility (ν) values for APDs 1-4. As backflashes
are not always generated at the same time after the
photodetection, they are detected by the backflashdetecting APD over a small range of times. Hence the
five time "slices" for each APD.

Table of Contents for the Digital Edition of The Bridge - February 2018