The Bridge - February 2018 - 25

Quantum Cryptography and Side Channel Attacks

Due to the design of the BB84 receiver (see Figure
1), all backflash photons acquire specific polarization
information as they exit their originating APD and
travel through a sequence of polarizing beamsplitters
and half-wave plates. For instance, APD1, being
parallel to its polarizing beamsplitter, is responsible
for measuring H-polarized photons. Therefore, when
backflash photons exit APD1, they return through
the polarizing beamsplitter where only H light is
transmitted, and photons of different polarizations
are reflected and lost.

Figure 3: Schematic of the experiment setup used by
the authors to investigate BB84 backflashes.

Those H photons then reach the main beamsplitter,
where half of them are transmitted through to the
quantum channel and the other half are lost. A
similar process occurs with the remaining APDs:
backflashes from APD2 reach the quantum channel
as V photons and APDs 3 and 4 also produce D and
A-polarized backflashes, respectively, as they exit the
polarizing beamsplitter and half-wave plate set to 45
degrees.
It is important to emphasize that there is no direct
coherence, classical or quantum, between the
incident photons from Alice and the backflash
photons. This is because the original photon is
completely destroyed in the measurement process
within the APDs. The resulting backflash creates a
pulse of light that is created simply as a result of hot
carrier relaxation. However, as those backflashes exit
the BB84 receiver, the key polarization information is
imprinted onto them by the various optical elements

used. Additionally, a single QKD photon may
create many backflash photons, allowing Eve to
measure the polarization of the QKD signal with
very high fidelity.
5.1 Experimental Apparatus
Figure 3 provides an overview of the experimental
setup. A function generator acts as a pulse generator
and serves as the timekeeper for the entire
apparatus by setting the reference time. It triggers
a 850 nm pulsed diode laser. A variable attenuator
significantly reduces the laser's
power so that it generates faint
pulses in an approximation of a
single photon source.
The resulting photons are
then directed through through
a linear polarizer set to 45
degrees with respect to the
fast and slow axes of the
immediately following quarter
wave plate. This transforms
the incoming photons into circular polarization.
This is necessary in order to ensure the balanced
distribution of all incoming photons among the four
BB84 detectors as polarized beamsplitters evenly
split circularly polarized light. The photons then pass
through a neutral density (ND) filter with an optical
density of 3.0 rotated ≈ 20 degrees off the beam
axis and then routed to the BB84 receiver (see
Figure 1 for details). Each BB84 measurement port
contains a Si APD for photodetection. Some of the
secondary photons resulting from the avalanches
within those APDs manage to escape into the BB84
receiver, becoming backflash photons. A portion of
those photons then exit the BB84 receiver (purple
beam in Figure 3) and find their way back to the
ND filter. The backflash photons reflect off the ND
filter and coupled into a multimode optical fiber and
detected by a silicon APD.
To permit the investigation of polarization correlations
between individual backflash signals and detection

HKN.ORG

25
http://www.HKN.ORG
Table of Contents for the Digital Edition of The Bridge - February 2018
