The Bridge - February 2018 - 23

Quantum Cryptography and Side Channel Attacks

is ultimately detected by a photodetector (out of
four total) that is associated with that polarization
state. Hence, when Bob receives a signal from a
photodetector associated with a certain polarization
state, he knows that he has received a photon in
that polarization state.
Figure 1 shows a typical BB84 receiver that achieves
this measurement of the photon state. It consists of
two arms, each with two single photon detectors.
Each arm is associated with one of the two BB84
bases (HV or AD), and is responsible for measuring
photons in its basis. The 50-50 beamsplitter that
connect the two arms performs the step of randomly
choosing the basis for Bob by sending the incoming
photons into either the HV or the AD arm with
equal probabilities.

continues straight through and is detected by an
avalanche photodiode (APD), a common type of
photodetector. Thus, when Bob sees this specific
APD fire, he knows that he has chosen the HV
basis, and the result of his measurement was H
(which can be translated to a bit value). Similarly, a
V polarized photon gets deflected by the polarizing
beamsplitter and is directed into the second APD in
the HV arm, registering as a V measurement.
On the other hand, if the photon is directed into the
second measurement arm by the first beamsplitter,
then Bob is measuring in the AD basis. As the
beamsplitters measure in the HV basis, Bob needs
to first rotate the polarization of the incoming AD
photon by π/4 using a half-wave plate (HWP) so
that a D photon becomes an H photon and an A
photon becomes a V
photon. The remainder
of the measurement
process is the same as in
the HV arm except that
when one of those APDs
fire, Bob knows he is
measuring in the
AD basis and records an
A measurement for one
and a D for the other.

Figure 1: Schematic of a typical BB84 receiver layout.
Figure credit: Stephen Pappas, MITRE & Colin Lualdi.

If the photon travels straight through the first
beamsplitter, it enters the HV arm and Bob is
measuring in this basis. The measurement process
begins when the photon passes through a polarizing
beamsplitter that directs the photon according to
its polarization. For example, an H polarized photon

Those APDs have design
flaw that could serve as
a source of information
leakage. First pointed out
by Kurtsiefer et al., this attack relies on the curious
fact that avalanche photodiodes are known to emit
backflash photons upon photonic stimulation due to
the physics of those detectors [6].
Their operating principle lies in the use of a
semiconductor p-n junction. Prior to single photon
detection, the reverse bias voltage of the p-n junction

ost commercial fiber-based QKD systems actually employ alternative encoding schemes due to the susceptibility of the polarization states
M
to disturbances from birefringence and other environmental effects that are difficult to manage outside of the laboratory setting [22].

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