The Bridge - February 2018 - 24

Feature
is raised to a breakdown voltage Vb. At this voltage,
absorption by even a single photon is sufficient
to create carriers in the conduction band of the
diode, which can then trigger an avalanche process,
creating a measurable current in the milliamp
range indicating photon detection.
Different semiconductor materials
are used for photons of different
wavelengths. Silicon (Si) APDs are
most sensitive to wavelengths in
the 400-1000 nm range (ideal
for laboratory experiments), while
alternative semiconductor materials
such as indium gallium arsenide are
well suited for telecommunication
wavelengths of 1300 nm or
1550 nm (ideal for long distance
transmission).
When photons strike the semiconductor material,
they cause electron excitations, creating electronhole pairs. The electrons function as charge carriers
(i.e. hot carriers or free carriers) and multiply, causing
the avalanche. Interestingly, a photoemission also
occurs as a result of those avalanches. Explanations
for those photoemissions are varied and include
radiative electron-hole recombination and carrier
energy relaxation (direct, phonon-assisted, or
Bremsstrahlung) [24, 25]. Those secondary photons
may be either absorbed in a quiescent region of the
semiconductor, initiating new avalanches, or they
may find their way out of the detector [4].
Photons that manage to escape the detector could
potentially be a source of information leakage when
APDs are used in a QKD implementation. Such
backflash photons could couple with the optical fiber
leading into the APDs, find their way through Bob's
QKD receiver, and out into the quantum channel
traveling in the opposite direction of the incident
photons from Alice. An eavesdropper could then
discreetly siphon off the backflash photons, leaving
the original photons untouched. If those backflash
photons carry meaningful information corresponding

THE BRIDGE

to the information carried by the original photons,
Eve could potentially perform measurements on
her backflash photons to gain information about the
results of Bob's measurements and, in extension,
the secret key.

Figure 2: Overview of a potential QKD attack exploiting
APD backflash emission.

Since the creation of backflash photons is a random
process, Alice and Bob have no knowledge of or
control over photons that escape Bob's receiver.
Thus, this attack allows Eve to obtain copies of
the photons used by Alice and Bob without their
knowledge, and thus leave the QBER unaffected.
Consequently, this represents a dangerous potential
QKD vulnerability.

5 DEMONSTRATION OF INFORMATION
LEAKAGE FROM APD BACK-FLASHES
We consider the hypothetical attack scenario shown
in Figure 2. Alice and Bob are using a fiber-based
BB84 QKD implementation with polarization
encoding. It is assumed that Eve has full access to
all classical communications between Alice and Bob.
We also propose that Eve is able to tap into the
quantum channel, perhaps with the use of an optical
circulator. This would allow all the photons sent by
Alice to Bob to travel uninterrupted, but diverts to
Eve the backflash photons traveling in the opposite
direction.

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