Cryptography Reference
In-Depth Information
the circulator (C) in D2. To implement the BB84 protocol, Alice's electronics
detects the incoming pulse and applies randomly a phase shift of 0 ,
π
,
π
or,
3
2 on the second pulse with a phase modulator PM A . Bob's electronics gen-
erates the laser pulses, and chooses the measurement basis upon applying a
0or
π/
2 phase shift on the first pulse on its way back. Finally, Bob gates the
detector when the photon is supposed to arrive and registers the click. Alice
and Bob's systems are connected to computers that communicate with each
other via ethernet or any other public classical channel.
The stability of the autoaligning interferometer has been tested in the
field [27]. Over terrestrial cables longer than 60 km and aerial cables longer
than 10 km, interference visibilities higher than 99.5% have been measured.
This means that the interference is almost perfect and does not introduce
significant bit errors. Indeed, almost all bit errors (or the quantum bit error
rate (QBER)) are due to the noise of the detectors. On one hand, the probability
that a photon arrives at Bob's decreases exponentially with the distance due
to the losses in the optical fiber; on the other hand, the dark count probability
is constant (a dark count is an event where a single-photon detector clicks
although there is no photon). Therefore the QBER increases exponentially
with distance, which limits the range of QKD, knowing that key distillation
becomes very inefficient above a QBER of 10%. At present, InGaAs
avalanche photodiode are used as photon counters for the 1550 nm telecom
window. These detectors have dark count probabilities of the order of 10 6
per gate (time window of about 1 ns), limiting QKD to distances of about
100 km. A couple of experiments have been performed over distances in this
range [27-29]. Today, pulse rates are in the order of a few MHz, leading to
net key creation rates in the order of 1500 and 50 Hz over 20 and 70 km,
respectively [27]. Again, the performance of the available photon counters
will eventually limit the maximal pulse rate. Nevertheless, faint laser QKD
has left the lab: commercial systems are available (Figure 2.6) [30] and it is
also being considered seriously from a military perspective, see Chapter 3 of
this topic.
π/
Figure 2.6
Commercial QKD-system based on the plug & play setup.
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