Cryptography Reference
In-Depth Information
2.4 Two-Photon Quantum Cryptography
As we have seen in Section 2.2, time-bin qubits realized using faint pulses can
be employed to encode and to transmit quantum information. The success
of the plug & play scheme (see Section 2.3) proves them to be particularly
well suited for long distance QKD via optical fibers. However, like any exper-
imental implementation, faint-pulse-based QKD schemes do not perfectly
meet the theoretical ideal for QKD. Nevertheless, this does not necessarily
render it unsecure. For instance, the approximation of single photons by faint
pulses only limits the maximum transmission span. First, in connection with
detector noise, the nonvanishing vacuum component leads to a higher QBER
compared to true single photons and thus to smaller maximum distance (see
the discussion at the end of Section 2.3.2). Second, the possibility of photon
number splitting attacks, based on the existence of pulses containing more
than one photon, places an upper bound on the tolerable transmission losses,
hence on the distance (see Section 2.5.2). Finally, even if a specific realiza-
tion of QKD works well for a particular setting, it might not be adapted to
different working conditions. For instance, it is difficult to generate true ran-
dom numbers and to implement the corresponding settings in a high-speed
system. In this section, we will show how photon-pair-based QKD systems,
although more complicated to implement, might help to overcome some lim-
its of faint-pulse-based systems. A further expansion of some of the ideas can
be found in Section 2.5.3, where we will elaborate on quantum teleportation
as a quantum relay.
2.4.1 Single-Photon Based Realizations
In order to get around one drawback of faint pulses, i.e., the high probability
of having zero photons in a pulse, a good idea is to replace the faint pulse
source by a SPDC-based photon-pair source (see Figure 2.7b) where one pho-
ton serves as a trigger to indicate the presence of the other [31,62]. In this case,
Alice can remove the vacuum component of her source, and Bob's detectors
are only activated whenever she sends at least one photon. In principle, it is
thus possible to achieve a probability of emitting a nonempty pulse equal to
one [90]. This leads to a higher sifted key rate (assuming the same trigger rate
as in the faint-pulse case) and a lower QBER for a given distance (for given
losses) and therefore to a larger maximum span. It is important to note that
the use of photon pairs created by SPDC does not avoid problems with mul-
tiphoton pulses. For a given mean number, the probabilities that a nonempty
pulse contains more than one photon, or pair, respectively, are essentially the
same [32]. Therefore, the possibility of multiphoton splitting eavesdropping
attacks exists as well.
2.4.2 Entanglement-Based Realizations
The potential of a source creating photon pairs is not restricted to creation of
two photons at the same time — one serving as a trigger for the other one.
