Cryptography Reference
In-Depth Information
on the BBN-BU link, in order continuously to exercise all parts of the system,
even though the resultant secret key yield is zero. In the near future, fiber
splices and perhaps detector upgrades should allow operation to BU with
mean photon numbers of 0.5.
The DARPA Quantum Network also contains Ali and Baba, the electron-
ics subsystems for a high-speed free-space QKD system designed and built
by the NIST. Ali and Baba run the BBN QKD protocols and are linked into
the overall network by key relay between Ali and Alice. It further contains
two new entanglement based nodes named Alex and Barb, built jointly by
BU and BBN, but these nodes are not yet fully operational.
4.3 Motivation for the DARPA Quantum
Network
QKD provides a technique for two distinct devices to come to agreement
upon a shared random sequence of classical bits, with a very low probability
that other devices (eavesdroppers) will be able to make successful inferences
as to those bits' values. Such sequences may then be used as secret keys for
encoding and decoding messages between the two devices. In short, it is a
cryptographic key distribution technique.
∗
Although QKD is an interesting
and potentially quite useful technique for key distribution, it is not the only
one — human couriers and algorithmic “one-way” functions such as the
Diffie-Hellman come immediately to mind — and thus it is important to
gauge QKD's strengths across a number of goals for key distribution systems
in general. Table 4.1 provides such an assessment of “classic” QKD techniques;
see [1] for a more extended treatment of this subject.
It can be seen that “classic” QKD, i.e., QKD performed by sending a sin-
gle quantum entity directly from source to destination, has areas of weakness
mixed with its strengths. As important guidelines of our overall research
agenda, we are working to strengthen QKD's performance in these weaker
areas. A surprising number of these weaknesses, as it turns out, can be re-
moved by weaving individual QKD links into an overall QKD network such
as the DARPA Quantum Network.
4.4 What Is a QKD Network?
Figure 4.3 depicts a typical stand-alone QKD system in highly schematic
form.
∗∗
In this example, Alice contains both a photon source and a mod-
ulator; in this case, Alice employs an attenuated laser and Mach-Zehnder
∗
Strictly speaking, it is a means for coming to agreement upon a shared key, rather
than a way to distribute a key, but we follow conventional QKD terminology in this
chapter.
∗∗
In fact, it depicts our “Mark 2” weak-coherent link but corresponding forms of
high-level schematics can be drawn for any quantum cryptographic system.
