Cryptography Reference
In-Depth Information
Table 4.2
Major Benefits of Quantum Cryptographic Networks
Benefit
Discussion
Longer Distances
QKD key relay can easily extend the geographic reach of
quantum cryptography. As one example, quantum
cryptography could be performed through
telecommunications fiber across a distance of 500 km by
interposing four relays between the QKD endpoints, with a
span of 100 km fiber between each relay node.
Heterogenous
Channels
QKD key relay can mediate between links based on differ-
ent physical principles, e.g., between free-space and fiber
links, or even between links based on entanglement and those
based on weak laser pulses. This allows one to “stitch
together” large networks from links that have been
optimized for different criteria.
Greater Robustness
QKD networks lessen the chance that an adversary could
disable the key distribution process, whether by active
eavesdropping or simply by cutting a fiber. When a given
point-to-point QKD link within the network fails — e.g., by
fiber cutting or too much eavesdropping or noise — that link
may be abandoned and another used instead. Thus QKD
networks can be engineered to be resilient even in the face of
active eavesdropping, fiber cuts, equipment failures, or other
denial-of-service attacks. A QKD network can be engineered
with as much redundancy as desired simply by adding more
links and relays to the mesh.
Cost Savings
QKD networks can greatly reduce the cost of large-scale
interconnectivity of private enclaves by reducing the
required (N
1)/2 point-to-point links to as few as
N links in the case of a simple star topology for the key
distribution network.
×
N
−
4.5 BBN's “Mark 2'' Weak
Coherent Systems
This section describes the four fiber based QKD systems currently running
in the DARPA Quantum Network. All became operational in October 2003;
we call these “Mark 2” systems because they replaced our first-generation
system, which started continuous operation in December 2002. These links
were inspired by a pioneering Los Alamos system [2].
Each Mark 2 link employs a highly attenuated telecommunications laser
(hence the term “weak coherent”) at 1550.12 nm, phase modulation via unbal-
anced Mach-Zehnder interferometers, and cooled avalanche photo detectors
(APDs). Most Mark 2 electronics are implemented by discrete components
such as pulse generators, though it would not be difficult to integrate all elec-
tronics onto a small custom board. Figure 4.5 depicts Anna and Boris, our first
rack-mounted versions of the Mark 2 hardware, before their deployment into
wiring closets at Harvard and Boston University as part of the metro network.
