More QKD WIP

chapter-qkd/chapter.tex

@@ -249,6 +249,28 @@ computers. Quantum Key Distribution systems use photons and only perform a handf
 between generation and measurement, with the vast majority of the state's lifetime spent in transit between the two
 endpoints of the QKD protocol.
 
+While QKD systems are easy to build and operationally robust compared to general quantum computers, at their core they
+still exchange information through quantum states that physically need to transit the distance from one endpoint to the
+other. For classical computer networks, bridging distances of several hundred kilometers is no big challenge. Using
+appropriate high-power transceivers, a single optical link can already bridge upwards of 100km. % FIXME cite
+Longer ranges can easily be achieved by either logically chaining multiple links, or by using optical amplifiers.
+
+In contrast, the quantum states at the core of QKD systems must necessarily be ``weak''. A single quantum state on the
+wire on average must consist of approximately a single photon. If the system's quantum states consisted of more than one
+photon carrying the same information, this would enable a \emph{Photon Number Splitting Attack}, in which an attacker
+extracts one of the state's photons for later analysis, and forwards the remaining photons to the receiver. The attacker
+can then later measure the captured photon to extract the same information that the receiver measured.
+
+The practical implication of this is that the optical brightness of a QKD system is directly proportional to the rate
+at which the system can prepare, and later measure the individual quantum states. With today's electronics, rates up to
+a few GHz are feasible. Alas, this brightness limit interacts poorly with the reality of optical communication,
+especially through fibers. Even modern, high-quality fiber-optic cables have attenuation in the order of 0.5 dB/km,
+which corresponds to roughly half of the signal being lost every 5 km. In classical optical networks, this can be
+compensated by increasing transmit power--i.e. packing more photons into each bit--or by optically amplifying the signal
+partway through the fiber. In QKD systems however, the signal cannot be amplified, and the system's bit rate
+exponentially decreases with distance due to absorption. Some QKD systems can reach ranges of several hundred kilometer,
+but the useable data rate (here called \emph{key rate}) of these systems usually is in the kilobits per second or worse.
+
 \section{Quantum Networking}
 
 \section{Securing QKD Networks with Inertial HSMs}
@@ -269,7 +291,7 @@ depth,
 meaning the QKD setup will at worst degrade to the same security a purely classical system would provide, never less.
 
 The second prediction we can make is that any practical QKD network will have to use trusted relays to bridge large
-distances. While in certain specialized applications such as the proposed financial QKD network in Swizerland
+distances. While in certain specialized applications such as the proposed financial QKD network in Switzerland
 % FIXME citation
 smaller, isolated networks are conceivable, in every telecommunication system from the telegraph through the telephone
 system and up to the internet it has been shown conclusively that there is a real demand for a unified, global