QKD: WIP, add more sources

chapter-qkd/Makefile

@@ -8,12 +8,15 @@ MAKEFLAGS += --no-builtin-rules
 
 VERSION_STRING := $(shell git describe --always --tags --long)
 
-all: chapter.pdf
+all: clean chapter.pdf
 
+# We need three runs for biblatex's defernumbers
 %.pdf: %.tex ../main.bib version.tex
 	pdflatex -shell-escape $<
 	biber $*
 	pdflatex -shell-escape $<
+	biber $*
+	pdflatex -shell-escape $<
 
 .PHONY: preview
 preview:

chapter-qkd/chapter.tex

@@ -52,6 +52,8 @@
     url=false, 
     doi=true,
     eprint=false,
+    % Make the split online / other resource bibliographies behave
+    defernumbers=true,
     ]{biblatex}
 \addbibresource{../main.bib}
 \DeclareSourcemap{
@@ -331,7 +333,7 @@ single, short key\footnote{
     data\cite{bhargavanPracticalSecurity64bit2016,}.
 }.
 
-\section{The Practical Security Implications of Quantum Computing}
+\subsection{The Practical Security Implications of Quantum Computing}
 \label{qc-practical-implications}
 
 Given that as of yet, noone has claimed to have a quantum computer powerful enough to pose a threat to current
@@ -435,7 +437,6 @@ various aspects of currents in superconducters as well as phonons\cite{berriosHi
 \todoplaceholder{Something is missing here.}
 
 \subsection{Practical Challenges}
-\todo{I don't like this paragraph.}
 The central challenge in general quantum computers is extending the lifetime of the quantum state encoding a qubit.
 Quantum states are extremely sensitive to disturbances, and despite the best efforts to shield them against external
 influence, their lifetime is still inconveniently short compared to the timescales required for quantum computation,
@@ -447,35 +448,53 @@ 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 hundreds or thousands of kilometers is no big challenge.
-Using appropriate high-power transceivers, a single optical link can already bridge upwards of 100km. \todo{Citation on
-distance} Longer ranges can easily be achieved by either logically chaining multiple links, or by using optical
+Using appropriate high-power transceivers, a single, \emph{unrepeatered} span of an optical link can bridge hundreds of
+kilometers while simultaneously achieving data rates of several terabits per second. Longer ranges are regularly
+achieved through the use of (analog!) optical amplifiers, with recent \emph{repeatered} systems approaching the petabit
+per second boundary
+\footnote{
+    cf.\ this encyclopedic entry \cite{JUNOSubmarineNetworks}, press releases by participating companies
+    \cite{NECBuildNew, NewCompanyBuilds}.
+}. These classical optical systems operate at hundreds of milliwatts of optical power, a limit resulting from nonlinear
+effects in the optical fibers used, power limitations of optical amplifiers, and limitations in power delivery to these
 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\cite{loSecureQuantumKey2014}. The attacker can then later measure the captured photons to extract the same
-information that the receiver measured. In practical QKD setups, attenuated pulsed lasers are often used, as there are
-no practical single-photon sources. The laser and its attenuator are tuned such that the average photon count of a pulse
-is in the order of $0.1$ \cite{loSecureQuantumKey2014}. For such setups, mitigations exist that prevent photon number
-splitting attacks\cite{wangBeatingPhotonNumberSplittingAttack2005}. However, while these mitigations patch this security
-weakness for weak, attenuated pulsed lasers, they still do not allow for higher transmit power.
+In contrast, QKD systems operate on signals that are weaker by several orders of magnitude. While classical optical
+signals use millions of photons per bit, the quantum states at the core of QKD systems must necessarily be ``weak''. A
+single quantum state in the fiber on average should 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\cite{loSecureQuantumKey2014}. The attacker can then later measure the captured photons
+to extract the same information that the receiver measured. In practical QKD setups, attenuated pulsed lasers are often
+used, as there are no practical single-photon sources. The laser and its attenuator are tuned such that the average
+photon count of a pulse is in the order of $0.1$ \cite{loSecureQuantumKey2014}. For such setups, mitigations exist that
+prevent photon number splitting attacks\cite{wangBeatingPhotonNumberSplittingAttack2005}. However, while these
+mitigations patch this security weakness for weak, attenuated pulsed lasers, they still do not allow for higher transmit
+power.
 
-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 \unit{\GHz} are feasible\cite{grunenfelderFastSinglephotonDetectors2023}. Alas, the brightness limit interacts
+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. The primary limitation is the speed and
+recovery time of the single-photon detector. In contrast to e.g.\ a simple photodiode that (mostly) linearly converts
+incident photons into electron flow, SPDs are designed to provide a large intrinsic gain. This improves their bandwidth
+as each photon's pulse must charge the detector's own parasitic capacitance as well as that of any wiring between it and
+the frontend preamplifier, but in many detector designs this intrinsic amplification process is also the origin of a
+long recovery time that limit's the detector's possible repetition rate. With today's electronics, repetition rates up
+to a few \unit{\GHz} are feasible\cite{grunenfelderFastSinglephotonDetectors2023}. Alas, the 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 \qty{0.5}{\dB\per\km}, which corresponds to roughly half of the signal being
-lost every \qty{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 kilometers, but the useable data rate (here called
-\emph{secret key rate}) of these systems usually is in the kilobits per second or worse.
-
-QKD signals cannot be amplified because their security rests on the fact that each transmitted quantum state on average
-only contains on the order of one photon each. Security rests on the Nomegabits Theorem, which implies that not just
-attackers, but even the system's operators are unable to duplicate the quantum state in flight without destroying it.
+cables have attenuation in the order of \qty{0.2}{\dB\per\km}\cite{chesnoyUnderseaFiberCommunication2015}, which
+corresponds to roughly half of the signal being lost every \qty{15}{\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. cIn QKD systems however, the signal's quantum states cannot be amplified both out of a
+concern of photon number splitting attacks and because of decoherence\footnote{
+    Note that this impossibility is not a consequence of the No-Cloning Theorem. The No-Cloning Theorem only asserts
+    that it is impossible to create a second, \emph{independent} copy of an arbitrary quantum state, which can then
+    independently be measured without disturbing the original state. Despite this, a hypothetical ``quantum amplifier''
+    could increase the quantum state's photon number, adding entangled photons that share the original quantum state.
+    Alas, doing this would not gain us much in a QKD system because an interaction of any of the quantum state's photons
+    with the fiber---that is, the same loss as before---would disturb the entire entangled state.
+}, and thus the system's bit rate decreases exponentially with distance due to absorption. Some QKD systems can reach
+ranges of several hundred kilometers, but the resulting payload data rate---usually called \emph{secret key rate}---of
+these long distance systems is measured in kilobits per second.
 
 When transmitted over a fiber, there are multiple effects that degrade the quantum-optical signal of a QKD system, which
 are collectively referred to as \emph{loss}. We can coarsely classify these degrading effects into two categories:
@@ -903,7 +922,14 @@ meshes do not have to rotate at the same rate of rotation. Instead, harmonic rat
 \section{Outlook}
 
 \newpage
-\printbibliography[heading=bibintoc]
+% TODO when breaking this out into a template for building both the whole thesis and individual chapters, we have to
+% decide whether we want to keep the bibliography per-chapter or only once for the whole thesis. In the latter case, we
+% probably want to replace subbibintoc with bibintoc, or add a custom "bibliography" chapter and adjust the second
+% bibliography's heading
+\newrefcontext[labelprefix={W}]
+\printbibliography[type={online},title={Web sources},heading=subbibintoc]
+\newrefcontext
+\printbibliography[nottype={online},resetnumbers,heading=subbibintoc]
 
 \appendix
 

chapter-qkd/figures/ihsm-secondary-mesh

@@ -1 +1 @@
-Subproject commit 601159904f4269366e29d85c2e90cbf000157f4f
+Subproject commit 3a7edbd1127cacc8f4c90376595b894105f3d479