ETSI GR QKD 003 V2.1.1 (2018-03)

Quantum Key Distribution (QKD); Components and Internal Interfaces

RGR/QKD-003ed2

General Information

• Status: Published
• Publication Date: 15-Mar-2018
• Technical Committee: QKD - Quantum Key Distribution
• Current Stage: 12 - Completion
• Due Date: 08-Mar-2018
• Completion Date: 16-Mar-2018

Standards Content (Sample)

GROUP REPORT
Quantum Key Distribution (QKD);
Components and Internal Interfaces
Disclaimer
The present document has been produced and approved by the Group Quantum Key Distribution (QKD) ETSI Industry
Specification Group (ISG) and represents the views of those members who participated in this ISG.
It does not necessarily represent the views of the entire ETSI membership.

2 ETSI GR QKD 003 V2.1.1 (2018-03)

Reference
RGR/QKD-003ed2
Keywords
interface, quantum key distribution

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3 ETSI GR QKD 003 V2.1.1 (2018-03)
Contents
Intellectual Property Rights . 5
Foreword . 5
Modal verbs terminology . 5
1 Scope . 6
2 References . 6
2.1 Normative references . 6
2.2 Informative references . 6
3 Definitions, symbols and abbreviations . 9
3.1 Definitions . 9
3.2 Symbols . 10
3.3 Abbreviations . 10
4 QKD systems . 11
4.1 Generic description. 11
4.2 Weak Laser Pulse QKD Implementations . 12
4.2.1 Generic Description . 12
4.2.2 One-Way Mach-Zehnder . 13
4.2.3 Send-and-return scheme (Mach-Zehnder) . 14
4.2.4 Phase-Intensity Modulator Implementation . 15
4.2.5 Coherent One-Way (COW) . 15
4.3 Entanglement-based QKD Implementations . 16
4.4 Continuous-Variable QKD Implementations . 17
4.4.1 Generic Description . 17
4.4.2 Transmitted Local Oscillator: TLO-CV-QKD scheme . 17
4.4.3 Local Local Oscillator: LLO-CV-QKD scheme . 19
5 Photon Detector . 20
5.1 Single-Photon Detector . 20
5.1.1 Generic Description and Parametrization . 20
5.1.2 InGaAs Single-Photon Avalanche Photodiodes. 23
5.1.2.1 Generic Description . 23
5.1.2.2 Gated-mode operation . 23
5.1.2.3 Free-running operation . 25
5.1.3 Superconducting nanowire single-photon detectors (SNSPDs) . 25
5.2 Photon Detector for a CV-QKD Set-up . 26
5.2.1 Coherent Detection . 26
5.2.2 Single-quadrature homodyne detection . 28
5.2.3 Dual-quadrature homodyne detection . 28
5.2.4 Heterodyne Detection . 28
5.2.5 CV-QKD Detector Parameters . 29
6 QKD Source . 30
6.1 Single-photon source . 30
6.1.1 Generic Description and Parametrization . 30
6.1.2 True Single-Photon Sources . 33
6.1.3 Weak Pulses . 34
6.1.3.1 Weak Laser . 34
6.1.3.2 Intensity-Modulated Weak Laser . 34
6.1.3.3 Phase-Coherent Weak Laser . 35
6.1.3.4 Composite Weak Laser . 35
6.1.4 Entangled-photon sources . 36
6.2 Continuous-Variable QKD Source . 37
7 Modulators . 37
Annex A: Discrete Variable Protocols . 40
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4 ETSI GR QKD 003 V2.1.1 (2018-03)
A.1 BB84. 40
A.1.1 Basic protocol . 40
A.1.2 Refinements . 40
A.1.2.1 State preparation - imperfections . 40
A.1.2.2 Multi-photon emission . 40
A.1.2.2.1 Security loophole . 40
A.1.2.2.2 Decoy state method . 41
A.1.2.2.3 SARG04 . 41
A.2 Entanglement-based . 41
A.2.1 Overview . 41
A.2.2 E91 . 41
A.2.3 BBM92 . 41
A.3 Distributed-phase reference protocols . 42
A.3.1 Overview . 42
A.3.2 Differential phase shift (DPS) . 42
A.3.3 Coherent One-Way (COW) . 42
A.4 Measurement-Device Independent (MDI) . 43
A.4.1 Overview . 43
Annex B: Continuous Variable Protocols . 44
B.1 Basic Protocols . 44
B.1.1 Basic protocols . 44
Annex C: Authors & contributors . 45
Annex D: Change History . 46
History . 47

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5 ETSI GR QKD 003 V2.1.1 (2018-03)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (https://ipr.etsi.org/).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Trademarks
The present document may include trademarks and/or tradenames which are asserted and/or registered by their owners.
ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
Foreword
This Group Report (GR) has been produced by ETSI Industry Specification Group (ISG) Group Quantum Key
Distribution (QKD).
Modal verbs terminology
In the present document "should", "should not", "may", "need not", "will", "will not", "can" and "cannot" are to be
interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.

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6 ETSI GR QKD 003 V2.1.1 (2018-03)
1 Scope
The present document is a preparatory action for the definition of properties of components and internal interfaces of
QKD Systems. Irrespective of the underlying technologies, there are certain devices that appear in most QKD Systems.
These are e.g. quantum physical devices such as photon sources and detectors, or classical equipment such as protocol
processing computer hardware and operating systems. For these components, relevant properties should be identified
that will subsequently be subject to standardization. Furthermore, a catalogue of relevant requirements for interfaces
between components should be established, to support the upcoming definition of internal interfaces.
2 References
2.1 Normative references
Normative references are not applicable in the present document.
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long term validity.
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] J. F. Dynes, Z. L. Yuan, A. W. Sharpe, and A. J. Shields: "Practical quantum key distribution over
60 hours at an optical fiber distance of 20km using weak and vacuum decoy pulses for enhanced
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[i.2] G. Ribordy, J-D. Gautier, N. Gisin, O. Guinnard and H. Zbinden: "Fast and user-friendly quantum
key distribution", J. Mod Opt. 47, 513-531 (2000).
[i.3] N. Gisin, G. Ribordy, W. Tittel, H. Zbinden, Quantum Cryptography, Rev. Mod. Phys. 74,
145-195 (2002).
[i.4] Y. Zhao, B. Qi, H.-K. Lo, L. Qian: "Security analysis of an untrusted source for quantum key
distribution: passive approach", New Journal of Physics, 12, 023024 (2010).
[i.5] L. Duraffourg, J.-M. Merolla, J.-P. Goedgebuer, Y. Mazurenko, W. T. Rhodes: "Compact
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[i.6] D. Stucki, N. Brunner, N. Gisin, V. Scarani, and H. Zbinden: "Fast and simple one-way quantum
key distribution" Applied Physics Letters 87(19); 194108, (2005).
[i.7] D. Stucki, N. Walenta, F. Vannel, R. T. Thew, N. Gisin, H. Zbinden, S. Gray, C. R. Towery,
S. Ten: "High rate, long-distance quantum key distribution over 250  km of ultra low loss fibres",
New J. Phys. 11(7), 75003 (2009).
[i.8] A. Poppe, A. Fedrizzi, R. Ursin, H. R. Böhm, T. Lorünser, O. Maurhardt, M. Peev, M. Suda,
C. Kurtsiefer, H. Weinfurter, T. Jennewein, and A. Zeilinger: "Practical quantum key distribution
with polarization entangled photons", Opt. Express 12(16), 3865-3871 (2004).
[i.9] A. Treiber, A. Poppe, M. Hentschel, D. Ferrini, T. Lorünser, E. Querasser, T. Matyus, H. Hübel
and A. Zeilinger: "A fully automated entanglement-based quantum cryptography system for
telecom fiber networks", New Journal of Physics 11, 045013 (2009).
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[i.10] Juan Yin, Yuan Cao, Yu-Huai Li, Ji-Gang Ren, Sheng-Kai Liao, Liang Zhang, Wen-Qi Cai,
Wei-Yue Liu, Bo Li, Hui Dai, Ming Li, Yong-Mei Huang, Lei Deng, Li, Qiang Zhang,
Nai-Le Liu, Yu-Ao Chen, Chao-Yang Lu, Rong Shu, Cheng-Zhi Peng, Jian-Yu Wang, and
Jian-Wei Pan: "Satellite-to-ground entanglement-based quantum key distribution", Phys. Rev.
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[i.11] S. Fossier, E. Diamanti, T. Debuisschert, A. Villing, R. Tualle-Brouri, P. Grangier: "Field test of a
continuous-variable quantum key distribution prototype", New J. Phys. 11(4), 045023 (2009).
[i.12] A. Leverrier & P. Grangier: "Unconditional Security Proof of Long-Distance Continuous-Variable
Quantum Key Distribution with Discrete Modulation", Phys. Rev. Lett. 102, 180504 (2009).
[i.13] Z. L. Yuan, B. E. Kardynal, A. W. Sharpe, A. J. Shields: "High speed single photon detection in
the near infrared", Appl. Phys. Lett. 91(4), 041114 (2007).
[i.14] M. A. Itzler, X. Jiang, B. Nyman, and K. Slomkowski: "InP-based negative feedback avalanche
photodiodes", Proceedings of SPIE 7222, 72221K (2009).
[i.15] B. Korzh, N. Walenta, T. Lunghi, N. Gisin, and H. Zbinden: "Free-running InGaAs single photon
detector with 1 dark count per second at 10% efficiency", Appl. Phys. Lett. 104, 081108 (2014).
[i.16] G. Boso, H. Zbinden, B. Korzh, and E. Amri: "Temporal jitter in free-running InGaAs/InP single-
photon avalanche detectors", Opt. Lett. 41(24), 5728-5731 (2016).
[i.17] C. M. Natarajan, M. G. Tanner, and R. H. Hadfield: "Superconducting nanowire single-photon
detectors - physics and applications", Supercond. Sci.Technol. 25, 063001 (2012).
[i.18] E. A. Dauler, M. E. Grein, A. J. Kerman, F. Marsili, S. Miki, S. W. Nam, M. D. Shaw, H. Terai,
V. B. Verma, and T. Yamashita: "Review of superconducting nanowire single-photon detector
system design options and demonstrated performance", Optical Engineering 53(8), 081907
(August 2014).
[i.19] S. Dorenbos, E. Reiger, N. Akopian, U. Perinetti, V. Zwiller, T. Zijlstra, and T. Klapwijk:
"Superconducting single photon detectors with minimised polarisation dependence". Appl. Phys.
Lett. 93, 161102 (2008).
[i.20] V. B. Verma, F. Marsili, S. Harrington, A. E. Lita, R. P. Mirin, and S. W. Nam: "A three-
dimensional polarization-insensitive superconducting nanowire avalanche photodetector". Appl.
Phys. Lett. 101, 251114 (2012).
[i.21] V. Burenkov, H. Xu, B. Qi, R. H. Hadfield, and H.-K. Lo: "Investigations of afterpulsing and
detection efficiency recovery in superconducting nanowire single-photon detectors", J. Appl. Phys.
113, 213102 (2013).
[i.22] D. Rosenberg, A. J. Kerman, R. J. Molnar, and E. A. Dauler: "High-speed and high-efficiency
superconducting nanowire single photon detector array", Opt. Exp. 21, 1440-1447 (2013).
[i.23] F. Marsili, V. B. Verma, J. A. Stern, S. Harrington, A. E. Lita, T. Gerrits, I. Vayshenker, B. Baek,
M. D. Shaw, R. P. Mirin, and S. W. Nam: "Detecting single infrared photons with 93% system
efficiency", Nature Photon. 7, 210-214 (2013).
[i.24] S. Miki, T. Yamashita, H. Terai, and Z. Wang: "High performance fiber-coupled NbTiN
superconducting nanowire single photon detectors with Gifford-McMahon cryocooler", Opt. Exp.
21, 10208-10214 (2013).
[i.25] J. Lodewyck & P. Grangier: "Tight bound on the coherent-state quantum key distribution with
heterodyne detection", Phys. Rev. A 76, 022332 (2007).
[i.26] S. Fossier, E. Diamanti, T. Debuisschert, R. Tualle-Brouri, P. Grangier: "Improvement of
continuous-variable quantum key distribution systems by using optical preamplifiers", J. Phys.:
Atomic, Molecular and Optical Physics 42, 114014 (2009).
[i.27] P. M. Intallura, M. B. Ward, O. Z. Karimov, Z. L. Yuan, P. See, P. Atkinson, D. A. Ritchie, A. J.
Shields: "Quantum communication using single photons from a semiconductor quantum dot
emitting at a telecommunication wavelength", J. Opt. A: Pure Appl. Opt., 11(5), 054005 (2000).
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8 ETSI GR QKD 003 V2.1.1 (2018-03)
[i.28] A. R. Dixon, J. F. Dynes, Z. L. Yuan, A. W. Sharpe, A. J. Bennett, A. J. Shields: "Ultrashort dead
time of photon-counting InGaAs avalanche photodiodes", Applied Physics Letters 94, 231113
(2009).
[i.29] W.-Y. Hwang: "Quantum Key Distribution with High Loss: Toward Global Secure
Communication", Phys. Rev. Lett. 91, 057901 (2003).
[i.30] X.-B. Wang: "Beating the Photon-Number-Splitting Attack in Practical Quantum Cryptography",
Phys. Rev. Lett. 94, 230503 (2005).
[i.31] H.-K. Lo, X. Ma, K. Chen: "Decoy state quantum key distribution", Phys. Rev. Lett. 94, 230504
(2005).
[i.32] P. G. Kwiat, K. Mattle, H. Weinfurter, A. Zeilinger, A. V. Sergienko, and Y. Shih: "New High-
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[i.33] A. Fedrizzi, T. Herbst, A. Poppe, T. Jennewein, A. Zeilinger: "A wavelength-tunable fiber-coupled
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[i.34] B. Blauensteiner, I. Herbauts, S. Bettelli, A. Poppe, H. Hübel: "Photon bunching in parametric
down-conversion with continuous wave excitation", Phys. Rev. A 79, 063846 (2009).
[i.35] J. F. Clauser, M. A. Horne, A. Shimony, and R. A. Holt: "Proposed Experiment to Test Local
Hidden-Variable Theories", Phys. Rev. Lett. 23, 880 (1969).
[i.36] C. H. Bennett and G. Brassard: "Quantum cryptography: Public key distribution and coin tossing.
Proceedings of IEEE International Conference on Computers Systems and Signal Processing",
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[i.37] P. W. Shor and J. Preskill: "Simple proof of security of the BB84 quantum key distribution
protocol", Phys. Rev. Lett., 85, 441 (2000).
[i.38] D. Mayers: "Unconditional security in Quantum Cryptography", JACM, 48(3), 351-406 (2001).
[i.39] D. Bruß: "Optimal Eavesdropping in Quantum Cryptography with Six States", Phys. Rev. Lett. 81,
3018 (1998).
[i.40] H-K. Lo: "Proof of Unconditional Security of Six-State Quantum Key Distribution Scheme",
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[i.41] K. Tamaki, M. Curty, G. Kato, H.-K. Lo, K. Azuma: "Loss-tolerant quantum cryptography with
imperfect sources", Phys. Rev. A 90, 052314 (2014).
[i.42] S. M. Barnett, B. Huttner, S.J.D. Phoenix: "Eavesdropping Strategies and Rejected-data Protocols
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[i.44] V. Scarani, A. Acin, G. Ribordy, N. Gisin: "Quantum cryptography protocols robust against
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[i.48] K. Inoue, E. Waks, Y. Yamamoto: "Differential-phase-shift quantum key distribution using
coherent light", Phys. Rev. A 68(2) , 022317 (2003).
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[i.49] T. Sasaki, Y. Yamamoto, M. Kaoshi: "Practical quantum key distribution protocol without
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[i.50] N. Walenta: "Concepts, components and implementations for quantum key distribution over
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Phys. Rev. Lett., 88(5), 057902 (2002).
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the following terms and definitions apply:
Alice: quantum information sender/transmitter in a QKD system
Bob: quantum information receiver in a QKD system
classical channel: communication channel that is used by two communicating parties for exchanging data encoded in a
form which may be non-destructively read and fully reproduced
Eve or eavesdropper: any adversary intending to intercept data in a quantum or classical channel
intensity modulator: device that can actively modulate its transmittance of optical signals passing through it
IQ modulator: device that can actively modulate both the in-phase component (denoted by 'I') and the quadrature
component (denoted by 'Q') of optical signals passing through it
phase modulator: device that can actively modulate the phase of optical signals passing through it
prepare-and-measure scheme: scheme where the quantum optical signals used for QKD are prepared by Alice and
sent to Bob for measurement
NOTE: Entanglement-based schemes where entangled states are prepared externally to Alice and Bob are not
normally considered "prepare-and-measure". Schemes where entanglement is generated within Alice can
still be considered "prepare-and-measure". Send-and-return schemes can still be "prepare-and-measure" if
the information content from which keys will be derived is prepared within Alice before being sent to
Bob for measurement.
quantum channel: communication channel for transmitting quantum signals
quantum photon source: optical source for carrying quantum information
random number generator: physical device outputting unpredictable binary bit sequences
send-and-return scheme: scheme where quantum optical signals are derived from optical signals previously sent in the
reverse direction along the quantum channel
NOTE: Such schemes are also referred to elsewhere as "plug-and-play". Many systems running other protocols
are auto-aligning and also able to deliver plug-and-play functionality so "send-and-return" will be used in
ETSI ISG QKD documents.
single-photon detector: device that transforms a single-photon into a detectable signal with finite probability
single-photon source: photon source that emits at most one photon at a time
weak laser pulse: optical pulse obtained through attenuating a laser emission
NOTE: A weak laser pulse typically contains less than one photon per pulse on average.
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3.2 Symbols
For the purposes of the present document, the following symbols apply:
C Maximum count rate
max
Δ electrical noise measurement variance accuracy
el
Δ total excess noise measurement variance accuracy
ξ
Δ shot-noise measurement variance accuracy
sn
η photon detection probability, photon detection efficiency
η(λ) detection efficiency (nm)
η(ν) detection efficiency (Hz)
η(t) photon detection probability profile
η(t,T) detector signal jitter
f electrical noise measurement variance stability
Δel
f total excess noise measurement variance stability
Δξ
f shot-noise measurement variance stability
Δsn
f gate repetition rate
gate
f optical pulse repetition rate
source
(2)
g second-order correlation coefficient
J timing jitter
source
L total receiver loss
RX
λ wavelength
Δλ spectral bandwidth
λr wavelength range
Mdf modulated degree of freedom
MaxDev maximal deviation values
μ mean photon number
N photon-number resolving depth
N number of photon-emitters in a multiple-source QKD transmitter
emitters
N vacuum noise variance
ν spectral frequency
Δν spectral bandwidth
Opr optical robustness
ξ total excess noise measurement variance
p after-pulse probability
after
p dark count probability
dark
p(n) photon number probability distribution]
P (t) emission temporal profile
emission
P mean optical power
mean
P(t) temporal profile
pulse
s electrical noise measurement variance
el
s spectral indistinguishability
ind
SNR supported signal-to-noise ratio
min
SNU shot-noise unit (1 SNU = vacuum noise variance, N )
t temporal indistinguishability
ind
t dead time
dead
t partial recovery time
partial_f
t recovery time
recovery
t rise and fall time
r/f
T temperature
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
AC Alternating Current
AMZI Asymmetric Mach-Zehnder Interferometer
APD Avalanche PhotoDiode
BB84 QKD protocol published by Bennett and Brassard in 1984 [i.36]
BNC Bayonet Neill-Concelman connector
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BW Band Width
CHSH Clauser-Horne-Shimony-Holt [i.35]
COW Coherent One-Way
CV Continuous Variable
CV-QKD Continuous Variable QKD
CW Continuous Wave
DAC Digital-to-Analogue Converter
DC Direct Current
DPS Differential Phase Shift
DSP Digital Signal Processor
DUT Device Under Test
DV Discrete Variable
ECL Emitter Coupled Logic
EPR Einstein-Podolsky-Rosen [after Einstein et al. Phys. Rev. 47(10), 777 (1935)]
FC/PC Ferrule Connector/Physical Contact
FPGA Field Programmable Gate Array
FW Full-width
FWHM Full-width at Half-maximum
GG02 QKD protocol published by Grosshans and Grangier in 2002 [i.52]
GM Gaussian Modulation
GMCS Gaussian Modulated Coherent State
LDPC Low Density Parity Check codes
LLO Local Local Oscillator
LO Local Oscillator
MDI Measurement-Device Independent
MM Multi-Mode
NFAD Negative Feedback Avalanche Photodiode
NIM Nuclear Instrumentation Module
PBS Polarising Beamsplitter
PDE Photon Detection Efficiency
PNS Photon Number Splitting
PSK Phase Shift Keying
QBER Quantum Bit Error Rate
QKD Quantum Key Distribution
QPSK Quadrature Phase Shift Keying
RRDPS Round Robin DPS
RX Receiver
SDE System Detection Efficiency
SM Single-Mode
SMA Sub-Miniature version A connector
SNR Signal-to-Noise Ratio
SNSPD Superconducting Nanowire Single-Photon Detector
SPAD Single-Photon Avalanche Photodiode
SPDC Spontaneous Parametric Down-Conversion
TAT Trap-Assisted Tunnelling
TLO Transmitted Local Oscillator
TTL Transistor-Transistor Logic
TX Transmitter
VOA Variable Optical Attenuator
WDM Wavelength Division Multiplexing
4 QKD systems
4.1 Generic description
A QKD system comprises a number of internal components. The purpose of the present document is to identify the
components which are common to many systems and their properties which may require calibration. The present
document also defines the interfaces between these common components.
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12 ETSI GR QKD 003 V2.1.1 (2018-03)
A survey of the literature reveals that many different types of QKD system have been proposed. Many of these have
been implemented physically with different levels of sophistication. At the most basic level, these systems utilize the
laws of quantum theory to make claims about the security levels of the shared key. Most commonly, they use signal
encoding upon quantum light states using several different bases which are non-orthogonal to one another. Quantum
theory dictates that it is impossible to gain full information of this encoding through measurement without prior
information about the encoding basis or post-selection of the basis used. This property is used to ensure that the
legitimate users of the system share more information than an eavesdropper can determine.
One convenient method of categorizing different types of QKD system is according to the photon source that they use.
Examples include true single-photon sources, entangled-photon pair sources and weak laser pulses. Common methods
for encoding the qubit information include controlling the phase or the polarization state of the transmitted photon. A
QKD system consists of two units which are physically separated at opposite ends of a pair of communication channels,
as illustrated by figure 4.1. The sending and receiving unit contain a source of randomness for use in the key generation
protocol. The source of randomness can be intrinsic, as in the case of sending entangled photons, or it can be an active
random number generator or a passive random selection component, such as a non-polarizing beamsplitter. Here, the
sending unit consists of a signal source and an encoder for the source, the receiving unit contains a component for signal
demodulation, i.e. for selecting the measurement basis, as well as one or more signal detectors. Control electronics, with
access to an independent random number generator, are necessary to generate the drive signals for these devices. The
detected signals are used by the control electronics to form the initial (or raw) shared key, which is then post-processed
(sifted, reconciled and privacy amplified) to achieve the final secure shared key.

Figure 4.1: Schematic of a generic QKD system showing internal interfaces and connections
Alice and Bob may exchange classical optical signals for clock synchronization/recovery and sifting and data
processing. These signals are transmitted through classical channels which may be on a separate fibre, or combined with
the quantum signal through the same fibre using wavelength- or time-division multiplexing. (In pure classical
communications, the channel used to perform management functions is called the signalling channel. It is the classical
communications equivalent of QKD synchronization and distillation channels).
4.2 Weak Laser Pulse QKD Implementations
4.2.1 Generic Description
In weak laser pulse QKD systems, the qubit values are encoded upon laser pulses attenuated to the single-photon level.
The sender (Alice) in a weak laser pulse QKD contains at least one weak laser source that is used as a quantum
information carrier. In implementations involving more than one weak laser source, the sources should be
indistinguishable from one another in every measurable attribute except the degree of freedom the quantum information
is encoded upon.
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The sender should contain a quantum encoder that encodes qubit information on each weak laser pulse. This encoder
should have a source of randomness that determines an encoding basis and an encoding bit value for each weak pulse.
The source of randomness should come from a random number generator.
The photon number splitting attack, and other such attacks, should be accounted for in the privacy amplification process
in a QKD session. To achieve this, the intensity and photon number statistics of each weak laser source should be
calibrated. The source stability should also be calibrated. In the case that the source is unstable, the worst case scenario
should be considered in the privacy amplification process.
In the following, a few example realisations of weak laser pulse QKD systems are presented.
4.2.2 One-Way Mach-Zehnder
Figure 4.2 shows an example of a QKD system using weak laser pulses as the signal carriers and Asymmetric Mach-
Zehnder Interferometers (AMZIs) to encode the quantum states, based on the paper by Dynes et al. [i.1]. The system
uses the decoy pulse protocol to obtain higher secure bit rates than are otherwise possible using weak laser pulses with
constant intensity. Intensity modulation is used to produce signal, decoy and vacuum pulses of differing intensities, as
well as strong reference pulses to enable active stabilization. The vacuum pulses could also be produced by omitting
trigger pulses to the signal laser. The signal, decoy and vacuum pulses are produced in a non-deterministic sequence
and have pre-determined relative occurrence probabilities assigned to them. The signal and decoy pulses are attenuated
to the single-photon level before entering the quantum channel implemented in standard single mode fibre.
Figure 4.2: Schematic of a one-way, weak-laser-pulse QKD system
The receiver's single-photon detectors are two InGaAs avalanche photodiodes (APDs), operated in gated Geiger mode.
This system uses active stabilization to lock the path phase difference in the sending and receiving AMZI. The strong
reference pulses are produced by the intensity modulator(s) at pre-determined times. These strong reference pulses are
either unmodulated, or modulated with pre-determined phase values by the phase modulator in the sending AMZI.
Detection rates of these reference pulses are used as a feedback to actively adjust a phase compensation component in
Bob, here a fibre stretcher, to compensate for the path phase difference. A similar active stabilization technique is used
to control the polarization state of photons entering Bob's AMZI.
In this implementation, the combination of the 1 550 nm laser diode, the intensity modulator and the attenuator forms
the photon source. Because only one laser diode is used for encoding all qubits, the indistinguishability of the source is
guaranteed. An intensity modulator is required to implement the decoy QKD protocol. Alice's AMZI is the encoder.
Standard single mode fibre is used as the quantum channel. In the receiving unit, the combination of the active
polarization recovery, active fibre stretcher and AMZI forms the decoder.
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Each control electronics unit can contain optoelectronics components, such as optoelectronics-based random-number
generators used as the sources of randomness.
Optical transceivers at Alice and Bob are used to provide signals for clock synchronization/recovery and classical
communications for sifting and data processing.
4.2.3 Send-and-return scheme (Mach-Zehnder)
Figure 4.3 depicts a typical send-and-return scheme (also called "plug-and-play") with a Mach-Zehnder architecture,
described in detail in [i.2] and [i.3]. Pulses emitted from the source S in Bob are directed by the circulator C to the
coupler BS1, where they split into two pulses. The pulse propagating along the short arm, Pshort has its polarisation
conditioned so that it is fully launched into the quantum channel by the polarisation splitter PS. The pulse propagating
along the long arm, P , also has its polarisation conditioned such that it is fully launched into the quantum channel at
long
PS (i.e. P and P are 90° out-of-phase with respect to each other at PS). The phase shifter Φ is inactive during the
long short 2
transit of P . At Alice, a beamsplitter BS2 reflects part of the incoming pulses to a detector D3:
long
i) providing a timing signal; and
ii) to monitor for so-called Trojan-horse attacks.
The transmitted pulses are reflected by a Faraday mirror (FM) which compensates for any birefringence in the quantum
channel, and returns the pulses to Bob orthogonally polarised with respect to their emitted states. An attenuator (AT)
reduces the intensity of the pulses to a suitably weak intensity (depending on the protocol used). Φ1 applies a phase
shift to P (but not to P ) to encode a bit value. At the receiving unit, P takes the short path and P takes the
long short long short
long path where Φ2 applies a phase shift to it to implement the measurement basis choice. Both pulses reach BS1
simultaneously with identical polarisation, leading to interference. Single-photon detectors D1 and D2 indicate which
output port is taken by the photon. The circulator C ensures isolation between the laser source and D1. With this
scheme, the security of a protocol has to be carefully investigated. In particular, without any knowledge of the state
Alice sends to Bob, the security is difficult to guarantee. Therefore, some monitoring has to be performed on the
outgoing pulses from Alice [i.4].

Figure 4.3: Schematic of a send-and-return scheme in a Mach-Zehnder system
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4.2.4 Phase-Intensity Modulator Implementation
Figure 4.4 depicts a simplified Single Sideband (SSB) system, according to L. Duraffourg et al.,[i.5]. The source S1 is
an attenuated pulsed laser diode operating at optical frequency ω (quantum signal). An unbalanced integrated Mach-
Zehnder modulator MZ1 modulates the intensity of the reference beam at Ω ≪ ω with a modulation depth m < 1. The
modulating signal is produced by a local oscillator (OS) that drives simultaneously a second integrated Mach-Zehnder
MZ2. The light emitted by the source S2 (synchronization signal), operating at optical frequency ω , is then modulated
s
at the same frequency Ω. Both optical signals are
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