High-speed photon counting techniques for broadband quantum key
distribution
D. Rogers, J.C. Bienfang, A. Mink, B.J. Hershman, A. Nakassis, L. Ma, X. Tang, D.H. Su, Charles
W. Clark, Carl J. Williams
National Institute of Standards and Technology, 100 Bureau Dr. Gaithersburg, MD, USA, 20899
ABSTRACT
Quantum Cryptography has demonstrated the potential for ultra-secure communications. However, with quantumchannel transmission rates in the MHz range, typical link losses and signal-to-noise ratios have resulted in keyproduction rates that are impractical for continuous one-time-pad encryption of high-bandwidth communications. We
have developed high-speed data handling electronics that support quantum-channel transmission rates up to 1.25 GHz.
This system has demonstrated error-corrected and privacy-amplified key rates above 1 Mbps over a free-space link.
While the transmission rate is ultimately limited by timing jitter in the single-photon avalanche photodiodes (SPADs),
we find the timing resolution of silicon SPADs sufficient to operate efficiently with temporal gates as short as 100 ps.
We have developed systems to implement such high-resolution gating in our system, and anticipate the attendant
reduction in noise to produce significantly higher secret-key bitrates.
Keywords: Single-photon detection, quantum key distribution, avalanche photodiode
1. INTRODUCTION
Quantum cryptography (QC) is emerging to provide a fundamental shift in cryptography. By relying on the quantum
mechanical properties of the physical communications layer rather than the mathematical complexity of the
communications process for its security, QC, and more specifically Quantum Key Distribution (QKD), can provide a
communications link that is provably immune to eavesdroppers, independent of their technological capabilities.
In broad terms, the security of a QKD system relies on two fundamental properties of quantum mechanics. The first,
the indivisibility of individual quanta, is a fundamental principle of quantum mechanics. Wooters and Zurek proved the
second idea, the so-called ‘no-cloning theorem,’ in 1982 [1], which states that an unknown quantum state cannot be
cloned without a measurement that risks changing that state. Together these results provide the premise for Quantum
Cryptography’s ‘unbreakable’ security.
2. THE CURRENT NIST QKD SYSTEM
We have established a QKD Testbed in order to develop and evaluate quantum cryptographic technologies. We have
constructed a free-space optical QKD communications link across 730 meters that operates up to 1.0 Mbps [2]. This
system has successfully demonstrated one-time-pad encrypted streaming video.
Advanced Photon Counting Techniques, edited by Wolfgang Becker, Proc. of SPIE
Vol. 6372, 637211, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.686188
Proc. of SPIE Vol. 6372 637211-1
Bob
Alice
Classical
730m
VCSEL BB84
VCSEL
TX
VCSEL
Optics
VCSEL
BB84
RX
Optics
APD
APD
APD
APD
Figure 1 - The NIST BB84 Free-Space QKD System
The key to the high bit rate of the NIST system lies in the custom PCI boards and parallel classical optical channel.
Each board has a field-programmable gate array (FPGA) and two four-channel gigabit Ethernet serializer/deserializers
(SerDes). The two SerDes, one for the primary classical channel and one for the quantum channel, use a 1.25 GBPS
data stream on each serial data channel and employ 8-bit/10-bit encoding to transmit a balanced clock signal over the
primary classical channel. This allows Bob to synchronize the receiver clock to the transmitter using an internal phaselocked loop (PLL). Using this technique we are able to continuously create 800 ps time bins to gate the quantum
channel receiver and thus enable high-speed quantum key transmission in a scalable manner.
The fundamental performance limitation of the current NIST QKD system is the timing jitter in the avalanche
photodiodes (APDs). Although we are able to synchronize the transmitter and receiver clock at 1.25 GBPS, the timing
jitter in the detectors limits us to lower clock rates. Figure 2 shows the count distribution of a typical detector. The
FWHM of 250 ps and long tail implies that, in order to avoid detection errors due to overlapping time bins, Alice must
transmit on alternating clock cycles. This effectively limits the transmission rate to 625 MHz.
Proc. of SPIE Vol. 6372 637211-2
ult
I\
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250 ps
I
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no
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Figure 2 - The current system is limited in transmission speed by the detector timing jitter.
Because we are limited by our detector performance, we are currently constructing a new free-space QKD system
designed around new detectors with improved timing resolution [3].
3. IMPROVED SINGLE-PHOTON DETECTORS
The central components behind the new NIST system are the improved single-photon detectors. These detectors are
based on APDs with reduced timing jitter. The timing resolution of an APD is significantly affected by the thickness of
the depletion region of the device. Since the output timing is strongly determined by the location at which the absorbed
photon generates the initial carrier, a larger physical volume over which the photon can be absorbed results in a larger
statistical distribution of delays between photon arrival and signal onset [4]. The new detectors have, among other
improvements, significantly thinner depletion regions and thus decrease the statistical variation in delay. Specifically,
these detectors exhibit a FWHM as low as 35 ps. The improved timing resolution is particularly significant in the tail of
the histogram, where the full width at 1/100 can be as low as 370 ps [5]. This should allow a bin width of 400 ps,
enabling an overall transmission rate of 2.5 GBPS – a four-fold improvement over our current system.
The higher timing resolution has the added benefit of strongly localizing the majority of the detection events to
a narrow temporal window. For example, Fig. 3a shows a histogram of detection events due to a 60 ps optical pulse.
Figure 3b shows the percentage of these counts contained within a given temporal window, or gate. In Fig. 3b it can be
seen that approximately 70% of the detection events occur within a 100 ps gate. With such strong localization it is
possible to enhance the signal-to-noise ratio on the QKD quantum channel with improved temporal gating.
Proc. of SPIE Vol. 6372 637211-3
6Ops 656nm pulse
FWHM / 656 nm source
1.2
I)
1
0.8
20.6
001
0.4
• II
p 0.2
I)
0001
-04
0
04
08
12
1.6
0
0
Time (ns)
0,1
0.2
0.3
0.4
0,5
0.6
Gate Width (ns)
(b)
(a)
Figure 3 – (a) Histogram from thin SPAD demonstrating strong temporal localization, and (b)
the percentage of counts in (a) that are contained within a given temporal gate width.
We have implemented a post-selection gate which operates up to 2.5 GHz repetition rate, and that is capable of
temporal gating as low as 100 ps. This gating system is demonstrated in Fig. 4, which shows a histogram of detection
events from optical pulses at a transmission rate of 625 MHz (black line) with a 1% background noise, as well as the
same system operating with 100 ps post-selection gating enabled at a repetition rate of 1.25 GHz (grey line). With this
gating system operating in conjunction with a 1.25 Gbps classical channel we expect that it will be possible to reduce
the minimum temporal gate on the single photon channel from 800 ps to 100 ps, and thereby reduce exposure to
background light by a factor of 8, without a significant reduction in signal. In a free-space system whose quantum-bit
error rate is dominated by solar background, such an improvement can have a significant effect on the ultimate rate of
secret key production.
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—625 MHz TX - Ungated
1
-0.4
0
04
0.8
1.2
1.6
2
Time (ns)
Figure 4 – Histogram of ungated (black) and gated (grey) detection events demonstrating 100
ps post-selection gating at a gate repetition rate of 1.25 GHz.
4. REFERENCES
[1] Wooters, W. K. and W. H. Zurek, “A Single Quantum Cannot be Cloned,” Nature, 299, 802 (1982).
[2] Bienfang, J. C., A. Gross, A. Mink, B. Hershman, A. Nakassis, X. Tang, R. Lu, D. Su, C. Clark, C. Williams, E.
Hagley, and J. Wen, "Quantum key distribution with 1.25 Gbps clock synchronization," Opt. Express, 12, 2011-2016
(2004).
[3] Cova, S., A. Longoni, and A. Andreoni, “Towards picosecond resolution with single-photon avalanche diodes,” Rev.
Sci. Instr., 52, 3 (1981).
[4] Cova, S., M. Ghioni, A. Lotito, I. Rech and F. Zappa, “Evolution and prospects for single-photon avalanche diodes
and quenching circuits,” J. Mod Opt., 15, 9-10, pp 1267-1288 (2004).
[5] Gulinatti, A., P. Maccagnani, I. Rech, M. Ghioni and S. Cova, “35 ps time resolution at room temperature with large
area single photon avalanche diodes,” Elec. Lett., 41, 5 (2005).
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