Stability of high bit rate quantum key distribution on installed fiber

doi:10.1364/OE.20.016339

Stability of high bit rate quantum key distribution on installed fiber

J. F. Dynes, I. Choi, A. W. Sharpe, A. R. Dixon, Z. L. Yuan, M. Fujiwara, M. Sasaki, and A. J. Shields »View Author Affiliations

Optics Express, Vol. 20, Issue 15, pp. 16339-16347 (2012)
http://dx.doi.org/10.1364/OE.20.016339

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▸ Article Outline
– Abstract
1. Introduction
2. QKD system over installed fiber
3. Results
4. Discussion- Long term stability of QKD systems
5. Conclusions
– References and links

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Abstract

We present a study of the effects of ambient fluctuations on the bit rate stability of a quantum key distribution (QKD) link operating on installed fiber. Secure quantum keys were distributed continuously for 60 hours over 45km of installed fiber. A total of 6.33 × 10¹⁰ bits were exchanged during this period, the largest volume of secure keys distributed on installed fiber in a single continuous session to date. We report a record bit-rate distance product for QKD of 13.2 × 10⁶ (bits/s)-km.

1. Introduction

Quantum key distribution (QKD) [1

C. H. Bennett and G. Brassard, “Quantum cryptography: public key distribution and coin tossing,” Proceedings of the IEEE International Conference on Computers Systems and Signal Processing, Bangalore India, 175–179 (1984).

] is a novel technology for providing secure keys between remote parties with a directly quantifiable security [2

N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, “Quantum cryptography,” Rev. Mod. Phys. 74(1), 145–195 (2002). [CrossRef]

V. Scarani, H. Bechmann-Pasquinucci, N. J. Cerf, M. Dušek, N. N. Lütkenhaus, and M. Peev, “The security of practical quantum key distribution,” Rev. Mod. Phys. 81(3), 1301–1350 (2009). [CrossRef]

]. Unlike conventional cryptography, its security does not rely on unproven mathematical complexity, but rather is based on laws of physics. Quantum mechanics guarantees that any eavesdropping activities can be detected. In addition, keys that are exchanged with QKD are future proof, even against the advancement of quantum computing. These unique features make QKD a prime candidate in providing a key distribution platform with utmost security.

During the past decade, rapid progress has been made towards the goal of a stable [4

D. Stucki, M. Legre, F. Buntschu, B. Clausen, N. Felber, N. Gisin, L. Henzen, P. Junod, G. Litzistorf, P. Monbaron, L. Monat, J.-B. Page, D. Perroud, G. Ribordy, A. Rochas, S. Robyr, J. Tavares, R. Thew, P. Trinkler, S. Ventura, R. Voirol, N. Walenta, and H. Zbinden, “Long-term performance of the SwissQuantum quantum key distribution network in a field environment,” New J. Phys. 13(12), 123001 (2011). [CrossRef]

P. Jouguet, S. Kunz-Jacques, T. Debuisschert, S. Fossier, E. Diamanti, R. Alléaume, R. Tualle-Brouri, P. Grangier, A. Leverrier, P. Pache, and P. Painchault, “Field test of classical symmetric encryption with continuous variable quantum key distribution,” arXiv:quant-ph/1201.3744v2 (unpublished).

], high bit rate [6

M. Sasaki, M. Fujiwara, H. Ishizuka, W. Klaus, K. Wakui, M. Takeoka, S. Miki, T. Yamashita, Z. Wang, A. Tanaka, K. Yoshino, Y. Nambu, S. Takahashi, A. Tajima, A. Tomita, T. Domeki, T. Hasegawa, Y. Sakai, H. Kobayashi, T. Asai, K. Shimizu, T. Tokura, T. Tsurumaru, M. Matsui, T. Honjo, K. Tamaki, H. Takesue, Y. Tokura, J. F. Dynes, A. R. Dixon, A. W. Sharpe, Z. L. Yuan, A. J. Shields, S. Uchikoga, M. Legré, S. Robyr, P. Trinkler, L. Monat, J. B. Page, G. Ribordy, A. Poppe, A. Allacher, O. Maurhart, T. Länger, M. Peev, and A. Zeilinger, “Field test of quantum key distribution in the Tokyo QKD Network,” Opt. Express 19(11), 10387–10409 (2011). [CrossRef] [PubMed]

–9

N. Namekata, H. Takesue, T. Honjo, Y. Tokura, and S. Inoue, “High-rate quantum key distribution over 100 km using ultra-low-noise, 2-GHz sinusoidally gated InGaAs/InP avalanche photodiodes,” Opt. Express 19(11), 10632–10639 (2011). [CrossRef] [PubMed]

] QKD system for real world applications [10

I. Choi, R. J. Young, and P. D. Townsend, “Quantum information to the home,” New J. Phys. 13(6), 063039 (2011). [CrossRef]

]. Most QKD demonstrations are fiber based and consequently can make use of installed fiber telecommunication networks. Such networks offer a practical and widely accessible infrastructure for deploying QKD [11

C. Elliott, A. Colvin, D. Pearson, O. Pikalo, J. Schlafer, and H. Yeh, “Current status of the DARPA quantum network,” in Quantum Information and Computation III, E. J. Donkor, A. R. Pirich, and H. E. Brandt, eds., Proc. SPIE 5815, 138–149 (2005); arXiv:quant-ph/0503058v2.

–15

A. Tanaka, M. Fujiwara, K. Yoshino, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “High-speed quantum key distribution system for 1-Mbps real-time key generation,” IEEE J. Quantum Electron. 48(4), 542–550 (2012). [CrossRef]

]. To make quantum security a reality, QKD must demonstrate its suitability for high speed encryption and its reliability for long term operations over installed fiber. In this paper, we present and discuss in detail the long term stability of a high-speed QKD link operating on installed fiber.

2. QKD system over installed fiber

2.1 Installed fiber characteristics

Figure 1 shows the geographic location of the installed fiber used in this experiment. The commercial JGN2plus fiber [5

] bundles have a total length of 45km, linking Otemachi in central Tokyo to Koganei in the western suburbs. They are relatively lossy as they consist of many splices and connectors. The fiber loss was measured to be about 0.3dB/km which is 50% higher than the industry specification for standard SMF-28 fiber of 0.2dB/km.

Fig. 1 Geographical locations of Otemachi and Koganei. The blue line with red dots indicates the approximate route for the 45km single mode fiber (SMF) link. (Note: the actual fiber route is more circuitous.) Also shown are in situ photographs of the transmitter Alice (at Otemachi) and the receiver Bob (at Koganei). (Map copyright: Google Maps 2012)

Furthermore, the fiber bundles are located above ground via utility poles for roughly half of their length. Fibers in this arrangement are highly susceptible to environmental fluctuations. In addition, fibers suffer from photon crosstalk due to close proximity with other fibers carrying normal classical traffic [16

M. Fujiwara, S. Miki, T. Yamashita, Z. Wang, and M. Sasaki, “Photon level crosstalk between parallel fibers installed in urban area,” Opt. Express 18(21), 22199–22207 (2010). [CrossRef] [PubMed]

2.2 Optics

The QKD system is based on a one-way, GHz clocked decoy-state BB84 scheme [7

A. R. Dixon, Z. L. Yuan, J. F. Dynes, A. W. Sharpe, and A. J. Shields, “Gigahertz decoy quantum key distribution with 1 Mbit/s secure key rate,” Opt. Express 16(23), 18790–18797 (2008). [CrossRef] [PubMed]

A. R. Dixon, Z. L. Yuan, J. F. Dynes, A. W. Sharpe, and A. J. Shields, “Continuous operation of high bit rate quantum key distribution,” Appl. Phys. Lett. 96(16), 1611021 (2010). [CrossRef]

]. Figure 2 shows the schematic diagram. In the transmitter (Alice), a distributed feedback (DFB) laser pulsed at 1GHz producing 1550nm photon pulses with 50ps width is used as the source. These pulses pass through an intensity modulator from which three different pulse intensities for the decoy state scheme are created. Signal pulses of 0.5 photons/pulse are sent with almost 99% probability and two different decoy pulses of 0.1 and 0.0007 photons/pulse are sent with less than 1% probability. Photon phase-encoding is achieved by using a Phase Modulator (PM) in an Asymmetric Mach-Zehnder Interferometer (AMZI). The photon pulses are attenuated to single photon level by an attenuator (A) before launching into 45km of installed single-mode fiber (SMF) from Otemachi to Koganei.

Fig. 2 Schematic of the GHz Quantum Key Distribution system. IM: intensity modulator; AMZI: asymmetric Mach-Zehnder interferometer; PM: phase modulator; PBC: polarization beam combiner; A: attenuator; M: power monitor; EPC: electronic polarization controller; PBS: polarization beam splitter; FS: fiber stretcher; SD-APD: self-differencing avalanche photodiode;

In the receiver (Bob), an electronic polarization controller (EPC) is used to correct for polarization drifts which occur in the installed fiber. A PM located in the lower arm of Bob’s AMZI is used for phase decoding and a fiber stretcher located in the upper arm for optical path length compensation. The photons from Bob’s AMZI are then detected by single photon detectors composed of Peltier cooled InGaAs avalanche photodiodes (APDs) gated in Geiger mode at 1GHz. The APDs outputs are analyzed using the self-differencing (SD) [17

Z. L. Yuan, B. E. Kardynal, A. W. Sharpe, and A. J. Shields, “High speed single photon detection in the near infrared,” Appl. Phys. Lett. 91(4), 041114 (2007). [CrossRef]

] technique, which enables suppression of afterpulses, allowing for operation at high gating speeds. The resulting detection efficiencies are ~19% with dark count rates of ~10kHz. A fiber Bragg grating at Alice is used to ensure the effects on the pulse width broadening caused by chromatic dispersion is compensated for at Bob’s detectors.

In both Alice and Bob, all electrical drives to the active components of the optical set-up are provided by Field Programmable Gate Arrays (FPGAs). FPGAs provide a cheap, small footprint and compact solution for realizing arbitrary modulation patterns required for high speed QKD.

2.3 Automated alignment

In a field deployment, the optical interferometers and installed fibers are sensitive to external environmental fluctuations and drifts. For the installed fibers this is particularly apparent when they are carried above ground via utility poles. Therefore, when designing a QKD system for real world application, including an automatic alignment system which compensates for these fluctuations and drifts allowing for continuous operation.

Single photon detector counts in the QKD system are used as a feedback signal to adjust the delay position of the detector gate, as well as the polarization controller state. The fiber stretcher is controlled by using the quantum bit error rate (QBER) as a feedback signal.

Accurate synchronization between the transmitter (Alice) and receiver (Bob) is of paramount importance when using gated detectors in QKD. We choose to wavelength multiplex all classical signals onto a single fiber neighboring the quantum fiber. Classical signals include a 1550nm optical clock driven by a square-wave voltage source and a conventional bi-directional communication channels required for sifting, error correction and privacy amplification. As the classical and quantum fibers are adjacent to each other, they experience similar ambient conditions, resulting in comparable temperature drifts and vibrations. The drift in the detectors’ temporal delays with respect to the synchronization pulses is slow and gradual; as such they are easily trackable.

2.4 Error correction and privacy amplification

Error correction (EC) and privacy amplification (PA) are performed using two multi-core processors located at the transmitter Alice and the receiver Bob. The EC routine is a multi-threaded application implementing the Cascade algorithm [18

G. Brassard and L. Salvail, “Secret-key reconciliation by public discussion,” Lect. Notes Comput. Sci. 765, 410–423 (1994). [CrossRef]

]. Each thread works on bit lengths of 1Mbit at a time. Tests using 4-core processors reveal EC speeds in excess of 5.5Mbps. These speeds were more than adequate for the QKD system over fiber distances with more than 10dB loss. PA was accomplished by a Toeplitz matrix approach [19

C. H. Bennett, G. Brassard, C. Crepeau, and U. M. Maurer, “Generalized privacy amplification,” IEEE Trans. Inf. Theory 41(6), 1915–1923 (1995). [CrossRef]

] which results in the secure key.

3. Results

3.1 Secure bit rate

Figure 3 shows the secure bit rate obtained for a duration of 60 hours of continuous operation. The average secure bit rate is 293kbit/s. This secure key rate is sufficient to support high bandwidth encryption applications, such as business conferencing video (typically 256kbit/s) continuously over 60 hours. This secure bit rate is much higher than the previous QKD field trial carried out by us in the SECOQC collaboration [13

M. Peev, C. Pacher, R. Alléaume, C. Barreiro, J. Bouda, W. Boxleitner, T. Debuisschert, E. Diamanti, M. Dianati, J. F. Dynes, S. Fasel, S. Fossier, M. Fürst, J.-D. Gautier, O. Gay, N. Gisin, P. Grangier, A. Happe, Y. Hasani, M. Hentschel, H. Hübel, G. Humer, T. Länger, M. Legré, R. Lieger, J. Lodewyck, T. Lorünser, N. Lütkenhaus, A. Marhold, T. Matyus, O. Maurhart, L. Monat, S. Nauerth, J.-B. Page, A. Poppe, E. Querasser, G. Ribordy, S. Robyr, L. Salvail, A. W. Sharpe, A. J. Shields, D. Stucki, M. Suda, C. Tamas, T. Themel, R. T. Thew, Y. Thoma, A. Treiber, P. Trinkler, R. Tualle-Brouri, F. Vannel, N. Walenta, H. Weier, H. Weinfurter, I. Wimberger, Z. L. Yuan, H. Zbinden, and A. Zeilinger, “The SECOQC quantum key distribution network in Vienna,” New J. Phys. 11(7), 075001 (2009). [CrossRef]

] where that system ran at a lower clock rate and over only 7.5dB (33km) of fiber loss. The average secure bit rate was 3.1kbit/s. The present work shows a two orders of magnitude of improvement in secure bit rate in this field trial is due mainly to the advance in self-differencing single photon detection technology [17

Z. L. Yuan, B. E. Kardynal, A. W. Sharpe, and A. J. Shields, “High speed single photon detection in the near infrared,” Appl. Phys. Lett. 91(4), 041114 (2007). [CrossRef]

] which greatly suppresses afterpulses and therefore, allow higher clock rates.

Fig. 3 Secure bit rate for the 45km fiber link between Otemachi and Koganei recorded over a continuous 60 hour operation. The average secure bit rate is 293kbits/s. The black arrow indicates the secure bit rate of 3.1kbit/s for the QKD demonstration over a 33km fiber link in the previous field trial in SECOQC [13

3.2 Secure bit rate-distance product

The total number of secure bits transferred over 60 hours of continuous operation is 6.33 × 10¹⁰ bits. This is >10% higher than the 5.46 × 10¹⁰ bits securely distributed by Stucki et al. over a continuous period of 273 days (SQ3 link from 2 Dec 2009 to 27 July 2010) as reported in [4

]. To our knowledge, this is a new record for the largest number of secure bits transmitted over installed fiber during a non-disrupted period.

In analogy with classical communication [20

G. P. Agrawal, Fiber-Optic Communication Systems, 3rd. ed. (John Wiley & Sons, 2002).

], a useful figure of merit for quantum communication system is the bit rate-distance product. The bit rate-distance product takes into account the two most important metrics for a QKD system: the secure bit rate and the key distribution distance. An ideal QKD system should support high secure bit rate and long key distribution distance. The bit rate-distance product provides a standard for a general performance comparison for QKD systems with different bit rate and distances used in demonstrations. Here, we report a record secure bit rate-distance product of 13.2 × 10⁶ (bits/s)-km. Table 1 shows a comparison of this secure bit rate-distance product as well as other important QKD parameters obtained in this paper with four other long duration QKD field trials. These other field trials are the SwissQuantum network [4

], the NEC field trial [15

], the SEQURE project [6

] and the Toshiba contribution to the SECOQC field trial [13

Table 1 Comparison of Recent Long Duration Field Trials

	This Work	SwissQuantum  [4 D. Stucki, M. Legre, F. Buntschu, B. Clausen, N. Felber, N. Gisin, L. Henzen, P. Junod, G. Litzistorf, P. Monbaron, L. Monat, J.-B. Page, D. Perroud, G. Ribordy, A. Rochas, S. Robyr, J. Tavares, R. Thew, P. Trinkler, S. Ventura, R. Voirol, N. Walenta, and H. Zbinden, “Long-term performance of the SwissQuantum quantum key distribution network in a field environment,” New J. Phys. 13(12), 123001 (2011). [CrossRef] ]	NEC* [15 A. Tanaka, M. Fujiwara, K. Yoshino, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “High-speed quantum key distribution system for 1-Mbps real-time key generation,” IEEE J. Quantum Electron. 48(4), 542–550 (2012). [CrossRef] ]	SEQURE  [6 M. Sasaki, M. Fujiwara, H. Ishizuka, W. Klaus, K. Wakui, M. Takeoka, S. Miki, T. Yamashita, Z. Wang, A. Tanaka, K. Yoshino, Y. Nambu, S. Takahashi, A. Tajima, A. Tomita, T. Domeki, T. Hasegawa, Y. Sakai, H. Kobayashi, T. Asai, K. Shimizu, T. Tokura, T. Tsurumaru, M. Matsui, T. Honjo, K. Tamaki, H. Takesue, Y. Tokura, J. F. Dynes, A. R. Dixon, A. W. Sharpe, Z. L. Yuan, A. J. Shields, S. Uchikoga, M. Legré, S. Robyr, P. Trinkler, L. Monat, J. B. Page, G. Ribordy, A. Poppe, A. Allacher, O. Maurhart, T. Länger, M. Peev, and A. Zeilinger, “Field test of quantum key distribution in the Tokyo QKD Network,” Opt. Express 19(11), 10387–10409 (2011). [CrossRef] [PubMed] ]	SECOQC  [13 M. Peev, C. Pacher, R. Alléaume, C. Barreiro, J. Bouda, W. Boxleitner, T. Debuisschert, E. Diamanti, M. Dianati, J. F. Dynes, S. Fasel, S. Fossier, M. Fürst, J.-D. Gautier, O. Gay, N. Gisin, P. Grangier, A. Happe, Y. Hasani, M. Hentschel, H. Hübel, G. Humer, T. Länger, M. Legré, R. Lieger, J. Lodewyck, T. Lorünser, N. Lütkenhaus, A. Marhold, T. Matyus, O. Maurhart, L. Monat, S. Nauerth, J.-B. Page, A. Poppe, E. Querasser, G. Ribordy, S. Robyr, L. Salvail, A. W. Sharpe, A. J. Shields, D. Stucki, M. Suda, C. Tamas, T. Themel, R. T. Thew, Y. Thoma, A. Treiber, P. Trinkler, R. Tualle-Brouri, F. Vannel, N. Walenta, H. Weier, H. Weinfurter, I. Wimberger, Z. L. Yuan, H. Zbinden, and A. Zeilinger, “The SECOQC quantum key distribution network in Vienna,” New J. Phys. 11(7), 075001 (2009). [CrossRef] ]
Duration of Trial	60 hours	>600 days	12 hours	Up to 7 months	24 hours
Uninterrupted operation	60 hours	237 days (2 Dec 2009 – 27 July 2010)	12 hours	85 days	24 hours
Distance (km)	45	3.7	45	17.7	33
Channel Loss (dB)	14.5	2.5	14.5	5.6	7.5
Total Secure Keys Distributed (Gigabit)	63.3	54.6	8.98	4.41	0.27
Secure Bit Rate (kbits/s) per channel	293	2.7	91.11 40.56 76.18	0.6	3.1
Bit Rate Distance Product (bit/s-km)	13.2 × 10⁶	9.9 × 10³	9.35 × 10⁶	10.6 × 10³	102.3 × 10³

*For the NEC results, their 3-channel combined bit rate of 207.85bit/s was used for all calculations

4. Discussion- Long term stability of QKD systems

4.1 Polarization drift in installed fiber

When photons are transmitted through SMF, the fiber birefringence causes the photons’ polarization to undergo a transformation, which varies as a function of time due to temperature or other environmental influences. As the QKD system comprises polarization sensitive components, it is important to recover the polarization of the transmitted photons before they enter the receiver’s interferometer. This is usually achieved using an electronic polarization controller in front of the receiver’s interferometer.

The impact of polarization drifts due to daily environmental fluctuations in the installed fiber from Koganei to Otemachi has been studied. Figure 4 shows the experimental setup. A 50ps 1GHz laser emits optical pulses at 1550nm. A polarizer is used to ensure all photons are linearly polarized before launching over the 45km of installed fiber. At the receiver side, a 1nm wide tunable filter centered at the laser wavelength is used to the reject any stray photons outside of the laser wavelength. A polarization beam splitter (PBS) is used to resolve the incoming photons into the orthogonal components. Both polarization components are recorded using a dual channel power meter.

Fig. 4 Measurement setup for polarization stability.

Figure 5 shows the optical powers measured for photons of two orthogonal polarizations taken over a three day period. The measurement was initialized such that one channel receives the majority of the incoming photons using the polarization controller. Then, the photon polarization is left uncontrolled and drifting freely. While the total power received by the power-meter remains constant, significant fluctuation in the receiving power appear within each individual channel. In some cases, photon polarization is changed completely.

Fig. 5 Polarization stability measurement over 72 hours. The two lines are orthogonal polarizations as measured by the dual channel power meter. Also shown are the periods of day and night (grey lines).

The fluctuation is a result of the polarization variations for photons transmitting through the 45km fiber channel. The fluctuation is especially apparent during the day. Fiber polarization changes are expected to be more pronounced during the day due to weather conditions such as heating from sunlight. During the first night of the experiment, heavy rain fell in Tokyo city. As discussed in Section 2.1, fiber cables hanging on utility poles are particularly susceptible to stress and vibrations. Heavy rain droplets fallen on the fiber cable could cause localized stress and vibrations which would disturb the polarization stability on a shorter time scale than those caused by the large day/night time temperature drifts. This can be seen in Fig. 5 where the polarization data taken during the first night is noisier than those taken during the second and third night.

In the present QKD system, as receiver Bob uses a polarization beam splitter to direct the corresponding orthogonal polarization components to their respective paths, only photons with correct polarization are aligned in time for interference, and hence, detection. Polarization drift that is uncorrected for would lead to a reduction in the secure key rate. For example, at hour 9 of Fig. 5, the secure key rate would be zero because the photons have the opposite polarization to the one initialized. As indicated by Fig. 5, the polarization can fluctuate on the order of minutes, especially during daytime operation. Careful polarization control is therefore required to maximize the photon count rates in the single photon detectors. A 4-channel electronic polarization controller (EPC) is used to compensate the polarization drifts over 60 hours of operation. The overall secure bit rate was kept reasonably constant over the 60 hour period (as shown in Fig. 3). This result indicates the EPC was compensating the rapid and unpredictable polarization drifts adequately.

4.2 Phase drift

To study the impact of environmental variation on the stability of the QKD system/link, we have monitored the fiber stretcher voltage (FSV) as a function of time. Located in Bob’s interferometer, the fiber stretcher is used to maintain the interference condition between Alice and Bob’s AMZIs by changing the optical path length. Local environmental fluctuation such as room heating or room air conditioning can induce phase drifts by changing the optical path length of the interferometers. These effects can be seen by examining Fig. 6 . The FSV continuously adjusts the optical path length in one arm of the interferometer of the receiver (Bob) in order to minimize the QBER.

Fig. 6 (a) Fiber stretcher voltage (FSV) variation as a function of time over the 60 hour period. Figure 6(b) shows a zoomed-in section over 4 hours.

Figure 6(a) shows the variation on FSV over the 60 hours of QKD operation. The variations are rapid and range over the full FSV extent of 0-19V, which corresponds to multiple integer number of 2π phase shifts. Figure 6(b) is a zoomed-in section of Fig. 6(a), showing a 4 hour period starting on a weekday morning at 4:30am and ending at 8:30am. For times before 7:30am, the fiber stretcher voltage has a number of resets characterized by the voltage being set back to 9.5V which is the default median FSV. There are 7 resets over this 3 hour period between 4:30am and 7:30am, giving an average of 2.3 resets/hour. In contrast, during the period from 7:30am – 8:30am, there are many more resets with 12 resets/hour. As Alice’s interferometer and Bob’s interferometer are located in different server rooms, geographically separated by approximately 45km, they experience different indoor environments. The interferometer optical path length changes can be induced by air conditioning or heating effect from day-time functioning equipment. The QKD system coped remarkably well with the combined effects of these variations as evidenced by the stable secure bit rate as shown in Fig. 3 over 60 hours.

4.3 Detector synchronization

As mentioned in section 2.3, the system synchronization pulses are sent through a fiber that is adjacent to the one which carries the quantum signal. As the two fibers are inside the same bundle, the optical path length difference between the two fibers caused by environmental fluctuation is expected to be minimal. Both detectors are gated with these synchronization pulses. Each detector has its own gate delay control and count rate. The individual count rate is used for feedback to adjust the gate delay of each detector. Therefore the gate delays for the two detectors are independent of each other.

Figure 7 shows the detector gate delay drift as a function of time. The detector gate delay is relatively stable during night time operation and gradually drifts down during the day. This is associated with the difference in the lengthening and shortening of the clock and quantum signal fibers as the air temperature changes. Although the two detectors track the synchronization pulses independently of each other, their gates drift by the almost the same amount with respect to the synchronization pulses. A routine continuously monitors the timing delay between detectors. The two detector gate drifts maintain a constant delay of 655ps (with ~18ps standard deviation) throughout the entire 60 hours of operation. The fact that the secure bit rate was roughly constant during this measurement confirms that the active detector gate delay tracking system has been successfully implemented.

Fig. 7 Detector gate delay drifts as a function of time.

5. Conclusions

The impact of environmental fluctuation on a high bit rate QKD system has been carefully analyzed. Local environmental fluctuations at Alice’s and Bob’s sites such as air conditioning and heating effect from day-time functioning equipment as well as the sunlight and vibration on installed fiber affect the stability of the QKD system. The influence of these fluctuations was compensated using several feedback systems to fix the interferometer phase offset, polarization and propagation delay of photons arriving at the receiver. The utility of this approach is evidenced by a high average secure bit rate of 293kbits/s transmitted over 60 hours of operation. This has allowed a record continuous secure bit transfer of 6.33 × 10¹⁰ bits and a record bit-rate distance product of 13.2 × 10⁶ (bits/s)-km. We expect to extend this 60 hours of stable continuous operation in future with further hardware and software improvements.

Acknowledgments

This research is partly supported by Research and Development of Secure Photonic Network Technologies, the Commissioned Research of National Institute of Information and Communications Technology (NICT), Japan. The authors would also like to acknowledge Dr Martin B. Ward for the helpful discussions.

References and links

1.	C. H. Bennett and G. Brassard, “Quantum cryptography: public key distribution and coin tossing,” Proceedings of the IEEE International Conference on Computers Systems and Signal Processing, Bangalore India, 175–179 (1984).
2.	N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, “Quantum cryptography,” Rev. Mod. Phys. 74(1), 145–195 (2002). [CrossRef]
3.	V. Scarani, H. Bechmann-Pasquinucci, N. J. Cerf, M. Dušek, N. N. Lütkenhaus, and M. Peev, “The security of practical quantum key distribution,” Rev. Mod. Phys. 81(3), 1301–1350 (2009). [CrossRef]
4.	D. Stucki, M. Legre, F. Buntschu, B. Clausen, N. Felber, N. Gisin, L. Henzen, P. Junod, G. Litzistorf, P. Monbaron, L. Monat, J.-B. Page, D. Perroud, G. Ribordy, A. Rochas, S. Robyr, J. Tavares, R. Thew, P. Trinkler, S. Ventura, R. Voirol, N. Walenta, and H. Zbinden, “Long-term performance of the SwissQuantum quantum key distribution network in a field environment,” New J. Phys. 13(12), 123001 (2011). [CrossRef]
5.	P. Jouguet, S. Kunz-Jacques, T. Debuisschert, S. Fossier, E. Diamanti, R. Alléaume, R. Tualle-Brouri, P. Grangier, A. Leverrier, P. Pache, and P. Painchault, “Field test of classical symmetric encryption with continuous variable quantum key distribution,” arXiv:quant-ph/1201.3744v2 (unpublished).
6.	M. Sasaki, M. Fujiwara, H. Ishizuka, W. Klaus, K. Wakui, M. Takeoka, S. Miki, T. Yamashita, Z. Wang, A. Tanaka, K. Yoshino, Y. Nambu, S. Takahashi, A. Tajima, A. Tomita, T. Domeki, T. Hasegawa, Y. Sakai, H. Kobayashi, T. Asai, K. Shimizu, T. Tokura, T. Tsurumaru, M. Matsui, T. Honjo, K. Tamaki, H. Takesue, Y. Tokura, J. F. Dynes, A. R. Dixon, A. W. Sharpe, Z. L. Yuan, A. J. Shields, S. Uchikoga, M. Legré, S. Robyr, P. Trinkler, L. Monat, J. B. Page, G. Ribordy, A. Poppe, A. Allacher, O. Maurhart, T. Länger, M. Peev, and A. Zeilinger, “Field test of quantum key distribution in the Tokyo QKD Network,” Opt. Express 19(11), 10387–10409 (2011). [CrossRef] [PubMed]
7.	A. R. Dixon, Z. L. Yuan, J. F. Dynes, A. W. Sharpe, and A. J. Shields, “Gigahertz decoy quantum key distribution with 1 Mbit/s secure key rate,” Opt. Express 16(23), 18790–18797 (2008). [CrossRef] [PubMed]
8.	A. R. Dixon, Z. L. Yuan, J. F. Dynes, A. W. Sharpe, and A. J. Shields, “Continuous operation of high bit rate quantum key distribution,” Appl. Phys. Lett. 96(16), 1611021 (2010). [CrossRef]
9.	N. Namekata, H. Takesue, T. Honjo, Y. Tokura, and S. Inoue, “High-rate quantum key distribution over 100 km using ultra-low-noise, 2-GHz sinusoidally gated InGaAs/InP avalanche photodiodes,” Opt. Express 19(11), 10632–10639 (2011). [CrossRef] [PubMed]
10.	I. Choi, R. J. Young, and P. D. Townsend, “Quantum information to the home,” New J. Phys. 13(6), 063039 (2011). [CrossRef]
11.	C. Elliott, A. Colvin, D. Pearson, O. Pikalo, J. Schlafer, and H. Yeh, “Current status of the DARPA quantum network,” in Quantum Information and Computation III, E. J. Donkor, A. R. Pirich, and H. E. Brandt, eds., Proc. SPIE 5815, 138–149 (2005); arXiv:quant-ph/0503058v2.
12.	T. E. Chapuran, P. Toliver, N. A. Peters, J. Jackel, M. S. Goodman, R. J. Runser, S. R. McNown, N. Dallmann, R. J. Hughes, K. P. McCabe, J. E. Nordholt, C. G. Peterson, K. T. Tyagi, L. Mercer, and H. Dardy, “Optical networking for quantum key distribution and quantum communications,” New J. Phys. 11(10), 105001 (2009). [CrossRef]
13.	M. Peev, C. Pacher, R. Alléaume, C. Barreiro, J. Bouda, W. Boxleitner, T. Debuisschert, E. Diamanti, M. Dianati, J. F. Dynes, S. Fasel, S. Fossier, M. Fürst, J.-D. Gautier, O. Gay, N. Gisin, P. Grangier, A. Happe, Y. Hasani, M. Hentschel, H. Hübel, G. Humer, T. Länger, M. Legré, R. Lieger, J. Lodewyck, T. Lorünser, N. Lütkenhaus, A. Marhold, T. Matyus, O. Maurhart, L. Monat, S. Nauerth, J.-B. Page, A. Poppe, E. Querasser, G. Ribordy, S. Robyr, L. Salvail, A. W. Sharpe, A. J. Shields, D. Stucki, M. Suda, C. Tamas, T. Themel, R. T. Thew, Y. Thoma, A. Treiber, P. Trinkler, R. Tualle-Brouri, F. Vannel, N. Walenta, H. Weier, H. Weinfurter, I. Wimberger, Z. L. Yuan, H. Zbinden, and A. Zeilinger, “The SECOQC quantum key distribution network in Vienna,” New J. Phys. 11(7), 075001 (2009). [CrossRef]
14.	I. Choi, R. J. Young, and P. D. Townsend, “Quantum key distribution on a 10Gb/s WDM-PON,” Opt. Express 18(9), 9600–9612 (2010). [CrossRef] [PubMed]
15.	A. Tanaka, M. Fujiwara, K. Yoshino, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “High-speed quantum key distribution system for 1-Mbps real-time key generation,” IEEE J. Quantum Electron. 48(4), 542–550 (2012). [CrossRef]
16.	M. Fujiwara, S. Miki, T. Yamashita, Z. Wang, and M. Sasaki, “Photon level crosstalk between parallel fibers installed in urban area,” Opt. Express 18(21), 22199–22207 (2010). [CrossRef] [PubMed]
17.	Z. L. Yuan, B. E. Kardynal, A. W. Sharpe, and A. J. Shields, “High speed single photon detection in the near infrared,” Appl. Phys. Lett. 91(4), 041114 (2007). [CrossRef]
18.	G. Brassard and L. Salvail, “Secret-key reconciliation by public discussion,” Lect. Notes Comput. Sci. 765, 410–423 (1994). [CrossRef]
19.	C. H. Bennett, G. Brassard, C. Crepeau, and U. M. Maurer, “Generalized privacy amplification,” IEEE Trans. Inf. Theory 41(6), 1915–1923 (1995). [CrossRef]
20.	G. P. Agrawal, Fiber-Optic Communication Systems, 3rd. ed. (John Wiley & Sons, 2002).

OCIS Codes
(270.0270) Quantum optics : Quantum optics
(270.5565) Quantum optics : Quantum communications
(270.5568) Quantum optics : Quantum cryptography

ToC Category:
Quantum Optics

History
Original Manuscript: April 16, 2012
Revised Manuscript: May 31, 2012
Manuscript Accepted: June 19, 2012
Published: July 3, 2012

Citation
J. F. Dynes, I. Choi, A. W. Sharpe, A. R. Dixon, Z. L. Yuan, M. Fujiwara, M. Sasaki, and A. J. Shields, "Stability of high bit rate quantum key distribution on installed fiber," Opt. Express 20, 16339-16347 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-15-16339

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References

C. H. Bennett and G. Brassard, “Quantum cryptography: public key distribution and coin tossing,” Proceedings of the IEEE International Conference on Computers Systems and Signal Processing, Bangalore India, 175–179 (1984).
N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, “Quantum cryptography,” Rev. Mod. Phys.74(1), 145–195 (2002). [CrossRef]
V. Scarani, H. Bechmann-Pasquinucci, N. J. Cerf, M. Dušek, N. N. Lütkenhaus, and M. Peev, “The security of practical quantum key distribution,” Rev. Mod. Phys.81(3), 1301–1350 (2009). [CrossRef]
D. Stucki, M. Legre, F. Buntschu, B. Clausen, N. Felber, N. Gisin, L. Henzen, P. Junod, G. Litzistorf, P. Monbaron, L. Monat, J.-B. Page, D. Perroud, G. Ribordy, A. Rochas, S. Robyr, J. Tavares, R. Thew, P. Trinkler, S. Ventura, R. Voirol, N. Walenta, and H. Zbinden, “Long-term performance of the SwissQuantum quantum key distribution network in a field environment,” New J. Phys.13(12), 123001 (2011). [CrossRef]
P. Jouguet, S. Kunz-Jacques, T. Debuisschert, S. Fossier, E. Diamanti, R. Alléaume, R. Tualle-Brouri, P. Grangier, A. Leverrier, P. Pache, and P. Painchault, “Field test of classical symmetric encryption with continuous variable quantum key distribution,” arXiv:quant-ph/1201.3744v2 (unpublished).
M. Sasaki, M. Fujiwara, H. Ishizuka, W. Klaus, K. Wakui, M. Takeoka, S. Miki, T. Yamashita, Z. Wang, A. Tanaka, K. Yoshino, Y. Nambu, S. Takahashi, A. Tajima, A. Tomita, T. Domeki, T. Hasegawa, Y. Sakai, H. Kobayashi, T. Asai, K. Shimizu, T. Tokura, T. Tsurumaru, M. Matsui, T. Honjo, K. Tamaki, H. Takesue, Y. Tokura, J. F. Dynes, A. R. Dixon, A. W. Sharpe, Z. L. Yuan, A. J. Shields, S. Uchikoga, M. Legré, S. Robyr, P. Trinkler, L. Monat, J. B. Page, G. Ribordy, A. Poppe, A. Allacher, O. Maurhart, T. Länger, M. Peev, and A. Zeilinger, “Field test of quantum key distribution in the Tokyo QKD Network,” Opt. Express19(11), 10387–10409 (2011). [CrossRef] [PubMed]
A. R. Dixon, Z. L. Yuan, J. F. Dynes, A. W. Sharpe, and A. J. Shields, “Gigahertz decoy quantum key distribution with 1 Mbit/s secure key rate,” Opt. Express16(23), 18790–18797 (2008). [CrossRef] [PubMed]
A. R. Dixon, Z. L. Yuan, J. F. Dynes, A. W. Sharpe, and A. J. Shields, “Continuous operation of high bit rate quantum key distribution,” Appl. Phys. Lett.96(16), 1611021 (2010). [CrossRef]
N. Namekata, H. Takesue, T. Honjo, Y. Tokura, and S. Inoue, “High-rate quantum key distribution over 100 km using ultra-low-noise, 2-GHz sinusoidally gated InGaAs/InP avalanche photodiodes,” Opt. Express19(11), 10632–10639 (2011). [CrossRef] [PubMed]
I. Choi, R. J. Young, and P. D. Townsend, “Quantum information to the home,” New J. Phys.13(6), 063039 (2011). [CrossRef]
C. Elliott, A. Colvin, D. Pearson, O. Pikalo, J. Schlafer, and H. Yeh, “Current status of the DARPA quantum network,” in Quantum Information and Computation III, E. J. Donkor, A. R. Pirich, and H. E. Brandt, eds., Proc. SPIE 5815, 138–149 (2005); arXiv:quant-ph/0503058v2.
T. E. Chapuran, P. Toliver, N. A. Peters, J. Jackel, M. S. Goodman, R. J. Runser, S. R. McNown, N. Dallmann, R. J. Hughes, K. P. McCabe, J. E. Nordholt, C. G. Peterson, K. T. Tyagi, L. Mercer, and H. Dardy, “Optical networking for quantum key distribution and quantum communications,” New J. Phys.11(10), 105001 (2009). [CrossRef]
M. Peev, C. Pacher, R. Alléaume, C. Barreiro, J. Bouda, W. Boxleitner, T. Debuisschert, E. Diamanti, M. Dianati, J. F. Dynes, S. Fasel, S. Fossier, M. Fürst, J.-D. Gautier, O. Gay, N. Gisin, P. Grangier, A. Happe, Y. Hasani, M. Hentschel, H. Hübel, G. Humer, T. Länger, M. Legré, R. Lieger, J. Lodewyck, T. Lorünser, N. Lütkenhaus, A. Marhold, T. Matyus, O. Maurhart, L. Monat, S. Nauerth, J.-B. Page, A. Poppe, E. Querasser, G. Ribordy, S. Robyr, L. Salvail, A. W. Sharpe, A. J. Shields, D. Stucki, M. Suda, C. Tamas, T. Themel, R. T. Thew, Y. Thoma, A. Treiber, P. Trinkler, R. Tualle-Brouri, F. Vannel, N. Walenta, H. Weier, H. Weinfurter, I. Wimberger, Z. L. Yuan, H. Zbinden, and A. Zeilinger, “The SECOQC quantum key distribution network in Vienna,” New J. Phys.11(7), 075001 (2009). [CrossRef]
I. Choi, R. J. Young, and P. D. Townsend, “Quantum key distribution on a 10Gb/s WDM-PON,” Opt. Express18(9), 9600–9612 (2010). [CrossRef] [PubMed]
A. Tanaka, M. Fujiwara, K. Yoshino, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “High-speed quantum key distribution system for 1-Mbps real-time key generation,” IEEE J. Quantum Electron.48(4), 542–550 (2012). [CrossRef]
M. Fujiwara, S. Miki, T. Yamashita, Z. Wang, and M. Sasaki, “Photon level crosstalk between parallel fibers installed in urban area,” Opt. Express18(21), 22199–22207 (2010). [CrossRef] [PubMed]
Z. L. Yuan, B. E. Kardynal, A. W. Sharpe, and A. J. Shields, “High speed single photon detection in the near infrared,” Appl. Phys. Lett.91(4), 041114 (2007). [CrossRef]
G. Brassard and L. Salvail, “Secret-key reconciliation by public discussion,” Lect. Notes Comput. Sci.765, 410–423 (1994). [CrossRef]
C. H. Bennett, G. Brassard, C. Crepeau, and U. M. Maurer, “Generalized privacy amplification,” IEEE Trans. Inf. Theory41(6), 1915–1923 (1995). [CrossRef]
G. P. Agrawal, Fiber-Optic Communication Systems, 3rd. ed. (John Wiley & Sons, 2002).

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