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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 20 — Sep. 26, 2011
  • pp: 19562–19571
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Afterpulse-like phenomenon of superconducting single photon detector in high speed quantum key distribution system

M. Fujiwara, A. Tanaka, S. Takahashi, K. Yoshino, Y. Nambu, A. Tajima, S. Miki, T. Yamashita, Z. Wang, A. Tomita, and M. Sasaki  »View Author Affiliations


Optics Express, Vol. 19, Issue 20, pp. 19562-19571 (2011)
http://dx.doi.org/10.1364/OE.19.019562


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Abstract

We discuss our estimates of the performance of a superconducting single photon detector (SSPD) in a high speed quantum key distribution (QKD) system. We find that at high repetition operation reflections from the readout circuit at room temperature causes an afterpulse-like phenomenon, and drastically increases the quantum bit error rate (QBER). Such effects are not seen during low frequency operation. By using an amplifier with a small reflection coefficient S11, we succeed in reducing the afterpulse-like phenomenon and increasing a secure key rate.

© 2011 OSA

1. Introduction

Quantum key distribution (QKD) [1

1. 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, Bangalore India, pp 175–179, December (1984).

,2

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

] is a representative candidate enabling critical issue to be protected in network service. QKD allows users to communicate with absolute security by combining it with Vernam’s one-time pad. Unconditional security is guaranteed by the fundamental laws of physics. In 2010, we demonstrated the world’s-first secure TV conferencing in a metropolitan QKD network with trusted nodes [3

3. 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]

], called the Tokyo QKD Network. Six different QKD systems were integrated into a mesh-type network. A gigahertz-clocked QKD system that has been developed by NEC and NICT in this network enabled us to demonstrate the information-theoretically secure TV conferencing over a distance of 45 km [3

3. 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]

5

5. A. Tanaka, M. Fujiwara, K. Yoshino, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “A scalable full quantum key distribution system based on colourless interferometric technique and hardware key distillation,” Proc. ECOC 2011, paper Mo.1.B.3 (2011).

]. The system is designed for a multi-channel QKD scheme with wavelength division multiplexing (WDM). Each channel is operated at a clock rate of 1.25 GHz, and a maximum of eight channels can be installed in our system. A laser diode on the transmitting side releases 1550 nm photon pulses with a 50 ps width at a repetition rate of 1.25 GHz at each channel. A 2-by-2 asymmetric Mach-Zehnder interferometer (MZI) made of a polarization free planar-lightwave-circuit (PLC) splits these pulses into pairs of double pulses with a 400 ps delay with time-bin encoding.

At the receiver side, the quantum and the synchronization signals are divided through a WDM filter, and the quantum signals are input to a 2-by-4 asymmetric and totally passive PLC-MZI, and these are then detected by four-channel detectors. Superconducting single photon detectors (SSPD) [6

6. G. Gol'tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, “Picosecond superconducting single-photon optical detector,” Appl. Phys. Lett. 79(6), 705–707 (2001). [CrossRef]

,7

7. A. Korneev, P. Kouminov, V. Matvienko, G. Chulkova, K. Smirnov, B. Voronov, G. N. Gol’tsman, M. Currie, W. Lo, K. Wilsher, J. Zhang, W. Słysz, A. Pearlman, A. Verevkin, and R. Sobolewski, “Sensitivity and gigahertz counting performance of NbN superconducting single photon detectors,” Appl. Phys. Lett. 84(26), 5338–5340 (2004). [CrossRef]

] have recently been widely used for quantum experiments [8

8. A. Tanaka, M. Fujiwara, S. W. Nam, Y. Nambu, S. Takahashi, W. Maeda, K. Yoshino, S. Miki, B. Baek, Z. Wang, A. Tajima, M. Sasaki, and A. Tomita, “Ultra-fast quantum key distribution over a 97 km installed telecom fiber with wavelength division multiplexing clock synchronization,” Opt. Express 16(15), 11354–11360 (2008). [CrossRef] [PubMed]

]. We have developed and demonstrated a three channel WDM QKD that is connected to two sets of SSPDs developed by NICT [9

9. S. Miki, M. Takeda, M. Fujiwara, M. Sasaki, and Z. Wang, “Compactly packaged superconducting nanowire single-photon detector with an optical cavity for multichannel system,” Opt. Express 17(26), 23557–23564 (2009). [CrossRef] [PubMed]

,10

10. S. Miki, T. Yamashita, M. Fujiwara, M. Sasaki, and Z. Wang, “Multichannel SNSPD system with high detection efficiency at telecommunication wavelength,” Opt. Lett. 35(13), 2133–2135 (2010). [CrossRef] [PubMed]

] and one set of semiconductor single photon detectors. In a single channel with SSPDs, the averaged sifted key and final secure key rates of our QKD system after 45 km transmission through a field installed fiber (14.5dB loss) are 268.9 kbps and 81.7 kbps respectively, under an averaged quantum bit error rate (QBER) of 2.7% [4

4. K. Yoshino et al., in preparation.

,5

5. A. Tanaka, M. Fujiwara, K. Yoshino, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “A scalable full quantum key distribution system based on colourless interferometric technique and hardware key distillation,” Proc. ECOC 2011, paper Mo.1.B.3 (2011).

]. The detection efficiency of the SSPDs themselves are about 15% at a dark count (DC) rate of 100 cps, but the total detection efficiency decreases to less than 10% when they are connected to the QKD system. One reason for this discrepancy is due to event selection with a time window of 400 ps. The active time window imposed on the time-bin signal cannot cover the whole pulse spreading after the fiber transmission. Moreover, the QBER increased drastically when the SSPD operated at high detection efficiency (DE) conditions (high bias current region).

This paper reports on the behavior of SSPDs when they are used in a high speed QKD system and the discrepancy between unit testing results and system performance. In contrast to the conventional belief that SSPDs are free of afterpulse phenomena, we observed an afterpulse-like phenomenon in SSPDs especially when they are operated at high repetition rate. We discuss its origin and present countermeasures against it deteriorating the improvements in QKD performance.

2. Afterpulse-like phenomenon of SSPD

It is noted that several approaches to improve the performances of SSPDs have been reported [13

13. B. Baek, A. E. Lita, V. Verma, and S. W. Nam, “Superconducting a-WxSi1-x nanowire single photon detector with saturated internal quantum efficiency from visible to 1850 nm,” Appl. Phys. Lett. 98(25), 251105 (2011). [CrossRef]

15

15. X. Hu, E. A. Dauler, R. J. Molnar, and K. K. Berggren, “Superconducting nanowire single-photon detectors integrated with optical nano-antennae,” Opt. Express 19(1), 17–31 (2011). [CrossRef] [PubMed]

], and our SSPDs are also on the way for further improvement. Namely, we will try to make SSPDs on other substrates i.e. Si or SiO2 to get higher affinity for fiber and silicon photonics. Moreover, DEs of our SSPDs have dependency on polarization of incident photons due to their meander structures. Amount of change in DE is about 3dB [16

16. S. Miki, T. Yamashita, M. Fujiwara, M. Sasaki, and Z. Wang, “Characterization of coupling efficiency and absorption coefficient for fiber-coupled SNSPD with an optical cavity,” IEEE Trans. Appl. Supercond. 21(3), 332–335 (2011). [CrossRef]

]. To apply SSPDs for a QKD system, we need polarization controllers in the QKD system. Or SSPDs with minimized polarization dependence [17

17. S. N. Dorenbos, E. M. Reiger, N. Akopian, U. Perinetti, V. Zwiller, T. Zijlstra, and T. M. Klapwijk, “Superconducting single photon detectors with minimized polarization dependence,” Appl. Phys. Lett. 93(16), 161102 (2008). [CrossRef]

] should be used.

Highly sensitive detectors are generally susceptible to grounding conditions. SSPDs are particularly vulnerable. When SSPDs are connected to the QKD system, we install an RF transformer to insulate the ground lines. Without this, the QKD system suffers from large noise of 50 Hz.

To measure the behavior of SSPDs at high detection rates, we use 50-ps-wide and 10-MHz-repetition periodic weak laser pulses at 1550 nm and a time interval analyzer (TIA: YOKOGAWA TA-520). The average photon number per pulse is adjusted from 0.1 to 1. Figure 3
Fig. 3 Temporal histograms for photon counting rates with and without illumination. (a) Biased at DC rate of 50-100 c/s, and (b) at DC rate of 800 −1000 c/s.
shows histograms of photon counting rates at DC rates of 50 −100 c/s and 800 −1000 c/s with and without illumination. The peaks in Fig. 3 correspond to detection events for input photon pulses. The temporal axis is opposite to the real time flow. The average photon number per pulse is 0.1, and DC levels are increased due to stray light. At DC rate of 50- 100 c/s, the floor levels are almost equal with and without illumination as seen in Fig. 3(a). Here, the counting rate of photons detection events is about 100k c/s. However, at a DC rate of 800 −1000 c/s, the floor level increases about tenfold when it is illuminated with counting a rate of about 150k c/s, which is shown in Fig. 3(b). Moreover, periodical fluctuations in the count rate can be observed. This is specifically an “afterpulse” phenomenon, which has been observed in our SSPD system at the first time. This phenomenon is consistent with the rapid increase of QBER in the QKD system in the region of high current bias.

To clarify the conditions for afterpulses, we provide histograms of the photon counting rate at a DC rate of 50 −100 c/s for various photon numbers per pulse in Fig. 4
Fig. 4 Temporal histograms for various photon numbers per pulse, (a) dark, (b) 0.1, (c) 0.7, and (d) 1.
. At least, when the average photon number exceeds 0.7, an afterpulse is observed. At that time, the photon detection rate is about 700k −1M c/s. Therefore, the high repetition rate of detection events induces afterpulses. To assess whether the high detection rate changes the characteristics of the superconductor itself, histograms are measured for various background photon levels from 50 to 2000 c/s, as shown in Fig. 5
Fig. 5 Temporal histograms for various back ground levels. Background photon counting rate levels are (a) 51 c/s, (b) 590 c/s, and (c) 2000 c/s. Average photon number per pulse is 0.7. Signal photon detection rate is about 700k c/s.
. Background photons are adjusted by changing brightness of a room light. The average photon number per pulse is 0.7 and bias is set at a DC rate of 50 c/s. The DCs (measured as counts between the pulses of the laser) increase to (a) 383, (b) 1220, and (c) 2725 c/s. The afterpulse–like phenomena are difficult to recognize. If the afterpulse emanates from the superconductor itself, i.e., the superconductor induces DCs by itself, the increase rate in the floor level would be liner to background photon numbers. However, the percentages for growth decrease when the background level increases. For that reason, we suspect the readout circuit as the cause of afterpulses.

3. Reflection from readout circuit

We have been used a series of two low noise amplifiers (RF Bay Inc., LNA-550 and LNA-1000) as the readout circuit of the SSPD system. To confirm interference to an SSPD, we measure the reflection coefficient S11 of this readout circuit with a network analyzer (Agilent E5061A).Before measuring S11, the influence of a cable is checked for lengths of 10, 30, and 50 cm. At the SSPD with low critical current (Ic) of around 10 µA, a QBER increases when the cable length is over 30cm. However, for the SSPD with Ic of around 20 µA, there is no change in the QBER. Therefore, we note below the performance of the SSPD with Ic of over 20 µA with 50 cm cable to eliminate the influence of attenuation in the cable. The frequency properties of S11 and waveforms with temporal histograms measured with an oscilloscope are shown in Fig. 6(a)
Fig. 6 Frequency properties of S11 and oscilloscope display: waveforms with temporal histograms of signals (blue lines) for (a) series of LNA-550 and LNA-1000, (b) series of 5840A and SHF 74B. Histograms indicate repetitions over threshold.
. Below 4 MHz, large reflections can be observed, and an afterpulse appears at about 175 ns (shown in Fig. 6(a): red circle in oscilloscope measurements) after a short dead time for the SSPD due to the band width of the read out circuit. Such a high S11 generally induces an undesirable standing wave effects. Moreover, back reflection voltage should influence the bias of the SSPD. Especially, a small increase of the bias would induce high DC in the region of high current bias. If this high S11 induces the afterpulse, we can suppress this phenomenon using an amplifier with low S11. We test the series of 5840A (Picosecond Lab.) and SHF 74B (SHF Communication Technology) amplifiers that are indicated in Fig. 1 (inside the dashed rectangle). The bandwidth of these amplifiers are around 10 GHz, therefore, two low frequency pass filters of 900 and 600 MHz are installed to reduce high frequency noise. Figure 6(b) shows the frequency properties and waveforms with a temporal histogram for this new combination. The value of S11 in the low frequency region is about −20dB and afterpulses are obviously suppressed. Figure 7
Fig. 7 Temporal histograms for counting rate using new readout circuit (series of 5840B and SHF 74A) for various input photon numbers.
shows temporal histograms of the new readout circuit for several input pulse intensities at a DC rate of 50 −100 c/s. When the average photon number is 1, despite the detection rate reaching 1M c/s, the increase of DC is suppressed to 130 c/s. To confirm the influence of the back reflection from the readout circuit, it would be desirable to see a change of the delay between the initial pulse from a photon and after-pulses by changing the length of the cable. However, this is not experimentally feasible, because QBER is strongly affected by attenuation in the cable. We measure a change of the delay of the reflection by increasing the cable length to 7 m. We observe a slight shift of the peak in the histogram, but cannot draw a definite conclusion, because the cable length of 7 m is too short to make a significant change in a broad peak of the 175 ns delay reflection, which corresponds to 35 m cable. Further increasing of a cable length induces serious attenuation of a signal pulse, and it hinders the measurement of meaningful data. Nevertheless, the fact that the afterpulse can be decreased by using a readout circuit with low S11 implies that afterpulse of a SSPD can be identified as the back reflections of a readout circuit.

4. Performance gain of the QKD system

5. Conclusion

We report the behavior of an SSPD when it is used in a gigahertz-clocked QKD system. An “afterpulse” appears in the SSPD when the repetition rate of detection increases near 1 MHz, and it produces the discrepancy between a unit testing result and system performance. Such phenomena are caused by reflection from a readout circuit, and the SSPD itself is free of afterpulse. The secure key generation rate of our QKD system will be improved more than 25% by using a low S11 readout circuit,

Acknowledgement

References and links

1.

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, Bangalore India, pp 175–179, December (1984).

2.

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

3.

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]

4.

K. Yoshino et al., in preparation.

5.

A. Tanaka, M. Fujiwara, K. Yoshino, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “A scalable full quantum key distribution system based on colourless interferometric technique and hardware key distillation,” Proc. ECOC 2011, paper Mo.1.B.3 (2011).

6.

G. Gol'tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, “Picosecond superconducting single-photon optical detector,” Appl. Phys. Lett. 79(6), 705–707 (2001). [CrossRef]

7.

A. Korneev, P. Kouminov, V. Matvienko, G. Chulkova, K. Smirnov, B. Voronov, G. N. Gol’tsman, M. Currie, W. Lo, K. Wilsher, J. Zhang, W. Słysz, A. Pearlman, A. Verevkin, and R. Sobolewski, “Sensitivity and gigahertz counting performance of NbN superconducting single photon detectors,” Appl. Phys. Lett. 84(26), 5338–5340 (2004). [CrossRef]

8.

A. Tanaka, M. Fujiwara, S. W. Nam, Y. Nambu, S. Takahashi, W. Maeda, K. Yoshino, S. Miki, B. Baek, Z. Wang, A. Tajima, M. Sasaki, and A. Tomita, “Ultra-fast quantum key distribution over a 97 km installed telecom fiber with wavelength division multiplexing clock synchronization,” Opt. Express 16(15), 11354–11360 (2008). [CrossRef] [PubMed]

9.

S. Miki, M. Takeda, M. Fujiwara, M. Sasaki, and Z. Wang, “Compactly packaged superconducting nanowire single-photon detector with an optical cavity for multichannel system,” Opt. Express 17(26), 23557–23564 (2009). [CrossRef] [PubMed]

10.

S. Miki, T. Yamashita, M. Fujiwara, M. Sasaki, and Z. Wang, “Multichannel SNSPD system with high detection efficiency at telecommunication wavelength,” Opt. Lett. 35(13), 2133–2135 (2010). [CrossRef] [PubMed]

11.

S. Miki, M. Fujiwara, M. Sasaki, B. Baek, A. J. Miller, R. H. Hadfield, S. W. Nam, and Z. Wang, “Large sensitive-area NbN nanowire superconducting single-photon detectors fabricated on single-crystal MgO substrates,” Appl. Phys. Lett. 92(6), 061116 (2008). [CrossRef]

12.

T. Yamashita, S. Miki, W. Qiu, M. Fujiwara, M. Sasaki, and Z. Wang, “Temperature dependent performances of superconducting nanowire single-photon detectors in an ultralow-temperature,” Appl. Phys. Express 3(10), 102502 (2010). [CrossRef]

13.

B. Baek, A. E. Lita, V. Verma, and S. W. Nam, “Superconducting a-WxSi1-x nanowire single photon detector with saturated internal quantum efficiency from visible to 1850 nm,” Appl. Phys. Lett. 98(25), 251105 (2011). [CrossRef]

14.

M. G. Tanner, C. M. Natarajan, V. K. Pottapenjara, J. A. O’Connor, R. J. Warburton, R. H. Hadfield, B. Baek, S. Nam, S. N. Dorenbos, E. B. Ureña, T. Zijlstra, T. M. Klapwijk, and V. Zwiller, “Enhanced telecom wavelength single-photon detection with NbTiN superconducting nanowires on oxidized silicon,” Appl. Phys. Lett. 96(22), 221109 (2010). [CrossRef]

15.

X. Hu, E. A. Dauler, R. J. Molnar, and K. K. Berggren, “Superconducting nanowire single-photon detectors integrated with optical nano-antennae,” Opt. Express 19(1), 17–31 (2011). [CrossRef] [PubMed]

16.

S. Miki, T. Yamashita, M. Fujiwara, M. Sasaki, and Z. Wang, “Characterization of coupling efficiency and absorption coefficient for fiber-coupled SNSPD with an optical cavity,” IEEE Trans. Appl. Supercond. 21(3), 332–335 (2011). [CrossRef]

17.

S. N. Dorenbos, E. M. Reiger, N. Akopian, U. Perinetti, V. Zwiller, T. Zijlstra, and T. M. Klapwijk, “Superconducting single photon detectors with minimized polarization dependence,” Appl. Phys. Lett. 93(16), 161102 (2008). [CrossRef]

18.

Japan Gigabit Network 2 plus, http://www.jgn.nict.go.jp/jgn2plus/english/index.html.

19.

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]

20.

H.-K. Lo, X. Ma, and K. Chen, “Decoy state quantum key distribution,” Phys. Rev. Lett. 94(23), 230504 (2005). [CrossRef] [PubMed]

OCIS Codes
(030.5260) Coherence and statistical optics : Photon counting
(040.0040) Detectors : Detectors

ToC Category:
Detectors

History
Original Manuscript: July 26, 2011
Revised Manuscript: August 19, 2011
Manuscript Accepted: August 23, 2011
Published: September 22, 2011

Citation
M. Fujiwara, A. Tanaka, S. Takahashi, K. Yoshino, Y. Nambu, A. Tajima, S. Miki, T. Yamashita, Z. Wang, A. Tomita, and M. Sasaki, "Afterpulse-like phenomenon of superconducting single photon detector in high speed quantum key distribution system," Opt. Express 19, 19562-19571 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-20-19562


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References

  1. 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, Bangalore India, pp 175–179, December (1984).
  2. N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, “Quantum cryptography,” Rev. Mod. Phys.74(1), 145–195 (2002). [CrossRef]
  3. 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]
  4. K. Yoshino et al., in preparation.
  5. A. Tanaka, M. Fujiwara, K. Yoshino, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “A scalable full quantum key distribution system based on colourless interferometric technique and hardware key distillation,” Proc. ECOC 2011, paper Mo.1.B.3 (2011).
  6. G. Gol'tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, “Picosecond superconducting single-photon optical detector,” Appl. Phys. Lett.79(6), 705–707 (2001). [CrossRef]
  7. A. Korneev, P. Kouminov, V. Matvienko, G. Chulkova, K. Smirnov, B. Voronov, G. N. Gol’tsman, M. Currie, W. Lo, K. Wilsher, J. Zhang, W. Słysz, A. Pearlman, A. Verevkin, and R. Sobolewski, “Sensitivity and gigahertz counting performance of NbN superconducting single photon detectors,” Appl. Phys. Lett.84(26), 5338–5340 (2004). [CrossRef]
  8. A. Tanaka, M. Fujiwara, S. W. Nam, Y. Nambu, S. Takahashi, W. Maeda, K. Yoshino, S. Miki, B. Baek, Z. Wang, A. Tajima, M. Sasaki, and A. Tomita, “Ultra-fast quantum key distribution over a 97 km installed telecom fiber with wavelength division multiplexing clock synchronization,” Opt. Express16(15), 11354–11360 (2008). [CrossRef] [PubMed]
  9. S. Miki, M. Takeda, M. Fujiwara, M. Sasaki, and Z. Wang, “Compactly packaged superconducting nanowire single-photon detector with an optical cavity for multichannel system,” Opt. Express17(26), 23557–23564 (2009). [CrossRef] [PubMed]
  10. S. Miki, T. Yamashita, M. Fujiwara, M. Sasaki, and Z. Wang, “Multichannel SNSPD system with high detection efficiency at telecommunication wavelength,” Opt. Lett.35(13), 2133–2135 (2010). [CrossRef] [PubMed]
  11. S. Miki, M. Fujiwara, M. Sasaki, B. Baek, A. J. Miller, R. H. Hadfield, S. W. Nam, and Z. Wang, “Large sensitive-area NbN nanowire superconducting single-photon detectors fabricated on single-crystal MgO substrates,” Appl. Phys. Lett.92(6), 061116 (2008). [CrossRef]
  12. T. Yamashita, S. Miki, W. Qiu, M. Fujiwara, M. Sasaki, and Z. Wang, “Temperature dependent performances of superconducting nanowire single-photon detectors in an ultralow-temperature,” Appl. Phys. Express3(10), 102502 (2010). [CrossRef]
  13. B. Baek, A. E. Lita, V. Verma, and S. W. Nam, “Superconducting a-WxSi1-x nanowire single photon detector with saturated internal quantum efficiency from visible to 1850 nm,” Appl. Phys. Lett.98(25), 251105 (2011). [CrossRef]
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