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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 15 — Jul. 16, 2012
  • pp: 16358–16365
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Simultaneous transmission of 20x2 WDM/SCM-QKD and 4 bidirectional classical channels over a PON

J. Mora, W. Amaya, A. Ruiz-Alba, A. Martinez, D. Calvo, V. García Muñoz, and J. Capmany  »View Author Affiliations


Optics Express, Vol. 20, Issue 15, pp. 16358-16365 (2012)
http://dx.doi.org/10.1364/OE.20.016358


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Abstract

We report the transmission of 40 quantum-key channels using WDM/SCM-QKD technology and 4 bidirectional classical channels over a PON. To our knowledge the highest number of quantum key channels simultaneously transmitted that has ever been reported. The quantum signal coexists with classical reference channel which is employed to process the qbits, but it has enough low power to avoid Raman crosstalk and achieving a high number of WDM-QKD channels. The experimental results allow us to determine the minimum rejection ratio required by the filtering devices employed to select each quantum channel and maximize the quantum key rate. These results open the path towards high-count QKD channel transmission over optical fiber infrastructures.

© 2012 OSA

1. Introduction

Quantum Key Distribution (QKD) techniques, which rely on exploiting the laws of quantum mechanics, can offer unconditional security without imposing any computational assumptions [1

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

, 2

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

] and at the same time are prone for practical implementation using standard photonic components. The functionality of QKD systems has been widely demonstrated and the last researches have been oriented to increase the secret key rate using well known multiplexing techniques by means of microwave photonics technologies [3

3. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]

].

Multiplexed QKD systems have two important goals: increasing the effective key rate and providing simultaneously secret keys to different users. The achievement of these goals is critical in order to spread the use of QKD to current deployed networks, especially in the access and metro areas. Among the reported contributions in the area of multiplexed QKD systems one can find Time Division Multiplexing (TDM)-QKD [4

4. J. Chen, G. Wu, L. Xu, X. Gu, E. Wu, and H. Zeng, “Stable quantum key distribution with active polarization control based on time-division multiplexing,” New J. Phys. 11(6), 065004 (2009). [CrossRef]

], Wavelength Division Multiplexing (WDM)-QKD [5

5. K. Yoshino, M. Fujiwara, A. Tanaka, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “A high-speed wavelength-division multiplexing quantum key distribution system,” Opt. Lett. 37(2), 223–225 (2012). [CrossRef]

] and Subcarrier Multiplexing (SCM)-QKD [6

6. J. Mora, A. Ruiz-Alba, W. Amaya, A. Martínez, V. García-Muñoz, D. Calvo, and J. Capmany, “Experimental demonstration of subcarrier multiplexed quantum key distribution system,” Opt. Lett. 37(11), 2031–2033 (2012). [CrossRef] [PubMed]

]. In principle the WDM-QKD approach is very attractive since it is compatible with the dominant techniques employed in the transmission of classical information channels along current optical networking infrastructures. However, the coexistence of classical and quantum signals, is not straightforward as the Raman effect that classical channels exert over the quantum channels pose a severe limitation. To overcome this limitation initially proposed WDM-QKD systems were configured by placing a single quantum channel in 1310 nm band while allocating the classical channels in 1550 nm band [7

7. P. Townsend, “Simultaneous quantum cryptographic key distribution and conventional data transmission over installed fibre using wavelength-division multiplexing,” Electron. Lett. 33(3), 188–190 (1997). [CrossRef]

]. However, in order to profit from the benefits of Dense WDM technology, alternative approaches have been proposed in which both classical and quantum channels are located in the 1550 band. For instance, the system proposed in [8

8. 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,” in Proc. 37th European Conference on Optical Communication, paper Mo.1.B.3, 1–3 (2011).

] reports up to 3 quantum channels with bit rate higher than 200 kb/s while in [9

9. 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]

] 4 bidirectional classical-quantum channels coexist in the C band, the classical channels transmitting at 10 Gb/s and quantum channels delivering keys at a rate of ~700 kb/s. In order to achieve these values it is proposed that the classical information and quantum channels travel separately from central node to a distribution point where they are combined and from which they travel together along the last mile to the end user. In this way, the deleterious contribution of the Raman effect is avoided. While this solution is technically feasible there are other QKD multiplexing schemes which require at least one classical reference traveling together with the quantum signal through the same fiber for management and control tasks such as clock recovery, polarization, delay and phase tracking etc. This is the case for instance of Time-bin QKD [5

5. K. Yoshino, M. Fujiwara, A. Tanaka, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “A high-speed wavelength-division multiplexing quantum key distribution system,” Opt. Lett. 37(2), 223–225 (2012). [CrossRef]

] or SCM-QKD [6

6. J. Mora, A. Ruiz-Alba, W. Amaya, A. Martínez, V. García-Muñoz, D. Calvo, and J. Capmany, “Experimental demonstration of subcarrier multiplexed quantum key distribution system,” Opt. Lett. 37(11), 2031–2033 (2012). [CrossRef] [PubMed]

] systems.

In this paper we report the design of a hybrid network which can transport classical channels and a high number of quantum channels coexisting with a classical channel which is employed as reference. We experimentally demonstrate the successful transmission of 4 bidirectional classical channels and 40 quantum channels based on SCM-QKD employing standard WDM 100 GHz ITU-Grid. In addition, a separate classical channel is also sent with the quantum channels for monitoring purposes. The transmission rate for the classical channels is 10 Gb/s while the secret key rate for each single quantum channels is 5 kb/s which is limited exclusively by the pulse source rate and the photon counter bandwidth. The quantum channels share fiber with the reference signal, which is launched with enough low power to avoid the Raman effect over the quantum channels.

2. Description of the hybrid DWDM quantum and classical transmission

The frequency plan shown in the lower part of the figure illustrates the spectral location of the classical, quantum and reference channels. The system incorporates coarse wavelength multiplexing (CWDM) to combine/separate the different optical bands. The CWDM multiplexer defines three optical bands with a 20 nm separation. The classical reference channel is centered at 1531 nm inside the erbium doped amplifier (EDFA) band as it requires further amplification. Indeed, we perform a double stage of amplification: a first optical stage before photodetection and other electrical stage after photodetection. The optical band centered at 1551 nm is used to transmit the quantum channels and the 1571 nm band is used to provide the US and DS channels. Initially, it could be better to place the classical channels in the 1550 nm band since the losses are lower than 1570 band. However, the FBG filtering stages, which were needed for each quantum channels were fabricated in this band, therefore, the classical channels had to be located in 1570 nm band.

The remote node is composed by two Coarse WDM (CWDM) devices one to split the incoming quantum signal from the reference channel and the other to combine the three bands again. Note that the remote node is a last mile solution, featuring a typical length below 2 km. Finally, in the distribution point (DP) there are a set of demultiplexers to split the incoming signals, one DWDM for the US/DS classical channels and another DWDM unit to demultiplex the quantum channels.

3. Experimental setup

As a consequence of the modulator concatenation, an interference single-photon signal is generated at each one of the sidebands of each subcarrier with a certain probability as shown in Fig. 2(b). For a given subcarrier, if Alice and Bob’s bases match, then the photon will be detected with probability 1 by either the detector placed after the filter centered at USB, or by the detector placed after the filter centered at LSB depending on whether a “0” or a “1” was encoded, respectively. If, on the contrary Bob and Alice’s bases do not match there will be an equal probability of ½ of detecting the single photon at any of the two detectors and this detection will be discarded in a subsequent procedure of public discussion. Note that Fig. 2(b) plots the measured bits at Bob’s side when the base choices match in all channels corresponding the bits “0” and “1” for the electrical subcarriers f1 and f2, respectively.

After 1 km fiber transmission, a demultiplexer separated the three bands providing for each user the US/DS channels (not shown in the Fig. 2 for more clarity), the quantum channels and the reference signal.

To demonstrate the successful performance of our system, the signal degradation for the classical and quantum channels was measured by employing the bit error rate (BER) as a performance parameter. For the experimental evaluation, DS and US channels were measured in presence of the quantum channels as shown in Fig. 3(a)
Fig. 3 BER measurement for DS and US channels with quantum channels (a) enabled and (b) disabled. DS channels are located at 1567.95 (●), 1568.11 (●), 1569.59 (●) and 1570.42 nm (●) and US channels are located at 1571.24 (■), 1572.06 (■), 1572.89 (■) and 1573.71 nm (■). Also, B2B is plotted for a DS (●) and US (■) channels.
. For comparison, optical back-to-back (B2B) measurements have been included in Fig. 3 which is similar to all US and DS channels. In Fig. 3(b), we show the corresponding BER measurement when the quantum channels are disabled. As we expected the quantum channels do not affect the classical channels. For both cases, the penalty is around 1.5 dB for the DS due to optical losses and double (3.5 dB) for US due to round trip propagation.

As shown in Fig. 2, we employed a DWDM to deliver the quantum channels to the final users and, in addition, each user has a filtering stage to select and detect each one of the optical sidebands sent from Alice [11

11. J. Mora, A. Ruiz-Alba, W. Amaya, V. Garcia-Muñoz, A. Martínez, and J. Capmany, “Microwave photonic filtering scheme for BB84 subcarrier multiplexed quantum key distribution,” in IEEE Topical Meeting on Microwave Photonics, pp. 286–289 (2007).

]. This filtering stage based on the cascade of Fiber Bragg Gratings (FBGs) has an extinction ratio (ER) around 20 dB. The extinction ratio composed of the DWDM and the FBG filtering stage determines the total ER and consequently, the quantum BER (QBER) of the system.

In order to evaluate the minimum ER at Bob’s side to obtain a suitable QBER, we used a programmable optical processor to provide a DWDM system simulation where the effective ER can be controlled. The procedure consists of the measurement of the QBER for the quantum channel corresponding to the worst case, i.e., the central channel, while adjacent quantum channels are added in function of the ER value imposed by the DWDM setup. In particular, we considered one single channel corresponding to the B2B case and the scenario when 3, 11 and 20 WDM quantum channels are simultaneously transmitted. The ER value of the DWDM channels is modified from 20 to 40 dB being the total ER from 40 to 60 dB.

In Fig. 4(a)
Fig. 4 QBER vs. the extinction ratio for the electrical subcarrier (a) 10 GHz and (b) 15 GHz and the (c) total secret key rate when a single quantum channel (■) and 3 (●), 11(▲) and 20 (▼) WDM quantum channels are enabled.
and 4(b), we show the QBER as a function of the DWDM ER for each electrical subcarrier corresponding to 10 and 15 GHz, respectively. For high ER values, the contribution of the adjacent channels is small and slight differences are found when 3, 11 or 20 WDM quantum channels are considered. In contrast, low ER values imply that a high number of quantum channels introduce a significant crosstalk increasing the QBER. However, a moderate change in the QBER is found between 3 and 20 quantum channels, which indicates that the first channels are more restrictive in the QBER measurement. Therefore, we can conclude from Fig. 4 that minimum ER to guarantee a QBER close to 2% is around 40 dB for each channel. Note that 40-45 dB can be high for commercial DWDM devices. Consequently, it can be interesting to consider the incorporation of FBGs filters as it occurs in our SCM-QKD systems by combining different FBGs filtering in cascade.

Note that we experimentally demonstrated moderate single (~5 kbps) and multiplexed (~200 kbps) key rates although the number of DWDM channels is very relevant. The capacity of the systems depends on the QKD terminal equipment used in the optical network which is limited to 1 MHz in our case due to the trigger and deadtime of the SPADs. Using 1 GHz-SPADs [14

14. 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]

], the individual and multiplexed secret key rates could be readily increased by 3 orders of magnitude, i.e., 5 Mbps and 200 Mbps, respectively. Obviously, each DWDM quantum channels has available a 50 GHz bandwidth and alternative QKD systems could be used.

4. Conclusion

We have proposed and implemented, as far as know for first time, an optical network to transport 4x10 Gb/s bidirectional classical channels and 40 quantum channels based on DWDM-SCM-QKD two-tier multiplexing. The transmission key rate was increased in a factor 20 from a SCM quantum channel with a secret key rate close to 5 kb/s. The quantum channels share fiber with the reference signal which travels with enough low power to avoid the Raman effect and they were transmitted with a QBER lower than 2% by controlling the ER of the demultiplexer which suitable value is close to 40 dB for each quantum channel.

Acknowledgments

The authors wish to acknowledge the financial support of the Spanish Ministry of Science & Innovation and the Generalitat Valenciana through projects CONSOLIDER INGENIO 2010 Quantum Information Technologies and PROMETEO GVA 2008-092 MICROWAVE PHOTONICS.

References and links

1.

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

2.

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

3.

J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]

4.

J. Chen, G. Wu, L. Xu, X. Gu, E. Wu, and H. Zeng, “Stable quantum key distribution with active polarization control based on time-division multiplexing,” New J. Phys. 11(6), 065004 (2009). [CrossRef]

5.

K. Yoshino, M. Fujiwara, A. Tanaka, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “A high-speed wavelength-division multiplexing quantum key distribution system,” Opt. Lett. 37(2), 223–225 (2012). [CrossRef]

6.

J. Mora, A. Ruiz-Alba, W. Amaya, A. Martínez, V. García-Muñoz, D. Calvo, and J. Capmany, “Experimental demonstration of subcarrier multiplexed quantum key distribution system,” Opt. Lett. 37(11), 2031–2033 (2012). [CrossRef] [PubMed]

7.

P. Townsend, “Simultaneous quantum cryptographic key distribution and conventional data transmission over installed fibre using wavelength-division multiplexing,” Electron. Lett. 33(3), 188–190 (1997). [CrossRef]

8.

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,” in Proc. 37th European Conference on Optical Communication, paper Mo.1.B.3, 1–3 (2011).

9.

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]

10.

B. Ortega, J. Mora, G. Puerto, and J. Capmany, “Symmetric reconfigurable capacity assignment in a bidirectional DWDM access network,” Opt. Express 15(25), 16781–16786 (2007). [CrossRef] [PubMed]

11.

J. Mora, A. Ruiz-Alba, W. Amaya, V. Garcia-Muñoz, A. Martínez, and J. Capmany, “Microwave photonic filtering scheme for BB84 subcarrier multiplexed quantum key distribution,” in IEEE Topical Meeting on Microwave Photonics, pp. 286–289 (2007).

12.

O. Guerreau, F. J. Malassenet, S. W. McLaughlin, and J. M. Merolla, “Quantum key distribution without a single-photon source using a strong reference,” IEEE Photon. Technol. Lett. 17(8), 1755–1757 (2005). [CrossRef]

13.

J. Capmany and C. R. Fernandez-Pousa, “Impact of third-order intermodulation on the performance of subcarrier multiplexed quantum key distribution,” J. Lightwave Technol. 29(20), 3061–3069 (2011). [CrossRef]

14.

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]

OCIS Codes
(060.4230) Fiber optics and optical communications : Multiplexing
(270.5568) Quantum optics : Quantum cryptography

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: May 1, 2012
Revised Manuscript: June 13, 2012
Manuscript Accepted: June 25, 2012
Published: July 3, 2012

Citation
J. Mora, W. Amaya, A. Ruiz-Alba, A. Martinez, D. Calvo, V. García Muñoz, and J. Capmany, "Simultaneous transmission of 20x2 WDM/SCM-QKD and 4 bidirectional classical channels over a PON," Opt. Express 20, 16358-16365 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-15-16358


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References

  1. N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, “Quantum criptography,” Rev. Mod. Phys. 74(1), 145–195 (2002). [CrossRef]
  2. V. Scarani, H. Bechmann-Pasquinucci, N. J. Cerf, M. Dušek, N. Lütkenhaus, and M. Peev, “The security of practical quantum key distribution,” Rev. Mod. Phys. 81(3), 1301–1350 (2009). [CrossRef]
  3. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]
  4. J. Chen, G. Wu, L. Xu, X. Gu, E. Wu, and H. Zeng, “Stable quantum key distribution with active polarization control based on time-division multiplexing,” New J. Phys. 11(6), 065004 (2009). [CrossRef]
  5. K. Yoshino, M. Fujiwara, A. Tanaka, S. Takahashi, Y. Nambu, A. Tomita, S. Miki, T. Yamashita, Z. Wang, M. Sasaki, and A. Tajima, “A high-speed wavelength-division multiplexing quantum key distribution system,” Opt. Lett. 37(2), 223–225 (2012). [CrossRef]
  6. J. Mora, A. Ruiz-Alba, W. Amaya, A. Martínez, V. García-Muñoz, D. Calvo, and J. Capmany, “Experimental demonstration of subcarrier multiplexed quantum key distribution system,” Opt. Lett. 37(11), 2031–2033 (2012). [CrossRef] [PubMed]
  7. P. Townsend, “Simultaneous quantum cryptographic key distribution and conventional data transmission over installed fibre using wavelength-division multiplexing,” Electron. Lett. 33(3), 188–190 (1997). [CrossRef]
  8. 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,” in Proc. 37th European Conference on Optical Communication, paper Mo.1.B.3, 1–3 (2011).
  9. 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]
  10. B. Ortega, J. Mora, G. Puerto, and J. Capmany, “Symmetric reconfigurable capacity assignment in a bidirectional DWDM access network,” Opt. Express 15(25), 16781–16786 (2007). [CrossRef] [PubMed]
  11. J. Mora, A. Ruiz-Alba, W. Amaya, V. Garcia-Muñoz, A. Martínez, and J. Capmany, “Microwave photonic filtering scheme for BB84 subcarrier multiplexed quantum key distribution,” in IEEE Topical Meeting on Microwave Photonics, pp. 286–289 (2007).
  12. O. Guerreau, F. J. Malassenet, S. W. McLaughlin, and J. M. Merolla, “Quantum key distribution without a single-photon source using a strong reference,” IEEE Photon. Technol. Lett. 17(8), 1755–1757 (2005). [CrossRef]
  13. J. Capmany and C. R. Fernandez-Pousa, “Impact of third-order intermodulation on the performance of subcarrier multiplexed quantum key distribution,” J. Lightwave Technol. 29(20), 3061–3069 (2011). [CrossRef]
  14. 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]

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