## Quantum key distribution on a 10Gb/s WDM-PON

Optics Express, Vol. 18, Issue 9, pp. 9600-9612 (2010)

http://dx.doi.org/10.1364/OE.18.009600

Acrobat PDF (1171 KB)

### Abstract

We present the first demonstration of quantum key distribution (QKD) on a multi-user wavelength division multiplexed passive optical network (WDM-PON) with simultaneous, bidirectional 10Gb/s classical channel transmission. The C-Band QKD system operates at a clock rate of 10GHz and employs differential phase shift keying (DPSK). A dual feeder fiber and band filtering scheme is used to suppress classical to quantum channel cross-talk generated by spontaneous Raman scattering, which would otherwise prevent secure key distribution. Quantum keys were distributed to 4 users with negligible Raman cross-talk penalties. The mean QBER value for 4 users was 3.5% with a mean raw key distribution rate of 1.3Mb/s, which decreased to 696kb/s after temporal windowing to reduce inter-symbol interference due to single photon detector timing jitter.

© 2010 OSA

## 1. Introduction

2. C. Marand and P. D. Townsend, “Quantum key distribution over distances as long as 30km,” Opt. Lett. **20**(16), 1695–1697 (1995). [CrossRef] [PubMed]

10. 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,” N. J. Phys. **11**(7), 075001 (2009). [CrossRef]

11. P. D. Townsend, S. J. D. Phoenix, K. J. Blow, and S. M. Barnett, “Design of quantum cryptography systems for passive optical networks,” Electron. Lett. **30**(22), 1875–1876 (1994). [CrossRef]

14. P. D. Kumavor, A. C. Beal, S. Yelin, E. Donkor, and B. C. Wang, “Comparison of Four Multi-User Quantum Key Distribution Schemes Over Passive Optical Networks,” J. Lightwave Technol. **23**(1), 268–276 (2005). [CrossRef]

## 2. Spontaneous Raman scattering theory

15. D. Subacius, A. Zavriyev, and A. Trifonov, “Backscattering limitation for fiber-optic quantum key distribution systems,” Appl. Phys. Lett. **86**(1), 011103 (2005). [CrossRef]

15. D. Subacius, A. Zavriyev, and A. Trifonov, “Backscattering limitation for fiber-optic quantum key distribution systems,” Appl. Phys. Lett. **86**(1), 011103 (2005). [CrossRef]

_{C}is the classical channel launch power, α is the fiber attenuation coefficient, β(λ,Δλ) is the linear spontaneous Raman scattering coefficient, which is dependent on wavelength (λ) and the measurement bandwidth (Δλ). Similarly, the forward-scattered Raman power can be described by Eq. (2) [19

19. N. A. Peters, P. Toliver, T. E. Chapuran, R. J. Runser, S. R. McNown, C. G. Peterson, D. Rosenberg, N. Dallmann, R. J. Hughes, K. P. McCabe, J. E. Nordholt, and K. T. Tyagi, “Dense Wavelength multiplexing of 1550nm QKD with strong classical channels in reconfigurable networking environments,” N. J. Phys. **11**(4), 045012 (2009). [CrossRef]

^{−1}(0.2dB km

^{−1}) and β=2.1×10

^{−9}km

^{−1}(see below) and assuming a value of P

_{C}=1mW (a typical practical value for WDM-PON applications) we calculate back- and forward-scattered powers as a function of fiber length. As we shall be concerned with the level of cross-talk noise that the Raman scattering generates in a given quantum channel, the results shown in Fig. 3 are represented in the form of a count rate, modelled by assuming an AWG measurement filter and a single photon detector with the same characteristics as those used in the experiments detailed later in this paper. For distances up to approximately 10km, which are relevant for WDM-PON applications, the backward and forward Raman levels are essentially identical. However for longer distances the backward Raman shows saturation behaviour, whilst the forward reaches a maximum value at 1/α (~21.7km) then decreases exponentially as the fiber attenuation reduces the scattered signal more quickly than it can be replenished by the classical channel [19

19. N. A. Peters, P. Toliver, T. E. Chapuran, R. J. Runser, S. R. McNown, C. G. Peterson, D. Rosenberg, N. Dallmann, R. J. Hughes, K. P. McCabe, J. E. Nordholt, and K. T. Tyagi, “Dense Wavelength multiplexing of 1550nm QKD with strong classical channels in reconfigurable networking environments,” N. J. Phys. **11**(4), 045012 (2009). [CrossRef]

^{th}quantum channel in the central node AWG due to back-scattering from the DS channels is given by Eq. (3):where N is the total number of DS channels (one per user), P

_{DS}is the DS channel launch power into the feeder fiber after the AWG in central node (assumed to be equal for all channels), L

_{F}and L

_{D}are, respectively, the feeder and drop fiber lengths, A is the AWG insertion loss and F is the additional loss arising for broadband light double passing the AWG filter. The latter is assumed to be Gaussian with full width half maximum (FWHM) equal to Δλ. For the 100GHz channel spacing AWGs used in the experiments Δλ = 0.4nm, F = 0.7527 and A = −4dB.

_{ij}(λ,Δλ) can be used to describe the scattering from the j

^{th}classical channel into the i

^{th}quantum channel. This assumption is supported by Fig. 4 where a Raman spectrum from 13 DS lasers in a WDM-PON configuration is shown. Evidently, across the AWG channel passband of 0.4nm, the accumulated Raman is relatively featureless and constant. Secondly, the AWG out-of-band crosstalk is negligible so that only Raman generated within the wavelength pass-band of the i

^{th}quantum channel contributes noise to the i

^{th}quantum channel receiver. These approximations are well-satisfied in practice. Equation (3) consists of two terms. The first term describes Raman generated in the feeder fiber, which is filtered once by the central node AWG. The second term describes the Raman generated in the drop fiber, which is doubly filtered; firstly by the remote node AWG and secondly by the central node AWG. It is evident from Eq. (3) that the dominant contribution to the back-scattered Raman comes from the feeder fiber, since all N DS channels contribute to this term. In contrast, the drop term contains a contribution from only one DS channel, because only Raman from the i

^{th}user’s drop fibre will reach the i

^{th}quantum channel receiver due to the filtering action of the remote node AWG. The drop term is also further attenuated with respect to the feeder term by the loss of the remote node AWG and transmission loss of the feeder fiber.

^{th}quantum channel due to forward-scattering from the US channels is given by Eq. (4):where P

_{US}is the US channel launch power and the other parameters are as defined above, with β

_{ik}(λ,Δλ) describing the scattering from the k

^{th}US classical channel into the i

^{th}quantum channel (note i=j=k=1 for user 1 and so on). The first term describes the Raman contribution from the drop fiber while the second term describes Raman from the feeder fiber. As is the case for Eq. (3), the feeder term is dominant since it contains contributions from all N US channels. This suggests that if the feeder fiber scattering can be suppressed, a significant reduction in Raman cross-talk from both US and DS channels is feasible. In the following sections we quantify the level of Raman cross-talk in a practical WDM-PON, show that such noise suppression is required for QKD and introduce a dual feeder fiber and band filtering scheme that allows it to be achieved.

## 3. Experimental set-up

_{USi}(1559.03nm-1571.17nm), λ

_{DSi}(1546.11nm-1558.16nm), and λ

_{QKDi}(1533.41nm-1545.36nm). As the anti-Stokes lines are weaker than the Stokes lines the quantum channels were placed on the shorter wavelength side of the DS or US channels to minimize Raman crosstalk [16

16. P. Toliver, R. J. Runser, T. E. Chapuran, S. McNown, M. S. Goodman, J. Jackel, R. J. Hughes, C. G. Peterson, K. McCabe, J. E. Nordholt, K. Tyagi, P. Hiskett, and N. Dallman, “Impact of Spontaneous Anti-Stokes Raman Scattering on QKD+DWDM Networking,”, in *Proceedings of Lasers and Electro-Optics Society (LEOS*), vol. **2**, pp. 491–492 (2004)

19. N. A. Peters, P. Toliver, T. E. Chapuran, R. J. Runser, S. R. McNown, C. G. Peterson, D. Rosenberg, N. Dallmann, R. J. Hughes, K. P. McCabe, J. E. Nordholt, and K. T. Tyagi, “Dense Wavelength multiplexing of 1550nm QKD with strong classical channels in reconfigurable networking environments,” N. J. Phys. **11**(4), 045012 (2009). [CrossRef]

_{QKDi}. A further dual stage R/B filter was used at the remote node to direct the QKD channels through the lower feeder fiber shown in Fig. 5, whilst the US and DS channels were carried by the upper feeder fiber. In the conventional single feeder configuration, this R/B filter was moved to the central node. At the central node, standard 40 × 100GHz channel spacing Gaussian pass-band, AWGs were used to multiplex/demultiplex the US, DS and QKD channels. The AWG filters used in experiments had channel isolations of >45dB between the QKD and the DS and US channels and polarization dependent losses <0.5dB. For the dual feeder scheme as shown in Fig. 5, for example, this gives a total isolation figure of >105dB for DS and >115dB for US, which is large enough to ensure that leakage from the DS or US channels into the QKD channels, either directly or via Rayleigh backscattering, is negligible (<0.1% of expected QKD counts) All AWGs were temperature controlled to maintain wavelength channel alignments.

^{7}-1 non-return-to-zero (NRZ) pseudo-random bit sequence (PRBS) generated by a pulse pattern generator (PPG). The US signal was detected by a PIN receiver (Rx) and a 10GHz clock recovery circuit. The DS channels were emulated by a bank of DFBs (13 were available), which were externally modulated using a Lithium Niobate Mach-Zehnder Modulator (MZM) driven by a 10Gb/s NRZ 2

^{31}-1 PRBS. The DS signal was detected at the user via a circulator and PIN Rx. The mean US and DS fiber launch powers were set to typical practical values of 0dBm per channel to ensure realistic levels of Raman cross-talk.

^{7}-1 NRZ PRBS to generate a sequence of 0 and π phase shifts in a differential phase shift keyed (DPSK) QKD scheme [9

9. H. Takesue, S. W. Nam, Q. Zhang, R. H. Hadfield, T. Honjo, K. Tamaki, and Y. Yamamoto, “Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors,” Nat. Photonics **1**(6), 343–348 (2007). [CrossRef]

## 4. Experimental results and discussion

_{ij}(λ,Δλ) in the range 1.8 - 2.2×10

^{−10}km

^{−1}(depending on channel wavelength) were found to give excellent fits to the observed data for the single feeder fiber case. In the dual feeder case, the β

_{ij}(λ,Δλ) values obtained for the single feeder experiment were used with the drop terms in Eqs. (3) and (4) to simulate the curves shown by the lower two solid lines in Fig. 7. Again excellent agreement is obtained with the measured data. Other parameter values were as given above, but with the fiber loss α set to 0.23dB/km to represents the mean measured value including connector and splice losses.

_{opt}, can be represented by the quantity κ=(1/2)(1−V), which shows that, with ideal phase modulation, the error contribution due to finite interference visibility should be less than 1%. However, in practice the optical system is further degraded by imperfect phase modulation, which arises from noise and distortion introduced by the 10Gb/s data generator, electrical drive amplifier and phase modulator. We include this contribution by means of an effective κ value, κ

_{eff}, which represents the performance of the complete interferometric system under dynamic modulation conditions. The quantity κ

_{eff}can be estimated from the DPSK data eye contrast ratio, C=1/κ

_{eff}, measured at the QKD Tx before attenuation (Fig. 8 ). This results in a value of C = 40(16dB) corresponding to a mean κ

_{eff}value and hence QBER

_{opt}= 2.5%. The second error contribution comes from the combined SSPD dark count rate, D. On average only half of the dark counts generate errors. For this experiment, D~200 counts per second, which is approximately 4 orders of magnitude smaller than the quantum channel photocount rate. As a result, QBER

_{Darkl}gives a negligible contribution to the total QBER. The final error contribution comes from Raman counts. On average, only half of the Raman counts contribute to errors. From Fig. 7, QBER

_{Raman}is estimated to be ~1%. Hence, QBER

_{Total}is expected to be ~3.5%. For completeness, we show the detailed contributions to each error term in Eq. (6). The total photocount is given by ημBR, where η is the quantum efficiency of the SSPDs used, μ is the mean photon number, B is the total system loss and R is the QKD system clock rate (10GHz). Raman counts are generated by the combined contribution of back- and forward- scattered Raman. The total received Raman power for the dual feeder case can be calculated by adding P

_{RBi}and P

_{RFi}for all channels from the drop terms from Eqs. (3) and (4). To obtain the Raman count rate, the summed power should be divided by the photon energies for the given quantum channel, E

_{i}, All other parameters are as defined above.Following Raman evaluation, the set-up was configured as shown in Fig. 6 with 8km feeder and 2km drop fibers and QKD experiments were performed. Results were obtained for emulated users 1-4 with QKD wavelengths of 1535.79nm, 1536.58nm, 1537.38nm and 1538.18nm, respectively. Figure 9 shows a typical example of a photocount histogram measured at the one (constructive interference for π phase shift) port of the DPSK demodulator over 78.7x10

^{6}PRBS pattern repetitions. The contrast between the maxima (one) and deepest minima (zero) levels is high indicating the potential for low QBER transmission. However, significant Inter-Symbol Interference (ISI) can be inferred from the relatively shallow minima observed between consecutive ones. This arises from the limited instrumental response time of the single photon detection system. The latter is dominated by ~40ps timing jitter of the SSPDs, which is a significant fraction of the 100ps bit period. To reduce the errors due to ISI, a windowing technique was used that rejected any counts occurring outside of the central τ ps of each bit period [9

9. H. Takesue, S. W. Nam, Q. Zhang, R. H. Hadfield, T. Honjo, K. Tamaki, and Y. Yamamoto, “Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors,” Nat. Photonics **1**(6), 343–348 (2007). [CrossRef]

9. H. Takesue, S. W. Nam, Q. Zhang, R. H. Hadfield, T. Honjo, K. Tamaki, and Y. Yamamoto, “Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors,” Nat. Photonics **1**(6), 343–348 (2007). [CrossRef]

^{−10}) was used to show the potential for low cost operation of the system. Relatively low power penalties of ≤ 1dB were observed and all channels were capable of error-free operation (defined as BER<10

^{−10}). The minimum received power was −9.9dBm for both US and DS and the measured system margins were >4dB and >5dB for US and DS channels respectively. This indicates that the US and DS channel launch powers are sufficient for error-free operation of the classical part of the system with adequate margin for aging effects, and hence, that the Raman levels were not underestimated in the experiments.

## 5. Summary and conclusion

## Acknowledgements

## References and links

1. | C. H. Bennett, and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing,” in |

2. | C. Marand and P. D. Townsend, “Quantum key distribution over distances as long as 30km,” Opt. Lett. |

3. | P. D. Townsend, “Simultaneous quantum cryptographic key distribution and conventional data transmission over installed fibre using wavelength-division multiplexing,” Electron. Lett. |

4. | C. Gobby, Z. L. Yuan, and A. J. Shields, “Quantum key distribution over 122 km of standard telecom fiber,” Appl. Phys. Lett. |

5. | R. J. Hughes, J. E. Nordholt, D. Derkacs, and C. G. Peterson, “Practical free-space quantum key distribution over 10 km in daylight and at night,” N. J. Phys. |

6. | D. Stucki, N. Gisin, O. Guinnard, G. Ribordy, and H. Zbinden, “Quantum key distribution over 67 km with a plug&play system,” N. J. Phys. |

7. | T. Schmitt-Manderbach, H. Weier, M. Fürst, R. Ursin, F. Tiefenbacher, T. Scheidl, J. Perdigues, Z. Sodnik, C. Kurtsiefer, J. G. Rarity, A. Zeilinger, and H. Weinfurter, “Experimental Demonstration of Free-Space Decoy-State Quantum Key Distribution over 144 km,” Phys. Rev. Lett. |

8. | R. Ursin, F. Tiefenbacher, T. Schmitt-Manderbach, H. Weier, T. Scheidl, M. Lindenthal, B. Blauensteiner, T. Jennewein, J. Perdigues, P. Trojek, B. Ömer, M. Fürst, M. Meyenburg, J. Rarity, Z. Sodnik, C. Barbieri, H. Weinfurter, and A. Zeilinger, “Entanglement-based quantum communication over 144 km,” Nat. Phys. |

9. | H. Takesue, S. W. Nam, Q. Zhang, R. H. Hadfield, T. Honjo, K. Tamaki, and Y. Yamamoto, “Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors,” Nat. Photonics |

10. | 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,” N. J. Phys. |

11. | P. D. Townsend, S. J. D. Phoenix, K. J. Blow, and S. M. Barnett, “Design of quantum cryptography systems for passive optical networks,” Electron. Lett. |

12. | P. D. Townsend, “Quantum cryptography on multi-user optical fibre networks,” Nature |

13. | V. Fernandez, R. J. Collins, K. J. Gordon, P. D. Townsend, and G. S. Buller, “Passive Optical Network Approach to Gigahertz-Clocked Multiuser Quantum Key Distribution,” IEEE J. Quantum Electron. |

14. | P. D. Kumavor, A. C. Beal, S. Yelin, E. Donkor, and B. C. Wang, “Comparison of Four Multi-User Quantum Key Distribution Schemes Over Passive Optical Networks,” J. Lightwave Technol. |

15. | D. Subacius, A. Zavriyev, and A. Trifonov, “Backscattering limitation for fiber-optic quantum key distribution systems,” Appl. Phys. Lett. |

16. | P. Toliver, R. J. Runser, T. E. Chapuran, S. McNown, M. S. Goodman, J. Jackel, R. J. Hughes, C. G. Peterson, K. McCabe, J. E. Nordholt, K. Tyagi, P. Hiskett, and N. Dallman, “Impact of Spontaneous Anti-Stokes Raman Scattering on QKD+DWDM Networking,”, in |

17. | N. I. Nweke, P. Toliver, R. J. Runser, S. R. McNown, T. E. Chapuran, M. S. Goodman, R. J. Hughes, C. G. Peterson, K. McCabe, J. E. Nordholt, K. Tyagi, P. Hiskett, and N. Dallmann, “Experimental Characterisation of Wavelength Separation for ‘QKD+WDM’ Co-existence,” in |

18. | T. J. Xia, D. Z. Chen, G. Wellbrock, and A. Zavriyev, A. Beal and K. M. Lee, “In-Band Quantum Key Distribution (QKD) on Fiber Populated by High-Speed Classical Data Channels,” in |

19. | N. A. Peters, P. Toliver, T. E. Chapuran, R. J. Runser, S. R. McNown, C. G. Peterson, D. Rosenberg, N. Dallmann, R. J. Hughes, K. P. McCabe, J. E. Nordholt, and K. T. Tyagi, “Dense Wavelength multiplexing of 1550nm QKD with strong classical channels in reconfigurable networking environments,” N. J. Phys. |

20. | ITU Standard G.694.1 (06/02), “Spectral grids for WDM applications: DWDM frequency grid,” International Telecommunication Union (2002). |

**OCIS Codes**

(060.0060) Fiber optics and optical communications : Fiber optics and optical communications

(270.5568) Quantum optics : Quantum cryptography

**ToC Category:**

Quantum Optics

**History**

Original Manuscript: January 15, 2010

Revised Manuscript: March 12, 2010

Manuscript Accepted: March 29, 2010

Published: April 23, 2010

**Citation**

Iris Choi, Robert J. Young, and Paul D. Townsend, "Quantum key distribution on a 10Gb/s WDM-PON," Opt. Express **18**, 9600-9612 (2010)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-9-9600

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### References

- C. H. Bennett, and G. Brassard, “Quantum cryptography: Public key distribution and coin tossing,” in Proceedings of the IEEE International Conference on Computers, Systems and Signal Processing, Bangalore, India, (IEEE New York), pp. 175–179 (1984)
- C. Marand and P. D. Townsend, “Quantum key distribution over distances as long as 30km,” Opt. Lett. 20(16), 1695–1697 (1995). [CrossRef] [PubMed]
- P. D. 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]
- C. Gobby, Z. L. Yuan, and A. J. Shields, “Quantum key distribution over 122 km of standard telecom fiber,” Appl. Phys. Lett. 84(19), 3762–3764 (2004). [CrossRef]
- R. J. Hughes, J. E. Nordholt, D. Derkacs, and C. G. Peterson, “Practical free-space quantum key distribution over 10 km in daylight and at night,” N. J. Phys. 4(43), 1–14 (2002). [CrossRef]
- D. Stucki, N. Gisin, O. Guinnard, G. Ribordy, and H. Zbinden, “Quantum key distribution over 67 km with a plug&play system,” N. J. Phys. 4(41), 1–8 (2002). [CrossRef]
- T. Schmitt-Manderbach, H. Weier, M. Fürst, R. Ursin, F. Tiefenbacher, T. Scheidl, J. Perdigues, Z. Sodnik, C. Kurtsiefer, J. G. Rarity, A. Zeilinger, and H. Weinfurter, “Experimental Demonstration of Free-Space Decoy-State Quantum Key Distribution over 144 km,” Phys. Rev. Lett. 98(1), 010504 (2007). [CrossRef] [PubMed]
- R. Ursin, F. Tiefenbacher, T. Schmitt-Manderbach, H. Weier, T. Scheidl, M. Lindenthal, B. Blauensteiner, T. Jennewein, J. Perdigues, P. Trojek, B. Ömer, M. Fürst, M. Meyenburg, J. Rarity, Z. Sodnik, C. Barbieri, H. Weinfurter, and A. Zeilinger, “Entanglement-based quantum communication over 144 km,” Nat. Phys. 3(7), 481–486 (2007). [CrossRef]
- H. Takesue, S. W. Nam, Q. Zhang, R. H. Hadfield, T. Honjo, K. Tamaki, and Y. Yamamoto, “Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors,” Nat. Photonics 1(6), 343–348 (2007). [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,” N. J. Phys. 11(7), 075001 (2009). [CrossRef]
- P. D. Townsend, S. J. D. Phoenix, K. J. Blow, and S. M. Barnett, “Design of quantum cryptography systems for passive optical networks,” Electron. Lett. 30(22), 1875–1876 (1994). [CrossRef]
- P. D. Townsend, “Quantum cryptography on multi-user optical fibre networks,” Nature 385(6611), 47–49 (1997). [CrossRef]
- V. Fernandez, R. J. Collins, K. J. Gordon, P. D. Townsend, and G. S. Buller, “Passive Optical Network Approach to Gigahertz-Clocked Multiuser Quantum Key Distribution,” IEEE J. Quantum Electron. 43(2), 130–138 (2007). [CrossRef]
- P. D. Kumavor, A. C. Beal, S. Yelin, E. Donkor, and B. C. Wang, “Comparison of Four Multi-User Quantum Key Distribution Schemes Over Passive Optical Networks,” J. Lightwave Technol. 23(1), 268–276 (2005). [CrossRef]
- D. Subacius, A. Zavriyev, and A. Trifonov, “Backscattering limitation for fiber-optic quantum key distribution systems,” Appl. Phys. Lett. 86(1), 011103 (2005). [CrossRef]
- P. Toliver, R. J. Runser, T. E. Chapuran, S. McNown, M. S. Goodman, J. Jackel, R. J. Hughes, C. G. Peterson, K. McCabe, J. E. Nordholt, K. Tyagi, P. Hiskett, and N. Dallman, “Impact of Spontaneous Anti-Stokes Raman Scattering on QKD+DWDM Networking,”, in Proceedings of Lasers and Electro-Optics Society (LEOS), vol. 2, pp. 491–492 (2004)
- N. I. Nweke, P. Toliver, R. J. Runser, S. R. McNown, T. E. Chapuran, M. S. Goodman, R. J. Hughes, C. G. Peterson, K. McCabe, J. E. Nordholt, K. Tyagi, P. Hiskett, and N. Dallmann, “Experimental Characterisation of Wavelength Separation for ‘QKD+WDM’ Co-existence,” in Proceedings of Conference on Lasers and Electro-Optics (CLEO), vol 2, pp.1503–1505 (2005), paper CW06.
- T. J. Xia, D. Z. Chen, G. Wellbrock, and A. Zavriyev, A. Beal and K. M. Lee, “In-Band Quantum Key Distribution (QKD) on Fiber Populated by High-Speed Classical Data Channels,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest (Optical Society of America, 2006), paper OTuJ7.
- N. A. Peters, P. Toliver, T. E. Chapuran, R. J. Runser, S. R. McNown, C. G. Peterson, D. Rosenberg, N. Dallmann, R. J. Hughes, K. P. McCabe, J. E. Nordholt, and K. T. Tyagi, “Dense Wavelength multiplexing of 1550nm QKD with strong classical channels in reconfigurable networking environments,” N. J. Phys. 11(4), 045012 (2009). [CrossRef]
- ITU Standard G.694.1 (06/02), “Spectral grids for WDM applications: DWDM frequency grid,” International Telecommunication Union (2002).

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