Source: https://docksci.com/quantum-metropolitan-optical-network-based-on-wavelength-division-multiplexing_5b01fd4ad64ab25e97047e35.html
Timestamp: 2019-04-19 16:23:38+00:00

Document:
Abstract: Quantum Key Distribution (QKD) is maturing quickly. However, the current approaches to its application in optical networks make it an expensive technology. QKD networks deployed to date are designed as a collection of point-to-point, dedicated QKD links where non-neighboring nodes communicate using the trusted repeater paradigm. We propose a novel optical network model in which QKD systems share the communication infrastructure by wavelength multiplexing their quantum and classical signals. The routing is done using optical components within a metropolitan area which allows for a dynamically any-to-any communication scheme. Moreover, it resembles a commercial telecom network, takes advantage of existing infrastructure and utilizes commercial components, allowing for an easy, cost-effective and reliable deployment. © 2014 Optical Society of America OCIS codes: (060.5565) Quantum communications; (270.5568) Quantum cryptography; (060.4265) Networks, wavelength routing.
References and links N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, “Quantum cryptography,” Rev. Mod. Phys 74, 145–195 (2002). ID Quantique SA, http://www.idquantique.com. Toshiba Research Europe Ltd., http://www.toshiba-europe.com/research/. MagiQ Technologies Inc., http://www.magiqtech.com. SeQureNet, http://www.sequrenet.com. AIT, http://www.ait.ac.at/epr. Swiss Quantum, http://swissquantum.idquantique.com. A. Mirza and P. Petruccione, “Realizing long-term quantum cryptography,” J. Opt. Soc. Am. B 27, A185–A188 (2010). 9. P. Jouguet, S. Kunz-Jacques, T. Debuisschert, S. Fossier, E. Diamanti, R. All´eaume, R. Tualle-Brouri, P. Grangier, A. Leverrier, P. Pache, and P. Painchault, “Field test of classical symmetric encryption with continuous variables quantum key distribution,” Opt. Express 20, 14030–14041 (2012). 10. C. Elliot, “Building the quantum network,” New J. Phys. 4, 46 (2002). 11. M. Peev, C. Pacher, R. All´eaume, C. Barreiro, J. Bouda, W. Boxleitner, T. Debuisschert, E. Diamanti, M. Dianati, J. F. Dynes, S. Fasel, S. Fossier, M. F¨urst, J.-D. Gautier, O. Gay, N. Gisin, P. Grangier, A. Happe, Y. Hasani, M. Hentschel, H. H¨ubel, G. Humer, T. L¨anger, M. Legr´e, R. Lieger, J. Lodewyck, T. Lor¨unser, N. L¨utkenhaus, 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, 1. 2. 3. 4. 5. 6. 7. 8.
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, 075001 (2009). D. Stucki, M. Legr´e, 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, 123001 (2011). 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. Legr´e, S. Robyr, P. Trinkler, L. Monat, J.-B. Page, G. Ribordy, A. Poppe, A. Allacher, O. Maurhart, T. L¨anger, M. Peev, and A. Zeilinger, “Field test of quantum key distribution in the Tokyo QKD Network,” Opt. Express 19, 10387– 10409 (2011). K. Kitayama, M. Sasaki, S. Araki, M. Tsubokawa, A. Tomita, K. Inoue, K. Harasawa, Y. Nagasako, and A. Takada, “Security in photonic networks: threats and security enhancement,” J. Lightwave Technol. 29, 3210– 3222 (2011). Y. Chen, M. T. Fatehi, H. J. La Roche, J. Z. Larsen, and B. L. Nelson, “Metro optical networking,” Bell Labs Tech. J. 4, 163–186 (1999). C.-H. Lee, W. V. Sorin, and B. Y. Kim, “Fiber to the home using a PON infrastructure,” J. Lightwave Technol. 24, 4568–4583 (2006). P. Townsend, S. Phoenix, K. Blow, and S. Barnett, “Design of quantum cryptography systems for passive optical networks,” Electron. Lett. 30, 1875–1877 (1994). P. Townsend, “Quantum cryptography on multiuser optical fibre networks,” Nature 385, 47–49 (1997). P. Townsend, “Experimental investigation of the performance limits for first telecommunications-window quantum cryptography systems,” IEEE Photonics Technol. Lett. 10, 1048–1050 (1998). D. Kumavor, C. Beal, S. Yelin, E. Donkor, and C. Wang, “Comparison of four multi-user quantum key distribution schemes over passive optical networks,” J. Lightwave Technol. 23, 268–276 (2005). 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, 130–138 (2007). W. Maeda, A. Tanaka, S. Takahashi, A. Tajima, and A. Tomita, “Technologies for quantum key distribution networks integrated with optical communication networks,” IEEE J. Sel. Top. Quantum Electron. 15, 1591–1601 (2009). D. Lancho, J. Mart´ınez, D. Elkouss, M. Soto, and V. Mart´ın, “QKD in standard optical telecommunications networks,” in 1st Int. Conf. on Quantum Communication and Quantum Networking (ICST, 2010), pp. 142–149. I. Choi, R. J. Young, and P. D. Townsend, “Quantum key distribution on a 10Gb/s WDM-PON,” Opt. Express 18, 9600–9612 (2010). J. Capmany and C. Fern´andez-Pousa, “Analysis of passive optical networks for subcarrier multiplexed quantum key distribution,” IEEE Trans. Microwave Theory Tech. 58, 3220–3228 (2010). I. Choi, R. J. Young, and P. D. Townsend, “Quantum information to the home,” New J. Phys. 13, 063039 (2011). M. Razavi, “Multiple-access quantum key distribution networks,” IEEE Trans. Commun. 60, 3071–3079 (2012). 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, 18790–18979 (2008). 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. Photon. 1, 1749– 4885 (2007). D. Stucki, N. Walenta, F. Vannel, R. T. Thew, N. Gisin, H. Zbinden, S. Gray, C. R. Towery, and S. Ten, “High rate, long-distance quantum key distribution over 250 km of ultra low loss fibres,” New J. Phys. 11, 075003 (2009). S. Wang, W. Chen, J.-F. Guo, Z.-Q. Yin, H.-W. Li, Z. Zhou, G.-C. Guo, and Z.-F. Han, “2 GHz clock quantum key distribution over 260 km of standard telecom fiber,” Opt. Lett. 37, 1008–1010 (2012). 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, 10632– 10639 (2011). P. Jouguet, S. Kunz-Jacques, A. Leverrier, P. Grangier, and E. Diamanti, “Experimental demonstration of longdistance continuous-variable quantum key distribution,” Nat. Photon. 7, 378–381 (2013). A. Treiber, A. Poppe, M. Hentschel, D. Ferrini, T. Lor¨unser, E. Querasser, T. Matyus, H. H¨ubel, and A. Zeilinger, “Fully automated entanglement-based quantum cryptography system for telecom fiber networks,” New J. Phys. 11, 045013 (2009). Recommendation ITU-T G.694.2, Spectral grids for WDM applications: CWDM frequency grid (2003). Recommendation ITU-T G.694.1, Spectral grids for WDM applications: DWDM frequency grid (2012). C. A. Brackett, “Dense wavelength division multiplexing networks: principles and applications,” IEEE J. Sel. Areas Commun. 8, 948–964 (1990).
38. P. Townsend, “Simultaneous quantum cryptographic key distribution and conventional data transmission over installed fibre using wavelength-division multiplexing,” Electron. Lett. 33, 188–190 (1997). 39. T. Xia, D. Chen, G. Wellbrock, A. Zavriyev, A. Beal, and K. Lee, “In-band quantum key distribution (QKD) on fiber populated by high-speed classical data channels,” in Optical Fiber Communication Conf. (IEEE, 2006), p. 3. 40. H. Rohde, S. Smolorz, A. Poppe, and H. Huebel, “Quantum key distribution integrated into commercial WDM systems,” in Optical Fiber Communication Conf. (IEEE, 2008), pp. 1–3. 41. 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, 105001 (2009). 42. B. Qi, W. Zhu, L. Qian, and H.-K. Lo, “Feasibility of quantum key distribution through dense wavelength division multiplexing network,” New J. Phys. 12, 18 (2010). 43. 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 1550 nm QKD with strong classical channels in reconfigurable networking environments,” New J. Phys. 11, 045012 (2009). 44. P. Eraerds, N. Walenta, M. Legr´e, N. Gisin, and H. Zbinden, “Quantum key distribution and 1 Gbps data encryption over a single fibre,” New J. Phys. 12, 063027 (2010). 45. K. A. Patel, J. F. Dynes, I. Choi, A. W. Sharpe, A. R. Dixon, Z. L. Yuan, R. V. Penty, and A. J. Shields, “Coexistence of high-bit-rate quantum key distribution and data on optical fiber,” Phys. Rev. X 2, 041010 (2012). 46. V. Scarani, H. Bechmann-Pasquinucci, N. J. Cerf, M. Dusek, N. L¨utkenhaus, and M. Peev, “The security of practical quantum key distribution,” Rev. Mod. Phys. 81, 1301–1350 (2009). 47. IEEE, IEEE standard for local and metropolitan area networks: overview and architecture (2002). 48. T. Ohara, H. Takara, T. Yamamoto, H. Masuda, T. Morioka, M. Abe, and H. Takahashi, “Over-1000-channel ultradense WDM transmission with supercontinuum multicarrier source,” J. Lightwave Technol. 24, 2311–2317 (2006). 49. R. Ramaswami, K. Sivarajan, and G. Sasaki, Optical Networks: A Practical Perspective, 3rd ed. (Morgan Kaufmann, 2009). 50. S.-J. Park, C.-H. Lee, K.-T. Jeong, H.-J. Park, J.-G. Ahn, and K.-H. Song, “Fiber-to-the-home services based on wavelength-division-multiplexing passive optical network,” J. Lightwave Technol. 22, 2582–2591 (2004). 51. B. Korzh, N. Walenta, R. Houlmann, and H. Zbinden, “A high-speed multi-protocol quantum key distribution transmitter based on a dual-drive modulator,” Opt. Express 21, 19579–19592 (2013). 52. H.-J. Briegel, W. D¨ur, J. I. Cirac, and P. Zoller, “Quantum repeaters: the role of imperfect local operations in quantum communication,” Phys. Rev. Lett. 81, 5932–5935 (1998). 53. L. Tian and H. Wang, “Optical wavelength conversion of quantum states with optomechanics,” Phys. Rev. A 82, 053806 (2010). 54. C. I. Osorio, N. Bruno, N. Sangouard, H. Zbinden, N. Gisin, and R. T. Thew, “Heralded photon amplification for quantum communication,” Phys. Rev. A 86, 023815 (2012). 55. P. Toliver, R. Runser, T. Chapuran, S. McNown, M. Goodman, J. Jackel, R. Hughes, C. Peterson, K. McCabe, J. Nordholt, K. Tyagi, P. Hiskett, and N. Dallmann, “Impact of spontaneous anti-Stokes Raman scattering on QKD+DWDM networking,” in 17th Annual Meeting of the IEEE Lasers and Electro-Optics Society (IEEE, 2004), pp. 491–492. 56. R. Runser, T. Chapuran, P. Toliver, M. Goodman, and J. Jackel, “Demonstration of 1.3 µm quantum key distribution (QKD) compatibility with 1.5 µm metropolitan wavelength division multiplexed (WDM) systems,” in Optical Fiber Communication Conf. (IEEE, 2005), p. 3. 57. D. Stucki, C. Barreiro, S. Fasel, J.-D. Gautier, O. Gay, N. Gisin, R. Thew, Y. Thoma, P. Trinkler, F. Vannel, and H. Zbinden, “Continuous high speed coherent one-way quantum key distribution,” Opt. Express 17, 13326– 13334 (2009). 58. J. Zhang, P. Eraerds, N. Walenta, C. Barreiro, R. Thew, and H. Zbinden, “2.23 GHz gating InGaAs/InP singlephoton avalanche diode for quantum key distribution,” Proc. SPIE 7681, 76810Z (2010). 59. N. Walenta, “Concepts, components and implementations for quantum key distribution over optical fibers,” Ph.D. thesis, Facult´e des Sciences de l’Universit´e de Gen`eve (2012). 60. D. Subacius, A. Zavriyev, and A. Trifonov, “Backscattering limitation for fiber-optic quantum key distribution systems,” Appl. Phys. Lett. 86, 011103 (2005). 61. Corning: SMF-28e+ LL optical fiber, http://www.corning.com/. 62. Flyin Optronics: splitter, circulators, CWDM filters and 1310/1550 WDM multiplexers, http://www.flyinoptronics.com/. 63. Polatis: optical switch Series 6000, http://www.polatis.com/datasheets/series-6000-192x192-low-loss-opticalswitch.pdf. 64. LG Nortel WPF 1132C (32-channels AWG). 65. V. Scarani and R. Renner, “Quantum cryptography with finite resources: Unconditional security bound for discrete-variable protocols with one-way postprocessing,” Phys. Rev. Lett. 100, 200501 (2008).
Quantum key distribution allows two distant parties to grow a secret key: an initial shared secret key can be made arbitrarily large while avoiding any information leakage. This is a information theoretic secure scheme based on the laws of quantum mechanics. The price to pay for such a high level of security is the usage of a symmetric key protocol with point-topoint connections . Both parties have to be connected through a quantum and a classical but authenticated channel, typically implemented by dedicated optical fiber links. The technology is mature enough for commercialization [2–6], and long-term practical settings have already been tested [7–9]. However, taking this concept to a network setup results in the need to use a completely separated optical infrastructure for QKD [10–14] which considerably increases its cost. Sharing an already deployed network and as much commercial technology as possible is then a must for the widespread adoption of QKD as a mainstream security technology. Nowadays, most telecom networks have adopted the optical paradigm . The use of passive optical technology is attracting interest in these networks since the absence of active components in the optical pathway, such as amplifiers or electro-optical converters, allows for a more robust and reliable network —albeit at the cost of some flexibility. From the quantum perspective this means that a unique, uninterrupted optical path can be set between two users and then used as quantum channel, i.e. quantum states can be transmitted in the network without being disrupted. Therefore, it opens the way for integrating QKD systems in commercial telecom networks; this has been a recurring issue in the last years [17–27]. It should be further mentioned that the discussed technology is mainly found in networks up to a metropolitan area scale (e.g. access networks and metro backbones), which in turn are the perfect market for QKD: they serve final users and the losses are compatible with the budget and key rate of actual QKD systems [28–34]. Furthermore, wavelength division multiplexing (WDM) [35, 36] is becoming a dominant technology in standard telecom networks. This allows to share efficiently a common optical infrastructure among multiple users . Ideally, a QKD system could communicate in these networks using a dedicated wavelength (i.e. a channel) for its quantum signal. Unfortunately, the transmission of single-photon pulses in a fiber together with strong, classical signals (carrying ≈ 107 photons per pulse) is disturbed by the noise generated by the latter. The coexistence of quantum and classical channels is thus limited to just a few of them [23, 38–45], especially when they operate in the same spectrum band. The objective of this work is to devise a technologically realistic and cost-effective QKD network, able to overcome the major roadblocks in the way towards a broader acceptance of QKD technology. The network design is inspired by the technologies and topologies of commercial telecom networks in order to use existing deployed infrastructures (e.g. dark fibers) and commercial components, such that the deployment and running costs are as low as possible and remain competitive with other high security network services. To this end, QKD devices are wavelength multiplexed in order to share resources. This includes quantum and classical signals, the latter being either generated for the stabilization of the quantum channel or for other QKD purposes like key distillation or encryption. Communications between QKD devices are routed using passive optical components in contrast to trusted repeaters . However, this fully passive version only works with static QKD links. In the case that an any-to-any scheme is required, optical switches must be added for dynamic routing. Finally, a network prototype based on the proposed model has been designed and deployed for testing purposes. The present approach focuses on prepare-and-measure QKD schemes that fall into two main classes according to the standard classification : discrete variables QKD and distributed phase-reference pulse QKD. Its extension to other QKD schemes such as continuous variable QKD and entangled photon-pairs QKD might be possible but lies beyond the scope of the present work.
The paper is organized as follows. Sec. 2 reviews the architecture and principle of operation of modern metropolitan optical networks. In Sec. 3 we discuss the proposed multiplexing scheme and the modifications required on the network nodes in order to use quantum signals. A prototype of a metropolitan QKD network is described and characterized in Sec. 4. Finally, we summarize the discussion and outline some future improvements in Sec. 5. 2.
Metropolitan networks aim to cover the area of cities, with a typical span from a few to several tens of kilometers . A common architecture of a metropolitan optical network (MON) foresees a division into core and access networks, as depicted in Fig. 1. It should be noted that, actually, the design and topologies in a MON could be more elaborated due to, for instance, external constraints, limited resources, or to the growing needs of the carrier company. However, for the sake of clarity we will stick to the network architecture just outlined as a typical one, denoting it as a canonical MON.
Fig. 1. Typical architecture and topology used in the canonical metropolitan optical network considered in this work. A core network, the backbone, with the highest capacity links set up in a ring, is connected with the final users through one-to-many passive access networks. The network component (NC, typically an splitter or multiplexing device) is usually located near the users (optical network units or ONUs) in order to minimize the amount of non-shared fiber used. The backbone uses (reconfigurable) optical add and drop modules, (R)OADM, to add or drop different signals to/from the access networks. In order to drive the access network and translate between its protocol and the one running in the core, an optical line terminator (OLT) is used. This usually means electro-optical conversion, which is disruptive for QKD. Finally, the backbone can be connected to other rings or long-haul networks.
composed of 18 channels spaced from 1270 to 1610 nm and each occupying 20 nm (O, E, S, C and L bands). DWDM is mainly limited to the 1550 nm region (S, C and L bands) and, depending on the chosen grid, channel separation ranges from 100 GHz (or multiples) down to 12.5 GHz (0.8-0.1 nm) to accommodate from 40 up to hundreds of channels . 2.1.
An access network follows a point-to-multipoint topology to connect many final users to the core using a simple fiber infrastructure of a few tens of kilometers . They are typically deployed in the so-called fiber-to-the-home (FFTH) architectures with cables containing multiple optical fiber strands and using passive optical technology (i.e. passive optical networks or PON). In a PON, an optical line terminator (OLT), with direct access to the backbone, is connected through a single fiber to a network component (NC) with N outputs, which in turn is connected through a non-shared fiber to N optical network units (ONU) located at the user’s premises. The NC is assumed to be close to the ONUs, thus reducing the amount of non-shared fiber used. Depending on the multiplexing technology used, communications between a particular ONU and the OLT, and vice versa, are addressed either using a specific wavelength or time slot (WDM or TDM, respectively) that differentiates it from its neighbors. In a typical TDM-based access network (e.g. Gigabit-capable PON or GPON), a beam splitter is used as the NC to connect multiple users. This introduces 3 dB of losses each time the number of users is doubled. Hence, a network of 32 users has a minimum of 15 dB losses in the NC. Instead, in a WDM-based approach (e.g. WDM-PON), the splitter is replaced by a wavelength multiplexer. This is typically an arrayed waveguide grating (AWG), which has less insertion losses than the splitter (e.g. a 32-channels AWG has ≈ 3 dB). Moreover, losses do not grow by much when adding more channels. This allows to increase the number of users while maintaining the same overall loss budget. Another key advantage of the AWG that will be used in the present approach is its cyclic behavior: through each output port, not only a single wavelength can be used, but also its periods in the upper and lower spectrum. Despite not being standardized, the common usage is to take advantage of this characteristic and use two spectrum bands to separate downstream and upstream signals . 2.2.
Different access networks are connected through a core network or backbone that in a MON is typically a ring. A first-level backbone is composed of M nodes covering all the metropolitan area, where each backbone node is connected to the OLT of one or more access networks. Signals within the ring are wavelength multiplexed and a (reconfigurable) optical add-drop multiplexer, (R)OADM, is used at the backbone node to add and drop different channels, i.e. add or extract wavelengths to/from the ring. The connection between core and access networks typically includes an electro-optical conversion, since the protocols and technologies can be very different. However, when the backbone and the access network are both based on optical technology and WDM, they can also be directly connected in the optical domain, thus opening the possibility to support quantum communications. This allows for a realistic network where QKD emitters can connect to different receivers (even if different QKD protocols are used ). Furthermore, a ROADM can also connect the core to a long-haul network in order to reach distant networks. However, we will not consider here this scenario, since the distance in these settings exceeds the loss budget of actual QKD systems. 3.
technology as a base to construct a QKD network. The creation and stabilization of a quantum channel is a challenging task that imposes strong requirements on the infrastructure. Quantum channels are easily degraded because of photon absorption or stray photons coming from classical signals in the same fiber. Moreover, technologies that could overcome these problems, like quantum repeaters, are still in their infancy [52–54]. Hence, the communication must follow a direct optical path, with always the same wavelength and within the loss budget of the QKD system. The objective of the proposed network is to provide an easy to deploy and maintain infrastructure, supporting many non-interfering quantum channels. By sharing the infrastructure among many users, QKD becomes more price-competitive and increases its potential market share. To this end, the network is designed to use in a shared way the very costly dark fiber that is already deployed and as much commercially available optical equipment as possible. It is to be noted that, in most settings, the cost of hiring or deploying from anew a dark fiber offsets by a long margin the cost of the QKD devices themselves. To limit the interference with classical communications signals, we define in principle the QKD network only for QKD purposes, i.e. at first only quantum and service signals will be allowed. By service signals, we mean the classical ones used to keep the QKD devices working (interferometer stabilization, synchronization, etc.). In this first approach, a pair of QKD devices only need a quantum channel and a service channel, both directed from emitter to receiver, in order to establish a quantum link. After studying the restrictions imposed by the service channels in Sec. 4, we will discuss the possibilities of adding further channels such as the ones for the classical key distillation protocols in QKD, for cipher-text transmission, or even for purely classical communications unrelated to the purposes of the QKD equipment. 3.1.
Fig. 2. (a) Spectrum of the proposed wavelength-multiplexing scheme. The spectrum is divided in two bands, quantum and service, separated well enough to minimize noise in the quantum band. The first band is located in the O band (13xx) and is used to transport the quantum channel. The second, mainly at the C band (15xx), carries the service channels needed to keep the quantum channel and cryptographic protocol working. Each of the two bands is divided in N subbands, named here Q1...N and S1...N , for quantum and service, respectively. A pair of quantum and service subbands will correspond to an access network. Each subband carries M channels, represented here as arrows. Channels are chosen in an ITU grid and periods of the AWG. Subbands are selected such that the corresponding wavelengths in the quantum and service band are in the same period, hence both will come out together in the same AWG port. (b) Experimental spectrum of the network prototype. To check the behavior of the network prototype, two signals were fed into the quantum and service subbands Q2 and S2. The subband structure is clearly seen. The different number of channels seen in both bands are due to the input signals used for the test. For a complete description, see Sec. 4.
wavelength was fed to the common port and an optical spectrum analyzer was used to measure the output port. In Fig. 3 we present the spectrum obtained summing the outputs 1, 8, 16, 24 and 32. Only output 16 is shown for the full range, including the 1340 to 1520 nm region that separates the quantum from the classical signals and is not used in the proposal. The figure clearly shows the periodicity used to route the corresponding pairs of quantum and service bands to the same destination. We define as a periodic set the set of channels that can be used through each output port of the AWG.
Fig. 3. Experimental data of the cyclic behavior of a 100 GHz 32-channels AWG, as the one used in the network prototype, in the range 1250-1620 nm. Only outputs 1, 8, 16, 24 and 32 are shown, and only output 16 is presented over the whole range. Channels from the same periodic set have the same color.
receiver’s side AWG, this limitation no longer exists and QKD emitters/receivers can be freely mixed and located at any port of any of the two access networks.
Fig. 4. Simplified network with two WDM-PON access networks. Only one switch is actually needed to allow a wavelength tunable QKD emitter to use any port of the AWG, and thus communicate in an all to all configuration with any receiver on the other AWG. If a switch is used on each side, as depicted, then emitters and receivers can be freely mixed in both access networks. See text.
Fig. 5. Backbone node: OADM designed for the QKD-MON. Built out of common network components, it drops the quantum and service subbands from the ring’s signal (input) to the access network, and adds any channel coming from the access network, no matter which subband it belongs to, to the ring (output).
Fig. 6. Proposed QKD-MON with three access networks. Colored dots are used to illustrate the communication over different paths. Each colored dot represents a pair of wavelengths: one in the quantum band and the corresponding periodic one in the service band that would come out of the same output port of the AWG. The main network components are represented in dashed-line boxes. These devices can be built with out of the shelf commercial components. Note how one access network can communicate simultaneously with the others in a non-blocking way: communications can be performed simultaneously since the network operates employing wavelength multiplexing.
(FWM) and crosstalk due to imperfect devices. However, we can eliminate the last two since, due to the separation between quantum and service bands, no signal generated by FWM from the service band will fall within the quantum band . Likewise, strong service signals that could produce too much crosstalk due to insufficient isolation in the devices can be easily filtered because they are also in other band. The only phenomenon that could spoil the quantum signals is Raman scattering, but then a band separation of approx. 150 nm is enough to attenuate it considerably [55, 60], as we will see in the next section. Table 1. Losses of typical optical network components. Values are from commercial models available in the market [61–64] that are used for the test-bed in Sec. 4.
Table 2. Calculated losses for the main network modules of the QKD-MON (according to Table 1). Using these theoretical values, we estimate the losses of different full optical paths in terms of OADMs and fiber length.
Fig. 7. QKD-MON test bed: Network prototype with three OADMs built following the design in Fig. 6. The total length of the fiber is approx. 16 km, a typical span for metro area. A relatively long fiber is used in the access network 1 to generate a high amount of Raman scattering, more than the average for access networks. Overlaid in black is the worst case path in the test bed with respect to losses and generated noise, hence this is the set-up used to perform the measurements.
Table 3. Measured losses in the quantum and service band for the OADMs and for the full optical path in the QKD-MON test bed (see Fig. 7).
Fig. 8. Noise measurement done using the setup in Fig. 7. A laser signal centered at 1520 nm and with power ranging from −30 to +2 dBm is fed at the access network #1. The forward noise produced in a quantum channel (1340 nm, triangles) is measured using a SPD at the access network #3. The backward noise (circles) is also measured by connecting the SPD to the access network #1. This allows assessing the amount of interference that would reach a QKD receiver coming from an emitter in the same AWG. As a further check, we also measure the forward noise in the service band (1530 nm, squares). To facilitate the comparison, values are normalized considering 1 ns gates. Besides, a quantum signal (mean photon number, µ = 0.1, and detector’s quantum efficiency, η = 0.1) detected at the access network #3 and the dark count rate of an SPD  are also presented. Using these data, a rough estimation of the QBER is shown for multiple points.
the backbone ring is directional: a signal originated in the receiver cannot be propagated back to the emitter using the same path. To do this, a signal traveling along the other part of the ring has to be used. Therefore, the receiver has to use a service channel assigned to the emitter. Since, by design, every device in the network has a pair of channels assigned, there is no extra addressing required for these return channels; they are already located in the channel plan. However, return channels require a different switch configuration and, thus, they cannot be used simultaneously with the corresponding service channel. This is because, in general, emitter and receiver are connected to different ports of their respective AWGs. Due to the number of signals that need to be generated to produce enough key material to get rid of finite key effects , the switching time is not a problem. However, if a simultaneous return channel is necessary, this can be easily taken into account. For instance, in a static version of the network, all channels (i.e. quantum, service and return) can be configured to belong to the same periodic set. If a dynamic addressable network is needed, then the simplest solution is to use different ports of the AWG for each direction. This means that a QKD device will be connected to the switch using two short fibers. This might not be the most economical use of the fiber, however it is not a technical problem since this is a short distance and most installations include spare fibers that could be used for this purpose. 5.
We have presented a quantum metropolitan network that is, in contrast to existing QKD networks, specifically designed to share infrastructure and use existing optical components in an attempt to make QKD a more cost-competitive technology and lower the barriers to a wider market adoption. We also show that the new modules needed can be built out of inexpensive, industrial grade and readily available components, without introducing unacceptable losses. The scheme is based on wavelength division multiplexing and addressing, whereby multiple QKD devices are simultaneously connected for transmitting quantum and classical signals. The architecture is a conventional one in metropolitan optical networks, comprising backbone and access networks, although these two segments are directly connected to provide uninterrupted optical paths between all users; a must in order to support a quantum channel. The network allows all to all QKD links and uses standard commercial WDM technology: CWDM for the backbone and DWDM for the access. Except the switches on the user side, needed only if all-to-all dynamic routing is required, the rest of the network is purely passive. This would potentially allow for a cheap, easy and reliable deployment. The scheme is limited by the loss budget of actual QKD systems (≈ 20-30 dB), but, as discussed above, this is enough for a backbone ring of 20 km with three access networks. This would allow to cover interesting regions in a city and its surroundings. The measurements performed on a prototype network demonstrate that it is capable of supporting at least 32 simultaneous QKD links, each one with a pair of a quantum and a service channel, whereby the latter can support traffic of up to 1.25 Gbps classical signals. This traffic could include key distillation communication or even cipher text transmission. Classical channels for other purposes could also be included when not all of the possible QKD links are installed. The estimate assumes 1 ns detector gates: more channels and a higher throughput would be possible if last generation, sub-ns gated detectors are used. We introduced the scheme with discrete, one-way QKD systems but it could, in principle, be extended to entangled pairs and continuous variables, although then the limits could possibly vary. We plan to address in near future the extension of the present scheme to cover all main QKD realizations.
Acknowledgments This work has been partially supported by projects QUITEMAD, Comunidad Aut´onoma de Madrid, and Quantum Hybrid Networks, Ministerio de Econom´ıa y Competitividad, Spain. GAP acknowledges financial support from NCCR-QSIT. AIT acknowledges support by the project QKD-Telco: Practical Quantum Key Distribution over Telecom Infrastructures, within the FIT-IT programme funded by the Austrian Federal Ministry for Transport, Innovation and Technology (BMVIT) in coordination with the Austrian Research Promotion Agency (FFG). The authors also thank P. Corredera for his assistance regarding the characterization of the AWG, and M. Soto and Telef´onica I+D for the loan of the OSA, EDFA and fiber prototype network used in this work.
Multicasting based optical inverse multiplexing in elastic optical network.
Effect of soil temperature on optical frequency transfer through unidirectional dense-wavelength-division-multiplexing fiber-optic links.
Orbital Angular Momentum-based Space Division Multiplexing for High-capacity Underwater Optical Communications.
A new material platform of Si photonics for implementing architecture of dense wavelength division multiplexing on Si bulk wafer.
Bidirectional multiband radio-over-fiber system based on polarization multiplexing and wavelength reuse.
Large efficiency at telecom wavelength for optical quantum memories.
Nonlinear inverse synthesis and eigenvalue division multiplexing in optical fiber channels.
Compressive sensing-based channel bandwidth improvement in optical wireless orthogonal frequency division multiplexing link using visible light emitting diode.
Enhancing the heralded single-photon rate from a silicon nanowire by time and wavelength division multiplexing pump pulses.
Wide-field high-speed space-division multiplexing optical coherence tomography using an integrated photonic device.
Improved wavelength coded optical time domain reflectometry based on the optical switch.
Design of DPSS based fiber bragg gratings and their application in all-optical encryption, OCDMA, optical steganography, and orthogonal-division multiplexing.
Transmission of O-band wavelength-division-multiplexed heralded photons over a noise-corrupted optical fiber channel.
Efficient multiuser quantum cryptography network based on entanglement.

References: V. 
 V. 
 V. 
 V. 
 V. 
 V.