Multi-party Agile Quantum Key Distribution Network with a Broadband Fiber-based Entangled Source

A reconfigurable, multi-party quantum key distribution scheme is experimentally demonstrated by utilizing a poled fiber-based source of broadband polarization-entangled photon pairs and dense wavelengthdivision multiplexing (DWDM). The large bandwidth (> 90 nm centered about 1555 nm) and highly spectrally-correlated nature of the entangled source is exploited to allow for the generation of more than 25 frequencyconjugate entangled pairs when aligned to the standard 200-GHz ITU grid. Such a network can serve more than 50 users simultaneously, allowing any one user on the network to establish a QKD link with any other user through wavelength-selective switching. The entangled pairs are delivered over 40 km of actual fiber (equivalent to 120 km of fiber based on channel-loss experienced), and a secure key rate of more than 20 bits/s per bi-party is observed. © 2015 Optical Society of America OCIS codes: (270.5568) Quantum cryptography; (060.1810) Buffers, couplers, routers, switches, and multiplexers; (190.4370) Nonlinear optics, fibers References and links 1. 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 decoystate quantum key distribution over 144 km,” Phys. Rev. Lett. 98, 010504 (2007). 2. R. Ursin, F. Tiefenbacher, T. Schmitt-Manderbach, H. Weier, T. Scheidl, M. Lindenthal, B. Blauensteiner, T. Jennewein, J. Perdigues, P. Trojek et al., “Entanglement-based quantum communication over 144 km,” Nature physics 3, 481–486 (2007). 3. A. Treiber, A. Poppe, M. Hentschel, D. Ferrini, T. 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Gerrits et al., “Photon-efficient quantum key distribution using time–energy entanglement with highdimensional encoding,” New Journal of Physics 17, 022002 (2015). Quantum key distribution (QKD) offers, in principle, an unconditionally secure method for two parties to generate a private cryptographic key. Many point-to-point (PTP) demonstrations using weak-coherent source-based [1] and entanglement-based [2, 3] quantum-cryptographic protocols such as the vaunted BB84 [4], BBM92 [5] and E91 [6] (respectively) have been demonstrated. However, PTP connections do not provide an efficient method to connect multiple users, and efforts thusfar in extending PTP links into multi-user networks have proven to be cumbersome at best. To that end, QKD networks based upon the hub-and-spoke model, where many end-users can be connected to a trusted node [7, 8, 9, 10], are used. Should any one end-user wish to communicate with another user secretly, a random secret key is first generated at the node, then QKD is performed between the node and each user separately; finally, the key is sent to each end-user classically using the QKD key material as a one-time pad. However, the trust model for such a network topology is inherently problematic, as any security vulnerability on the node will compromise the entire network. Additionally, time-multiplexing is often required in such circumstances to service multiple users [11, 12], and only one end-user can perform QKD with the central node at a time. Other schemes [13, 14] involving wavelength-division multiplexing (WDM) have been introduced where individual users on a PTP network can perform QKD with one another simply by addressing each other at different laser wavelengths. All users are equivalent on the network, and no central trusted node is required. However, each user must have both single photon detectors and laser sources at their disposal, and the incremental cost of adding a new user to an N-user network requires increasing the number of laser wavelengths available to each user to N + 1. In contrast, we show that a reconfigurable multi-user QKD network utilizing the distribution of polarization-entangled photon pairs from a (potentially untrustworthy) central office can be realized efficiently using a single broadband polarization-entangled source (Fig. 1). The entangled photon pairs generated from the source are highly spectrally-anticorrelated (Fig. 2), which allows for multiple (N) frequency-conjugate pairs to be carved out of the spectrum and distributed to N (> 25) pairs of users simultaneously. With a 2N × 2N wavelength-selective switch (WSS) to perform dynamic spectrum allocation [15], and without any additional hardware resource, any user on the network can perform QKD with any other user. Additionally, our scheme allows multiple users (2N > 50) to perform QKD simultaneously; when combined with time-multiplexing, such a scheme can extend the number of users well beyond what the bandwidth of the source can accommodate. Previous works [16, 17] demonstrated the distribution of polarization-entangled photon pairs using WDM technology. However, QKD was not performed in either of those cases. Fiber spans of 10 km were used in [16], while [17] did not distribute the entangled photon pairs over any appreciable length (> 500 metres) of fiber. Additionally, both works employed PPLN-based correlated photon pair sources combined interferometrically (Sagnac loop in [16], and type-0 sandwich in [17]) to create entangled pairs, which are markedly more complex experimentally than the poled fiber source [18] used in this work. The usable bandwidths of both sources (10