Boosting the secret key rate in a shared quantum and classical fibre communication system

ARTICLE https://doi.org/10.1038/s42005-019-0238-1 OPEN 1234567890():,; Boosting the secret key rate in a shared quantum and classical fibre communication system Davide Bacco 1,6*, Beatrice Da Lio1,6*, Daniele Cozzolino 1, Francesco Da Ros1, Xueshi Guo2, Yunhong Ding 1, Yusuke Sasaki3, Kazuhiko Aikawa3, Shigehito Miki4, Hirotaka Terai 4, Taro Yamashita5, Jonas S. Neergaard-Nielsen 2, Michael Galili1, Karsten Rottwitt1, Ulrik L. Andersen2, Toshio Morioka1 & Leif K. Oxenløwe 1 During the last 20 years, the advance of communication technologies has generated multiple exciting applications. However, classical cryptography, commonly adopted to secure current communication systems, can be jeopardised by the advent of quantum computers. Quantum key distribution (QKD) is a promising technology aiming to solve such a security problem. Unfortunately, current implementations of QKD systems show relatively low key rates, demand low channel noise and use ad hoc devices. In this work, we picture how to overcome the rate limitation by using a 37-core fibre to generate 2.86 Mbit s−1 per core that can be space multiplexed into the highest secret key rate of 105.7 Mbit s−1 to date. We also demonstrate, with off-the-shelf equipment, the robustness of the system by co-propagating a classical signal at 370 Gbit s
1 , paving the way for a shared quantum and classical communication network. 1 CoE SPOC, Dep. Photonics Eng., Technical University of Denmark, Kgs. Lyngby 2800, Denmark. 2 CoE bigQ, Dep. of Physics, Technical University of Denmark, Kgs. Lyngby 2800, Denmark. 3 Advanced Technology Laboratory, Fujikura Ltd., 1440, Mutsuzaki, Sakura, Chiba 285-8550, Japan. 4 Advanced ICT Res. Inst., National Institute of Information and Communications Technology, 588-2 Iwaoka, Nishi-ku, Kobe 651-2492, Japan. 5 Department of Electronics, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan. 6These authors contributed equally: Davide Bacco, Beatrice Da Lio. *email: ; COMMUNICATIONS PHYSICS | (2019)2:140 | https://doi.org/10.1038/s42005-019-0238-1 | www.nature.com/commsphys 1 ARTICLE COMMUNICATIONS PHYSICS | https://doi.org/10.1038/s42005-019-0238-1 O ur society is based on the continuous exchange of billions of data, and most of them travel in optical fibres. Nonetheless, most of the exchanged data are not protected against upcoming threats, i.e., new algorithms able to break current cyphers and the expected availability of quantum computers1. A quantum computer is a machine based on the laws of quantum physics, which will be able to crack some of the current cryptosystems, whose security relies on the limited computational power of an eavesdropper2. As derived by Shannon, a way to achieve information theoretical secure communications is to use OneTime-Pad encryption, which requires a pre-shared key of the same size as the message to be sent3. However, other symmetric key algorithms based on computationally hard problems are also used, such as the Advanced Encryption Standard, since they require keys of constant length (e.g. 128 or 256 bits)4. The symmetric key used by these cryptosystems must be exchanged between the two parties, and this is usually achieved through public-key algorithms, two of the most widely used being the Rivest–Shamir–Adler and the elliptic curve cryptography5–7. A quantum computer can however break both algorithms, leaving the task of distributing keys to either post-quantum cryptography8 or to alternative methods such as quantum cryptography. Within the latter, quantum key distribution (QKD) addresses this challenge by relying on the laws of quantum physics to provide the required information-theoretic security9. During the last 30 years, multiple demonstrations of free-space, underwater and fibre-based QKD systems have shown the feasibility of such a technology10–16. Nevertheless, multiple factors are limiting the global deployment of QKD systems: the low information rate, the short propagation distance and the compatibility with the existing network infrastructure. Indeed, most of QKD implementations have been realised in low noise environments (e.g. dark fibres), attesting to the difficulty of integration with the bright signals used in classical communications17–23. In this work, we show how to overcome the low rate and compatibility limitations by exploiting a 37-core multicore fibre (MCF) as a technology for quantum communications24. This technology allows for efficient key generation, enabling the highest secret key rate presented to date. Moreover, we co-propagate in all the 37 cores simultaneously a high-speed classical signal, showing that the quantum communication is only weakly perturbed by it, paving the way for a full-fleshed implementation in current communication infrastructures. reducing the overall power budget, thus decreasing the total power consumption25–27. As reported in Fig. 1a, through SDM the achievable rate of an N-core MCF corresponds theoretically to N times the achievable rate of a single mode fibre28. To achieve these high rates, a low cross-talk between the different channels is of extreme importance for SDM schemes. Indeed, cross-talk is the measure of the leakage from one channel to another, leading therefore to a larger insertion loss in the former and, more importantly, to an extra noise source in the latter. As an example, high-speed classical communication at 10:16 Pbit s
1 has been demonstrated thanks to SDM combined with dense wavelength division multiplexing, polarisation modulation and advanced coding29. Likewise, both multicore and few-mode fibres have been exploited for encoding and transmitting high-dimensional (Hi-D) quantum states30–32. In particular, the cores or the modes of these special fibres have been used to increase the dimensionality of the Hilbert space, thus allowing higher photon information efficiency. Although Hi-D quantum states are suitable for high rate quantum communications, in specific channel conditions (low noise channel), it was experimentally demonstrated that SDM is suitable for very high rate secure communications33. The principle is based on the concatenation of parallel keys acquired in each core/mode as shown in Fig. 1b33. As a support of this statement, Fig. 2 shows a theoretical analysis of the secret key rate as a function of the error rate for Hi-D and parallel encoding when assuming transmission in a lossy bosonic Gaussian channel (e.g. a fibre link)28. As a matter of fact, in the low noise regime, the performance achieved by Hi-D protocols is lower compared to the one obtained with the multiplexing of parallel encoding (SDM). Note that, similarly to SDM in classical communications, the generation of parallel keys is achieved when each core/mode is used to transmit independent quantum states. The receiver measures each core/mode individually and then after distillation and reconciliation processes a longer key is available between the two users. In our demonstration (...truncated)