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arXiv:1911.05360 · Communications Physics

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

Davide Bacco, Beatrice Da Lio, Daniele Cozzolino, Francesco Da Ros, Xueshi Guo, Yunhong Ding, Y. Sasaki, Kazuhiko Aikawa, Shigehito Miki, Hirotaka Terai, Taro Yamashita, Jonas Schou Neergaard-Nielsen, Michael Galili, Karsten K. Rottwitt, Ulrik L. Andersen, Toshio Morioka, Leif Katsuo Oxenløwe

Published Nov 11, 2019• 88 citations
arXivHTMLPDF

Abstract

Abstract 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 $${}^{-1}$$ − 1 , paving the way for a shared quantum and classical communication network.

DOI
10.1038/s42005-019-0238-1

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Abstract 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{}^{-1}−1 − 1 , paving the way for a shared quantum and classical communication network.
Quantum key distribution (QKD) guarantees unconditional security and has thus been intensely studied both from a theoretical and an experimental point of view [1]. However, limitations due to transmission loss and noise are still detrimental for these systems performance in terms of reachable distance and key rates, having so far confined QKD implementations to the use of dedicated channels and hence reduced a successful integration with the classical network, with few exceptions [2–6]. In this work we demonstrate a step forward in the co-existence of QKD with classical bright signals, exploiting a 7.9-km long 37-core heterogeneous multicore fiber (MCF) [7], and a space division multiplexing (SDM) technique to achieve a total of 87 Mbit/s secret key generation rate (SKR) in the finite key scenario with a simultaneous co-propagating classical signal of 10 Gbit/s.
Abstract Secret key rate is a core performance indicator in implementing quantum key distribution, which directly determines the transmission rate of enciphered data. Here we demonstrate a high-key-rate quantum key distribution system over mature telecom components. The entire framework of quantum key distribution over these components is constructed. The high-rate low-noise Gaussian modulation of coherent states is realized by a classical electro-optic IQ modulator. High-baud low-intensity quantum signals are received by a commercial integrated coherent receiver under the shot-noise limit. A series of digital signal processing algorithms are proposed to achieve accurate signal recovery and key distillation. The system has yield a secret key rate of 10.37 Mbps, 1.61 Mbps, 337.82 kbps, and 58.06 kbps under the standard telecom fiber of 20 km, 50 km, 70 km, and 100 km, respectively. Our results represent the achieved highest secret key generation rate for quantum key distribution using continuous variables at a standard telecom wavelength. Moreover, it breaks the isolation between quantum communication and classical optical communication in terms of components, and opens the way to a high-speed and cost-effective formation of metropolitan quantum secure communication networks.
Quantum key distribution protocols are easily the most advanced quantum information technology up to date, driving the whole field of quantum communications towards actual systems implementations and flourishing theoretical advances and experimental demonstrations. Exploiting the consolidated telecommunications industry knowledge, optimized equipment and already existing broad infrastructure, fiber-based quantum key distribution schemes are in particular a promising direction in which the scientific research efforts are addressed, for the final goal of building a quantum network. However, three main challenges are still preventing the large-scale industrial deployment of fiber-based quantum key distribution schemes: the limited achievable secret key generation rate, when compared to those reached by classical communications; the bounded propagation distances, due to intrinsic channel losses; and the struggle of the integration with classical data rates, which are additional sources of noise from the point of view of a quantum channel. During my Ph.D., we have focused on two of these problems, proposing new efficient designs for fiber-based high-dimensional protocols and space division multiplexing schemes to increase the secret key rate of quantum systems, and investigating the optimization of wavelength multiplexing setups for the integration with classical communications.
Quantum Key Distribution (QKD) is currently receiving much attention as it provides a secure source of encryption keys. Discrete-Variable QKD (DV-QKD) is possible for single photon transmission in QKD to-coexist with and encode classical wavelength division multiplexed (WDM) data with appropriate system design. Nevertheless, previous QKD field trials adopted either or both of key relay via trusted nodes and transparent link via optical switching. The former requires guaranteed physical security of the relay nodes, but can expand key distribution distance arbitrarily. The latter can realize key establishment for more users with less complexity of key management over an untrusted network. To realise the adaption of the QKD system for future high speed and long distance metropolitan world exploitation at lower cost, there has to be investigations on existing fibre infrastructures. Prior to this work, previous researches over similar distances feature extremely low secure key rates. For example, the Swiss Quantum Network between three sites displayed secure bit rates of 2.5 kbps at a fibre length of 17km. Quantum Key distribution within the 25km Cambridge Quantum Network have demonstrated the highest long-term secure key rates yet demonstrated in a field trial of at least 2.5Mb/s which is the fastest and much higher than 0.8 kbps which was reached over the similar channel loss field trial up to date. Additional field trials have been performed on the UK Quantum Network using a 66km path having 16dB loss. Combined wavelength division multiplexed 2 x 100 Gb/s traffic encrypted using QKD co-existing on the same fibres has operated for several months, with a long-term key rate of 80kb/s that is also faster than any other similar long-term QKD trial systems. In addition to this advanced commercial QKD system, there have been secure key rate analysis comparisons between laboratory fibre coils and practical field trials more than field trials only conducted before.These comparisons help to identify factors that limit future QKD network scale in both quantity and quality aspects. Also, the limit for the highest secure key rate at longest fibre length QKD in the multiplexing environment is discussed and determined in this research thesis. Nevertheless, in this thesis, improvements have been made to minimise the corresponding negative effects by investigations on the dependence of temperature have been done in order to ensure system operation environment effects. It was found from the trial results that there exists a relationship between temperature and secure key rate and further study has been done to evaluate the system sensitivity to operating temperature. Although the conventional DV-QKD system, original BB84 coding scheme, was designed to exploit the quantum properties of single photon polarization states, the trial equipment operates based upon the phase coding schemes. These coding schemes are based on the properties of interferometers and the coding is implemented by changing the relative optical path lengths or phase between the internal arms of the interferometer, while in the real transmission environment, temperature or polarization variation happens unpredictably. The existing polarisation controllers operate at relative low speed align within the interferometer, which slows to operation environment such as a punch to fibre causing phase difference. Therefore, in this project, there has been an improvement in the QKD-WDM system performance by adding an external polarization controller to minimize the Raman noise and increase the secure key rate at the longest fibre length up to date. In Summary, transmitting quantum keys over a coil of fibre in the lab differs a lot from actually putting it in the ground. This work contrasts the world fastest QKD system at the longest distance in field trials with lab fibre reels and then characterises and identifies two of the key factors, temperature and polarizations, influencing performance in practical wavelength-multiplexed secure communication systems. This is a significant step towards the coexistence of the quantum and conventional data channels on the same fibre for metropolitan networks and paves a way for an information-secure communication infrastructure.
Current technological progress is driving Quantum Key Distribution towards a commercial and worldwide scale expansion. Its capability to deliver secure communication regardless of the computational power of the attackers will be a fundamental feature in the next generations of telecommunication networks. Nevertheless, demonstrations of QKD implementation in a real operating scenario and their coexistence with the classical telecom infrastructure are of fundamental importance for reliable exploitation. Here we present a Quantum Key Distribution application implemented over a classical fiber-based infrastructure. We exploit a 50 MHz source at 1550 nm, a single 13 km-long fiber cable for both the quantum and the classical channel, and a simplified receiver scheme with just one single-photon detector. In this way, we achieve an error rate of approximately 2% and a secret key rate of about 1.7 kbps, thus demonstrating the feasibility of low-cost and ready-to-use Quantum Key Distribution systems compatible with standard classical infrastructure.