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Coexistence of High-Bit-Rate Quantum Key Distribution and Data on Optical Fiber

Phys. Rev. X 2, 041010 – Published 20 November 2012
K. A. Patel, J. F. Dynes, I. Choi, A. W. Sharpe, A. R. Dixon, Z. L. Yuan, R. V. Penty, and A. J. Shields
Physics logo See Synopsis: Finding Quantum Keys in Noisy Fibers


Quantum key distribution (QKD) uniquely allows the distribution of cryptographic keys with security verified by quantum mechanical limits. Both protocol execution and subsequent applications require the assistance of classical data communication channels. While using separate fibers is one option, it is economically more viable if data and quantum signals are simultaneously transmitted through a single fiber. However, noise-photon contamination arising from the intense data signal has severely restricted both the QKD distances and secure key rates. Here, we exploit a novel temporal-filtering effect for noise-photon rejection. This allows high-bit-rate QKD over fibers up to 90 km in length and populated with error-free bidirectional Gb/s data communications. With a high-bit rate and range sufficient for important information infrastructures, such as smart cities and 10-Gbit Ethernet, QKD is a significant step closer toward wide-scale deployment in fiber networks.

DOI: http://dx.doi.org/10.1103/PhysRevX.2.041010

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  • Received 22 June 2012
  • Published 20 November 2012

Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society


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Finding Quantum Keys in Noisy Fibers

Published 20 November 2012

“Quantum keys” can be sent long distances on high-traffic optical fibers.

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Authors & Affiliations

K. A. Patel1,2, J. F. Dynes1, I. Choi1, A. W. Sharpe1, A. R. Dixon1, Z. L. Yuan1,*, R. V. Penty2, and A. J. Shields1,†

  • 1Toshiba Research Europe Limited, Cambridge Research Laboratory, 208 Cambridge Science Park, Milton Road, Cambridge, CB4 0GZ, United Kingdom
  • 2Cambridge University Engineering Department, 9 J J Thomson Avenue, Cambridge, CB3 0FA, United Kingdom

  • *zhiliang.yuan@crl.toshiba.co.uk
  • andrew.shields@crl.toshiba.co.uk

Popular Summary

In the world of traditional communications, it is impossible for one person who receives a set of encryption keys from another to know whether or not the keys have been seen by the eyes of an eavesdropper. However, quantum key distribution (QKD), which encodes keys in the quantum states of a medium, can achieve the impossible by exploiting the fundamental quantum property that any eavesdropping on a quantum state always leaves a trace. In practice, secure QKD requires the use of only faint light pulses to exchange secret digital encryption keys over optical telecommunication fibers and has to, therefore, operate typically in a low-noise environment such as an expensive, dedicated fiber link. This is a severe bottleneck to wider adoption of QKD on a commercially viable scale. New methods that can integrate QKD into existing fiber infrastructures where data signals, which are millions of times brighter, are transmitted will represent breakthroughs. This paper demonstrates experimentally a paradigm-shifting method that allows fast and reliable quantum key distribution to go hand in hand with the transmission of bright optical data in conventional fibers over a record distance of 90 km.

A main difficulty for quantum key and optical data transmission sharing the same fiber arises from signal cross contamination, not unlike the difficulty of communicating over a long distance using only a faint torchlight under broad daylight. The intense data signals generate stray light which may spectrally overlap with the QKD signal and therefore cannot be filtered out by spectral filters. As the severity of contamination increases significantly with communication distance, past demonstrations of QKD were limited to a distance of 50 km, which is inadequate for extensive metropolitan coverage. Our approach to overcoming such signal contamination shifts from the spectral-filtering paradigm to a temporal-filtering one using fast-gated photo detectors. Noise photons enter the detectors at random times, while quantum-key signals arrive regularly. By gating the detectors only during the arrivals of quantum-key signals, noise photons arising from the data signal can be filtered out with a rejection efficiency nearing 90%. With this approach, we have achieved QKD in Gbit-per-second data fibers with lengths up to 90 km and with a transmission rate of secure keys that is more than 3 orders of magnitude higher than the current rates.

We expect that the temporal filtering will become a technique indispensable to integrating quantum-information technologies into today’s data communication networks.

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