Quantum key distribution with an efficient countermeasure against correlated intensity fluctuations in optical pulses

Quantum key distribution (QKD) allows two distant parties to share secret keys with the proven security even in the presence of an eavesdropper with unbounded computational power. Recently, GHz-clock decoy QKD systems have been realized by employing ultrafast optical communication devices. However, security loopholes of high-speed systems have not been fully explored yet. Here we point out a security loophole at the transmitter of the GHz-clock QKD, which is a common problem in high-speed QKD systems using practical band-width limited devices. We experimentally observe the inter-pulse intensity correlation and modulation-pattern dependent intensity deviation in a practical high-speed QKD system. Such correlation violates the assumption of most security theories. We also provide its countermeasure which does not require significant changes of hardware and can generate keys secure over 100 km fiber transmission. Our countermeasure is simple, effective and applicable to wide range of high-speed QKD systems, and thus paves the way to realize ultrafast and security-certified commercial QKD systems

Quantum key distribution over multicore fiber

We present the first quantum key distribution (QKD) experiment over multicore fiber. With space division multiplexing, we demonstrate that weak QKD signals can coexist with classical data signals launched at full power in a 53 km 7-core fiber, while showing negligible degradation in performance. Based on a characterization of intercore crosstalk, we perform additional simulations highlighting that classical data bandwidths beyond 1Tb/s can be supported with high speed QKD on the same fiber.S.J.K. acknowledges support from the EPSRC CDT in Photonic Systems Development

Quantum networks in the UK

We describe recent progress in quantum secured optical networks in the UK. The Cambridge Quantum Network has been operating for several years with 3 nodes separated by between 5-10 km of installed fibre. All links are secured by QKD systems operating with secure key rates in excess of 1 Mb/s, the highest recorded long term key rates in a deployed network. The network operates in the presence of 100Gb/s classical traffic with no significant reduction of secure key generation rate. In addition, the Bristol Quantum Network has four nodes 1-3km apart connected in a mesh protected by two pairs of QKD systems. The network is designed to be very dynamic, switching both QKD and WDM classical traffic to enable rapid reconfiguration and is used as a testbed for QKD protected dynamic applications. The two metropolitan networks are being connected by a 410 km QKD link, with 4 spans, the longest of which operates over 129km of fibre with an attenuation of 28dB achieving secure key rates of 2.7kb/s, the longest and highest loss QKD field trial to date. A 120km extension of the UK quantum network from Cambridge to BT Labs, Adastral Park operates with fully commercially available components and is an important testbed comprising 3 intermediate nodes and operates with 5 x 100Gb/s classical channels. This helps determine how to proceed with a large-scale commercial deployment of QKD