Photonic integrated circuits (PICs) are considered as the way to make photonic systems or subsystems cheap and ubiquitous. PICs still are several orders of magnitude more expensive than their microelectronic counterparts, which has restricted their application to a few niche markets.

Future-proofing current fibre networks with quantum key distribution (QKD) is an attractive approach to combat the ever growing breaches of data theft. To succeed, this approach must offer broadband transport of quantum keys, efficient quantum key delivery and seamless user interaction, all within the existing fibre network. However, quantum networks to date either require dark fibres and/or offer bit rates inadequate for serving a large number of users. Here we report a city wide high-speed metropolitan QKD network-the Cambridge quantum network-operating on fibres already populated with high-bandwidth data traffic. We implement a robust key delivery layer to demonstrate essential network operation, as well as enabling encryption of 100 Gigabit per second (Gbps) simultaneous data traffic with rapidly refreshed quantum keys. Network resilience against link disruption is supported by high-QKD link rates and network link redundancy. We reveal that such a metropolitan network can support tens of thousands of users with key rates in excess of 1 kilobit per second (kbps) per user. Our result hence demonstrates a clear path for implementing quantum security in metropolitan fibre networks.npj Quantum Information (2019) 5:101 ; https://doi.

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Directly modulated multimode-fiber (MMF) based radio-over-fiber links are demonstrated which support RF carrier frequencies up to 20GHz. This is believed to be the highest frequency yet reported for RF signal transmission using a directly modulated laser. The DFB laser diode introduces less than 1% excess EVM up to 16GHz, while both high-and low-bandwidth MMF links introduce very little penalty at carrier frequencies in excess of 20GHz for distances of 1000m and 575m, respectively.Index Terms -Error vector magnitude, microwave communication, optical fiber communication, radio-overfiber (RoF), semiconductor lasers.

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