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.
For a future 5G Ethernet-based fronthaul architecture, 100G trunk lines of a transmission distance up to 10 km standard single mode fiber (SSMF) in combination with cheap grey optics to daisy chain cell site network interfaces are a promising cost-and power-efficient solution. For such a scenario, different intensity modulation and direct detect (IMDD) formats at a data rate of 112 Gb/s, namely Nyquist four-level pulse amplitude modulation (PAM4), discrete multi-tone transmission (DMT) and partial-response (PR) PAM4 are experimentally investigated, using a low-cost electro-absorption modulated laser (EML), a 25G driver and current state-of-the-art high speed 84 GS/s CMOS digital-to-analog converter (DAC) and analogto-digital converter (ADC) test chips. Each modulation format is optimized independently for the desired scenario and their digital signal processing (DSP) requirements are investigated. The performance of Nyquist PAM4 and PR PAM4 depend very much on the efficiency of pre-and post-equalization. We show the necessity for at least 11 FFE-taps for pre-emphasis and up to 41 FFE coefficients at the receiver side. In addition, PR PAM4 requires an MLSE with four states to decode the signal back to a PAM4 signal. On the contrary, bit-and power-loading (BL, PL) is crucial for DMT and an FFT length of at least 512 is necessary. With optimized parameters, all modulation formats result in a very similar performances, demonstrating a transmission distance of up to 10 km over SSMF with bit error rates (BERs) below a FEC threshold of 4.4E-3, allowing error free transmission.
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