Scientists from multiple Chinese institutions have achieved a new world record by successfully reaching a quantum distribution of keys without relay over a distance of 1002km. The team accomplished this feat through the use of the quantum distribution of keys with a double field and sending-not-sending protocol, a fiber cable with ultra-low losses, and superconducting single-photon detectors with an ultra-low noise level. This development is a significant step forward towards high-speed inter-city quantum communication networks.
The quantum distribution of keys (QKD) allows for the safe transmission of keys between two remote sides using the principles of quantum mechanics. When used with the “disposable notebook” encryption method, QKD provides a high level of security for confidential communication. However, QKD’s applicability is limited by channel loss and system noise.
The team employed the Sending-Not-Sending (SNS) protocol with the TF-QKD scheme to improve the speed dependence of the key on the channel’s bandwidth from a linear η to a square root η. This change enables greater security than traditional QKD protocols.
In order to achieve long-distance QKD, the research group collaborated with Yangtze Optical Fibre and Cable Joint Stock Limited Company (YOFC) to use a fiber cable with ultra-low losses based on pure silica core technology, ensuring minimal attenuation at 0.16 dB/km. The team also developed superconducting single-photon detectors with super-hammered noise, reducing noise to about 0.02 counts per second (CPS) by using filters at temperatures of 40k and 2.2k to suppress false signals.
To further reduce noise caused by spontaneous combination scattering (Raman effect), the team developed a two-band phase assessment scheme, lowering systemic noise to less than 0.01 Hz. These technological developments culminated in the team successfully reaching a TF-QKD at a record distance of 1002 km, at a key speed of 0.0034 bits/s.
This achievement confirms the implementation of the SNS-TF-QKD scheme at extremely long distances and demonstrates its ability to deliver high keys in practical scenarios, providing a promising solution for promoting safe quantum communication. The results of the study were published in the journal Physical Review Letters.