Study Achieves A New Record Fiber QKD Transmission Distance of Over 509 km

According to a new study published in Physical Review Letters, researchers have achieved a new record quantum key distribution (QKD) transmission distance of over 509 kilometers. Phys.Org reports: "Using the sending-or-not-sending twin-field (SNS-TF) protocol, we realized secure quantum key distribution (QKD) over 509 km, which is a new record secure distance in QKD over fiber," Qiang Zhang, one of the researchers who carried out the study, told Phys.org. "One of the important objectives of our study was to successfully break the absolute key-rate limit of repeater-less QKD, with any measurement device." The SNS-TF protocol was developed and introduced in a previous research effort by some of the researchers who wrote the recent paper. In previous studies, the protocol proved to be highly advantageous for QKD applications, particularly for achieving long distance transmission. In their recent work, Zhang and his colleagues implemented the SNS-TF QKD protocol by eliciting a single-photon level, first-order interference between two independent lasers. These two independent lasers were combined with a remote frequency locking technique, ultimately enabling QKD transmission over unparalleled distances. In their experiments, the researchers also used superconducting single-photon detectors with high count rate and detection efficiencies. Using their SNS-TF approach, the researchers achieved a secure key rate at 509 km, over seven times higher than the relative repeaterless bound QKD, and with the same detection loss. Remarkably, the key rate they achieved is also higher than that achieved by more traditional QKD protocols running on a perfect repeaterless QKD device.

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