- The paper demonstrates secure key distribution over 250 km with secret bit rates from 6 kbits/s to 15 bits/s and low QBER (0.85%-1.9%) using the COW protocol.
- It utilizes superconducting single photon detectors in a cryogen-free system to improve quantum efficiency and extend the QKD range.
- Ultra low loss Corning SMF-28 fibre reduces attenuation to 0.164 dB/km, minimizing channel loss and enabling practical long-distance quantum communications.
High Rate, Long-Distance Quantum Key Distribution Over 250 km of Ultra Low Loss Fibres
The paper presents a sophisticated prototype for Quantum Key Distribution (QKD), achieving notable bit rates over distances up to 250 km using ultra-low-loss (ULL) fibres. The research addresses the prevailing challenge in QKD—maintaining high bit rates over extended distances—through innovative protocol applications, advanced photon detection systems, and optimized optical fibres.
QKD Protocol and Prototype
Central to the paper is the implementation of the Coherent One Way (COW) protocol, a robust QKD protocol tailored for fibre-based quantum communication. This approach employs pulses with a mean photon number of 0.5, allowing for non-orthogonal states and enhancing security. The prototype leverages field-programmable gate arrays (FPGAs) and embedded systems to control communication between the transmitter (Alice) and receiver (Bob), conduct secret key distillation, and manage other critical system operations.
Superconducting Single Photon Detectors (SSPDs)
A notable technological advancement in the prototype is the integration of superconducting single photon detectors (SSPDs). These detectors, which operate at minimal dark count noise levels and high quantum efficiency, significantly extend the potential range of QKD. Their implementation in a cryogen-free, closed-cycle cryostat system represents a step toward practical deployment. The detectors' performance is optimized by adjusting the bias current and input polarization, accommodating environmental variations that might affect detection efficiency.
Ultra Low Loss Fibres
The research employs Corning SMF-28 ULL fibre, which boasts a superior low attenuation rate of 0.164 dB/km at 1550 nm. This choice facilitates the extension of QKD over greater distances by minimizing channel loss, a critical factor in long-distance quantum communication. Achieving an overall transmission loss of 42.6 dB for a 250-km link, the paper demonstrates the practicality of these fibres in both terrestrial and potentially submarine applications.
Experimental Results
The experimental phase yielded secret bit rates from 6 kbits/s at 100 km to 15 bits/s at 250 km, with Quantum Bit Error Rates (QBER) ranging from 0.85% to 1.9%. The privacy of the key exchange is maintained through precise monitoring of Eve's potential information and adjustments facilitated by high-quality interference visibility. The prototype's ability to sustainably generate secret keys over significant timelines suggests stability and reliability for practical applications.
Conclusions and Implications
This investigation illustrates the convergence of theoretical and applied physics, telecommunications, and advanced materials science to push the boundaries of QKD capabilities. By achieving secure key distribution over 250 km with a significant secret bit rate, the research moves close to realizing quantum key exchanges over inter-city distances in a secure, high-efficiency manner.
The implications for secure communication networking are profound, opening pathways for future developments in quantum cryptography and potentially driving down the cost of large-scale implementations. As fibre technology and detector efficiencies continue to evolve, the prospect of securing communication over even longer distances with meaningful secret bit rates becomes increasingly feasible.
The paper provides a definitive step forward in addressing the challenges associated with long-distance QKD, laying the groundwork for future exploration and development in the intersection of quantum physics and communication technologies.