Record-breaking quantum key distribution transmission distance achieved alongside classical channels

Quantum key distribution (QKD) harnesses the power of quantum mechanics to securely transmit confidential information. When an outside source eavesdrops on a QKD transmission, the quantum states are affected. This dependably alerts the receiver and sender that the transmission is no longer secure.

Unfortunately, there have thus far been limitations in implementing QKD technology. Telecom networks require QKD and classical data to share fiber infrastructure to reduce costs enough to be feasible on a large scale and classical data channels introduce noise that limits the distance and performance of QKD transmissions. Many solutions have been proposed and tested, such as extra filtering or dedicated wavelengths, but these still complicate integration into existing telecom networks.

Now, researchers from Denmark and the Czech Republic may have a better solution that, when tested, broke the record for the longest transmission achieved with QKD and classical data.

Their work, published in Physical Review Letters, focuses on continuous-variable (CV) QKD, which uses coherent states, instead of the single-photon sources used in discrete-variable (DV) QKD.

Using a local-oscillator (LO) CV-QKD system with Gaussian-modulated coherent states and ultra-low-loss fiber, the team achieved record-long CV-QKD transmissions of 120 km in the asymptotic regime and 100 km in the finite-size regime, coexisting with fully populated classical channels.

“Secure keys are generated in the finite-size regime at 100 km, while asymptotic keys are achieved at 120 km. This record transmission distance is enabled by the intrinsic mode-filtering properties of the LO and the optimization of modulation variance to suppress phase noise-induced excess noise,” the study authors explain.

The team compared noise levels with and without classical channels and found no significant increase in excess noise or drop in secure key rate due to classical channels. They then compared their setup to a commercial DV-QKD system, and found that CV-QKD outperformed the DV-QKD system, which failed in similar noise environments.

The study authors write, “Our results highlight that CV-QKD provides a plug-and-play solution for long-haul optical links without requiring additional filtering techniques or specific wavelength allocation.”

This stands out over past solutions that required additional filtering and wavelength dedication techniques and shows that quantum security can be added to current networks with minimal disruption. Furthermore, the team says their system can be improved upon by increasing symbol rates and improving error correction to enable even longer distances and higher key rates, and digital methods for polarization and clock management can further simplify integration in the future.

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