Researchers Combine Two Quantum Keys across Fibre and Open Air

A key rate of approximately 1.43 megabits per second was achieved through the first simultaneous operation of Continuous-Variable and Discrete-Variable Quantum Key Distribution systems on a shared optical channel. The joint operation utilised wavelength-division multiplexing with colleagues at Sorbonne University and Paris Saclay University in France. Data transmission succeeded across both seventy kilometers of optical fibre and a six hundred and twenty metre daylight free-space link without detectable performance reduction.

Two distinct methods of secure communication, Continuous Variable Quantum Key Distribution and Discrete Variable Quantum Key Distribution, now combine onto a shared fibre optic cable without reducing performance. Wavelength-division multiplexing, a technique already common in conventional networks, enables short and long distance connections to share resources within quantum communications systems. Successfully combining these two distinct methods represents a sharp step towards practical quantum communication networks by transmitting data securely on the same infrastructure.

Continuous-Variable QKD encodes information within properties like brightness and phase, akin to modulating radio waves but using quantum mechanics for security; conversely, Discrete-Variable QKD uses individual particles of light, similar to Morse code where each flash represents a bit of data. This integration enabled wavelength-division multiplexing which splits the optical fibre into multiple ‘lanes’ using different colours of light, much like several roads running parallel to one another.

Simultaneous CV and DV QKD overcomes 7.56dB channel loss with sustained key

Channel loss previously limited simultaneous operation of Continuous-Variable (CV) and Discrete-Variable (DV) Quantum Key Distribution systems. A joint fibre optic and free-space transmission now delivers approximately 1.43 Mbit/s for both protocols, crossing over at 7.56 dB channel loss. This threshold represents an advance because prior attempts to combine CV-QKD, suited for short distances with high data rates, and DV-QKD, ideal for long reach but slower speeds, hampered differing noise sensitivities impacting performance.

Approximately 1.43 Mbit/s achieved for each system utilising both Continuous-Variable (CV) and Discrete-Variable (DV) Quantum Key Distribution protocols simultaneously over a shared fibre optic channel despite combined signal loss of 7.56 dB. The joint operation extended to a 620 metre free-space link in urban daylight conditions where key rates remained stable despite fluctuating atmospheric attenuation affecting the signals.

Analysis revealed no detrimental effects on either protocol’s performance due to multiplexing; specifically, there was no measurable increase in error rate or excess noise affecting security. Wavelength-division multiplexing using standard C-band filters at precise wavelengths of 1550.12nm for CV-QKD and 1545.32nm for DV-QKD enabled this combined transmission.

Concurrent Continuous-Variable and Discrete-Variable Quantum Key Distribution via Wavelength Allocation

Wavelength-division multiplexing proved central to enabling simultaneous operation by splitting a single optical fibre into multiple ‘lanes’ utilising different colours of light, much like several roads running parallel to one another. By assigning specific wavelengths, 1550.12nm for CV and 1545.32nm for DV, potential interference between the systems’ differing noise sensitivities was circumvented; these fall within the standard C-band used in optical communications. This technique allows both short-range high throughput links favoured by CV-QKD and long reach, high loss links suited to DV-QKD, to share a single infrastructure, potentially reducing deployment costs.

Hybrid Quantum Key Distribution advances towards practical long-distance deployment

Successfully merging continuous-variable (CV) and discrete-variable (DV) Quantum Key Distribution onto shared fibre offers a pathway toward more flexible quantum networks capable of serving diverse users with varying needs. Scaling these combined systems presents considerable challenges as real world deployments demand operation over significantly longer distances within complex urban environments; this initial demonstration relied on carefully controlled laboratory conditions and a relatively short 620 metre free-space link. This proof of principle establishes the potential for building flexible quantum networks simultaneously supporting both high-throughput local connections and long-distance backbone links using existing fibre infrastructure. These protocols encode information differently, with CV-QKD utilising properties like brightness while DV-QKD utilises individual light particles. Achieving simultaneous operation across standard fibres represents a step towards practical quantum networks. Operating through open air also represents a step toward practical quantum networks establishing hybrid CV-DV wavelength division multiplexing as a viable solution enabling metropolitan high-throughput connections alongside long-distance links within the same infrastructure; future work will focus on extending range and improving durability in challenging environments.

This research demonstrated the first successful joint operation of continuous-variable (CV) and discrete-variable (DV) Quantum Key Distribution systems using shared optical fibre and a 620 metre free-space link. The two protocols, operating at wavelengths of 1550.12nm and 1545.32nm, achieved key rates around 1.43 Mbit/s where their ranges overlapped at 7.56 dB channel loss. This coexistence allows for flexible quantum networks capable of supporting both short-range high throughput users and long-distance links on a single infrastructure; authors intend to extend range and improve durability in future work.