Researchers Achieve 0.93 Mbit/sec Key Rate Via Squeezed-State QKD

Researchers at University of Denmark alongside collaborators in Italy, Austria and Czech Republic have achieved continuous-variable quantum key distribution (CV-QKD) using both coherent and squeezed states over a deployed hybrid channel consisting of 620 metres of free-space link combined with two kilometres of optical fibre.

The team transmitted secure information utilising continuous-variable quantum key distribution through both optical fibres and open air simultaneously; CV-QKD typically focuses on employing either one medium or the other. The demonstration combined 620 metres of free space with two kilometres of optical fibre, achieving up to 20 decibels of signal loss. By shifting complex adjustments into software processing, the researchers simplified how future networks could be deployed. Dnan A. E. CV-QKD is a method of sending encryption keys by modulating properties of light waves instead of relying on simple on/off signals. This demonstration successfully transmitted secure information over a hybrid channel consisting of 620 metres of open air linked with two kilometres of optical fibre, experiencing up to 20 decibels of signal loss.

The team simplified future network deployment by shifting complex adjustments into software processing rather than hardware modifications. This approach is akin to focusing all attention onto hearing quiet sounds while slightly ignoring background noise, enhancing sensitivity in one area at the expense of another. Concatenated fibre and free-space quantum communication demonstrates record key rates Squeezed-state continuous-variable quantum key distribution (CV-QKD) attained an asymptotic secret-key rate of 0.93 Mbit per second; this represents an improvement over the previously recorded 0.42 Mbit/s using coherent states under identical conditions and marks successful operation across concatenated media for the first time. CV-QKD systems were largely confined to either optical fibre or free space transmission environments until now, but combining these channels presented significant challenges in maintaining both security and performance due to differing characteristics. Adaptive post-processing techniques implemented by researchers at Technical University of Denmark shift channel adaptation from complex optics adjustments into software data analysis, effectively unifying protocols applicable to both fibre and atmospheric links. An improved key rate was achieved by scientists at Technical University of Denmark utilising both coherent and squeezed states. The coherent state protocol yielded an asymptotic secret-key rate of 0.42 Mbit per second while employing adaptive post-processing techniques that account for varying conditions between fibre optic cable and free space transmission. Covariance-matrix averaging, a method reducing errors caused by signal fluctuations within the atmosphere, further enhanced performance, recovering up to 19% more potential key material alone. The deployed system combined a 620-metre free-space link with two kilometres of optical fibre, experiencing total losses reaching 20 dB; however, these rates were obtained under ideal laboratory settings and do not yet reflect practical considerations such as atmospheric turbulence or component imperfections encountered in real-world deployments. Local oscillator limitations and future network interoperability Integrating optical fibre with the flexibility of free-space transmission is essential for establishing secure communication across varied terrains; nevertheless, researchers acknowledge their current system relies on locally generated local oscillators for encryption, a deliberate limitation within this study. This approach potentially excludes other viable configurations utilising different oscillator sources which may offer advantages in larger or more complex networks. Achieving truly universal interoperability between diverse quantum devices and infrastructures will likely require broader compatibility to facilitate seamless integration. A key step towards building flexible networks was achieved through successful integration of fibre optic cables with free-space transmission. Data processing can adapt to varying channel conditions rather than requiring custom optical setups for each medium as demonstrated by the team. Previously focused largely on either optical fibre or free space individually, continuous-variable quantum key distribution now establishes a pathway toward practical quantum networks uniting disparate communication infrastructures. Shifting adaptation from physical optics into software processing simplified network deployment, enabling secure communication across both two kilometres of fibre optic cable and six hundred and twenty metres of open air simultaneously. The researchers successfully combined continuous-variable quantum key distribution over both 620m of free-space and 2km of optical fibre with up to 20 dB loss. This demonstrates that data processing can be adapted to account for differing transmission media rather than requiring bespoke optical configurations.

The team achieved secret-key rates of 0.42 Mbit per sec using a coherent-state protocol and 0.93 Mbit per sec utilising squeezed states through unified adaptive post-processing. They note future work may focus on broader compatibility between diverse quantum devices to facilitate network interoperability. 👉 More information🗞 Squeezed- and coherent-state quantum key distribution over a deployed hybrid fibre-free-space channel✍️ Dnan A. E. Hajomer, Huy Q. Nguyen, Ivan Derkach, Andreas B. Kidmose, Edoardo Rossi, Mattia Sabatini, Yoann Pietri, Marco Avesani, Francesco Vedovato, Michael Hentschel, Radim Filip, Giuseppe Vallone, Vladyslav Usenko, Tobias Gehring, Soren Forchhammer, Paolo Villoresi and Ulrik L. Andersen🧠 ArXiv: https://arxiv.org/abs/2608.20088 Stay currentSee today’s quantum computing news on Quantum Zeitgeist for the latest breakthroughs in qubits, hardware, algorithms, and industry deals. Tags: