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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Rydberg Atomic Microwave-to-Optical Converter in a Free-Space Continuous-Wave Operation Mode

Sebastian Borówka, Uliana Pylypenko, Mateusz Mazelanik, Michal P. Parniak

Published Jun 26, 2023• 0 citations
Publisher

Abstract

The task of bridging energetically distant domains of microwave (MW) and optical photons presents an outstanding challenge to modern photonics and quantum communication science. From enabling hybrid quantum networks [1] to giving the basis for next-generation sensors in fundamental and observational astronomy [2], to allowing for coherent GHz and THz imaging, a plethora of applications emerges from photonic conversion.

DOI
10.1109/cleo/europe-eqec57999.2023.10231908

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The task of bridging energetically distant domains of microwave (MW) and optical photons presents an outstanding challenge to modern photonics and quantum communication science. From enabling hybrid quantum networks [1] to giving the basis for next-generation sensors in fundamental and observational astronomy [2], to allowing for coherent GHz and THz imaging, a plethora of applications emerges from photonic conversion.
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Quantum key distribution (QKD) promises information-theoretically secure communication, but future networks must bridge fiber and free-space links that naturally employ different photonic encodings, namely time-bin in fiber and polarization in free space. Here we demonstrate a complete hybrid fiber and free-space QKD link that bridges both media within a single end-to-end protocol, converting between the two encodings entirely in the optical domain. Using the decoy-state BB84 protocol operating at 1550 nm, we demonstrate continuous secure-key generation over a 90 m outdoor free-space link. The system operates across atmospheric conditions spanning more than two orders of magnitude in the refractive-index structure parameter Cn^2, from strong daytime turbulence to quiescent nighttime conditions, and we further validate photon-level operation over a 750 m free-space extension. Throughout, the link maintains a session-mean quantum bit error rate (QBER) of 5.6-6.8%, well below the 11% BB84 security threshold. The encoding conversion is performed entirely in the optical domain without measurement or state reconstruction, preserving the security assumptions of the BB84 protocol. Consequently, the time-bin-to-polarization (T2P) and polarization-to-time-bin (P2T) converters remain part of the untrusted quantum channel rather than trusted intermediate nodes. These results establish secure photonic encoding conversion as a practical interface between fiber and free-space quantum communication platforms, providing a building block for future quantum networks applications.