Intermodal quantum key distribution over an 18 km free-space channel with adaptive optics and room-temperature detectors

Abstract

Intermodal quantum key distribution at telecom wavelengths provides a hybrid interface between fiber connections and free-space links, both essential for the realization of scalable and interoperable quantum networks. Although demonstrated over short-range free-space links, long-distance implementations of intermodal quantum key distribution remain challenging, due to turbulence-induced wavefront aberrations which limit efficient single-mode fiber coupling at the optical receiver. Here, we demonstrate a real-time intermodal quantum key distribution field trial over an 18 km free-space link, connecting a remote terminal to an urban optical ground station equipped with a 40 cm-class telescope. An adaptive optics system, implementing direct wavefront sensing and high-order aberration correction, enables efficient single-mode fiber coupling and allows secure key generation of 200 bit/s using a compact state analyzer equipped with room-temperature detectors. We further validate through experimental data a turbulence-based model for predicting fiber coupling efficiency, providing practical design guidelines for future intermodal quantum networks.

Figures (5)

• Figure 1: Testbed illustration.a Aerial view of the free-space channel of 18 km. Data from Google Earth [©2025 Google]. b Image of the transmitter node located on top of Monte Grande in the Colli Euganei area, including a photo of the enclosure and the sketches of the optical setup. c Image of the receiver node showing the telescope's dome on the DEI rooftop, the 3D illustration of the $0.41$ m telescope equipped with the breadboard, the optical scheme of the AO bench, highlighting the primary focus and pupil planes of reference (blue and yellow stars, respectively), and the photo of the Luxor laboratory where the QKD receiver is placed. VOA: variable optical attenuator; DWDM: dense wavelength-division multiplexer; WDM: wavelength-division multiplexer; 90:10: 90:10 fiber beam-splitter; PM: power meter; CAM: camera; CL: calibration laser; FLM: flip-mirror; DFM: deformable mirror; WFS: wavefront sensor; BS: beam-splitter; SMF: single-mode fiber.
• Figure 2: QKD results and atmospheric conditions during experiments.a Temperature, b wind velocity and c Fried parameter measured on two consecutive days during QKD experiments with SPADs and SNSPDs. d Signal and noise dection rate, e QBER and f secret key rate generated during the experiments.
• Figure 3: Results of the adaptive optics system.a Aberration variances of Zernike modes with AO-OFF/ON. In the absence of AO correction, the aberrations induced by turbulence follow the Kolmogorv spectrum as described by Eq. \ref{['eq:eq_fit']}. b Comparison between the coupling efficiency estimated from wavefront sensor data and the corresponding values obtained from direct power measurements.
• Figure 4: Channel parameters estimation due to turbulence:a Expected waist for the received beam after 18 km of free-space propagation. b Expected collection efficiency for our telescope of $D_{\rm Rx}$ aperture with an obstruction of $D_{\rm Obs}$.
• Figure 5: Simulations of the different efficiency terms contributing to $\eta_{\rm SMF}$. Common parameters used in the simulations are the link distance $L=18$ km, the receiver telescope diameter $D_{\rm Rx} = 410$ mm, and the wavelength $\lambda = 1555$ nm. We vary the Fried parameter $r_0$ in the range 3 to 15 cm, as expected in the real experiment, corresponding to a $D_{\rm Rx}/r_0$ ratio between 3 and 14. a Efficiency $\eta_S$ due to scintillation as a function of the Fried parameter $r_0$. b Efficiency $\eta_{\varphi(J)}$ due to residual spatial phase variance after having perfectly corrected for $J$ modes --- Eq. \ref{['eta:spatial:J']} --- as a function of the Fried parameter $r_0$. Case $J=2$ implies just tip-tilt correction, $J=35$ is our design baseline, and $J=100$ is shown for comparison. c Efficiency $\eta_\tau$ as a function of the Fried parameter $r_0$ by assuming different wind conditions.