The global-scale quantum communication network of the future will use optical fibers and free-space channels for different link distances. Free-space segments must support daytime operation, compatibility with fiber infrastructure, and compact devices for space-to-ground links. Current free-space QKD is limited compared to fiber-based systems. This paper addresses key requirements for free-space QKD: (i) full-day functionality, (ii) compatibility with telecom wavelengths (around 1550 nm), and (iii) stable coupling to single-mode fibers (SMFs). Daytime operation is challenging due to sunlight noise; previous work often used wavelengths in the 700-900 nm range. Using the telecom C-band (around 1550 nm) offers advantages: compatibility with existing fiber-based systems and silicon photonics, enabling compact, low-power devices suitable for portable transmitters and satellite payloads. Stable SMF coupling requires compensating for atmospheric turbulence, typically done via active correction of aberrations. This paper introduces the QCoSOne system, which performs free-space daylight QKD at 1550 nm using an integrated silicon-photonics chip for state encoding. The chip implements a 3-state 1-decoy QKD protocol with decoy- and polarization-modulation. The system utilizes commercially available components like wavelength filters and SNSPDs. The system achieved stable SMF coupling over a 145m free-space link using an active correction for first-order aberrations. QKD was successfully performed during daylight (11:00 to 20:00 CEST), resulting in a QBER around 0.5% and a secret key rate up to 65 kbps (after finite-key analysis). This is the lowest QBER reported for free-space QKD and the longest daylight demonstration at 1550 nm using a chip-based encoder in a field trial, representing progress toward a seamless satellite-fiber quantum network at telecom wavelengths.

Quantum Key Distribution (QKD) is a crucial application of quantum information science, with continuous improvements in protocols and experimental implementations. The goal of QKD is secure communication between any two points on Earth, achieved through fiber-based or free-space quantum communication (QC) depending on distance. While satellite-to-ground links have been demonstrated, free-space QKD technology lags behind fiber-based counterparts. For a continental-scale quantum network, free-space QC requires full-day functionality, compatibility with telecom wavelengths (to integrate with fiber infrastructure), and stable single-mode fiber coupling. Daylight operation is a major challenge due to background noise from sunlight. Studies have explored daylight QKD, often using wavelengths in the 700–900 nm band, but the telecom C-band (around 1550 nm) offers advantages of compatibility with existing fiber technology and with integrated silicon photonics. Silicon photonics is promising for creating lightweight, compact, scalable, and low-power devices for portable QKD transmitters and satellite payloads. Atmospheric turbulence introduces aberrations, necessitating active compensation to ensure stable coupling to single-mode fibers.

The QCoSOne system was developed to meet the requirements outlined above. The system utilizes a silicon-based photonic integrated circuit (PIC) as a compact state encoder, implementing a 3-state 1-decoy QKD protocol. This chip integrates intensity and polarization modulation, enabling the generation of three states required by the protocol. The chip, approximately 5 mm x 5 mm, is packaged into a 1.2 cm x 1.5 cm x 1.2 cm unit. A detailed description of the PIC components and fabrication process is provided. A gain-switched distributed feedback (DFB) laser source at 1550 nm provides a 50 MHz stream of phase-randomized pulses. The pulses are coupled into and out of the PIC via an 8-channel SMF array. The PIC utilizes interferometric structures with multi-mode interference (MMI) devices, thermo-optic modulators (TOMs), and carrier-depletion modulators (CDMs) for amplitude and polarization modulation. An extinction ratio of 30 dB was achieved. The output pulses are spectrally filtered, attenuated, and split; one portion monitors pulse intensity, while the other is directed to a transmitting telescope. The experimental setup includes a closed-loop feedback control system using a fast-steering mirror (FSM) to compensate for lower-order aberrations (tip-tilt) caused by atmospheric turbulence. The system uses a 1064 nm beacon laser and a position-sensitive detector (PSD) to track and correct beam wander. The average SMF coupling efficiency was approximately 8.5%. Bob’s receiver comprises a Cassegrain reflector telescope, a 40m SMF connecting to the state analyzer, dense WDM filter for spectral selection, a fiber beam splitter, automatic polarization controllers (APCs), polarizing beam splitters (PBSs), and four superconducting nanowire single-photon detectors (SNSPDs). A synchronization algorithm using GPS modules at both ends coordinates the transmitter and receiver. Data acquisition employs a time-to-digital converter (TDC) with 81 ps resolution. A post-processing algorithm compensates for GPS clock drift. The QKD runs were conducted during daylight hours with clear sky conditions. Measurements of QBER, signal-to-noise ratio (SNR), and detection rate were taken, and the impact of atmospheric turbulence was analyzed using a model that takes into account the variances of Zernike modes.

The QCoSOne system successfully performed daylight QKD over a 145-meter free-space link for eight consecutive hours on April 18th, 2019. The total detection rate (TDR) averaged around 100 kHz, the signal-to-noise ratio (SNR) was about 400, and the QBER was consistently below 0.75%. The QBER was remarkably low (around 0.5%), among the lowest ever reported for free-space QKD, and comparable to fiber-based systems. The secret key rate (SKR) achieved was up to 65 kbps after finite-key analysis, with an average of 33 kbps during the eight-hour experiment. This experiment demonstrated that the system could generate a secret key even with 17 dB of additional losses, which translates to a potential link distance of approximately 70 km based on beam diffraction. Further QKD runs over three consecutive days showed consistent performance with sifted bit rates from 50 to 150 kbps and SKR from 20 to 70 kbps. The results, obtained under diverse daylight conditions (including the sun at its maximum elevation), demonstrate the resilience and robustness of the QCoSOne system. Comparison with previous QKD experiments at 1550 nm in daylight shows that the QCoSOne system outperforms earlier implementations up to 23 dB of fixed attenuation.

The results demonstrate the feasibility of chip-based free-space QKD in daylight at telecom wavelengths. The low QBER and high SKR achieved, even under challenging daylight conditions, underscore the effectiveness of the integrated silicon-photonics approach, the active aberration correction system, and the efficient QKD protocol. The compact and portable nature of the system is a significant advancement for developing portable QKD terminals and satellite payloads. This work addresses several limitations of previous free-space QKD systems, particularly in terms of operating wavelength and environmental robustness. The use of the 1550nm wavelength improves compatibility with existing fiber-optic infrastructure, potentially enabling the development of hybrid free-space/fiber quantum networks. The demonstration of daylight QKD with a low QBER and a high secret key rate paves the way for practical, real-world applications of QKD, potentially enabling the development of large-scale quantum networks.

This paper demonstrated a chip-based prototype for free-space quantum key distribution (QKD) operating at 1550 nm in daylight conditions. The system achieved record-low QBER and a secret key rate of tens of kbps, even with the sun at its zenith. The integrated intensity and polarization modulation on a single chip, combined with effective background noise mitigation strategies, highlight the maturity of daylight QKD technology. Future improvements could include increasing the system clock rate and employing adaptive optics for even longer link distances. This research paves the way for practical applications in global-scale quantum communication networks.

The current experimental setup is limited to a relatively short 145m free-space link. While simulations suggest the system could operate over longer distances, further experimental validation is needed. The atmospheric turbulence correction is limited to first-order aberrations (tip-tilt); higher-order aberrations were not actively compensated. The impact of adverse weather conditions, beyond the modeled rain and snow scenarios, requires additional investigation. The system's security relies on the assumptions of the 3-state 1-decoy QKD protocol and the performance of the employed components.