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Optica Quantum

Time-bin encoding quantum key distribution in free-space horizontal links during nighttime and daytime

Sebastiano Cocchi, Domenico Ribezzo, Giulia Guarda, Pietro Centorrino, Tommaso Occhipinti, Alessandro Zavatta, Davide Bacco

Published May 23, 2025• 10 citations
Publisher

Abstract

Free-space quantum key distribution (QKD) represents a groundbreaking advancement in secure communication, enabling secure key exchange over vast distances and offering robust encryption for the future quantum internet. However, the compatibility between fiber and free-space infrastructures continues to pose challenges for QKD protocols. Indeed, free-space and fiber-based networks commonly use different wavelengths and qubits encoding schemes. On the one hand, free-space QKD typically exploits visible light for its beneficial beam divergence compared with longer wavelengths, and polarization encoding for its robustness against turbulence. On the other hand, fiber-based QKD employs infrared light, particularly the C-band, because it shows the minimum losses with silica fibers, and time-bin encoding, due to polarization instability in optical fibers. In our study, we demonstrate the viability of a time-bin encoded QKD protocol operating in the C-band through horizontal turbulent free-space channels. We test the setup into a 50-m- and a 500-m-long link, respectively achieving an average secure key rate of 793 kbps and 40 kbps over several hours of measurements. The results encourage further exploration of the interoperability between free-space and fiber-based infrastructures, opening new possibilities for connecting terminal users with satellites in hybrid infrastructures.

DOI
10.1364/opticaq.553977

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Free-space quantum key distribution (QKD) represents a groundbreaking advancement in secure communication, enabling secure key exchange over vast distances and offering robust encryption for the future quantum internet. However, the compatibility between fiber and free-space infrastructures continues to pose challenges for QKD protocols. Indeed, free-space and fiber-based networks commonly use different wavelengths and qubits encoding schemes. On the one hand, free-space QKD typically exploits visible light for its beneficial beam divergence compared to longer wavelengths, and polarization encoding for its robustness against turbulence. On the other hand, fiber-based QKD employs infrared light, particularly the C-band, because it shows the minimum losses with silica fibers, and time-bin encoding, due to polarization instability in optical fibers. In our study, we demonstrate the viability of a time-bin encoded QKD protocol operating in the C-band through horizontal turbulent free-space channels. We test the setup into a 50 m and a 500 m long links, achieving an average secure key rate of, respectively, 793 kbps and 40 kbps over several hours of measurements. The results encourage further exploration of the interoperability between free-space and fiber-based infrastructures, opening new possibilities for connecting terminal users with satellites in hybrid infrastructures.

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Quantum key distribution (QKD) with deterministic single photon sources has been demonstrated over intercity fiber and free-space channels. The previous implementations relied mainly on polarization encoding schemes, which are susceptible to birefringence, polarization-mode dispersion and polarization-dependent loss in practical fiber networks. In contrast, time-bin encoding offers inherent robustness and has been widely adopted in mature QKD systems using weak coherent laser pulses. However, its feasibility in conjunction with a deterministic single-photon source has not yet been experimentally demonstrated. In this work, we construct a time-bin encoded QKD system employing a high-brightness quantum dot (QD) single-photon source operating at telecom wavelength. Our proof-of-concept experiment successfully demonstrates the possibility of secure key distribution over fiber link of 120 km, while maintaining extraordinary long-term stability over 6 h of continuous operation, that is highest secure key rate among the time-bin QKDs based on single-photon sources. This work provides the first experimental validation of integrating a QD single-photon source with time-bin encoding in a telecom-band QKD system. This development signifies a substantial advancement in the establishment of a robust and scalable QKD network based on solid-state single-photon technology.

•Jan 1, 2026•0 citations
Free-space quantum key distribution (QKD) represents a groundbreaking advancement in secure communication, enabling secure key exchange over vast distances and offering robust encryption for the future quantum internet. However, the compatibility between fiber and free-space infrastructures continues to pose challenges for QKD protocols. Indeed, free-space and fiber-based networks commonly use different wavelengths and qubits encoding schemes. On the one hand, free-space QKD typically exploits visible light for its beneficial beam divergence compared with longer wavelengths, and polarization encoding for its robustness against turbulence. On the other hand, fiber-based QKD employs infrared light, particularly the C-band, because it shows the minimum losses with silica fibers, and time-bin encoding, due to polarization instability in optical fibers. In our study, we demonstrate the viability of a time-bin encoded QKD protocol operating in the C-band through horizontal turbulent free-space channels. We test the setup into a 50-m- and a 500-m-long link, respectively achieving an average secure key rate of 793 kbps and 40 kbps over several hours of measurements. The results encourage further exploration of the interoperability between free-space and fiber-based infrastructures, opening new possibilities for connecting terminal users with satellites in hybrid infrastructures.
Free-space quantum key distribution (QKD) represents a groundbreaking advancement in secure communication, enabling secure key exchange over vast distances and offering robust encryption for the future quantum internet. However, the compatibility between fiber and free-space infrastructures continues to pose challenges for QKD protocols. Indeed, free-space and fiber-based networks commonly use different wavelengths and qubits encoding schemes. On the one hand, free-space QKD typically exploits visible light for its beneficial beam divergence compared to longer wavelengths, and polarization encoding for its robustness against turbulence. On the other hand, fiber-based QKD employs infrared light, particularly the C-band, because it shows the minimum losses with silica fibers, and time-bin encoding, due to polarization instability in optical fibers. In our study, we demonstrate the viability of a time-bin encoded QKD protocol operating in the C-band through horizontal turbulent free-space channels. We test the setup into a 50 m and a 500 m long links, achieving an average secure key rate of, respectively, 793 kbps and 40 kbps over several hours of measurements. The results encourage further exploration of the interoperability between free-space and fiber-based infrastructures, opening new possibilities for connecting terminal users with satellites in hybrid infrastructures.
Satellite-based quantum key distribution (QKD) holds the potential to establish global quantum communication networks. While satellite QKD has been demonstrated using polarization encoding, time-bin encoding offers distinct advantages, such as simplifying polarization-maintaining telescope designs and being insensitive to satellite-ground relative motion. In this work, we developed a 625-MHz QKD light source using a robust Sagnac-interferometer-based modulation scheme and subsequently demonstrated time-bin QKD over a 7 km urban terrestrial free-space channel. This experiment successfully operated over a channel traversing an equivalent atmospheric thickness and loss exceeding that of typical satellite-to-ground links, achieving a low quantum bit error rate of 0.87% and a secure key rate of 134 bps@51.4 dB. Furthermore, by using a half-wave plate to simulate the polarization-basis-rotation inherent in such links, we verified the robustness of time-bin encoding for satellite scenarios. The results validate time-bin encoding as a compelling alternative and lay the technical foundation for future satellite QKD applications.
Quantum key distribution (QKD), in which security is ensured by physical quantum laws, is shown as the most secure communication between two distant users [1]–[3]. Recent efforts in global research focus on providing the infrastructure that can support QKD and extending applications to large scale of users and distances. Three types of links are explored up to date: terrestrial free-space links (limited by visible distance), fiber-based links (repeater-free range limited to a few 100 km [1]) and finally satellite links, which allow to bridge intercontinental distances. For each of these scenarios, different types of hardware are involved and state encoding (polarization-, time- [1]–[2], spatial- and frequency-based [3]) is tailored to mitigate link impairments. Additionally, the effort towards higher QKD rates is given [1]–[3] that future global quantum communication networks support a wider range of use cases.
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.
Quantum key distribution is a quantum communication protocol which seeks to address potential vulnerabilities in data transmission and storage. One of the main challenges in the field is achieving high rates of secret key in lossy and turbulent free-space channels. In this scenario, most experimental demonstrations have used the polarization of photons as their qubit carriers, due to the relative robustness of polarization in free-space propagation. Time-bin or phase-based protocols are considered less practical due to the wave-front distortion caused by atmospheric turbulence. However, demonstrations of novel free-space interferometer designs are enabling interferometers to measure multimodal signals with high visibility. That means it is now viable to consider the prospects of implementing time-bin or phase-based protocols, which have demonstrated high key rates and long transmission distances in optical fiber. In this work, we present the possibilities of implementing time-bin protocols in turbulent free-space channels, using the coherent one-way protocol as the example. We present an analysis of the secret key rate and quantum bit error rate of the system, providing the errors due to noise counts, and the extinction ratio of the pulses. Finally, we developed a model to quantify the expected losses for a turbulence free-space channel, specifically for a free-space satellite-to-ground station channel.
Quantum key distribution (QKD) with deterministic single photon sources has been demonstrated over intercity fiber and free-space channels. The previous implementations relied mainly on polarization encoding schemes, which are susceptible to birefringence, polarization-mode dispersion and polarization-dependent loss in practical fiber networks. In contrast, time-bin encoding offers inherent robustness and has been widely adopted in mature QKD systems using weak coherent laser pulses. However, its feasibility in conjunction with a deterministic single-photon source has not yet been experimentally demonstrated. In this work, we construct a time-bin encoded QKD system employing a high-brightness quantum dot (QD) single-photon source operating at telecom wavelength. Our proof-of-concept experiment successfully demonstrates the possibility of secure key distribution over fiber link of 120 km, while maintaining extraordinary long-term stability over 6 h of continuous operation, that is highest secure key rate among the time-bin QKDs based on single-photon sources. This work provides the first experimental validation of integrating a QD single-photon source with time-bin encoding in a telecom-band QKD system. This development signifies a substantial advancement in the establishment of a robust and scalable QKD network based on solid-state single-photon technology.