Quantum key distribution exploits the principles of quantum mechanics to create and distribute cryptographic keys. The quantum communication channel can be either fiber-based or free space-based. Fiber-based quantum key distribution has evolved from lab demonstrations to commercially available systems, but its infrastructure dependency and limitations on achievable distance, due to optical fiber loss, do not allow for the realization of a global-scale quantum communication network. Free space quantum key distribution addresses this problem with features such as low loss, high data rates, and ease of deployment. This paper presents an experimental demonstration of free space quantum key distribution using the differential phase shift protocol in an urban location during daylight hours. We established a secure link, achieving a key rate of 10 kbps.
Quantum key distribution exploits the principles of quantum mechanics to create and distribute cryptographic keys. The quantum communication channel can be either fiber-based or free space-based. Fiber-based quantum key distribution has evolved from lab demonstrations to commercially available systems, but its infrastructure dependency and limitations on achievable distance, due to optical fiber loss, do not allow for the realization of a global-scale quantum communication network. Free space quantum key distribution addresses this problem with features such as low loss, high data rates, and ease of deployment. This paper presents an experimental demonstration of free space quantum key distribution using the differential phase shift protocol in an urban location during daylight hours. We established a secure link, achieving a key rate of 10 kbps.
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.
Quantum key distribution (QKD) is a promising technology that aims to solve the security problem arising from the advent of quantum computers. While the main theoretical aspects are well developed today, limited performances, in terms of the achievable link distance and the secret key rate, are preventing the deployment of this technology on a large scale. More recent QKD protocols, which use multiple degrees of freedom for encoding of the quantum states, allow enhancement of the system performances. Here, we present the experimental demonstration of the differential phase-time shifting protocol up to 170 km of the fiber link. We compare its performance with the well-known coherent one-way and differential phase shifting protocols, demonstrating a higher secret key rate up to 100 km. Moreover, we propagate a classical signal in the same fiber, proving the compatibility of quantum and classical light.
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.
Abstract Quantum communication has seen rapid progress towards practical large-scale networks, with quantum key distribution (QKD) spearheading this development. While fibre-based systems have been shown to be well suited for metropolitan scales, suitable fibre infrastructure may not always be in place. Here, we make the case for an entanglement-based free-space quantum network as a practical and efficient alternative for metropolitan applications. We developed a deployable free-space QKD system and demonstrated its use in realistic scenarios. For a representative 1.7-km free-space link, we showcase its ad hoc deployability and achieve secure key rates of up to 5.7 kbps, with 2.5 kbps in direct noon sunlight. By extrapolating experimental data, we show that kbps key rates are achievable even for 10-km distances and multi-user scenarios. We anticipate that our work will establish free-space networks as a viable solution for metropolitan applications and an indispensable complementary building block in the future global quantum internet.
Abstract We demonstrate a free-space quantum key distribution (QKD) system based on phase-time encoding by reconfiguring a commercially available fiber-based QKD platform. The main goal of this work is to develop a free-space QKD system with minimal modifications, primarily through the implementation of a free-space multi-mode (MM) delay interferometer at the receiver. The use of a MM interferometer is crucial for enabling phase encoding over atmospheric channels, where direct coupling into single-mode fibers suffers from prohibitive losses due to atmospheric turbulence and beam distortions. We review existing methods for detecting spatially distorted quantum states, propose technological improvements, and experimentally evaluate several interferometer designs. A free-space MM Michelson interferometer with polarization routing is identified as the optimal solution, offering high interference visibility, low loss, and ease of integration into the system. A laboratory QKD experiment conducted with the modified system achieved a quantum bit error rate of 4.5% and a stable secret key rate of hundreds bit s−1, demonstrating the feasibility of secure key distribution over free-space links using adapted fiber-based technology. Detailed theoretical and experimental studies of chromatic and modal dispersion effects confirm that their impact can be effectively managed. This approach provides a practical and scalable route for extending quantum-secure communications into scenarios where optical fiber deployment is impractical or impossible.
Quantum key distribution (QKD) is the most established approach in quantum communication. However, long-distance free-space implementations, particularly satellite links, remain challenging, especially during the day due to daylight background noise. Quantum keyless private communication (QKPC) is a quantum communication protocol that enables information-theoretic security with simpler system requirements, improved robustness against noise, and without the need for secret key distribution. QKPC and QKD are complementary, with QKPC enabling free-space links where QKD is impractical, while QKD provides channel monitoring for applications that require eavesdropping detection. Here, we report a complete implementation of QKPC in a daylight free-space experiment over a 90 m rooftop link, using an experimentally simple setup. Our demonstration includes all stages of the protocol, from encoding and synchronization to message decoding, operates entirely without auxiliary classical synchronization channels, and is implemented offline through post-processing. This work demonstrates the feasibility of practical and scalable quantum communication over high-noise daylight links and highlights the potential of QKPC as a complementary solution to QKD for future ground-based and space-based communication systems.