Free-space quantum key distribution (QKD) is important to realize a global-scale quantum communication network. However, performing QKD in daylight against the strong background light noise is a major challenge. Here, we develop the stochastic parallel gradient descent (SPGD) algorithm with a deformable mirror to improve the signal-to-noise ratio (SNR). We then experimentally demonstrate free-space QKD in the presence of urban daylight. The final secure key rate of the QKD is 98∼419 bps throughout the majority of the daylight hours.
Free-space quantum key distribution (QKD) is important to realize a global-scale quantum communication network. However, performing QKD in daylight against the strong background light noise is a major challenge. Here, we develop the stochastic parallel gradient descent (SPGD) algorithm with a deformable mirror to improve the signal-to-noise ratio (SNR). We then experimentally demonstrate free-space QKD in the presence of urban daylight. The final secure key rate of the QKD is 98∼419 bps throughout the majority of the daylight hours.
Satellite-based quantum communication is a promising approach for realizing global-scale quantum networks. For free-space quantum channel, single-mode fiber coupling is particularly important for improving the signal-to-noise ratio of daylight quantum key distribution (QKD) and compatibility with standard fiber-based QKD. However, achieving a highly efficient and stable single-mode coupling efficiency under strong atmospheric turbulence remains experimentally challenging. Here, we develop a single-mode receiver with an adaptive optics (AO) system based on a modal version of the stochastic parallel gradient descent (M-SPGD) algorithm and test its performance over an 8 km urban terrestrial free-space channel. Under strong atmospheric turbulence, the M-SPGD AO system obtains an improvement of about 3.7 dB in the single-mode fiber coupling efficiency and a significant suppression of fluctuation, which can find its applications in free-space long-range quantum communications.
One of the challenges of implementing free-space quantum key distribution (QKD) systems working in daylight is to remove unwanted background noise photons from sunlight. Elaborate elimination of background photons in the spectral, temporal, and spatial domains is an indispensable requirement to decrease the quantum bit error rate (QBER), which guarantees the security of the systems. However, quantitative effects of different filtering techniques and performance optimization in terms of the secure key rate have not been investigated. In this study, we quantitatively analyze how the performance of the QBER and the key rates changes for different combinations of filtering techniques in a free-space BB84 QKD system in daylight. Moreover, we optimize the conditions of filtering techniques in order to obtain the maximum secure key rate.
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 (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.
Current satellite-based quantum key distribution (QKD) is limited to nighttime operations, and the reliance on microwave communication for key distillation leads to significant delays, often spanning several days. These challenges collectively hinder the establishment of a practical global-scale quantum network. Here, by developing a 625-MHz inherently robust decoy-state light source and daytime noise suppression close to the Fourier transform limitation, we achieve QKD covering all the 24 h of the day over a 20-km terrestrial free-space channel, resulting in an average secure key rate of approximately 495 bps. Additionally, bidirectional laser communication is integrated into the QKD transmitter and the ground station to enable real-time key distillation, improving the timeliness from days to real time. This comprehensive verification lays a solid foundation and paves the way for all-day real-time QKD with quantum satellites.
Quantum key distribution (QKD) has matured in recent years from laboratory proof-of-principle demonstrations to commercially available systems. One of the major bottlenecks is the limited communication distance in fiber networks due to the exponential signal damping. To bridge intercontinental distances, low Earth orbit satellites transmitting quantum signals over the atmosphere can be used. These free-space links, however, can only operate during the night, as the sunlight otherwise saturates the detectors used to measure the quantum states. For applying QKD in a global quantum internet with continuous availability and high data rates, operation during daylight is required. In this work, we model a satellite-to-ground quantum channel for different quantum light sources to identify the optimal wavelength for free-space QKD under ambient conditions. Daylight quantum communication is possible within the Fraunhofer lines or in the near-infrared spectrum, where the intrinsic background from the sun is comparably low. The highest annual secret key length considering the finite key effect is achievable at the Hα Fraunhofer line. More importantly, we provide the fundamental model that can be adapted, in general, to any other specific link scenario taking into account the required modifications. We also propose a true single-photon source based on a color center in hexagonal boron nitride coupled to a microresonator that can implement such a scheme. Our results can also be applied in roof-to-roof scenarios and are, therefore, relevant for near-future quantum networks.