Experimental realization of entanglement-based quantum key distribution (QKD) in a daylight uplink (ground-to-satellite) communication channel is highly challenging due to the very low signal-to-noise ratio (SNR) of this configuration and it has yet to be successfully implemented to date. While current research efforts focus on weak coherent pulse (WCP) sources for daylight QKD, we present a proof-of-principle demonstration of uplink-daylight QKD in a free-space-fiber hybrid configuration using polarization-entangled photon pairs generated via a spontaneous parametric down-conversion (SPDC) process. The simulated uplink channel attenuation reaches up to 50 dB, equivalent to low-earth-orbit (LEO) distances. Furthermore, the detected noise level (up to a few MHz) in the receiver telescope is representative of daylight conditions. The enhancement of SNR is achieved by implementing rigorous filtering on spatial, spectral, and temporal modes. Our ultra-bright source of entangled photons, characterized by a narrow spectral bandwidth of 0.54 nm FWHM, ensures minimal signal attenuation. We also propose a novel theoretical model of entanglement-based QKD link that fits our experimental data perfectly and is consistent with the prevailing models in the field. Our model considers a non-ideal entangled photon source and is applicable to both continuous wave (CW) and pulse-pumped sources. Using this model, we demonstrate that entanglement-based QKD is feasible under uplink daylight conditions in LEO satellites, but only up to a distance of 400 km, utilizing current state-of-the-art single-mode fiber (SMF) coupling technology.
Experimental realization of entanglement-based quantum key distribution (QKD) in a daylight uplink (ground-to-satellite) communication channel is highly challenging due to the very low signal-to-noise ratio (SNR) of this configuration and it has yet to be successfully implemented to date. While current research efforts focus on weak coherent pulse (WCP) sources for daylight QKD, we present a proof-of-principle demonstration of uplink-daylight QKD in a free-space-fiber hybrid configuration using polarization-entangled photon pairs generated via a spontaneous parametric down-conversion (SPDC) process. The simulated uplink channel attenuation reaches up to 50 dB, equivalent to low-earth-orbit (LEO) distances. Furthermore, the detected noise level (up to a few MHz) in the receiver telescope is representative of daylight conditions. The enhancement of SNR is achieved by implementing rigorous filtering on spatial, spectral, and temporal modes. Our ultra-bright source of entangled photons, characterized by a narrow spectral bandwidth of 0.54 nm FWHM, ensures minimal signal attenuation. We also propose a novel theoretical model of entanglement-based QKD link that fits our experimental data perfectly and is consistent with the prevailing models in the field. Our model considers a non-ideal entangled photon source and is applicable to both continuous wave (CW) and pulse-pumped sources. Using this model, we demonstrate that entanglement-based QKD is feasible under uplink daylight conditions in LEO satellites, but only up to a distance of 400 km, utilizing current state-of-the-art single-mode fiber (SMF) coupling technology.
Experimental realization of entanglement-based quantum key distribution (QKD) in a daylight-uplink (ground-to-satellite) communication channel is highly challenging due to the very low signal-to-noise ratio (SNR) of this configuration, and it has yet to be successfully implemented to date. While current research efforts focus on weak coherent pulse (WCP) sources for daylight QKD, we present a proof-of-principle laboratory demonstration emulating daylight-uplink QKD in a free-space-fiber hybrid configuration using polarization-entangled photon pairs generated via a spontaneous parametric down-conversion (SPDC) process. The emulated uplink channel attenuation reaches up to 50 dB, equivalent to low-earth-orbit (LEO) distances. Furthermore, the detected noise level (up to a few MHz) in the receiver telescope is representative of daylight conditions. The enhancement of SNR is achieved by implementing rigorous filtering on spatial, spectral, and temporal modes. Our ultra-bright source of entangled photons, characterized by a narrow spectral bandwidth of 0.54 nm FWHM, ensures minimal signal attenuation. Using a theoretical model, we estimate the conditional feasibility of entanglement-based QKD for daylight-uplink LEO satellite links up to approximately 400 km, subject to the experimentally motivated system assumptions adopted in this work, including efficient single-mode fiber (SMF) coupling performance.
In recent years, quantum key distribution (QKD) has seen the first proof-of-concept demonstrations from space. Next on the agenda towards a full-blown global quantum internet is to address the more practical aspects, such as efficiency, flexibility, and accessibility of QKD services. One of the main challenges that remains to be solved in this regard is to enable operation in the presence of daylight noise. Here, we present a complete framework for modelling daylight QKD from an orbiting satellite. We include the effects of atmospheric turbulence and adaptive optics correction at the receivers. We consider single- and multi-mode fibre coupling as a means of spatial filtering, for which we derived simple formulas for estimating coupling efficiencies of signal as well as noise. Using our framework, we identify the most critical system parameters for daylight operation and discuss the choice of signal wavelength and detection technology. Finally, we provide simulation results for various parameter combinations in a hypothetical daylight QKD between Berlin and Munich via a satellite in a low Earth orbit. The results show a clear advantage of 800 nm signal wavelength over 1550 nm with the currently available technology. Moreover, we show the relevance of single-mode fibre coupling and the importance of detectors with low timing jitters. We anticipate our work will provide valuable insight and tools to aid the future feasibility studies of daylight QKD in dual-downlink configurations. Additional presentation content can be accessed on the supplemental content page.
Satellite based quantum communication has been proven as a feasible way to achieve global scale quantum communication network. Very recently, a low-Earth-orbit (LEO) satellite has been launched for this purpose. However, with a single satellite, it takes an inefficient 3-day period to provide the worldwide connectivity. On the other hand, similar to how the Iridium system functions in classic communication, satellite constellation (SC) composed of many quantum satellites, could provide global real-time quantum communication. In such a SC, most of the satellites will work in sunlight. Unfortunately, none of previous ground testing experiments could be implemented at daytime. During daytime, the bright sunlight background prohibits quantum communication in transmission over long distances. In this letter, by choosing a working wavelength of 1550 nm and developing free-space single-mode fibre coupling technology and ultralow noise up-conversion single photon detectors, we overcome the noise due to sunlight and demonstrate a 53-km free space quantum key distribution (QKD) in the daytime through a 48-dB loss channel. Our system not only shows the feasibility of satellite based quantum communication in daylight, but also has the ability to naturally adapt to ground fibre optics, representing an essential step towards a SC-based global quantum network.
A promising use of quantum networking is quantum key distribution (QKD), which can provide information-theoretic security unattainable by classical means. While optical fiber-based QKD networks suffer from exponential loss, satellite-assisted quantum communication offers a scalable solution for long-distance secure key exchange. In this work, we propose and evaluate a satellite-based QKD setup covering the Iberian Peninsula, linking Madrid with Barcelona, Bilbao, and Lisbon. Our proposed setup uses a Low-Earth-Orbit (LEO) state-of-the-art satellite equipped with a spontaneous parametric down-conversion (SPDC) source to distribute entangled photon pairs to ground stations. Considering vibrations in the satellite, we optimize the beam waist to enhance the transmission probability and improve the secret key rate (SKR). Our results show that key rates sufficient for real-world applications, such as secure communication between hospitals, using hybrid classical-quantum protocols, are feasible with existing protocols. Our results highlight the viability of near-term satellite-based QKD networks for national-scale secure communications.
Abstract Quantum key distribution (QKD) is a pioneering quantum technology on the brink of widespread deployment. Nevertheless, the distribution of secret keys beyond a few 100 km at practical rates remains a major challenge. One approach to circumvent lossy terrestrial transmission of entangled photon pairs is the deployment of optical satellite links. Optimizing these non-static quantum links to yield the highest possible key rate is essential for their successful operation. We therefore developed a high-brightness polarization-entangled photon pair source and a receiver module with a fast steering mirror capable of satellite tracking. We employed this state-of-the-art hardware to distribute photons over a terrestrial free-space link with a distance of 143 km, and extracted secure key rates up to 300 bits per second. Contrary to fiber-based links, the channel loss in satellite downlinks is time-varying and the link time is limited to a few minutes. We therefore propose a model-based optimization of link parameters based on current channel and receiver conditions. This model and our field test will prove helpful in the design and operation of future satellite missions and advance the distribution of secret keys at high rates on a global scale.
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.