Free-space optical communication links promise better security and increase bandwidths but can suffer from noise in daylight. This is particularly detrimental in quantum communications where current mitigation techniques, such as spectral, temporal, and spatial filtering, are not yet sufficient to make daylight tolerable for satellite quantum key distribution (SatQKD). As all current SatQKD systems are polarization-encoded, polarization filtering has not been investigated. However, by using time- and phase-encoded SatQKD, it is possible to filter in polarization in addition to existing domains. Scattered daylight can be more than 90% polarized in the visible band, yielding a reduction in detected daylight between 3 dB and 13 dB, such that polarization filtering can reduce the brightness of 780 nm daylight to below the unfiltered equivalent at 1550 nm. Simulations indicate that polarization filtering increases the secure key rate and allows for SatQKD to be performed at dawn and dusk. This could open the way for daylight SatQKD utilizing shorter near-infrared wavelengths and retaining their benefits.
Free-space optical communication links promise better security and increase bandwidths but can suffer from noise in daylight. This is particularly detrimental in quantum communications where current mitigation techniques, such as spectral, temporal, and spatial filtering, are not yet sufficient to make daylight tolerable for satellite quantum key distribution (SatQKD). As all current SatQKD systems are polarization-encoded, polarization filtering has not been investigated. However, by using time- and phase-encoded SatQKD, it is possible to filter in polarization in addition to existing domains. Scattered daylight can be more than 90% polarized in the visible band, yielding a reduction in detected daylight between 3 dB and 13 dB, such that polarization filtering can reduce the brightness of 780 nm daylight to below the unfiltered equivalent at 1550 nm. Simulations indicate that polarization filtering increases the secure key rate and allows for SatQKD to be performed at dawn and dusk. This could open the way for daylight SatQKD utilizing shorter near-infrared wavelengths and retaining their benefits.
Free-space quantum key distribution (QKD) systems are often designed to implement polarization-encoding protocols. Alternatively, time-bin/phase-encoding protocols are considerably more challenging to perform over a channel experiencing atmospheric turbulence. However, over the last decade, new and improved optical platforms have revived the interest in them. In this paper, we present a free-space multi-protocol receiver designed to work with three different time-bin/phase-encoding protocols highlighting its interoperability with different systems and architectures for potential satellite-based communications. We also present a detailed analysis of different experimental configurations when implementing the coherent one-way (COW) protocol in a free-space channel, as well as a polarization filtering technique showing how time-bin/phase-encoding protocols could be used for QKD applications in daylight conditions. We demonstrate secret key rates of several kbps for channels with a total 30 dB attenuation even with moderately high QBERs of ≈3.5%. Moreover, a 2.6 dB improvement in the signal to noise ratio is achieved by filtering background light in the polarization degree of freedom, a technique that could be used in daylight QKD.
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.
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.
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.
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.