Atmospheric turbulence can significantly degrade the performance of free-space optical communication links. Beam wander, intensity scintillations, beam broadening and angle of arrival fluctuations give rise to signal fading with communication channel drop-outs. In this work the performance of the potentially beneficial combination of a retro reflective link operating at 1535 nm in conjunction with a single-photon quantum key distribution link at 850 nm was studied. The general idea is that the weak light beam of the quantum channel can be steered and controlled by the 1535 nm signal. A dual-channel laser unit, with output beams at 850 and 1535 nm, was used to characterise turbulence effects along an atmospheric path close above ground. The laser beams were retro reflected from a corner cube and detected in the same laser unit. The 1535 nm beam was monitored with a position sensitive detector while the 850 nm beam (low intensity channel) was aligned with the 1535 nm beam and detected separately but along the same optical axis of the laser unit. Effects of beam wander, angular fluctuations and intensity scintillations were studied experimentally at different turbulence strengths and weather conditions. The experimental results are discussed with respect to theoretical models.
Atmospheric turbulence can significantly degrade the performance of free-space optical communication links. Beam wander, intensity scintillations, beam broadening and angle of arrival fluctuations give rise to signal fading with communication channel drop-outs. In this work the performance of the potentially beneficial combination of a retro reflective link operating at 1535 nm in conjunction with a single-photon quantum key distribution link at 850 nm was studied. The general idea is that the weak light beam of the quantum channel can be steered and controlled by the 1535 nm signal. A dual-channel laser unit, with output beams at 850 and 1535 nm, was used to characterise turbulence effects along an atmospheric path close above ground. The laser beams were retro reflected from a corner cube and detected in the same laser unit. The 1535 nm beam was monitored with a position sensitive detector while the 850 nm beam (low intensity channel) was aligned with the 1535 nm beam and detected separately but along the same optical axis of the laser unit. Effects of beam wander, angular fluctuations and intensity scintillations were studied experimentally at different turbulence strengths and weather conditions. The experimental results are discussed with respect to theoretical models.
We present a quantum-correlation-based free-space optical(FSO) link over 250 m using an outdoor active reflector 125 m from the transceiver station. The performance of free-space optical communication can be significantly degraded by atmospheric turbulence effects, such as beam wander and signal fluctuations. We used a 660 nm tracking laser to reduce atmospheric effects, by analyzing the fast beam wander and slow temporal beam drift, using this information to correct the quantum channel alignment of the 810 nm signal photons. In this work, the active reflector consisted of a mirror, a 6-axis hexapod stage, and a long-range wireless bridge. The slow drift of the beam path due to outdoor temperature changes was steered and controlled using wireless optical feedback between the receiver units and the active reflector. Our work provides useful knowledge for improved control of beam paths in outdoor conditions, which can be developed to ensure high quality quantum information transfer in real-world scenarios, such as an unmanned FSO link for urban quantum communication or retro-reflective quantum communication links.
We devoted to analyzing information transfer in quantum channels in terms of the performance of quantum key distribution (QKD) free-space optical (FSO) communication systems through Bennett and Brassard 84 (BB84) protocol by taking into account the combined effect of attenuation, turbulence, pointing error, and angle of arrival (AOA) fluctuations. Path loss is modeled by Beer-Lambert law, and the turbulent channel is assumed to be Fisher-Snedecor F distributed. The pointing error and AOA fluctuations are modeled by Hoyt and Rayleigh distributions, respectively. The error and sift probabilities, quantum bit error rate, and secret key rate of a QKD FSO system are obtained in analytical form, and results are presented depending on various parameters. It is shown that increased beam waist, beam deviation, AOA fluctuations, and propagation distance cause performance degradation and less secure information transmission. However, an increase in the ratio of horizontal and vertical beam misalignments, field of view, average photon number per pulse, visibility of the atmospheric environment, and optical beam wavelength impact the performance of QKD FSO systems positively. The effect of the turbulence structure constant is observed to be modest on the performance of the QKD FSO system.
We investigate the performance of relativistic quantum key distribution (RQKD) over turbulent free-space optical channels for both single-photon and coherent-state implementations. A theoretical model is developed to describe the effect of atmospheric turbulence on interferometric visibility and secret-key generation by incorporating turbulence-induced phase fluctuations into the key-rate analysis. Numerical simulations are performed under realistic channel conditions to evaluate the achievable communication distance and secret-key rate. The results show that atmospheric turbulence degrades the protocol performance by reducing interference visibility, with the severity of the degradation determined by the ratio between the interferometer-arm separation r (or temporal separation of signal and reference states ΔT) and Fried's parameter. The single-photon protocol remains operational over all investigated turbulence regimes, whereas the coherent-state implementation exhibits a critical dependence on the interferometer geometry (especially on ΔT), leading to a threshold beyond which secure key generation is no longer possible. These findings quantify the impact of atmospheric turbulence on relativistic quantum key distribution and provide practical guidelines for the design and optimization of free-space RQKD systems.
One of the most rapidly developing communication technologies is quantum communication. Quantum Free-Space Optical Communication (FSO) provides unlimited bandwidth for data transmission. A key barrier to FSO links is associated with the losses caused by atmospheric absorption and scattering of molecules and aerosols. In the present study, we analyze the performance of a free-space classical link for 650 nm, 785 nm, 850 nm and 1550 nm wavelengths for a link range of 0.5 Km and 50 Km. The losses due to atmospheric components such as rain, fog, snow and turbulence are predicted for different wavelengths. Also, we presented similar analysis for free space quantum link using difference frequency generation(DFG) method for both short range (0.5 Km) and long-range link (50 Km) under diverse weather conditions. It is done by combining weak wavelengths of 650 nm, 785 nm, 850 nm and 1550 nm, along with a tunable pump LASER (Krypton) to generate IR B waves (1400 nm - 3000 nm). The total atmospheric losses with DFG is compared with the losses generated without using DFG.
Compared to fiber continuous-variable quantum key distribution (CVQKD), atmospheric link offers the possibility of a broader geographical coverage and more flexible transmission. However, there are many negative features of the atmospheric channel that will reduce the achievable secret key rate, such as beam extinction and a variety of turbulence effects. Here we show how these factors affect performance of CVQKD, by considering our newly derived key rate formulas for fading channels, which involves detection imperfections, thus form a transmission model for CVQKD. This model can help evaluate the feasibility of experiment scheme in practical applications. We found that performance deterioration of horizontal link within the boundary layer is primarily caused by transmittance fluctuations (including beam wandering, broadening, deformation, and scintillation), while transmittance change due to pulse broadening under weak turbulence is negligible. Besides, we also found that communication interruptions can also cause a perceptible key rate reduction when the transmission distance is longer, while phase excess noise due to arrival time fluctuations requires new compensation techniques to reduce it to a negligible level. Furthermore, it is found that performing homodyne detection enables longer transmission distances, whereas heterodyne allows higher achievable key rate over short distances.