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Laser Communication and Propagation through the Atmosphere and Oceans

Transmission of single photons through atmospheric turbulence with spatial diversity

Andres I. Seguel, Jaime A. Anguita, Carlos S. Pirela

Published Oct 3, 2024• 0 citations
Publisher

Abstract

A multiple-input single-output configuration is used to study the effectiveness of spatial diversity in reducing single-photon losses caused by atmospheric turbulence in quantum communications over free-space. The system consists of two transmitters and a single receiver, where two parallel beams are transmitted, each comprising a 660-nm beacon and a 515-nm collinear single-photon signal. The beacon is intended to compare the turbulence conditions on each path to determine the most suitable transmitter for the single-photon signal. An outdoor experimental campaign with a propagation distance of 500 meters was conducted to obtain results under real turbulence conditions. First, the correlation between the 515-nm and 660-nm collinear signals was measured at 0.82, showing that the beacon signal accurately represents the state of turbulence affecting the single-photon signal most of the time. The spatial correlation was measured at separation distances between the transmitters of 12 and 20 centimeters, yielding correlation coefficients of 0.35 and 0.17, respectively. As expected, the spatial correlation decreases with increasing separation distance. Finally, a slight improvement in the detection of the single-photon signal was observed when using spatial diversity, but further improvements to the system and longer propagation distances are needed to obtain more promising results.

DOI
10.1117/12.3027814

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A multiple-input single-output configuration is used to study the effectiveness of spatial diversity in reducing single-photon losses caused by atmospheric turbulence in quantum communications over free-space. The system consists of two transmitters and a single receiver, where two parallel beams are transmitted, each comprising a 660-nm beacon and a 515-nm collinear single-photon signal. The beacon is intended to compare the turbulence conditions on each path to determine the most suitable transmitter for the single-photon signal. An outdoor experimental campaign with a propagation distance of 500 meters was conducted to obtain results under real turbulence conditions. First, the correlation between the 515-nm and 660-nm collinear signals was measured at 0.82, showing that the beacon signal accurately represents the state of turbulence affecting the single-photon signal most of the time. The spatial correlation was measured at separation distances between the transmitters of 12 and 20 centimeters, yielding correlation coefficients of 0.35 and 0.17, respectively. As expected, the spatial correlation decreases with increasing separation distance. Finally, a slight improvement in the detection of the single-photon signal was observed when using spatial diversity, but further improvements to the system and longer propagation distances are needed to obtain more promising results.
Spatial diversity is a technique widely used in wireless communications to enhance the signal quality at the receiver. We propose a multiple-input single-output system that utilizes this technique to enhance a free-space optics quantum communication link by reducing the amount of photon losses caused by atmospheric turbulence, thus increasing the capacity of the quantum channel. The system consists of two transmitters with uncorrelated optical paths, and a single receiver. A 515-nm quantum signal is transmitted through the transmission path with the highest gain, dynamically chosen by comparing the signal distortions of a 660-nm classical signal. Preliminary experiments with a single transmitter have been conducted in a laboratory environment with atmospheric turbulence generated via heat guns. We observed that the single-photon channel is highly correlated with the fluctuations of the 660-nm classical signal, so that an improvement in the former is expected when selecting the path with highest gain. The number of photon counts received was compared with the turbulence-free scenario, revealing that the mean number of counts decreased, and its standard deviation increased when turbulence is present, as expected.
Quantum entanglement is critical to build the backbone of all quantum technologies. Quantum networks, quantum computations, and quantum communication networking is based on long-range distribution of entangled photons and teleportation of photon qubit states. In order to understand quantum entanglement, characterization of atmospheric turbulence and its effects on propagating quantum states in free-space is essential. One method of photon entanglement is using a photon’s polarization. In this paper, we report results using polarization entangled signal and idler photons. The results may be applicable to support various quantum computing, encryption, and other qubit based high-performance communication protocols. Classically, the degradation of beam quality occurs due to many factors but primarily due to the distortion of spatial and temporal fields of refractive index. However, behavior of single photons through similar turbulent media creates a different set of challenges pointing to integrity of quantum states during propagation. We study this behavior by analyzing quantum states and the degree of entanglement in real-time and correlating it to known atmospheric models, (refractive index structure parameter), and relevant propagation path parameters. This experimental study was performed initially in a controlled laboratory environment, and then devised to be implemented outdoors over a 100-meter free space communication link.
In order to study the change regularity of photon states in atmospheric turbulence,the photon state detection probability was derived based on facts that transverse spatial wave function of Laguerre-Gaussian beam is the eigenfunction of a photon and transverse spatial wave function of Laguerre-Gaussian beam propagation in turbulent atmosphere approaches to superimposition of all photon state eigenfunctions.Effect of atmospheric turbulence,propagation length,quality factor on detection probability was analyzed.The results present that the change of the photon state largely relies on beam quality factor. With the same quality factor,the measurement weight at the receiving plane is nearly the same.In addition,the total measurement weight from initial photon to state at which the orbital angular momentum increases to a certain value,is equivalent to the total measurement weight of the corresponding decrease.The stronger the turbulence is,the smaller the photon state measurement weight of the initial photon state at the receiving side is.That is also true of the propagating distance.However, distance puts much less effect on photon states than atmospheric turbulence.
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.
Beam wander caused by atmospheric turbulence can significantly degrade the performance of horizontal free-space quantum communication links. Classical beam wander correction techniques cannot be applied due to the stronger requirements of transmitting single photons. One strategy to overcome this limitation consists in using a separate wavelength from that of the quantum signal to analyze the beam wander and use this information for its correction. For this strategy to work adequately, both wavelengths should be affected equally by atmospheric turbulence, i.e. no chromatic effects should be originated from beam wander. In this letter, a series of experiments are performed to prove that this is the case for λ ∼ 850 nm as the quantum signal and λ ∼ 1550 nm as the tracking signal of a quantum communication system. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1362–1365, 2016
The physical processes that contribute to the single photon fading and loss need to be carefully analyzed in the theory of quantum light transmission through the atmospheric turbulence. Here we propose a single photon receiving model, and analyze the influence of the atmospheric turbulence on the single photon transmission. We simulate and analyze the photon counting and the final key generation rate under different intensity of the atmospheric turbulence. Our model can be used to reflect the optical coupling and loss between transmitter and receiving terminal.