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Propagação de feixes ópticos de correlação em atmosfera turbulenta

Marcelo Vitor da Cunha Pereira

Published Jul 25, 2014• 0 citations
OpenAlex

Abstract

The turbulent mixing of air at different temperatures turns the refractive index of air in the atmosphere into a randomly varying quantity. Together with scattering by fog, this fluctuation figures among the greatest obstacles in information transmission through free-space optics. In this thesis, a passive method is introduced that attenuates the distortions created by the atmosphere through the use of correlation beams: entities which describe photon correlations and propagate formally as regular beams. The correct choice of correlation mitigates some of the most severe aberrations and allows for information coding immune to the fluctuations up to good approximation. The models are demonstrated via Monte Carlo simulations and through an experiment in which turbulence is generated in the lab with a turbulence emulating chamber and the correlation beam is created using spontaneous parametric down-conversion. One of the correlation beam's spatial properties, parity, is shown to be particularly robust against the fluctuations and an experimental proposal is presented that exploits this capability. Investigations are done concerning other implications of a robust correlation beam, which indicate at least one drawback: a reduction in the overall signal collected by a receiver.

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The turbulent mixing of air at different temperatures turns the refractive index of air in the atmosphere into a randomly varying quantity. Together with scattering by fog, this fluctuation figures among the greatest obstacles in information transmission through free-space optics. In this thesis, a passive method is introduced that attenuates the distortions created by the atmosphere through the use of correlation beams: entities which describe photon correlations and propagate formally as regular beams. The correct choice of correlation mitigates some of the most severe aberrations and allows for information coding immune to the fluctuations up to good approximation. The models are demonstrated via Monte Carlo simulations and through an experiment in which turbulence is generated in the lab with a turbulence emulating chamber and the correlation beam is created using spontaneous parametric down-conversion. One of the correlation beam's spatial properties, parity, is shown to be particularly robust against the fluctuations and an experimental proposal is presented that exploits this capability. Investigations are done concerning other implications of a robust correlation beam, which indicate at least one drawback: a reduction in the overall signal collected by a receiver.
Scattering by atmospheric turbulence is one of the main challenges in creating long free-space optical links, and specifically links of entangled photons. Classical compensation methods are hard to apply to entangled photons, due to inherently low signal to noise ratios and the fragility of entanglement. We have recently shown that we can use a bright laser beam that pumps spontaneous parametric down conversion to control the spatial correlations between entangled photons for compensating their scattering. In this work, we apply the pump-shaping technique to compensate for the scrambling of correlations between entangled photons that scatter by emulated atmospheric turbulence. We use a spatial light modulator and Kolmogorov’s turbulence model to emulate atmospheric turbulence in the lab, and enhance the entangled photons’ signal by a factor of fifteen using pump optimization. We show this for both a static and dynamic emulated atmosphere, and also demonstrate the compensation of the scattering of a higher-order mode. Our results can open the door towards realizing free-space quantum links with entangled photons, used in applications such as quantum key distribution.
A major field of research at the current time is that of implementing Quantum Key Distribution over large distances using satellites. If this protocol works with this technology, it will have huge implications on future information security. In order for a satellite to implement this idea, there are many aspects that must be taken into account. One of the big issues that comes up for this type of system is that of propagating light through the turbulent atmosphere and its effects on the acquisition, pointing and tracking system. The projects studied in this thesis study some of the effects of the atmosphere on certain detectors, try to develop pointing schemes for better accuracy as well as develop knowledge in free space propagation of other single photon experiments. \n \nIn the first experiment, I study the spatial correlations of the daughter photons created in spontaneous parametric down conversion. I look at the effect of altering the pump beam on the positions of the down converted photons and see if the pump can be manipulated in a way to control the directions of the daughter photons. I begin to utilize a deformable mirror and Shack-Hartmann wavefront sensor which are generally used in adaptive optics, but we plan to use them to alter the pump beam in the spontaneous parametric down conversion process to analyze the correlations between the pump and down converted photons. \n \nThe second experiment investigates the effects of laser scintillation on the performance of a possible tracking device that could be implemented on a satellite. This quad sensor tracks the position of a beam and a system will be developed to move the sensor to keep the beam in the center where there is a hole for the quantum single photons to stream through. In order to create the effects of scintillation, a turbulence simulator box was built and characterized. This box combines wind turbulence with a heat gradient to mimic atmospheric turbulence on a small scale. \n \nFinally, my contributions to a large scale, long distance free space quantum optics experiment are explained and the overall goal of the experiment is discussed. This experiment exposed me to actual free space transmission issues as well as many fundamental techniques for performing long distance optics experiments. In this experiment there was no correction for atmospheric turbulence, but in the future, techniques could be implemented which might increase the efficiencies of the free space links.
Quantum entanglement is an essential element for building the backbone of quantum information systems. Our particular interest lies in long-range distribution of entangled photons to facilitate secure data transfer in free space. To achieve this, we rely on photon pairs generated in such a way that their polarization characteristics represent the corresponding qubit states and have a high degree of correlation in measurements. The main focus of this paper is integrity of the quantum states in free-space channels. When transmitted in atmosphere, classical signals suffer from wave front distortions caused by the spatial and temporal fields of the refractive index. However, this mechanism does not have the same bearing on qubit values and their correlation. We study the effects of turbulence on quantum states by utilizing a laboratory testbed that includes an atmospheric chamber developed by the AFRL. It uses a system of controlled components capable of creating various turbulence conditions. When polarized signals are passed through the atmospheric chamber, we analyze the corresponding quantum states and evaluate the degree of entanglement using our mathematical models and existing metrics.
The primary focus of this paper is high-performance quantum communication systems that facilitate secure data transfer via free-space links. We consider an approach that uses correlated photon pairs generated in such a way that their polarizations are entangled and can be used to support quantum encryption protocols. However, when deployed in free space, these links can be affected by channel distortion, primarily via the spatial and temporal fields of the refractive index along the propagation path. In classical links, these fields alter the optical wave front characteristics; however, this mechanism does not directly apply to the quantum states utilized in single-photon or entangled photon protocols. Transmitting signals with quantum-based encryption creates a realm of problems, not related to wave front distortions, but rather to integrity of the quantum states after the signals propagate over free-space channels. We study these phenomena by implementing a laboratory testbed capable of creating a turbulent environment using atmospheric chambers developed by the AFRL. It is then used for experimental investigation of quantum entanglement after photon pairs are propagated both collinearly and via separate paths.
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.