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Applied Physics Letters

The invariance and distortion of vectorial light across a real-world free space link

Cade Peters, Mitchell Arij Cox, Alice V. Drozdov, Andrew Forbes

Published Jul 10, 2023• 31 citations
Publisher

Abstract

Vectorial structured light, where the polarization is inhomogeneously distributed in space, has found a myriad of applications in both 2D and 3D optical fields. Here, we present an experimental study of the invariance and distortion of vectorial light through a real-world medium of atmospheric turbulence. We show that the amplitude and polarization structure are both severely distorted by the turbulent medium, yet the non-separability of these two degrees of freedom remains invariant. We monitor this invariance under a range of beam types and atmospheric conditions, over extended time periods, revealing the unitary nature of atmospheric turbulence in our experiment. Our results provide conclusive evidence that invariance and distortion are not mutually exclusive and that the degree of classical entanglement remains unaltered through such channels, and will be of interest to the large community interested in classical and quantum communication in free space.

DOI
10.1063/5.0152065

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Vectorial structured light, where the polarization is inhomogeneously distributed in space, has found a myriad of applications in both 2D and 3D optical fields. Here, we present an experimental study of the invariance and distortion of vectorial light through a real-world medium of atmospheric turbulence. We show that the amplitude and polarization structure are both severely distorted by the turbulent medium, yet the non-separability of these two degrees of freedom remains invariant. We monitor this invariance under a range of beam types and atmospheric conditions, over extended time periods, revealing the unitary nature of atmospheric turbulence in our experiment. Our results provide conclusive evidence that invariance and distortion are not mutually exclusive and that the degree of classical entanglement remains unaltered through such channels, and will be of interest to the large community interested in classical and quantum communication in free space.
Structured light offers a promising solution for the increasing data demands of modern optical networks, opening up degrees of freedom that can be leveraged for greater channel capacity and more bits per photon. However, its implementation is hindered by real-world distortions, for example, atmospheric turbulence in free space, with severe and rapidly evolving phase perturbations that alter the amplitude, phase, and vectorial polarization structure of the beam. Here, we demonstrate that optical topologies in the form of skyrmions are highly resilient to the effects of real-world atmospheric turbulence. We create and transmit these particle-like topologies of light through a 270-meter free-space optical link, revealing their robustness across a wide variety of conditions and turbulence strengths. While we observe severe distortion in the states' underlying degrees of freedom, we show that the topological numbers are preserved in all cases. We account for fast changes to the medium, where the channel produces statistically averaged outcomes, by probing the state's decoherence, showing that while the degree of polarization consequently decays, the topology remains intact. Using topology, we show that information can be transmitted through the channel with almost perfect fidelity (>98%) in most cases, only decreasing to 86% in the most severe conditions tested. This work idemonstrates the potential for optical topologies as reliable and robust information carriers in a real-world environment and points to the potential for other complex channels too, offering attractive features for classical and quantum communication alike.
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.
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.
Abstract The degradation of classical and quantum structured light induced by complex media constitutes a critical barrier to its practical implementation in a range of applications, from communication and energy transport to imaging and sensing. Atmospheric turbulence is an exemplary case due to its complex phase structure and dynamic variations, driving the need to fnd invariances in light. Here we construct classical and quantum optical skyrmions and pass them through experimentally simulated atmospheric turbulence, revealing the embedded topological resilience of their structure. In the quantum realm, we show that while skyrmions undergo diminished entanglement, their topological characteristics maintain stable. This is paralleled classically, where the vectorial structure is scrambled by the medium yet the skyrmion remains stable by virtue of its intrinsic topological protection mechanism. Our experimental results are supported by rigorous analytical and numerical modelling, validating that the quantum-classical equivalence of the topological behaviour is due to the non-separability of the states and one-sided nature of the channel. Our work blurs the classical-quantum divide in the context of topology and opens a new path to information resilience in noisy channels, such as terrestrial and satellite-to-ground communication networks.
In this paper, we focus on quantum communication systems that facilitate either secure data transfer or quantum key distribution via free-space links. Unlike classical channels where the effects of turbulent media on the optical wave front is well known and can be predicted with existing theoretical models, the mechanism described in the latter cannot be directly applied to quantum states. In our approach that relies on emitting correlated photon pairs with polarization entanglement, another realm of problems is encountered, which is not related to wave front distortions, but rather to integrity of the quantum states. Proper response of the detection system to non-classical features of light requires that photon pairs with proper polarization arrive to the receiver and their correlation characteristics are still preserved. Therefore, it is necessary to research a wide array of operating conditions corresponding to different levels of turbulence and finding proper mechanisms to replicate those on our laboratory testbed. In this paper, we present a system that integrates an atmospheric chamber developed by the AFRL, a link emulating quantum communication and analysis instrumentation. A system is developed that allows scaling the experiments over different ranges and quantitative analysis of entanglement characteristics of the received signals. Integrity of the quantum states is evaluated under practical operating conditions.