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Quantum Communications and Quantum Imaging

Experimental characterization of entanglement characteristics in free-space quantum communication links

Vladimir V. Nikulin, Vijit Bedi, Kathy-Anne Soderberg, Paul M. Alsing, Laura Wessing, Peter A. Ricci, John W. Heinig, William Lipe

Published Sep 5, 2023• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2682030

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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.
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.
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.
This thesis is focused on the development of advanced techniques for quantum communications in free-space channels. At present there are several demonstrations of quantum communication protocols at long distances, many of these exploit optical fiber as transmission channel. The optical fiber is very advantageous in that it is scarcely influenced by external conditions and allows to connect remote localities that are not in direct line of sight. On the other hand, it presents strong limitations in transmission distance because of dispersion and attenuation. It is therefore necessary to explore new transmission channels in order to allow a global spread of quantum communication. The free-space channels are good candidates, both the vertical ones (between earth and space) and the horizontal ones. The study of these is of fundamental importance for the extension of quantum communication over a global scale. Since the eighties to present day have been defined several quantum communication protocols such as Bennet-Brassard 84 (BB84), Bennet 92 (B92), Ekert 91, Decoy State, Dense Coding, etc.. These protocols present different characteristic such as reliability, security and key rate. These three parameters are very important for the characterization of a communication protocol and are directly related to each other. The development of new quantum communication protocols that maximize these parameters is very important for future progress of quantum communications. This thesis is divided into three parts. In the first part we consider the characteristics and the behaviour of a free-space horizontal quantum channel. We study the impact of atmospheric turbulence in the case of single beam propagation and twin-beam propagation. We then measure the losses of the channel and analyse the effect of turbulence on photon statistics. Finally, we propose a method that exploits the turbulence to improve the signal to noise ratio (SNR) of the channel. In the second part we take into consideration the security and efficiency of Quantum Key Distribution (QKD) protocols . We propose the experimental demonstration of the B92 protocol with non-maximally entangled states . Using this kind of states allow us to improve the security and the key rate of this protocol. Finally, in the last part, we presented a possible way to improve the quantum channel capacity exploiting hyperentangled photon pairs. We designed and experimentally tested a system that can be used to transmit hyperentangled states at long distances.
Atmospheric channels are a promising candidate to establish secure quantum communication on a global scale. However, due to their turbulent nature, it is crucial to understand the impact of the atmosphere on the quantum properties of light and examine it experimentally. In this paper, we introduce a method to probe atmospheric free-space links with quantum light on a laboratory scale. In contrast to previous works, our method models arbitrary intensity losses caused by turbulence to emulate general atmospheric conditions. This allows us to characterize turbulent quantum channels in a well-controlled manner. To implement this technique, we perform a series of measurements with different constant attenuations and simulate the fluctuating losses by combining the obtained data. We directly test the proposed method with an on-chip source of nonclassical light and a time-bin-multiplexed detection system. With the obtained data, we characterize the nonclassicality of the generated states for different atmospheric noise models and analyze a postselection protocol. This general technique in atmospheric quantum optics allows for studying turbulent quantum channels and predicting their properties for future applications.
This paper describes high-performance optical circuits operating on hyper-entangled quantum states. The concept is based on correlated photon pairs generated in such a way that their polarization, frequency or propagation paths are entangled and can be used to support quantum encryption protocols. Detection and processing of such signals requires hyper-spectral optical circuits capable of responding to non-classical features of light. Processing capabilities of these quantum circuits are straight-forward in theory, but physical implementation can still present a challenge for integrity of the quantum states. In this paper, we address experimental verification of entanglement characteristics at each step of building the quantum optical circuits and present a formal quantitative approach based on the quantum state tomography.