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Journal of Xidian University

Study of quantum communication systems used for free space

Lei Zhang

Published Jan 1, 2007• 2 citations
OpenAlex

Abstract

An experimental system of quantum communication that operated for free space is presented.This system is composed of two subsystems,which use attenuated laser or entangled photon pairs as the photon source,respectively.It can be used for experimental investigation in atmospheric characteristics of the system using a single photon source or entangled photon pairs,respectively,along with a comparison of characteristics between the two systems.The results show that the atmosphere in the lower layer plays an important role in affecting the quantum communication,and that the efficiency of the quantum key distribution is higher in the system using a single photon source.

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We report free-space distribution of entangled photon pairs over a noisy ground atmosphere of 13 km. It is shown that the desired entanglement can still survive after both entangled photons have passed through the noisy ground atmosphere with a distance beyond the effective thickness of the aerosphere. This is confirmed by observing a spacelike separated violation of Bell inequality of 2.45+/-0.09. On this basis, we exploit the distributed entangled photon source to demonstrate the Bennett-Brassard 1984 quantum cryptography scheme. The distribution distance of entangled photon pairs achieved in the experiment is for the first time well beyond the effective thickness of the aerosphere, hence presenting a significant step towards satellite-based global quantum communication.

arXiv:quant-ph/0412218•Physical Review Letters•Apr 20, 2005•302 citations
An experimental system of quantum communication that operated for free space is presented.This system is composed of two subsystems,which use attenuated laser or entangled photon pairs as the photon source,respectively.It can be used for experimental investigation in atmospheric characteristics of the system using a single photon source or entangled photon pairs,respectively,along with a comparison of characteristics between the two systems.The results show that the atmosphere in the lower layer plays an important role in affecting the quantum communication,and that the efficiency of the quantum key distribution is higher in the system using a single photon source.
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.
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.
The results of physical simulation of various optical disturbances in the atmospheric segment of a quantum wireless communication system are presented. Experimental setups for evaluating changes in the characteristics of the quantum key distribution system in an atmospheric communication channel in the presence of optical inhomogeneities in free-space (thermal zone disturbance, scattering water aerosol, incoming airflow) are described. Dynamic changes in quantum bit error rate (QBER) and the quantum sifted key generation rate under the influence of interfering disturbances in the atmospheric channel were obtained. It is determined that the strongest fluctuations in the characteristics of the quantum channel occurred when the source of disturbances was placed directly near the optical collimator connected to the quantum key distribution unit, which housed the single photon source.
Today, perhaps more than ever before, the security of information and communication is increasingly important. The cryptographic protocols used in classical cryptography are based on mathematical algorithms and can be breached. On the other hand, the laws of quantum physics enable quantum key distribution (QKD) and it is possible to achieve absolute security of communication based on the fundamental laws of nature. Anyone who would eavesdrop QKD-based communication must perform a measurement, which implies an impact on the system that can be detected by the sender and receiver. In this paper, we present a brief overview of QKD implementations and our experimental setup for generation of quantum key. Using this setup, we measured violation of the Bell's inequality of 114 standard deviations from the classical boundary, which confirms the non-local behavior of the entangled photon pairs. One can also use this source to create hybrid connections in combination with both optical fibers and free space.
Subject of study. This study examined the applicability of quantum key distribution (QKD) technology under free-space atmospheric conditions. Aim of study. The objective was the development of a hardware and software system for investigating the applicability of QKD technology in wireless communication systems based on commercially available modules. The system includes optical blocks for atmospheric optical communication and a training setup for studying QKD in fiber-optic communication lines. Method. The characteristics of a quantum communication channel in free-space conditions were studied through experiments. The channel was formed by the QKD units of the EMQOS 1.0 scientific and educational complex, integrated with optical blocks for atmospheric optical communication. The systems were also upgraded to minimize losses in the quantum channel. Main results. A hardware and software system was developed, within which the transmission of a quantum key through free-space atmospheric conditions over a distance of 180 m was successfully demonstrated. The quantum bit error rate did not exceed 6%. This system enabled the study of the effects of weather conditions on optical losses in the quantum communication channel and the quantum key generation rate. Practical significance. The developed hardware and software system allows the monitoring of the parameters of both classical and quantum communication channels in free space under various weather conditions. Its modular structure enables the integration of other quantum equipment for comparative testing of the performance of quantum communication systems under real atmospheric transmission conditions.
We report free-space distribution of entangled photon pairs over a noisy ground atmosphere of 13 km. It is shown that the desired entanglement can still survive after both entangled photons have passed through the noisy ground atmosphere with a distance beyond the effective thickness of the aerosphere. This is confirmed by observing a spacelike separated violation of Bell inequality of 2.45+/-0.09. On this basis, we exploit the distributed entangled photon source to demonstrate the Bennett-Brassard 1984 quantum cryptography scheme. The distribution distance of entangled photon pairs achieved in the experiment is for the first time well beyond the effective thickness of the aerosphere, hence presenting a significant step towards satellite-based global quantum communication.