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Quantum Communications and Quantum Imaging

Investigation of propagation characteristics of laser light and squeezed light in fog

Genta Masada

Published Oct 4, 2022• 1 citation
Publisher

Abstract

There is a great deal of interest in highly sensitive sensing technology that utilizes the quantum entanglement of quantum light. Light wave sensing based on the quantum illumination theory is expected to be used outdoors with atmospheric disturbances. For the realization of such technology, it is extremely important to know the fundamental properties of quantum light propagation under various atmospheric conditions. In this study, we first investigated the propagation characteristics of laser light in fog. Next, we investigated the effects of fog on the quantum properties of single-mode squeezed light. It was found that when the fog density increases, not only the light energy is attenuated, but also the noise is superimposed on the intensity distribution. It was also found that the effect of fog on the squeezed light is mainly energy attenuation.

DOI
10.1117/12.2633384

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There is a great deal of interest in highly sensitive sensing technology that utilizes the quantum entanglement of quantum light. Light wave sensing based on the quantum illumination theory is expected to be used outdoors with atmospheric disturbances. For the realization of such technology, it is extremely important to know the fundamental properties of quantum light propagation under various atmospheric conditions. In this study, we first investigated the propagation characteristics of laser light in fog. Next, we investigated the effects of fog on the quantum properties of single-mode squeezed light. It was found that when the fog density increases, not only the light energy is attenuated, but also the noise is superimposed on the intensity distribution. It was also found that the effect of fog on the squeezed light is mainly energy attenuation.
Quantum illumination is a technology that aims to improve the detection sensitivity of a target by utilizing quan- tum entangled state of light. We are studying quantum illumination using two-mode squeezed light. In quantum illumination, one light wave in a quantum entangled state is emitted toward a target during atmospheric disturbance. Therefore, it is extremely important to investigate the propagation characteristics of light waves under various atmospheric conditions toward the realization of quantum illumination. As an example of atmospheric disturbances, we have investigated the effect of uniform fog on light wave propagation in previous studies. It has been found that the effect of uniform fog on the propagation of laser light is mainly energy attenuation. Here we measure the propagation characteristics of single-mode squeezed light in a fog. We succeeded in observing the squeezed light that passed through the fog, and confirmed that the noise level of the squeezed light was degraded by the influence of the fog. It was found that the degradation of the noise level can be explained by considering that the influence of fog is mainly the attenuation of energy.
Quantum illumination utilizing two-mode squeezed light beams is a promising target detection technology. It is expected that the error probability of target detection will be improved even under atmospheric disturbance such as loss and noise because of the non-classical property of the light source. One of the entangled beam, called the signal beam, is reflected by a target in atmospheric disturbance and then detected by homodyne measurement at the receiver. To improve detection efficiency, it is important to study optical wave propagation at various atmospheric conditions. We focused on fog as an example of atmospheric disturbance and investigated the effect of fog on near-infrared laser beam. We studied beam propagation using Mach-Zehnder interferometer including a chamber filled with a uniform fog. It was observed that the visibility of interference decreased with increasing fog density. Our experimental results and theoretical analysis indicate that the effect of uniform fog is mainly energy loss and does not change the spatial modes of light wave.
We aim to study quantum illumination using two-mode squeezed light. When quantum illumination is applied to outdoor radar applications, one of the light waves in the quantum entangled state is emitted toward the atmospheric disturbance. Therefore, it is extremely important to investigate the effect of atmospheric disturbance on the propagation characteristics and quantum nature of the lightwave. In this study, we focus on thermal fluctuations and air ow as atmospheric disturbances. First, the effect of atmospheric disturbance on the propagation characteristics and interferometry of light waves are investigated using laser light. Then, the effect of atmospheric disturbance on the quantum properties of single-mode squeezed light is investigated. We found that when the squeezed light passes through thermal fluctuations, the optical axis and intensity distribution are affected and the observed squeezing level fluctuates significantly. This may be due to the fact that the propagation of the squeezed light was affected by the thermal fluctuation and the interference efficiency with the local oscillator light fluctuated. On the other hand, when the squeezed light passed through the air ow, the propagation was affected very little. The observed squeezing levels remained almost unchanged.
We started to research quantum illumination radar and quantum imaging by utilizing high quality continuous-wave two-mode squeezed light source as a quantum entanglement resource. Two-mode squeezed light is a macroscopic quantum entangled state of the electro-magnetic field and shows strong correlation between quadrature phase amplitudes of each optical field. One of the most effective methods to generate two-mode squeezed light is combining two independent single-mode squeezed lights by using a beam splitter with relative phase of 90 degrees between each optical field. As a first stage of our work we are developing two-mode squeezed light source for exploring the possibility of quantum illumination radar and quantum imaging. In this article we introduce current development of experimental investigation of single-mode squeezed light. We utilize a sub-threshold optical parametric oscillator with bow-tie configuration which includes a periodically-polled potassium titanyl phosphate crystal as a nonlinear optical medium. We observed the noise level of squeezed quadrature −3.08±0.13 dB and anti-squeezed quadrature at 9.29±0.13 dB, respectively. We also demonstrated the remote tuning of squeezing level of the light source which leads to the technology for tuning the quantum entanglement in order to adapt to the actual environmental condition.
Two-mode squeezed light is an effective resource for quantum entanglement and shows a non-classical correlation between each optical mode. We are developing a two-mode squeezed light source to explore the possibility of quantum radar based on the quantum illumination theory. It is expected that the error probability for discrimination of target presence or absence is improved even in a lossy and noisy environment. We are also expecting to apply two-mode squeezed light source to quantum imaging. In this work we generated two-mode squeezed light and verify its quantum entanglement property towards quantum radar and imaging. Firstly we generated two independent single-mode squeezed light beams utilizing two sub-threshold optical parametric oscillators which include periodically-polled potassium titanyl phosphate crystals for the second order nonlinear interaction. Two single-mode squeezed light beams are combined using a half mirror with the relative optical phase of 90◦ between each optical field. Then entangled two-mode squeezed light beams can be generated. We observes correlation variances between quadrature phase amplitudes in entangled two-mode fields by balanced homodyne measurement. Finally we verified quantum entanglement property of two-mode squeezed light source based on Duan’s and Simon’s inseparability criterion.
Two-mode squeezed light is a macroscopic quantum entangled state of electro-magnetic fields and shows non-classical correlation between quadrature phase amplitudes in each optical mode. In this work the author is developing a high-quality two-mode squeezed light source for exploring the possibility of a quantum radar system based on a quantum illumination method and also expecting to apply it to quantum imaging. Two-mode squeezed light can be generated by combining two independent single-mode squeezed light beams using a beam splitter with a relative optical phase of 90 degrees between them. In current experimental progress the author developed two sub-threshold optical parametric oscillators to generate single-mode squeezed light beams. In the actual quantum radar or quantum imaging system, a turbulent atmosphere degrades quantum entanglement of a light source and affects performance of target detection. An optical loss is one of the simplest and most probable examples of environmental factors. In this work an evaluation method for quantum entanglement of two-mode squeezed light source is developed with consideration for the optical loss based on Duan's inseparability criteria.