Quantum limits of the resolution of imaging and lidar systems

BI Si-wen

BI Si-wen, Hailong Zhu, Jingshi Du +5 more

As a combination of laser technology and modern photoelectric detection technology, lidar (laser radar) has notable advantages compared with traditional radar such as high resolution, strong anti-interference ability, and good low-altitude detection performance. However, with continuous application in aerospace, military, artificial intelligence and other fields, lidar has reached the limits of classical physics and can’t meet the requirements in detection, measurement and imaging. In order to break the bottleneck, worldwide researchers have developed a new type of radar-quantum lidar, which is based on the combination of traditional radar technology and quantum information technology. Its main principle is to get imaging through squeezed light that is below the noise limit of classical physics and then manipulated by the quantum state to achieve imaging with higher quality. Quantum lidar in the paper injects quantum squeezed light to receiver end and uses quantum phase-sensitive amplifier technology to compensate the photon loss that caused by insufficient quantum power to further improve resolution. In the squeezed light, the generation of quantum squeezed light and detection theory are studied, the resonant cavity locking principle and balanced homodyne detection principle are analyzed, and a mode cleaner with narrow line width is designed to improve beam transverse mode quality of laser and filter high frequency noise. In order to reduce quantum noise and improve resolution, quantum lidar imaging system is developed and the theoretically derivation and numerical simulation results show that the resolution of quantum lidar imaging is 1.71 times higher than that of lidar imaging.

BI Si-wen, Zhe Sheng, Zhiying Zhou +3 more

Lidar (laser radar) offers advantages over conventional radar in terms of higher resolution, greater anti-interference capability and ultra-low altitude detection performance. However, Lidar has now reached a bottleneck in terms of detection range and is no longer able to correctly identify targets beyond 500 meters, thus failing to meet the requirements for detection, measurement and imaging. To break this bottleneck, we have developed a Quantum Lidar with a detection range of up to one kilometre, which is based on a quantum vacuum squeezed laser and can compress the photon fluctuation by 5.6dB in the vacuum state through quasi-phase matching in non-linear crystals, reducing the quantum noise of laser which accounts for 73% of the lidar, thus reducing the overall noise of the quantum lidar by 3.27dB and improves the phase stability of the laser (from 10 minutes to more than 120 minutes) to achieve higher accuracy measurements, with a Signal-Noise Ratio (SNR) twice that of conventional lidar, and therefore a sensitivity twice as high. The basic technical implementation is to use a 20W power 1064nm wavelength continuous wave laser (CW, M2 less than or equal to 1.05) to irradiate a target located one kilometre away. By detecting a very weak reflected laser (power=10μW ± 5μW) which is exponentially attenuated by distance, We use noiseless amplification by a quantum vacuum-squeezed laser with the same power magnitude to obtain a higher power (7dB amplification) and higher signal-noise ratio (4.77dB SNR improvement). A reflective lens set (250mm aperture) of our design is used to receive the laser diffusion due to the 4mrad transmission angle of one kilometre. In the squeezed laser system, the resonant cavity locking efficiency is improved to 99.7%, and the re-locking time is less than 0.3s, it is directly switchable with the balance homodyne detection system. The mode cleaner improves the transverse mode quality(the mode bandwidth is reduced by 67%) while filtering high-frequency noise. The quantum lidar imaging system has been developed, and the experimental results show that the resolution of the laboratory-simulated long-range decay experiment is 3 times higher than that of traditional lidar is a promising development. This means that the system can detect and image smaller objects and features with greater accuracy and precision. The use of a self-developed quantum enhancement algorithm to obtain a 5 times higher contrast lidar image is also a significant improvement, and the use of a quantum denoising algorithm to reduce graphic distortion and image scatter caused by atmospheric pollution and environmental interference is a critical development. These issues can significantly degrade the quality of lidar images, so being able to mitigate their effects is a significant step forward. Through a self-developed LiDAR 3D reconstruction algorithm, the reconstruction efficiency is increased by 3 times and the resolution is increased by 2.5 times, which means that the reconstructed 3D model is faster, more detailed and accurate. In the future, based on one kilometre's work, achieving a quantum lidar with a detection distance of 10, 20 or even 50 kilometres with much clearer images would require advancements by steadily improving the resolution, improving the signal-noise ratio of the system, improving the laser power and optimising the algorithm.

Mikhail I. Kolobov

Haochen Li, Kaimin Zheng, Rui Ge +18 more

Quantum-inspired imaging techniques have been proven to be effective for LiDAR with the advances of single photon detectors and computational algorithms. However, due to the disturbance of background noise and the varies of signal in outdoor environment, the performance of LiDAR is still far from its ultimate limit set by the quantum fluctuations of coherent probe light. In this work, we propose and demonstrate a LiDAR from the detection perspective for approaching the standard quantum-limited performance. The photon numbers of echo signals are recorded by a photon-number-resolving detector and applied to overcome heavy background noise through an active photon number filter in the LiDAR. It can approach the standard quantum limit in intensity estimation in a wide photon-flux range, and achieve a Fisher information of only 0.04 dB less than the quantum Fisher information when the mean signal photon number is 10. Experimentally, a noise-free target reconstruction and imaging is demonstrated in the daytime by the proposed LiDAR. It also performs better in reflectivity resolution when taking only 1/1000 of the measurements based on on/off detection. This work provides a fundamental strategy for constructing a LiDAR to quickly extract targets and identify materials in complex environments, which is important for intelligent agents such as autonomous vehicles.

Zachary Dutton, John J. Myers