Classical optical imaging requires that the light source, object, imaging lens, and imaging plane are in a collinear position, and the first-order correlation property of the light field is employed to obtain the object information for imaging. Different from classical optical imaging, quantum imaging divides the imaging optical path into the signal optical path and the reference optical path and separates the process into the detection process and imaging process, which can complete imaging in the case that classical optical imaging cannot be achieved. As an important development field of quantum information science, quantum precision measurement studies how to utilize quantum effects to measure physical quantities. Compared with traditional measurement techniques, quantum precision measurement has more advantages in measurement accuracy, sensitivity, and security. In the medical and aerospace fields, it has caught widespread attention in China and abroad, with great development potential. Although quantum imaging is based on photon arrival time series received on signal and reference optical paths for coincidence measurement or intensity correlation to achieve imaging, with strong anti-interference ability compared with classical optical imaging, there are still some problems. In the reference optical path, it is necessary to adopt digital micromirror device (DMD) for two-dimensional spatial scanning to obtain spatial information. Pixel-by-pixel scanning is required to obtain accurate coincidence values, which limits imaging efficiency. To improve the efficiency of entangled optical quantum imaging, our study adopts the two-step coincidence counting method to quickly obtain target imaging information and reduce the time overhead of entangled optical quantum imaging.

First, the pump light generated by the laser is modulated by the combination of the lens and wave plate to improve the efficiency of spontaneous parametric down-conversion of the periodically poled KTiOPO4 (PPKTP) crystal. Second, the ranging region is selected by the DMD to construct the difference value of the single-photon time pulse sequence. Third, this difference value is leveraged to complete local coincidence counting to obtain the time difference between signal and reference optical paths. Fourth, by controlling the DMD, the imaging region is selected; the single-photon time pulse sequence is corrected, and then global coincidence counting is completed by this corrected sequence. Finally, the quantum image of the target is obtained by mapping the coincidence counting value into the gray value.

As the distance between the light source and the target increases, the image quality decreases (Figs. 4 and 5). Although the imaging quality of the proposed method is slightly lower than that of the classical quantum imaging method, the required imaging time overhead is significantly reduced. The average imaging time overhead of the classical quantum imaging method is 179.1807 s, while that of our method is only 0.2412 s. Our method can significantly improve imaging efficiency and obtain the distance between the light source and the target, and the corresponding average ranging error is 0.039 m (Table 1). With the increasing ranging area size, the imaging quality improves. When the ranging region is greater than 16 pixel×16 pixel, the imaging quality of our proposed method is better than that of the classical quantum imaging method. The main reason is that the classical quantum imaging method only employs the photon information of a single pixel position to estimate the delay difference, while our method processes the photon information of all pixels in the ranging region. The increase in photon information can obtain more accurate coincidence values and better imaging quality (Figs. 6 and 7). Compared with the imaging time overhead of the classical quantum imaging method (with an average imaging time overhead of 180.1317 s), the imaging efficiency of our method is significantly improved (with an average imaging time overhead of 1.1326 s), and meanwhile, accurate target ranging can be achieved spontaneously (Table 2). With the rising single pixel exposure time (SPET), the number of photons received at each pixel increases, and then more accurate coincidence values can be obtained. When SPET are 1, 2, and 3 s, the PSNRs of the proposed method are 4.2914, 14.6427, and 17.8427 respectively, while the PSNRs of the classical quantum imaging method are 3.1075, 12.8154, and 17.7154 respectively. As the SPET increases, the image quality also improves (Table 3). In addition, a real optical path is built to conduct actual experiments based on the above simulation analysis. The associated results are shown in Table 4, Table 5, and Table 6, and are consistent with simulation results.

A new entangled optical quantum imaging method based on two-step coincidence counting is proposed. This method adaptively acquires and corrects the delay difference between signal and reference optical paths when the target location is unknown to obtain more accurate coincidence values and realize quantum imaging and distance estimation of the target. Compared with the classical quantum imaging method that corrects coincidence counting for each pixel, the proposed method employs the DMD to select local areas for coincidence counting and then utilizes the delay difference obtained during the imaging to correct the photon arrival time series. This method can reduce repeated operations in the classical quantum imaging method and improve the imaging speed on the premise of ensuring certain imaging quality. In addition, with the decreasing distance between the light source and the target and the increasing ranging area, the imaging quality can be effectively improved. Furthermore, an actual quantum imaging optical path is built, and the corresponding experimental results are consistent with simulation results, verifying the effectiveness of the proposed method.