Experimental device and method for realizing quantum imaging by using sunlight

Abstract

The invention discloses an experimental device and method for realizing quantum imaging by using sunlight. The device comprises a sunlight shaping system, a quantum correlation photon pair generation unit and a wide spectrum quantum imaging system, wherein the sunlight shaping system is used for shaping collected small-spot sunlight and polarizing the small-spot sunlight into a pump beam polarized in the horizontal direction, so that the pump beam can completely penetrate through the quantum correlated photon pair generation unit; the quantum correlation photon pair generation unit is used for receiving the pumping light beam and splitting the light beam to generate quantum correlation photon pairs under a wide spectrum; the polarization directions of the quantum correlation photon pair are respectively vertical polarization and horizontal polarization; a wide-spectrum quantum imaging system and the like. According to the invention, collection, transmission and shaping of sunlight can be realized, a spontaneous parametric down-conversion process is carried out on the wide-spectrum light beam nonlinear crystal, enough wide-spectrum down-conversion two-photon pairs for quantum imaging are generated, and a set of experimental device is provided for application research of sunlight in the aspect of quantum imaging.

Description

Technical Field

[0001] The present invention relates to the technical field of quantum imaging, and in particular to an experimental device and method for realizing quantum imaging by using sunlight. Background Technique

[0002] Ghost Imaging (GI) is generally considered a technique for realizing imaging by using light that has never physically interacted with the object to be measured. The classical GI scheme was first realized by the Shih research group in 1995 [T. B. Pittman, Y. H. Shih, D. V. Strekalov, et al, Phys. Rev. A 52, R3429 (1995)]. Its core feature is that only a single-pixel detector or a bucket detector is required to collect the object's reflected / transmitted light signal, while a reference detector with spatial resolution ability records the light source information. This experiment revealed how to construct a GI system by using the spatial correlation characteristics between the signal-idler photon pairs generated by the spontaneous parametric down-conversion (SPDC) process - this process is usually realized by using a laser nonlinear crystal.

[0003] In many studies of classical GI, an innovative experiment used sunlight as the light source: first, its narrow spectral lines were filtered out, and then the intensity correlation characteristics were measured, successfully realizing the imaging of an object with a double-hole structure [X.-F. Liu, X.-H. Chen, X.-R. Yao, et al, Opt. Lett. 39, 2314 (2014)]. Although the contrast of the double-hole image is low, the success of this experiment marks the first step of classical GI technology with daily sunlight as the illumination light source towards practical applications. It should be noted that the non-zero background in classical GI makes the image contrast have an upper limit in theory. Due to the ultra-short coherence length of sunlight in the transverse light field, how to improve the imaging contrast of sunlight GI has become the core challenge in its practical application.

[0004] It is worth noting that some researchers have used commercial light-emitting diodes (LEDs), a completely incoherent pumping source, to induce the SPDC process and systematically characterized the correlation properties of two-photon pairs [G. Tamoˇsauskas, J. Galinis, A. Dubietis, et al, Opt. Express 18, 4310(2010)]. In addition, by modulating the laser phase into a partially spatially coherent beam (also known as pseudo-thermal light) through a rotating diffuser, the generation of spatially [L. Hutter, G. Lima, and S. P. Walborn, Phys. Rev. Lett. 125, 193602 (2020)] and polarization [Y. Ismail, S. Joshi, and F. Petruccione, Scientific Reports 7 (2017).] entangled photon pairs has been successfully achieved experimentally. Studies have shown that SPDC photon pairs based on partially coherent pumping have the following characteristics: they show stronger robustness to atmospheric turbulence effects [Y. Qiu and W. She, Appl. Phys. B 108, 683 (2012)]; they can enhance the two-photon entanglement degree [L. Hutter, G. Lima, and S. P. Walborn, Phys. Rev. Lett. 125, 193602 (2020)].

[0005] Although the above work has confirmed the feasibility of generating photon pairs by incoherent light pumping of SPDC, the experimental implementation technology for ideal incoherent light sources such as sunlight is still blank. This leads to a key question: Can we truly use sunlight to pump and generate SPDC photon pairs and achieve ghost imaging experimentally? The core challenges of sunlight-pumped SPDC are: how to stably couple the "dynamic" sunlight into the nonlinear crystal and achieve measurable photon counting under the condition of stable docking of the detector fiber. Its advantages are reflected in that it can be achieved by using natural light sources, and at the same time, the imaging contrast exceeds that of the classical GI system. This breakthrough provides a key technical path for constructing a space-based quantum imaging system without lasers or external power supply. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to propose an experimental device and method for realizing quantum imaging using sunlight, which can collect, transmit, and shape sunlight, and thus enable the spontaneous parametric down-conversion process to occur in a broadband spectral beam nonlinear crystal, generating sufficient broadband down-converted two-photon pairs for quantum imaging, and providing a set of experimental devices for the application research of sunlight in quantum imaging.

[0007] According to one aspect of the present invention, an experimental device for realizing quantum imaging by using sunlight is provided, including: A sunlight shaping system, a quantum correlated photon pair generation unit, and a wide-spectrum quantum imaging system; wherein, The sunlight shaping system is used to shape and polarize the collected small-spot sunlight into a pump beam polarized in the horizontal direction, so that the pump beam can completely pass through the quantum correlated photon pair generation unit; The quantum correlated photon pair generation unit is used to receive the pump beam and split the beam to generate quantum correlated photon pairs under a wide spectrum; the polarization directions of the quantum correlated photon pairs are vertically polarized and horizontally polarized respectively; The wide-spectrum quantum imaging system is used to receive the quantum correlated photon pairs, project the vertically polarized photons onto the object to be measured, and collect the first light intensity signal; project the horizontally polarized photons onto the target area, and collect the second light intensity signal point by point in the target area; extract the second light intensity signal associated with the first light intensity signal based on the light intensity signal time stamp, and input it into an analysis device for point-by-point imaging based on the dot matrix.

[0008] In the above technical solution, by using sunlight to pump a nonlinear crystal to generate wide-spectrum down-converted photon pairs, quantum imaging based on sunlight is realized. Further, this device demonstrates the feasibility of using sunlight as a light source to generate entangled photons, and is expected to promote the application of establishing a sunlight-pumped quantum light source in outer space in the future, with broad application prospects. Specifically: The sunlight shaping system shapes and polarizes the collected small-spot sunlight into a pump beam polarized in the horizontal direction, so that it can completely pass through the quantum correlated photon pair generation unit. By shaping and polarizing the sunlight, a pump beam with suitable conditions for subsequent generation of quantum correlated photon pairs is provided, ensuring that the characteristics and direction of the beam meet the experimental requirements. This is the initial light source processing link in the entire quantum imaging process. The pump beam processed in this way can completely pass through the quantum correlated photon pair generation unit, providing a stable and suitable light source for subsequent generation of quantum correlated photon pairs.

[0009] The quantum-correlated photon pair generation unit receives a pump beam, splits the beam to generate quantum-correlated photon pairs under a broad spectrum, and the polarization directions of the quantum-correlated photon pairs are vertically polarized and horizontally polarized respectively. Generating quantum-correlated photon pairs using a pump beam is a key step in quantum imaging. Through a specific nonlinear optical process (such as spontaneous parametric down-conversion, etc.), pump photons are converted into two photon pairs with quantum correlation in a broad spectral range, and by control, their polarization directions are made different, providing a light source with specific quantum properties for the subsequent imaging process. In this process, due to the properties of materials such as nonlinear optical crystals, the two generated photons are correlated in quantum states such as energy, momentum, and polarization. Among them, the polarization direction of one photon is vertically polarized, the polarization direction of the other photon is horizontally polarized, and the spectral range of these two photons is relatively wide, covering multiple bands from visible light to near-infrared, that is, the broad-spectrum property.

[0010] The broad-spectrum quantum imaging system receives quantum-correlated photon pairs, projects the vertically polarized photons onto the object to be measured, and collects the first light intensity signal; projects the horizontally polarized photons onto the target area, and collects the second light intensity signal point by point in the target area; extracts the second light intensity signal correlated with the first light intensity signal based on the time stamp of the light intensity signal, and inputs it into an analysis device for point-by-point imaging based on a dot matrix. This is the core part of realizing quantum imaging. After the broad-spectrum quantum imaging system receives quantum-correlated photon pairs, it projects the vertically polarized photons onto the object to be measured. When these photons irradiate the object to be measured, they will interact with the object, and some photons are reflected or scattered by the object, thus carrying the structure and information of the object. The system collects the light intensity signals of these reflected or scattered photons, denoted as the first light intensity signal. At the same time, the horizontally polarized photons are projected onto the target area. During the propagation of these photons in the target area, they will also interact with the substances in this area, and the system collects the light intensity signals of these photons point by point, denoted as the second light intensity signal. Due to the temporal correlation between quantum-correlated photon pairs, through the light intensity signal time stamp technology, the second light intensity signal (from the target area) correlated with the first light intensity signal (from the object to be measured) can be extracted. These correlated light intensity signals are input into the analysis device, and the analysis device performs point-by-point imaging according to the dot matrix method, and finally reconstructs the image of the object to be measured.

[0011] Sunlight is a natural, widely existing and energy-rich light source. Using sunlight as a pump light source eliminates the need for additional artificial light source equipment, reducing the experimental cost and equipment complexity. At the same time, sunlight has a wide spectral range, containing rich spectral information, which is conducive to realizing wide-spectrum quantum imaging and improving the quality and resolution of imaging. Previous quantum imaging experiments usually used artificial light sources such as lasers, while this device innovatively uses sunlight as a light source, opening up a new way for quantum imaging light sources and providing new ideas and methods for the development of quantum imaging technology. Wide-spectrum quantum imaging can obtain information about an object in multiple wavelength ranges. Compared with narrow-spectrum imaging, it can provide more abundant information about the object's structure and characteristics. For example, in biomedical imaging, wide-spectrum quantum imaging can simultaneously obtain characteristics such as absorption and scattering of cells or tissues under different spectra, helping to diagnose diseases more accurately; in materials science, it can study the optical property changes of materials under different spectra, providing more comprehensive data support for the research, development and application of materials.

[0012] In some embodiments, the quantum-correlated photon pair generation unit includes a nonlinear crystal and a nonlinear metasurface.

[0013] In the above technical solution, under the action of the pump beam, the nonlinear crystal can convert a high-energy pump photon into two low-energy quantum-correlated photon pairs through the spontaneous parametric down-conversion (SPDC) process. This process follows the laws of energy conservation and momentum conservation, that is, the total energy of the two generated photons is equal to the energy of the pump photon, and the total momentum is equal to the momentum of the pump photon. By optimizing the parameters of the crystal, such as the thickness, temperature, and intensity of the pump light, the generation efficiency of quantum-correlated photon pairs can be further improved, thereby increasing the signal intensity and imaging quality of quantum imaging. And the nonlinear crystal can generate quantum-correlated photon pairs in a relatively wide spectral range. For example, a BBO crystal can generate quantum-correlated photon pairs in the visible to near-infrared spectral range, which makes wide-spectrum quantum imaging possible, enabling quantum imaging to obtain information about an object in multiple wavelength ranges and improving the resolution and information content of imaging.

[0014] In some embodiments, the object to be measured includes a transmissive object or a reflective object that can reflect photons.

[0015] In the above technical solution, a transmissive object refers to an object that can allow photons to pass through. In quantum imaging, the internal structure and properties of a transmissive object will affect the propagation of photons, such as absorption, scattering, refraction, etc. These effects will change the properties of photons such as light intensity, phase, polarization, etc., thereby carrying the internal information of the object. A reflective object refers to an object that can reflect photons back. In quantum imaging, the surface properties of a reflective object will affect the reflection of photons, such as reflectivity, reflection angle, polarization, etc. These effects will change the properties of photons such as light intensity, phase, polarization, etc., thereby carrying the surface information of the object. Both of the above two objects can be applied to the solution of the present invention.

[0016] In some embodiments, the device further includes a sunlight collection system for collecting sunlight and reducing the spot size and then inputting it into the sunlight shaping system.

[0017] In the above technical solution, the main function of the sunlight collection system is to collect sunlight. After processing the collected sunlight, the sunlight collection system inputs it into the sunlight shaping system. By collecting and reducing the spot size, the sunlight collection system provides a light source with a suitable spot size and light intensity distribution for the sunlight shaping system. This enables the sunlight shaping system to better shape and polarize the sunlight, providing a high-quality pump beam for the subsequent generation of quantum correlated photon pairs and the quantum imaging process.

[0018] In some embodiments, the sunlight collection system includes a condenser lens, and an adjustment device that holds the focusing lens and the optical fiber and controls their rotation angles; the condenser lens focuses sunlight onto the end face of the optical fiber, and couples the light to the sunlight shaping system through the optical fiber.

[0019] In the above technical solution, as an optional solution, the condenser lens is the core optical component of the sunlight collection system, and its main function is to focus sunlight. By designing appropriate lens parameters, such as focal length, aperture, etc., the condenser lens can focus a large area of sunlight onto a small point, thereby increasing the light intensity and energy density. The condenser lens usually has a large aperture to collect more sunlight; at the same time, its focal length needs to be designed according to actual application requirements to ensure that the size of the focused light spot and the light intensity distribution meet the requirements of the subsequent system. The main function of the adjustment device is to hold the condenser lens and the optical fiber and control their rotation angles. Since the position of the sun changes over time, in order to ensure that the condenser lens can always accurately focus sunlight onto the end face of the optical fiber, it is necessary to adjust the angles of the condenser lens and the optical fiber in real time through the adjustment device. It should be noted that the sun tracking rotation device can refer to the prior art such as patent document EP2546975B1, which will not be elaborated here. After the sunlight passes through the condenser lens, the light is focused onto a very small point. This focal point is usually located on the end face of the optical fiber, and the light is transmitted to the subsequent sunlight shaping system through the optical fiber. The focal length and aperture of the condenser lens determine the size of the focused light spot and the light intensity distribution. By optimizing the lens parameters, efficient light collection and focusing can be achieved. Due to the equipped adjustment device, the sunlight collection system can flexibly adjust the angles of the condenser lens and the optical fiber according to the position change of the sun, ensuring that the sunlight can always be accurately focused onto the end face of the optical fiber. This flexible adjustment ability enables the system to work stably under different time and season conditions, improving the reliability and practicality of the system.

[0020] In some embodiments, the sunlight shaping system sequentially includes, along the optical path direction: At least one filter for filtering sunlight into a pump beam; the central wavelength of the pump beam is adapted to the central operating wavelength of the quantum correlated photon pair generation unit; At least one polarizer for polarizing the pump beam into a horizontally polarized state; At least one objective lens for focusing the pump beam.

[0021] In the above technical solution, the main function of the filter is to filter sunlight into a pump beam. The spectral range of sunlight is relatively wide, covering multiple bands from ultraviolet to infrared, while the quantum correlated photon pair generation unit usually requires light within a specific wavelength range as the pump light source. By selectively transmitting light within a specific wavelength range, the filter filters out light of other wavelengths in sunlight, thereby obtaining a pump beam that is adapted to the central operating wavelength of the quantum correlated photon pair generation unit. The main function of the polarizer is to polarize the pump beam into horizontal polarization. During the generation of quantum correlated photon pairs, the polarization state of photons has an important impact on the generation and properties of photon pairs. By using a polarizer to polarize the pump beam into horizontal polarization, it can be ensured that the polarization state of the pump beam matches the requirements of the quantum correlated photon pair generation unit, thereby improving the generation efficiency and quality of quantum correlated photon pairs. The main function of the objective lens is to focus the pump beam. The pump beam processed by the filter and the polarizer needs to be focused by the objective lens onto a specific position of the quantum correlated photon pair generation unit to achieve efficient photon pair generation.

[0022] In some embodiments, before the filter, there is also included: At least one objective lens and a diaphragm, which are used to collimate the collected sunlight and control the divergence angle.

[0023] In the above technical solution, the main function of the objective lens is to collimate the collected sunlight. After passing through the sunlight collection system, sunlight usually has a certain degree of divergence. The objective lens can convert the divergent beam into a parallel beam, that is, a collimated beam, through its optical characteristics. The optical path directions of the collimated beam are consistent and the light intensity distribution is uniform, which is beneficial for subsequent beam processing and shaping. The diaphragm can be used in conjunction with the objective lens to further optimize the divergence angle of the beam so that it meets the requirements of the subsequent filter and the quantum correlated photon pair generation unit. In quantum imaging experiments, high-quality pump beams are required to stimulate the generation of quantum correlated photon pairs. The use of the objective lens and the diaphragm can ensure that the collimation and divergence angle of the pump beam meet the requirements, thereby improving the generation efficiency and quality of quantum correlated photon pairs.

[0024] In some embodiments, the broadband quantum imaging system sequentially includes along the optical path direction: At least one filter, which is used to filter out the unsplit pump beam; An imaging lens group, which is used to project the generated quantum correlated photon pairs completely; A polarization beam splitter, which is used to project photons polarized in the vertical direction to a first direction and project photons polarized in the horizontal direction to a second direction; And, The object to be measured and the first fiber optic coupling module arranged in sequence along the first direction; wherein, the first fiber optic coupling module is used to collect the first light intensity signal and couple it to the photon counting system; The second fiber optic coupling module arranged in the second direction and with a controllable relative position, which is used to collect the second light intensity signal point by point in the target area and couple it to the photon counting system.

[0025] In the above technical solution, the main function of the filter is to filter out the unsplit pump beam. During the quantum imaging process, the pump beam is used to excite the generation of quantum-correlated photon pairs, but the unsplit pump beam may interfere with the imaging result. By using the filter, the unsplit pump beam can be filtered out to ensure that only the quantum-correlated photon pairs participate in the imaging process. The main function of the imaging lens group is to completely image the generated quantum-correlated photon pairs. After the quantum-correlated photon pairs are generated, they need to be imaged by the imaging lens group to obtain the image information of the object. The imaging lens group needs to ensure the integrity and accuracy of the quantum-correlated photon pairs and avoid aberration and energy loss during the imaging process. The main function of the polarization beam splitter is to project the photons polarized in the vertical direction to the first direction and project the photons polarized in the horizontal direction to the second direction. Through the action of the polarization beam splitter, the quantum-correlated photon pairs can be separated according to the polarization direction.

[0026] According to another aspect of the present invention, there is provided an experimental method for realizing quantum imaging by using sunlight, and the method includes: Set the object to be measured and the dot matrix parameters for point-by-point acquisition; Project the photons polarized in the vertical direction onto the object to be measured, and collect the first light intensity signal at the rear end of the object to be measured; project the photons polarized in the horizontal direction onto the target area, and collect the second light intensity signal point by point based on the dot matrix parameters in the target area; Compare the timestamps of the first light intensity signal and the second light intensity signal, screen out the second light intensity signal associated with the first light intensity signal, and perform point-by-point imaging based on the dot matrix and the intensity of the second light intensity signal.

[0027] In the above technical solution, in the experiment, it is necessary to select a suitable object to be measured according to the research purpose and place it in a suitable position to ensure that photons can effectively interact with it. The dot matrix parameters collected point by point are used to determine the acquisition position and order of the light intensity signal. By setting the dot matrix parameters, the target area can be divided into multiple sampling points to achieve high-resolution acquisition of the light intensity signal. The dot matrix parameters include the spacing and arrangement of the sampling points, etc., and these parameters can be optimized according to the experimental requirements and imaging resolution. Photons polarized in the vertical direction are used as probes for quantum imaging and are projected onto the object to be measured. These photons interact with the object to be measured, such as scattering and reflection, and carry the information of the object. A detector is set at the back end of the object to be measured to collect the first light intensity signal modulated by the object. This signal contains the structure and information of the object and is one of the basic data for imaging. Photons polarized in the horizontal direction are also used as probes for quantum imaging and are projected onto the target area. According to the preset dot matrix parameters, the second light intensity signal is collected point by point in the target area. By collecting point by point, the detailed light intensity distribution information of the target area can be obtained. By comparing the timestamps of the first light intensity signal and the second light intensity signal, the correlation between the two can be determined. Since the quantum-correlated photon pairs have temporal correlation, the second light intensity signal corresponding to the first light intensity signal can be screened out by timestamp comparison. By associating the light intensity signal of each sampling point with the corresponding dot matrix position, the image of the object can be reconstructed. Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0029] Figure 1 is a schematic optical path diagram of an embodiment of the experimental device of the present invention; Figure 2 is a diagram of the experimental results of an embodiment of the experimental device of the present invention; wherein, Figure 2 (a) is the light intensity distribution diagram pumped by sunlight throughout the day, Figure 2 (b) is the single-channel count diagram and coincidence count distribution diagram pumped by sunlight throughout the day, Figure 2 (c) is the spectral distribution diagram of laser light and sunlight after passing through the filter, Figure 2 (d) is the one-dimensional scanning experimental result diagram of the double-slit pumped by laser light and sunlight; Figure 3 is a schematic flow diagram of an embodiment of the experimental method of the present invention; Figure 4These are the results of the ghost face experiment of solar quantum imaging at different days of time consumption in an embodiment of the experimental method of the present invention. Detailed implementation manners

[0030] The present invention will be further described in detail below in conjunction with the drawings and embodiments. It should be specifically noted that the following embodiments are only used to illustrate the present invention, but do not limit the scope of the present invention. Similarly, the following embodiments are only partial embodiments of the present invention rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0031] The present invention provides an experimental device and method for realizing quantum imaging by using sunlight, which can collect, transmit and shape sunlight, and pump a non-linear crystal with a broadband spectral beam to generate a spontaneous parametric down-conversion process, generating sufficient broadband down-converted two-photon pairs for quantum imaging, and providing a set of experimental devices for the application research of sunlight in quantum imaging.

[0032] Embodiment 1 Please refer to Figure 1 , an experimental device for realizing quantum imaging by using sunlight.

[0033] The sun 1 irradiates sunlight to the sunlight collector 2 on the ground surface. In this embodiment, the sunlight collection system is used to provide an indoor light source. By using a photosensitive chip to calibrate the position and a low-loss plastic optical fiber to conduct the light beam, the power and stability of the pump light intensity are improved. Further, the sunlight collector 2 automatically adjusts the pitch angle of the device according to the current time and geographical coordinates, and uses a Fresnel lens with a diameter of 10 cm to focus sunlight (it should be noted that other ordinary lenses can also be applicable). Further, the diameter of the plastic optical fiber 3 is 2.5 mm and the length is 20 m to realize the introduction of sunlight from the sunlight collector 2 into the indoor laboratory. It should be noted that as long as the sunlight can be collected and shaped into a non-linear crystal or other devices that can generate correlated photon pairs, the sun tracking rotation device can refer to the prior art such as the patent document EP2546975B1, which will not be elaborated here.

[0034] In this embodiment, a beam shaping system (first objective lens 4, first aperture 5, first filter 6, polarizer 7, second objective lens 8) is sequentially arranged along the first direction A. After the sunlight passes through the sunlight collection system and the beam shaping system, it then passes through a wide-spectrum quantum imaging system (nonlinear crystal 9, second filter 10, first convex lens 11, second aperture 12, second convex lens 13, polarization beam splitter 14), and down-converted photon pairs are generated at the nonlinear crystal 9, and the sunlight is filtered out by the second filter 10; the vertically polarized down-converted photons are reflected to a second direction B, pass through the object under test 16 and the third objective lens 17, and are coupled into the first optical fiber 18; the horizontally polarized down-converted photons are transmitted to a third direction C, pass through the two-dimensional electric displacement stage 15, and enter the second optical fiber 19; then both the first optical fiber 18 and the second optical fiber 19 are connected to a photon counting system (single-photon detector 20, coincidence counter 21), and finally input to the computer 22.

[0035] In this embodiment, the first objective lens 4, the first aperture 5, and the second objective lens 8 in the beam shaping system (it should be noted that the main purpose of the beam shaping system is to shape the collected beam to match the size of the crystal used, so that the beam can pass through the crystal completely. As long as the above conditions are met, an example is given here) shape the sunlight into a pump beam with a diameter of about 0.8 mm and a numerical aperture of 0.1 to meet the size requirements of the nonlinear crystal 9 (it should be noted that meeting the size requirements means that the beam needs to pass through the crystal completely. In this embodiment, the size of the crystal used is 1 mm in length and 5 mm. Therefore, the beam of 0.8 and the numerical aperture of 0.1 describe the divergence angle, which can ensure that it can still pass through the crystal after propagating 5 mm with an incident size of 0.8 mm. In addition, in this embodiment, the nonlinear crystal used is a periodically poled KTP crystal, and there are no excessive restrictions on the size, which can be selected according to actual needs. If a large beam is incident, a large-size crystal can be used to realize a quantum imaging system with a larger field of view. At the same time, in addition to the nonlinear crystal, a nonlinear metasurface can be used instead, as long as the system can generate quantum correlated photon pairs. It should be noted that if a metasurface is used, it is imaging of the nonlinear metasurface.) Further, the sunlight passes through the first filter 6 and becomes a pump beam with a central wavelength of about 405 nm and a bandwidth of 10 nm for wide-spectrum quantum imaging (it should be noted that this point value is for illustrative purposes in this embodiment. The values of the central wavelength and bandwidth depend on the wavelength band matched by the crystal. A wide wavelength band is also possible, as long as the crystal meets the wavelength requirements. At the same time, regarding the selection of the nonlinear crystal, as long as it can generate correlated photon pairs, other types and other wavelength bands of crystals can also be used. The advantage of sunlight quantum imaging compared to lasers is that it can utilize the characteristics of a wide spectrum (wide wavelength band) to achieve position-correlated quantum imaging. Compared with the prior art that uses a single-wavelength bandwidth (such as 1.5 nm) light to pump the crystal, while the present invention uses a wide-spectrum bandwidth (~10 nm) light to pump the crystal, and it can also achieve quantum imaging. It should be pointed out that the coherence of the laser is relatively strong, and the coherence of sunlight is relatively weak. The efficiency of pumping the nonlinear crystal with monochromatic laser is high. However, if considering the same power, the yield of incoherent light wide-spectrum position-correlated photon pairs can be similar to that of single-frequency coherent laser. Therefore, from this perspective, in fact, the wide spectrum makes up for the disadvantage of poor coherence. In addition, because the nonlinear crystal is quasi-phase-matched, the spectral range is limited, and an extremely wide spectrum is not possible, which depends on the selection of the crystal.) Further, the polarizer 7 is adjusted to the horizontal direction so that the pump light is horizontally polarized to meet the requirements of type-II quasi-phase matching of the nonlinear crystal 9.

[0036] In this embodiment, the wide-spectrum quantum imaging system is a key part for realizing quantum imaging using sunlight, and is used to image down-converted photon pairs with different polarization directions to corresponding positions. Further, a spontaneous parametric down-conversion process occurs at the nonlinear crystal 9, satisfying energy conservation and momentum conservation. One high-frequency pump photon is split into two low-frequency photons, and these two photons are a pair of down-converted photons, with polarization directions being vertically polarized and horizontally polarized respectively. Further, the second filter 10 filters out the wide-spectrum pump beam while retaining the wide-spectrum down-converted light for subsequent wide-spectrum quantum imaging. Further, the first convex lens 11 and the second convex lens 13 form an imaging system, which is used to image the cross-section at the center of the nonlinear crystal 9, and is paired with a polarization beam splitter 14 (it should be noted that the purpose of the imaging system is to ensure that the position-correlated two photons generated by sunlight at the crystal cross-section can be completely imaged onto an object or a detector collection device. More precisely, one of the photons in the position-correlated two photons is imaged onto the object, and the other photon is imaged onto the detector. The lens system should ensure that the photons can cover the size of the object and the parameters of the detector. Therefore, the imaging system can be selected according to actual needs and will not be limited here), and respectively image the vertically polarized down-converted photons to the object to be measured 16 at a second direction B (it should be noted that the selection of the object to be measured is not restricted. Both transmissive objects and reflective objects are acceptable as long as photons can be reflected from the object. For an opaque object, only the transmissive part will be imaged, not only the edge contour, but also the transmissive middle area. In the present invention, a transmissive object to be measured is used to improve efficiency). The horizontally polarized down-converted photons are imaged to the end face of the second optical fiber 19 at a third direction C, so that there is a position correlation characteristic between the two down-converted photons at the object to be measured 16 and the end face of the second optical fiber 19 respectively. Further, the second aperture 12 is used to filter out uncorrelated down-converted photon pairs to ensure the position correlation of the imaging photons.

[0037] In this embodiment, the object to be measured 16 is a black-and-white object to be detected. After passing through the object to be measured 16, a part of the down-converted photons are blocked, and the remaining down-converted photons are all coupled into the first optical fiber 18 by the third objective lens 17 and finally enter the photon counting system. Further, the two-dimensional electric displacement stage 15 is controlled by the computer 22 to move the end face of the second optical fiber 19 on a two-dimensional plane to collect the transmitted down-converted photons point by point, and conduct them into the photon counting system through the second optical fiber 19.

[0038] In this embodiment, the photon counting system is used to convert an optical signal into an electrical signal, and based on the time difference between electrical signals, it determines whether two photons are correlated, thereby extracting a correlation signal. Further, for the photons conducted from the first optical fiber 18 and the second optical fiber 19, the single-photon detector 20 converts each photon into an electrical pulse signal one by one and outputs it to the coincidence counter 21. Further, the coincidence counter 21 compares the input electrical pulse signals on the time axis, and the time window is set to 1 nanosecond. If the time stamp difference between the two electrical pulse signals is within the time window, these two signals are considered correlation signals; otherwise, they are not. The basis for this is that when spontaneous parametric down-conversion occurs at the nonlinear crystal 9, a pair of photons will be generated simultaneously. After experiencing similar optical paths, these two photons arrive at the single-photon detector 20 almost simultaneously, are converted into electrical signals, and the time stamps are compared by the coincidence counter 21, and finally a coincidence counting signal is output.

[0039] In this embodiment, the computer 22 performs subsequent processing on the coincidence counting signal to obtain information about the object 16 to be measured, and finally realizes quantum imaging. Further, the second optical fiber 19 collects down-converted photons point by point on a two-dimensional plane. Considering that the down-converted photon pairs maintain the position correlation characteristic and there is a spatial correspondence relationship, when the end face of the second optical fiber 19 moves to a certain position, if this position allows the down-converted photons to pass through at the corresponding position on the plane where the object 16 to be measured is located (i.e., the light-transmitting position of the object 16 to be measured), then both photons in the down-converted photon pair can be detected by the coincidence counter 21, and a coincidence counting signal will be generated at this position; otherwise, no coincidence counting signal will be generated at the corresponding light-blocking position of the object 16 to be measured. Further, for each scanned spatial point position, there is a corresponding coincidence counting signal value. Based on this, the spatial distribution of the coincidence counting can be obtained, and due to the correlation characteristic of the down-converted photon pairs, this spatial distribution can reflect the structural information of the object 16 to be measured. It should be noted that both the plane of the object 16 to be measured and the end face of the optical fiber 19 are image planes of the central cross-section of the nonlinear crystal 9. Therefore, the distance from the object 16 to be measured to the polarization beam splitter 14 is the same as the distance from the end face of the optical fiber 19 to the polarization beam splitter 14. The position of the object 16 to be measured placed along the optical path direction should be in the image plane of the central cross-section of the nonlinear crystal 9.

[0040] The theoretical basis of this embodiment is as follows: (1) Adopt an experimental device for realizing quantum imaging by using sunlight as described in one of the embodiments. Place a power meter at the nonlinear crystal 9 to measure the light intensity distribution of the sunlight pumping beam throughout the day, as Figure 2 shown in (a).

[0041] (2) Without placing the object under test 16, replace the two-dimensional electric displacement stage 15 with an objective lens identical to the third objective lens 17, and adjust the coupling heads of the first optical fiber 18 and the second optical fiber 19 to appropriate positions respectively, so that the coincidence count is maximized, and measure the single-channel count and coincidence count distribution of the sunlight pump beam throughout the day, as Figure 2 shown in

[0042] (b). Figure 2 (3) Use a spectrometer to measure the spectral distribution of sunlight after the first filter 6, and then replace the light source with a laser, and continue to measure the spectral distribution of the laser after the first filter 6, as

[0043] shown in Figure 2 (c).

[0044] (4) Use a double-slit structure object as the object under test 16, with a double-slit spacing of 0.75 mm and a slit width of 0.15 mm. Use the two-dimensional electric displacement stage 15 to move the position of the coupling head of the second optical fiber 19 to achieve one-dimensional point scanning. The scanning time for each point is 13 minutes, the scanning step size is 0.05 mm, and the scanning length is 1.3 mm to achieve one-dimensional imaging of the double-slit object under sunlight pumping. Further, replace the light source with a laser with a wavelength of 405 nm, couple the laser to a plastic optical fiber (replacing the sunlight collection system), and keep the other experimental parameters the same. Given that the peak light intensity of sunlight pumping is about 3 μW, attenuate the laser intensity to about 3 μW to keep the pumping light intensity consistent, and repeat the scanning process to achieve one-dimensional imaging of the double-slit object under laser pumping, as

[0045] shown in (d). (5) Based on the one-dimensional imaging results of the double-slit object, calculate that the double-slit imaging contrast under both pumping methods reaches about 95%, and the coincidence counts of the two are at a similar level, proving that even with spectral broadening, the two-photon position correlation is still effectively maintained.

[0046] The working principle of this device is as follows: and respectively represent the signal optical field and the idler optical field. The three-dimensional spatial coordinates of the optical field are defined as , and ; is the transverse coordinate of the optical field; is the transmission distance of the optical field along the optical axis. is the polarization angle of the optical field; is the transverse wave vector of the optical field; is the wave vector of the optical field. represents having a polarization angle and the transverse wave vector annihilation operator of the mode.

[0047] The two-photon cross-spectral density function is:

[0048] where is a physical constant, is the wave vector of the pump light field, represents the ensemble average, is the cross-spectral density function of the Gaussian Schell-model beam, which is characterized by the transverse correlation length in the momentum basis in the plane and the beam waist width and can be expressed as:

[0049] where is a constant, is the effective spectral width, is the beam waist of the beam at the plane . Thus, at the plane , the positive-frequency parts of the signal light field and the idler light field are in the form of:

[0050] Substituting into the two-photon cross-spectral density function, we can obtain:

[0051] Furthermore, substituting the cross-spectral density function of the Gaussian Schell-model beam and performing the integration, we finally obtain the two-photon cross-spectral density function:

[0052] Considering that the objective lens is placed behind the plane where the object to be measured 16 is located to collect photons, the normalized coincidence count of the two-photon state at another imaging plane (the end face of the second optical fiber 19) can be expressed as:

[0053] where , is the transfer function of the 4f system from the crystal plane to the image plane , is the imaging magnification of the 4f system. As can be seen from the above equation, the spatial distribution of coincidence counts contains information about the object to be measured 16, which means that imaging of the object can be achieved by scanning the spatial position. Its physical meaning can be explained as follows: when a pair of correlated photons at a specific image plane position can penetrate or be reflected by the object to be measured, coincidence counts can be detected at the corresponding position; otherwise, no coincidence counts will occur. It should be noted that if a multi-frequency light beam is used to pump the nonlinear crystal, non-degenerate broadband down-converted photon pairs will be generated, and each single-frequency pumping component can individually meet the conditions of the equation. Therefore, all these down-converted photon pairs will contribute to object imaging. The key difference from a typical single-frequency laser pumping device is that the latter usually places a narrowband filter in front of the detector to ensure that the signal photons and idler photons have the same frequency, while the present invention omits the narrowband filter in front of the detector. This design enables broadband position-correlated photon pairs to be used for quantum imaging.

[0054] Compared with the prior art, the advantages of the present invention are as follows: The present invention has successfully constructed an innovative experimental device, and for the first time, used sunlight - the most common natural light source - to pump a nonlinear crystal to generate down-converted photon pairs, and made a breakthrough: under the quasi-phase matching condition of the crystal, the broadband spectral characteristics of sunlight can generate the same number of down-converted photon pairs as those generated by single-frequency laser pumping. This phenomenon is of core significance to the ghost imaging technology based on the position correlation of down-converted photon pairs; not only has it achieved imaging with a contrast ratio of up to 95% for a double-slit structure (comparable to the performance of laser quantum ghost imaging), but it has also completed the correlation imaging of a two-dimensional complex ghost face pattern for the first time. The present invention achieves high-contrast imaging through the direct pumping of sunlight by the spontaneous parametric down-conversion mechanism without pre-spectral filtering, filling the technical gap between classical ghost imaging and quantum ghost imaging. This invention breakthrough provides a new path for the development of passive ghost imaging light sources in space environments. Its device achieves measurable photon counts by stably coupling dynamic sunlight to the nonlinear crystal, marking the practical application of space-based quantum imaging technology.

[0055] Embodiment 2 Please refer to Figure 3 , an experimental method for realizing quantum imaging using sunlight, based on the device described in Embodiment 1.

[0056] The method includes the following steps: S1. Set the object to be measured and the lattice parameters for point-by-point acquisition. S2. Photons polarized in the vertical direction are projected onto the object to be measured, and the first light intensity signal is collected at the rear end of the object to be measured; photons polarized in the horizontal direction are projected onto the target area, and the second light intensity signal is collected point by point based on the lattice parameters in the target area. S3. Compare the timestamps of the first optical intensity signal and the second optical intensity signal, filter out the second optical intensity signal associated with the first optical intensity signal, and perform point-by-point imaging based on the dot matrix and the intensity of the second optical intensity signal.

[0057] The experimental results of the ghost face experiment of sunlight quantum imaging are as Figure 4 shown.

[0058] (1) Replace the object to be measured 16 with a ghost face pattern, and adjust the position of the ghost face so that it is at the center of the light beam.

[0059] (2) Under sunlight pumping, use the two-dimensional electric displacement stage 15 to move the position of the coupling head of the second optical fiber 19 to achieve two-dimensional point scanning. The scanning time for each point is 40 seconds per day, the scanning step size is 0.1 mm, the scanning range is 2.3 * 2.3 square millimeters, and the ghost face images collected over ten days are superimposed to achieve sunlight quantum imaging. It should be noted that the core diameter of the multimode optical fiber used in this example is 0.1 mm. Therefore, a scanning step size of 0.1 mm is selected. The scanning range of 2.3 * 2.3 square millimeters can just cover the object to be measured - the ghost face, which is slightly larger than the actual size of the ghost face. The specific scanning parameters can be set according to actual needs.

[0060] The present invention discloses a set of convenient devices that converge sunlight to the laboratory for quantum exploration. This not only realizes the generation of correlated photon pairs using sunlight for the first time but also further conducts ghost imaging experiments. The research results of the present invention show that the designed device can not only image simple objects but also image complex objects with high visibility. The present invention provides a cutting-edge solution to the challenges of photon manipulation and opens the door to exploring the application of sunlight in the field of quantum information. Given that sunlight is a generally available and free natural light source, the present invention demonstrates the feasibility of using sunlight as a light source to generate entangled photons. This technology has broad application prospects and is expected to promote the future application of establishing a sunlight-pumped quantum light source in outer space.

[0061] The above are only some embodiments of the present invention, and thus do not limit the protection scope of the present invention. Any equivalent device or equivalent process transformation made using the content of the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, shall be similarly included in the patent protection scope of the present invention.

Claims

1. An experimental device for realizing quantum imaging by using sunlight, characterized in that, The device includes: A sunlight shaping system, a quantum correlated photon pair generation unit, and a wide-spectrum quantum imaging system. Among them, The sunlight shaping system is used to shape and polarize the collected small-spot sunlight into a pump beam polarized in the horizontal direction, so that the pump beam can completely pass through the quantum correlated photon pair generation unit; The quantum correlated photon pair generation unit is used to receive the pump beam and split the beam to generate quantum correlated photon pairs in a wide spectrum; the polarization directions of the quantum correlated photon pairs are vertically polarized and horizontally polarized respectively; The wide-spectrum quantum imaging system is used to receive the quantum correlated photon pairs, project the vertically polarized photons onto the object to be measured, and collect the first light intensity signal; project the horizontally polarized photons onto the target area, and collect the second light intensity signal point by point in the target area; extract the second light intensity signal correlated with the first light intensity signal based on the light intensity signal time stamp, and input it into an analysis device for point-by-point imaging based on the dot matrix.

2. An experimental device for realizing quantum imaging using sunlight according to claim 1, characterized in that The quantum correlated photon pair generation unit includes a nonlinear crystal and a nonlinear metasurface.

3. An experimental device for realizing quantum imaging using sunlight according to claim 1, characterized in that The object to be measured includes a transmissive object or a reflective object that can reflect photons.

4. An experimental device for realizing quantum imaging using sunlight according to claim 1, characterized in that The device further includes a sunlight collection system for collecting sunlight, reducing the spot size, and then inputting it into the sunlight shaping system.

5. An experimental device for realizing quantum imaging using sunlight according to claim 4, characterized in that The sunlight collection system includes a condenser lens, and an adjustment device that holds the focusing lens and the optical fiber and controls its rotation angle; the condenser lens focuses the sunlight onto the end face of the optical fiber, and couples the light to the sunlight shaping system through the optical fiber.

6. An experimental device for realizing quantum imaging using sunlight according to claim 1, characterized in that The sunlight shaping system sequentially includes along the optical path direction: At least one filter for filtering the sunlight into a pump beam; the central wavelength of the pump beam is adapted to the central working wavelength of the quantum correlated photon pair generation unit; At least one polarizer for polarizing the pump beam into a horizontal polarization; At least one objective lens for focusing the pump beam.

7. An experimental device for realizing quantum imaging using sunlight according to claim 6, characterized in that Before the filter, there is also: At least one objective lens and a diaphragm for collimating the collected sunlight and controlling the divergence angle.

8. An experimental device for realizing quantum imaging by using sunlight, as described in claim 1, characterized in that, The wide-spectrum quantum imaging system sequentially includes along the optical path direction: At least one filter for filtering out the unsplit pump beam; An imaging lens group for completely imaging the generated quantum correlated photon pairs and filtering out non-correlated photons; A polarization beam splitter for projecting the vertically polarized photons to a first direction and the horizontally polarized photons to a second direction; And The object to be measured and the first fiber optic coupling module arranged in sequence along the first direction; wherein, the first fiber optic coupling module is used to collect the first light intensity signal and couple it to the photon counting system; The second fiber optic coupling module arranged in the second direction and with a controllable relative position, which is used to collect the second light intensity signal point by point in the target area and couple it to the photon counting system.

9. An experimental method for realizing quantum imaging by using sunlight, characterized in that, The method includes: Setting the object to be measured and the dot matrix parameters for point-by-point acquisition; Photons polarized in the vertical direction are projected onto the object to be measured, and the first light intensity signal is collected at the rear end of the object to be measured; photons polarized in the horizontal direction are projected onto the target area, and the second light intensity signal is collected point by point based on the dot matrix parameters in the target area; Comparing the timestamps of the first light intensity signal and the second light intensity signal, screening out the second light intensity signal associated with the first light intensity signal, and performing point-by-point imaging based on the dot matrix and the intensity of the second light intensity signal.

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