Method and apparatus for preparing entangled light source based on grating and perfect vortex metasurface

Abstract

The application relates to a preparation method of an entangled light source based on a grating and a perfect vortex super surface, which comprises the following steps: providing a Gaussian light beam, linearly polarizing the Gaussian light beam along a specific polarization direction, and then generating signal light and idle light through a periodical polarization potassium titanyl phosphate crystal; collimating the photons, changing the polarization states of the signal light and the idle light through a 1 / 4 wave plate, and converting the signal light and the idle light into right-handed and left-handed circular polarizations respectively; making the Gaussian light beam pass through a perfect vortex super surface to obtain a spatially-varying geometric phase; then passing through a one-dimensional grating, and separating the right-handed and left-handed perfect vortex lights to output an overlapping part of an entangled state; and coupling the photons in the two paths into optical fibers respectively, and transmitting the photons to single-photon detectors to record coincidence numbers.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of quantum entanglement source emission, in particular to a preparation method and device of an entangled light source based on a grating and a perfect vortex super surface. BACKGROUND

[0002] An entangled photon source is a core resource in the fields of quantum communication, quantum computing and quantum cryptography. An entangled photon pair has non-classical quantum correlation characteristics and can realize quantum teleportation, quantum key distribution and other applications.

[0003] Traditional entangled photon sources are mainly realized by using a nonlinear crystal to realize phase matching between pump light and down-converted photons, and then using a single-photon detector to detect signal photons and idler photons, and verifying the entanglement characteristics of the photon pair through coincidence counting technology. Although the traditional SPDC technology has been widely used in the generation of entangled photon sources, there are still some limitations: the nonlinear process conversion efficiency of the crystal is low, high-power light is required, and the phase matching condition is sensitive to temperature, humidity and other environmental conditions, and the angle range of the entangled photons is also limited. Therefore, a new entangled light resource generation scheme is needed.

[0004] Based on this, the present application is proposed. SUMMARY

[0005] To solve the above technical problems, the purpose of the present application is to provide a preparation method and device of an entangled light source based on a grating and a perfect vortex super surface.

[0006] In the first aspect of the present application, a preparation method of an entangled light source based on a grating and a perfect vortex super surface is provided, which includes the following steps under a general inventive concept:

[0007] A Gaussian light beam is provided, and the Gaussian light beam is linearly polarized along a specific polarization direction and generates signal light and idler light through a periodically poled potassium titanyl phosphate crystal;

[0008] After collimating the photons, the polarization state of the signal light and the idler light is changed through a 1 / 4 wave plate, and the signal light and the idler light are converted into right-handed and left-handed circular polarization respectively;

[0009] The Gaussian light beam passes through a perfect vortex super surface to obtain a spatially varying geometric phase;

[0010] After passing through a one-dimensional grating, the left-handed and right-handed perfect vortex lights are separated to output an overlapping part of an entangled state; the photons in the two paths are coupled into optical fibers respectively and transmitted to a single-photon detector to record the coincidence number.

[0011] Specifically, a Gaussian beam is first provided, linearly polarized along a specific polarization direction, and focused by a first lens onto a periodically polarized potassium titanate crystal to achieve spontaneous parametric down-conversion under nonlinear optical effects, generating signal and idler light. A second lens collimates the signal and idler light so that they overlap in the output plane. A quarter-wave plate changes the polarization states of the signal and idler light, changing the signal light from horizontal polarization to right-hand circular polarization and the idler light from vertical polarization to left-hand circular polarization. A perfect vortex metasurface is provided, wherein the perfect vortex metasurface... The optical axis of the surface periodically varies along the angular direction. The Gaussian beam, after passing through a quarter-wave plate, passes through a perfect vortex metasurface to obtain a spatially varying geometric phase. Specifically, the signal light is transformed from a right-hand circularly polarized Gaussian beam to a left-hand circularly polarized Bessel-Gaussian beam, and the idle light is transformed from a left-hand circularly polarized Gaussian beam to a right-hand circularly polarized Bessel-Gaussian beam. A third lens performs Fourier transforms on both the left-hand and right-hand circularly polarized Bessel-Gaussian beams, resulting in a perfectly vortex beam that coincides on the focal plane. A one-dimensional grating is provided, with its optical axis direction along... The axial direction changes periodically, and the signal light and idle light pass through. This generates momentum differences along the positive and negative y-axis in real space, respectively. This allows them to separate along the y-direction, resulting in the output of the prepared entangled state.

[0012] In a further embodiment of this application, the Gaussian beam is linearly polarized along a specific polarization direction, including: providing a Glan polarizer and a half-wave plate to adjust the Gaussian beam to be horizontally polarized, and fine-tuning the polarization state of the Gaussian beam by means of the half-wave plate.

[0013] In a further embodiment of this application, in step S20, the first lens is a focusing lens to focus the light beam onto the center of the periodically polarized potassium titanate crystal; the second lens is a collimating lens, and the first lens and the second lens constitute a 4f system.

[0014] After passing through a periodically polarized potassium titanate crystal, the signal light and idle light generated, collimated by a second lens, are represented as follows:

[0015]

[0016]

[0017]

[0018]

[0019]

[0020]

[0021] in, and Indicates signal light and idle light. The Jones vector representation of the light field. and yes direction and The polarization component of the direction, Indicates transpose. The spatial distribution of the complex amplitude of the Gaussian beam. , , The position of a point in space in three directions. This can represent the propagation distance, where k is the wave vector. Let z be the half-width at any z-position. , Indicates the radius of curvature of the wavefront. This indicates the phase of Guei.

[0022] In a further embodiment of this application, step S40 further includes: step S41, using a beam splitter to split the entangled photons after passing through a one-dimensional grating into two paths; step S42, setting a fourth lens in one path and a fifth lens in the other path to collimate the entangled photons in the two paths respectively.

[0023] In a further embodiment of this application, the method for preparing the entangled light source further includes: step S51, setting a first bandpass filter in one path to remove photons of non-measurement wavelengths in the beam after passing through the fourth lens; setting a second bandpass filter in the other path to remove photons of non-measurement wavelengths in the beam after passing through the fifth lens; step S52, using an optical fiber coupler to couple the signal photons and idle photons in the two paths into an optical fiber respectively, and transmitting them to a single-photon detector to record the coincidence number.

[0024] In a further embodiment of this application, the perfect vortex metasurface possesses Phase delay, period is One-dimensional gratings possess Phase delay, with a period of n mm, preferably 300 mm;

[0025] The first metasurface optical axis and The included angle of the axis is:

[0026]

[0027] in, It is polar coordinates. It is the polar radius. It is the polar angle. For complex arguments, It is the electric field distribution of a 0th-order Bessel-Gaussian beam, and .

[0028] It is a 0th-order Bessel-Gaussian beam. The complex amplitude of the Bessel-Gaussian beam is:

[0029]

[0030] in, It is a natural exponential function. It is a Bessel function of the first kind of order l. It is the polar radius. It is the polar angle. , exist Components in direction, The imaginary unit, It is the topological charge number. ;

[0031] The second metasurface optical axis and The included angle of the axis is:

[0032]

[0033] in, For metasurface periodicity, This represents the phase gradient.

[0034] In a further aspect of this application, the ideal model of the perfect vortex beam used is as follows:

[0035]

[0036] in, Let r be the Dirac function and r be the polar radius. Let be the beam radius, i be the imaginary unit, and l be the topological charge of the perfect vortex beam.

[0037] By performing Fourier transforms on both the left-handed and right-handed circularly polarized Bessel-Gaussian beams using a third lens, a perfectly overlapping vortex beam is obtained on the focal plane, represented as follows:

[0038]

[0039] in, The radius of the annular light spot is... Let f be the beam waist of the Gaussian beam on the focal plane of the lens, where f is the focal length of the lens and k is the wave number. wave number Projection along the direction parallel to the metasurface.

[0040] Signal light and idle light pass through After that:

[0041]

[0042]

[0043] wherein, represents the optical field of the signal light after passing through the grating, represents the optical field of the idler light after passing through the grating, represents the complex amplitude of the perfect vortex beam, is the base of the natural logarithm, is the imaginary unit, is the direction geometric phase factor;

[0044] The final prepared entangled state output is:

[0045]

[0046] wherein, represents the left-handed state represents the right-handed state is an entangled state representation;

[0047] The second aspect of the application also provides a metasurface-based entangled light source device, comprising: a focused light path device, including a laser light source, a Glan polarizer, a half-wave plate, a first lens, and a periodically poled potassium titanyl phosphate crystal, the laser light source can emit a Gaussian beam, the Glan polarizer is used to adjust the polarization state of the Gaussian beam, after fine adjustment of the polarization state of the Gaussian beam by the half-wave plate, the Gaussian beam is focused by the first lens and incident into the periodically poled potassium titanyl phosphate crystal to complete spontaneous parametric down-conversion under nonlinear optical effect, generating signal light and idler light; a signal light and idler light collimating light path device, including a second lens and a 1 / 4 wave plate, the second lens collimates the signal light and the idler light to make them overlap with each other on an output plane, and the 1 / 4 wave plate changes the polarization state of the signal light and the idler light, wherein the signal light changes from horizontal polarization to right circular polarization, and the idler light changes from vertical polarization to left circular polarization; a metasurface-based perfect vortex beam generation light path device, including a perfect vortex metasurface and a third lens, wherein the optical axis direction of the perfect vortex metasurface changes periodically along the angular direction, the Gaussian beam after the 1 / 4 wave plate passes through the perfect vortex metasurface to obtain a spatially varying geometric phase, the signal light changes from the right circularly polarized Gaussian beam to the left circularly polarized Bessel-Gaussian beam, and the idler light changes from the left circularly polarized Gaussian beam to the right circularly polarized Bessel-Gaussian beam; the third lens is used to perform Fourier transform on the left circularly polarized Bessel-Gaussian beam and the right circularly polarized Bessel-Gaussian beam to obtain a perfect vortex beam that overlaps on a focal plane; a metasurface-based light separation light path device, including a one-dimensional grating, a light splitting prism, a fourth lens, a fifth lens, a first band-pass filter, and a second band-pass filter, the optical axis direction of the one-dimensional grating changes periodically along the x-axis direction; the signal light and the idler light pass through the one-dimensional grating and the light splitting prism to generate a momentum difference along the positive and negative directions of the y-axis in real space respectively, so as to separate them along the y direction, the light splitting prism divides the entangled photons after the one-dimensional grating into two paths, the fourth lens and the first band-pass filter are arranged in one path, and the fifth lens and the second band-pass filter are arranged in the other path; a measurement light path device of the entangled photons, including two fiber couplers and a single-photon detector connected with the fiber couplers, the two fiber couplers are arranged on the end light paths of the two first band-pass filters and the second band-pass filter respectively to record the coincidence number.

[0048] In a further scheme of the application, the first lens is a focusing lens to focus the light beam to the center of the periodically poled potassium titanyl phosphate crystal; the second lens is a collimating lens, and the first lens and the second lens constitute a 4f system.

[0049] After passing through the periodically poled potassium titanyl phosphate crystal, the generated signal light and idler light are collimated by the second lens, and are expressed as:

[0050] ​​​

[0051]

[0052]

[0053]

[0054]

[0055]

[0056] ;

[0057]

[0058]

[0059] .

[0060] In a further embodiment of this application, the perfect vortex metasurface possesses Phase delay, period is One-dimensional gratings possess Phase delay, period n mm, preferably 300 mm; the optical axis of the first metasurface is... The included angle of the axis is:

[0061]

[0062] in, middle It is polar coordinates. It is the polar radius. It is the polar angle. and These are metasurface parameters;

[0063] It is a 0th-order Bessel-Gaussian beam. The complex amplitude of the Bessel-Gaussian beam is:

[0064]

[0065] in, It is the polar radius. It is the polar angle. It is an l-order Bessel function It is Wave Arrow exist Components in direction, The imaginary unit, It is the topological charge number. yes ;

[0066] The second metasurface optical axis is perpendicular to the first metasurface optical axis. The angle between the axis and the optical axis is:

[0067]

[0068] wherein, is the period of the metasurface.

[0069] In summary, the preparation method of the entangled light source based on the grating and the perfect vortex metasurface according to the general inventive concept has at least the following technical effects:

[0070] 1. A new entangled source is generated by the metasurface, which realizes the replacement of the BBO crystal with the periodically poled potassium titanyl phosphate crystal, and the environmental conditions such as temperature and humidity are less restricted, the environmental robustness is improved, quasi-phase matching (QPM) is realized, and the nonlinear process is more efficient.

[0071] 2. The present application can realize efficient entangled photon pair generation with low-power pump light, while the traditional method needs high-power pump light to achieve the same efficiency, and high-power pump light may cause thermal effect and system instability, and the present application can make the nonlinear process conversion efficiency higher.

[0072] 3. Unlike the traditional entangled photon source preparation, the present application can prepare left-handed and right-handed entangled states, which are not sensitive to the rotation of the optical system, so it is more robust in free-space quantum communication, and the circularly polarized state can be more conveniently combined with other optical elements to realize more complex photon state manipulation.

[0073] 4. The introduction of the metasurface makes the whole device more compact and integrated.

[0074] Other features and advantages of the embodiments of the present application will be described in the following specific embodiment part. BRIEF DESCRIPTION OF DRAWINGS

[0075] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0076] Fig. 1 The flowchart of the entangled light source method provided by the present application;

[0077] Fig. 2 The structural schematic diagram of the entangled light source device provided by the present application. DETAILED DESCRIPTION

[0078] The terms "second direction", "first direction", "third direction", "inner", "outer" and the like that appear below indicate the description of the orientation or positional relationship, if no special description, it is understood as the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0079] In addition, as the features appearing with "first", "second" are only for the purpose of description, they cannot be understood as indicating or implying the relative importance or implicitly indicating the number of the indicated technical features. The features limited by "first", "second" can explicitly or implicitly include at least one of the features limited by "first", "second". As the description of "multiple" appears, the general meaning is at least two, such as two, three, etc., unless otherwise explicitly specified.

[0080] In the present application, unless otherwise explicitly specified and limited, the terms such as "mounting", "connecting", "connecting", "fixing" and the like should be understood in a broad sense. For example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected, it can be directly connected, or it can be indirectly connected through an intermediate medium, it can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0081] In the description of the present application, the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" and the like mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In the present description, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine the different embodiments or examples described in the present description and the features of the different embodiments or examples without contradiction.

[0082] Reference Figs. 1-2 As shown in the drawings, the present application first provides a preparation method of an entangled light source based on a grating and a perfect vortex super surface, comprising:

[0083] Step S10, providing a Gaussian beam, linearly polarizing the Gaussian beam along a specific polarization direction, and focusing the Gaussian beam into a periodically poled potassium titanyl phosphate crystal through a first lens to generate signal light and idler light through spontaneous parametric down-conversion under nonlinear optical effect;

[0084] The Gaussian beam has a light intensity distribution in the form of a Gaussian function, with the central light intensity being the maximum and gradually decreasing outward; the Gaussian beam provided here is an initial light source for subsequent nonlinear optical processes. Linear polarization means that the polarization direction of the light is limited to a specific direction (e.g., horizontal or vertical direction); the polarizer is used to adjust the Gaussian beam to a linearly polarized light in a specific direction, such as horizontal polarization or vertical polarization.

[0085] The first lens is used to focus the linearly polarized Gaussian beam, so that the energy is concentrated. The focused beam is incident into the periodically poled potassium titanyl phosphate crystal. Due to the nonlinear optical properties of the crystal, under spontaneous parametric down-conversion, one high-energy photon (pump light) can spontaneously split into two low-energy photons, i.e., signal light and idler light.

[0086] Step S20, collimating the signal light and the idler light through a second lens to make them overlap on an output plane, and changing the polarization states of the signal light and the idler light through a 1 / 4 wave plate, wherein the signal light changes from horizontal polarization to right circular polarization, and the idler light changes from vertical polarization to left circular polarization;

[0087] After the signal light and the idler light exit from the crystal, they are collimated by the second lens; the signal light and the idler light overlap on the output plane, and the polarization states of the signal light and the idler light are changed by the 1 / 4 wave plate: the signal light changes from horizontal polarization to right circular polarization; the idler light changes from vertical polarization to left circular polarization; the circularly polarized light has the characteristic that the electric field vector of the light rotates with time, and is divided into left and right circularly polarized light.

[0088] Step S30, providing a perfect vortex super surface (first PB phase super surface), wherein the optical axis direction of the perfect vortex super surface changes periodically along the angular direction, and the Gaussian beam after the 1 / 4 wave plate passes through the perfect vortex super surface to obtain a spatially varying geometric phase, wherein the signal light changes from the right circularly polarized Gaussian beam to the left circularly polarized Bessel-Gaussian beam, and the idler light changes from the left circularly polarized Gaussian beam to the right circularly polarized Bessel-Gaussian beam; the third lens is used to perform Fourier transform on the left circularly polarized Bessel-Gaussian beam and the right circularly polarized Bessel-Gaussian beam to obtain a perfect vortex beam that overlaps on a focal plane;

[0089] The polarization state and spatial distribution of the signal light and the idle light change after passing through the perfect vortex super surface: the signal light changes from a right circularly polarized Gaussian beam to a left circularly polarized Bessel Gaussian beam; the idle light changes from a left circularly polarized Gaussian beam to a right circularly polarized Bessel Gaussian beam; the transformed Bessel Gaussian beams are subjected to Fourier transform through a third lens; on the focal plane, the signal light and the idle light coincide to form a perfect vortex beam. The perfect vortex beam has a spiral phase structure and carries orbital angular momentum (OAM).

[0090] In step S40, a one-dimensional grating (second PB phase super surface) is provided, and the optical axis direction of the one-dimensional grating periodically changes along the z-axis direction. The signal light and the idle light pass through the one-dimensional grating to generate momentum differences in the positive and negative directions of the y-axis in real space, respectively. So that they are separated along the y direction to output the prepared entangled state.

[0091] The optical axis direction of the one-dimensional grating periodically changes along the z-axis direction, which is different from the angular change of the perfect vortex super surface. After the signal light and the idle light pass through the second PB phase super surface, momentum differences in the positive and negative directions of the y-axis are generated in real space, and the momentum differences cause the signal light and the idle light to be separated in space and propagate along the y direction; finally, the signal light and the idle light are separated and output to form an entangled photon source.

[0092] In summary, the preparation method of the entangled light source based on the grating and the perfect vortex super surface according to the general inventive concept has at least the following technical effects:

[0093] 1. A new entangled source is generated by the super surface, which realizes the replacement of the BBO crystal with the periodic poled potassium titanyl phosphate crystal, and the environmental conditions such as temperature and humidity are less restricted, the environmental robustness is improved, quasi-phase matching (QPM) is realized, and the nonlinear process is more efficient;

[0094] 2. The present application only needs low-power pump light to realize efficient entangled photon pair generation; compared with the traditional method which needs high-power pump light to achieve the same efficiency, the high-power pump light may cause thermal effect and system instability, and the present application can make the nonlinear process conversion efficiency higher;

[0095] 3. Unlike the traditional entangled photon source preparation, the present application can prepare left-handed and right-handed entangled states, which is not sensitive to the rotation of the optical system, so it has more robustness in free space quantum communication, and the circularly polarized state can be more conveniently combined with other optical elements to realize more complex photon state manipulation.

[0096] In step S10, the Gaussian beam is linearly polarized along a specific polarization direction, including;

[0097] A Glan polarizer and a half-wave plate are provided to adjust the Gaussian beam to horizontal polarization, and the polarization state of the Gaussian beam is fine-tuned through the half-wave plate.

[0098] In this step, the Glan polarizer converts the Gaussian beam into horizontally polarized light, and if the horizontally polarized light output by the Glan polarizer does not fully meet the experimental requirements, the polarization direction can be further adjusted by rotating the angle of the half-wave plate; for example, the horizontally polarized light is adjusted to linearly polarized light at a certain angle with the x-axis. After adjustment by the Glan polarizer and the half-wave plate, the Gaussian beam is accurately adjusted to linearly polarized light in a specific direction; the linearly polarized light is then focused by the first lens and incident into the periodically poled potassium titanyl phosphate crystal to perform a spontaneous parametric down-conversion (SPDC) process.

[0099] In step S20, the first lens is a focusing lens to focus the beam to the center of the periodically poled potassium titanyl phosphate crystal; the second lens is a collimating lens, and the first lens and the second lens form a 4f system.

[0100] The first lens is a focusing lens for focusing the Gaussian beam to the center of the periodically poled potassium titanyl phosphate crystal (PPKTP); the purpose of focusing is to increase the energy density of the beam, thereby improving the efficiency of nonlinear optical processes (such as spontaneous parametric down-conversion, SPDC); the second lens is a collimating lens for recollimating the signal light and idler light after passing through the periodically poled potassium titanyl phosphate crystal. The first lens and the second lens form a 4f system: the first lens focuses the Gaussian beam to the center of the periodically poled potassium titanyl phosphate crystal (object plane), and the second lens recollimates the signal light and idler light and realizes the overlap of the signal light and idler light on the output plane (image plane). In this 4f system, the object plane and the image plane are located at the front focal plane and the back focal plane of the system, respectively.

[0101] After passing through the periodically poled potassium titanyl phosphate crystal, the generated signal light and idler light are collimated by the second lens, represented as

[0102]

[0103]

[0104]

[0105] wherein, , , is the waist width of the Gaussian beam, is the propagation distance.

[0106] In summary, after passing through the periodically poled potassium titanyl phosphate crystal and being collimated by the second lens, the electric field distribution of the signal light and idler light can be described in the form of a Gaussian beam; the collimated signal light and idler light overlap on the output plane.

[0107] In a further solution, step S40 further comprises:

[0108] Step S41, using a beam splitter prism to divide the entangled photons after the one-dimensional grating into two paths;

[0109] Step S42, setting a fourth lens in one of the paths and a fifth lens in the other path to collimate the entangled photons in the two paths respectively.

[0110] In this step, the beam splitter prism divides the entangled photons after the one-dimensional grating into two paths; specifically, after passing through the one-dimensional grating, due to the periodic variation of the optical axis direction of the metasurface along the axial direction, the signal light and the idle light produce momentum differences in the positive and negative directions of the y-axis in real space, causing them to separate in space, and the beam splitter further divides the two separated beams into two paths, which enter different optical paths respectively. The fourth lens and the fifth lens are used to collimate the entangled photons in the two paths respectively. The beam splitter divides the entangled photons into two paths, achieving spatial separation and independent processing; the fourth lens and the fifth lens collimate the entangled photons in the two paths respectively, ensuring high-quality output of the light beams.

[0111] The method for preparing the entangled light source further comprises:

[0112] Step S51, setting a first band-pass filter in one of the paths to remove photons of non-measurement wavelengths in the light beam after the fourth lens; and setting a second band-pass filter in the other path to remove photons of non-measurement wavelengths in the light beam after the fifth lens;

[0113] Step S52, using a fiber coupler to couple the signal photons and the idle photons in the two paths into optical fibers respectively and transmit them to a single-photon detector to record the coincidence counts.

[0114] In the first path, the first band-pass filter is used to filter out photons of non-measurement wavelengths in the light beam after the fourth lens, and only the signal photons of the target wavelength are retained; in the second path, the second band-pass filter is used to filter out photons of non-measurement wavelengths in the light beam after the fifth lens, and only the idle photons of the target wavelength are retained, which improves the signal-to-noise ratio of the detection signal, reduces the influence of background noise on the experimental results, and ensures that only photons of the target wavelength enter the subsequent fiber coupling and detection process, thereby improving the accuracy of the experiment. In the first path, a fiber coupler is used to couple the signal photons into an optical fiber, and in the second path, a fiber coupler is used to couple the idle photons into an optical fiber; the coupled photons are transmitted to a single-photon detector through the optical fiber, and the single-photon detector is used to detect and record the arrival events of individual photons. By recording the coincidence counts of the signal photons and the idle photons, the generation and characteristics of the entangled photon pairs can be verified.

[0115] In this application, the perfect vortex metasurface has Phase delay, period is n mm, preferably 300 mm; the optical axis of the first metasurface (i.e. perfect vortex metasurface) and the optical axis of the one-dimensional grating are at an angle of: ; the one-dimensional grating has Phase delay, period is n mm, preferably 300 mm; the optical axis of the first metasurface (i.e. perfect vortex metasurface) and the optical axis of the one-dimensional grating are at an angle of: Axis angle is:

[0116]

[0117] Wherein, is a 0-order Bessel-Gauss beam.

[0118] The complex amplitude of the 0-order Bessel-Gauss beam is:

[0119]

[0120] Wherein, the optical axis of the one-dimensional grating and the optical axis of the perfect vortex metasurface are at an angle of: Axis angle is:

[0121]

[0122] Wherein, is the period of the metasurface.

[0123] Wherein, the ideal model of the perfect vortex beam used is:

[0124]

[0125] The left-handed circularly polarized Bessel-Gauss beam and the right-handed circularly polarized Bessel-Gauss beam are Fourier transformed by the third lens, and a perfect vortex beam is obtained on the focal plane, which is expressed as:

[0126]

[0127] The signal light and the idler light pass through After that:

[0128]

[0129]

[0130] The final prepared entangled state output is:

[0131]

[0132] Specifically as follows:

[0133] In an ideal state, the distribution of the complex amplitude of the perfect vortex beam on the cross section can be expressed as

[0134]

[0135] in It represents the polar coordinates of a point on the cross-section of the light beam. It is the Dirac function. It is the radius of the annular light spot. It is the topological charge number.

[0136] This device generates a perfect vortex beam by using a third lens to perform a Fourier transform on a higher-order Bessel-Gaussian beam.

[0137] The incident light passes through a periodically polarized potassium titanate crystal, and the resulting signal light and idle light, after being collimated by a lens, are represented as follows:

[0138]

[0139]

[0140]

[0141] in , , , It is the waist width of the Gaussian beam. It refers to the transmission distance.

[0142] The optical axis is parallel to The Jones matrix of the 1 / 4 waveplate of the axis is expressed as:

[0143]

[0144] The signal light and idle light pass through this quarter-wave plate, represented as:

[0145]

[0146]

[0147] Then, the Gaussian beam passes through a perfect vortex metasurface, whose Jones matrix is ​​expressed as:

[0148]

[0149]

[0150]

[0151] in, Let be the polar coordinates of a point on the hypersurface. for The first-order Bessel function of the first kind, The beam waist width of the Gaussian beam on the metasurface. wave number Projection in the direction of the parallel super surface. The signal light and the idler light are denoted as

[0152]

[0153]

[0154] The complex amplitude of the Bessel-Gauss beam at the focal plane of the lens is given by

[0155]

[0156] where is the polar coordinate of a point at the focal plane of the lens, is the is the first kind of modified Bessel function of order is the waist width of the Gaussian beam at the focal plane of the lens, is the radius of the annular beam. The approximate expression of the above equation is

[0157]

[0158] The intensity distribution at the focal plane of the lens is given by

[0159]

[0160] The signal light and the idler light are denoted as

[0161]

[0162]

[0163] After that, the perfect vortex beam passes through a one-dimensional grating (i.e., a one-dimensional grating), and the angle between the optical axis of the one-dimensional grating and the axis is where is the period of the super surface of the one-dimensional grating.

[0164] Therefore, the Jones matrix of the super surface is given by

[0165]

[0166] The signal light and the idler light are denoted as

[0167]

[0168]

[0169] Geometric phase along the spatial distribution Momentum difference is generated:

[0170]

[0171] where left-handed state corresponds to right-handed state corresponds to The two have a small angle, and the circular ring spot is separated.

[0172] The photons at the two intersection points of the circular ring spot are in the entangled state of left-handed state and right-handed state :

[0173]

[0174] That is the final output of the device.

[0175] The second aspect of the application also provides an entangled light source device 100 based on a metasurface, comprising:

[0176] The focusing light path device 10 comprises a laser light source 11, a Glan polarizer 12, a half-wave plate 13, a first lens 14, and a periodically poled potassium titanyl phosphate crystal 15. The laser light source 11 can emit a Gaussian beam. The Glan polarizer 12 is used to adjust the polarization state of the Gaussian beam. After fine adjustment of the polarization state of the Gaussian beam by the half-wave plate 13, the Gaussian beam is focused by the first lens 14 and incident on the periodically poled potassium titanyl phosphate crystal 15 to complete spontaneous parametric down-conversion under nonlinear optical effect, generating signal light and idler light.

[0177] The signal light and idler light collimating light path device 20 comprises a second lens 21 and a 1 / 4 wave plate 22. The second lens 21 collimates the signal light and idler light to make them overlap each other in the output plane. The 1 / 4 wave plate 22 changes the polarization state of the signal light and idler light, wherein the signal light changes from horizontal polarization to right-handed circular polarization, and the idler light changes from vertical polarization to left-handed circular polarization.

[0178] The perfect vortex beam generation light path device 30 based on a metasurface comprises a perfect vortex metasurface 31 and a third lens 32. The optical axis direction of the perfect vortex metasurface 31 changes periodically along the angular direction. The Gaussian beam after the 1 / 4 wave plate 22 passes through the perfect vortex metasurface 31 to obtain a spatially varying geometric phase. The signal light changes from a right-handed circularly polarized Gaussian beam to a left-handed circularly polarized Bessel-Gaussian beam, and the idler light changes from a left-handed circularly polarized Gaussian beam to a right-handed circularly polarized Bessel-Gaussian beam. The third lens 32 is used to perform Fourier transform on the left-handed circularly polarized Bessel-Gaussian beam and the right-handed circularly polarized Bessel-Gaussian beam to obtain a coincident perfect vortex beam on the focal plane.

[0179] The super-surface-based light separation optical path device 40 comprises a one-dimensional grating 41, a light splitting prism 42, a fourth lens 43, a fifth lens 44, a first band-pass filter 45, and a second band-pass filter 46, the one-dimensional grating has an optical axis direction along the y-axis direction periodically changes along the z-axis direction; the signal light and the idle light pass through the one-dimensional grating to generate momentum differences in the positive and negative directions of the y-axis in the real space so as to be separated along the y-axis direction, the light splitting prism 42 divides the entangled photons after passing through the one-dimensional grating 41 into two paths, the fourth lens 43 and the first band-pass filter 45 are arranged in one path, and the fifth lens 44 and the second band-pass filter 46 are arranged in the other path; the measurement optical path device 50 of the entangled photons comprises two fiber couplers 51 and a single-photon detector 52 connected with the fiber couplers, the two fiber couplers 51 are arranged at the ends of the two paths of the first band-pass filter 45 and the second band-pass filter 46 respectively to record coincidence counts.

[0180] The first lens is a focusing lens to focus the light beam to the center of the periodically poled potassium titanyl phosphate crystal; the second lens is a collimating lens, and the first lens and the second lens form a 4f system;

[0181] After passing through the periodically poled potassium titanyl phosphate crystal, the signal light and the idle light collimated by the second lens are represented as

[0182]

[0183]

[0184]

[0185] wherein, , , is the waist width of the Gaussian light beam, is the propagation distance.

[0186] The perfect vortex super-surface has a phase delay with a period of ;

[0187] The one-dimensional grating has a phase delay with a period of n mm, preferably 300 mm;

[0188] The first super-surface optical axis and the z-axis have an included angle of:

[0189]

[0190] is a 0-order Bessel-Gaussian light beam. The complex amplitude of the m-order Bessel-Gaussian light beam is:

[0191]

[0192] The second metasurface optical axis is at an angle of 45° to the optical axis of the first metasurface. The angle between the axis and the optical axis of the first metasurface is 45°.

[0193]

[0194] wherein, n is the refractive index of the material of the metasurface, is the period of the metasurface.

[0195] The technical features described above can be combined arbitrarily. Although all possible combinations of the technical features are not described, any combination of the technical features should be considered to be covered by the present specification, as long as there is no contradiction in such a combination.

[0196] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand: it can still adjust the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part or all of the technical features; and these adjustments or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing an entangled light source based on a grating and a perfect vortex metasurface, characterized in that, The method comprises the following steps of: S10, providing a Gaussian beam, linearly polarizing the Gaussian beam along a specific polarization direction, and focusing the Gaussian beam into a periodically poled potassium titanyl phosphate crystal through a first lens to generate signal light and idler light through spontaneous parametric down-conversion under a nonlinear optical effect; S20, collimating the signal light and the idler light through a second lens to make them overlap with each other on an output plane, and changing polarization states of the signal light and the idler light through a 1 / 4 wave plate, wherein the signal light is changed from horizontal polarization to right circular polarization, and the idler light is changed from vertical polarization to left circular polarization; S30, providing a perfect vortex super surface, wherein an optical axis direction of the perfect vortex super surface changes periodically in an angular direction, and the Gaussian beam after the 1 / 4 wave plate passes through the perfect vortex super surface to obtain a spatially-varying geometric phase, wherein the signal light is changed from the right circularly polarized Gaussian beam to a left circularly polarized Bessel-Gaussian beam, and the idler light is changed from the left circularly polarized Gaussian beam to a right circularly polarized Bessel-Gaussian beam; and performing Fourier transform on the left circularly polarized Bessel-Gaussian beam and the right circularly polarized Bessel-Gaussian beam through a third lens to obtain a perfect vortex beam that overlaps on a focal plane; Step S40, providing a one-dimensional grating, the optical axis direction of the one-dimensional grating periodically changes along the x-axis direction The signal light and the idle light pass through the one-dimensional grating, respectively generating momentum difference along the positive and negative directions of the y-axis in the real space So as to separate them along the y-axis direction, and output the prepared entangled state.

2. The method of claim 1, wherein the method further comprises: The linear polarization of the Gaussian beam along a specific polarization direction comprises the following steps of: providing a Glan polarizer and a half wave plate to adjust the Gaussian beam to horizontal polarization, and finely adjusting the polarization state of the Gaussian beam through the half wave plate.

3. The method of claim 1, wherein the method further comprises: In the step S20, the first lens is a focusing lens to focus the light beam to be incident at the center of the periodically poled potassium titanyl phosphate crystal; and the second lens is a collimating lens, and the first lens and the second lens constitute a 4f system; the signal light and the idler light generated after passing through the periodically poled potassium titanyl phosphate crystal are collimated by the second lens and represented as: , , , , , where, and denote the signal light and the idler light, is the Jones vector representation of the light field, and are the polarization components of the directions, denotes the transpose, is the spatial distribution of the complex amplitude of the Gaussian beam, , , is the position of a point in space in three directions, represents the propagation distance, k is the wave vector, is the half-width at any z position, , denotes the wavefront curvature radius, denotes the Gouy phase.

4. The method of claim 1, wherein the method further comprises: The method further comprises the following steps of: S41, using a beam splitter prism to divide the entangled photons after passing through a one-dimensional grating into two paths; S42, setting a fourth lens in one of the two paths and setting a fifth lens on the other path to collimate the entangled photons in the two paths, respectively.

5. The method of claim 4, wherein the method further comprises: The preparation method of the entangled light source further comprises the following steps of: S51, setting a first band-pass filter in one of the two paths to remove photons of non-measurement wavelengths in the light beam after passing through the fourth lens; and setting a second band-pass filter in the other path to remove photons of non-measurement wavelengths in the light beam after passing through the fifth lens; S52, using a fiber coupler to respectively couple the signal photons and the idler photons in the two paths into optical fibers and transmit them to a single-photon detector to record coincidence counts.

6. The preparation method of the entangled light source based on a grating and a perfect vortex super surface according to claim 1, wherein the perfect vortex beam is a Bessel-Gaussian beam. Perfect vortex metasurface has Phase delay, period is ; The one-dimensional grating has a phase delay of 300 mm; The first metasurface optical axis and The angle between the axis and wherein, in is a polar coordinate, is a polar radius, is a polar angle, is a complex argument, is a 0th order Bessel-Gauss beam electric field distribution, and is a super surface optical axis initial orientation angle; is a 0th order Bessel-Gauss beam, the complex amplitude of the pth order Bessel-Gauss beam is: wherein is the natural exponential function, is the first kind Bessel function of order l, is the polar radius, is the polar angle, is the Bessel function of order l, is the wave vector in the direction, is the imaginary unit, is the topological charge, ; One-dimensional grating and The angle between the shafts is: wherein, is the super surface period, y is the position of a point in space in the y direction.

7. The preparation method of the entangled light source based on a grating and a perfect vortex super surface according to claim 1, wherein the perfect vortex beam is a Bessel-Gaussian beam. wherein The ideal model of the perfect vortex beam is: wherein is the Dirac function and r is the radial distance; ； The Fourier transform of the left circularly polarized Bessel-Gaussian beam and the right circularly polarized Bessel-Gaussian beam through the third lens to obtain the perfect vortex beam that overlaps on the focal plane is represented as: wherein, is the beam waist width of a Gaussian beam on the lens focal plane, where f is the focal length of the lens and k is the wave number, is the wave number projection in the direction of the parallel super surface; The signal light and the idler light pass through After that: , wherein, represents the optical field after the signal light has passed through the grating, represents the optical field after the idler light has passed through the grating, represents the complex amplitude of the perfect vortex beam, is the base of the natural logarithm, is the imaginary unit, is directional geometric phase factor; The final prepared entangled state output is: ； wherein represents a left-handed state represents a right-handed state is a kind of entangled state representation.

8. An entangled light source device based on a grating and a perfect vortex metasurface, characterized in that, The method comprises the following steps of: The focusing light path device comprises a laser light source, a Glan polarizer, a half-wave plate, a first lens and a periodically poled potassium titanyl phosphate crystal, the laser light source is capable of emitting a Gaussian beam, the Glan polarizer is used for adjusting the polarization state of the Gaussian beam, the half-wave plate is used for fine-tuning the polarization state of the Gaussian beam, and the Gaussian beam is focused into the periodically poled potassium titanyl phosphate crystal via the first lens to generate signal light and idler light through spontaneous parametric down-conversion under a nonlinear optical effect; The signal light and the idler light collimation light path device comprises a second lens and a 1 / 4 wave plate, the second lens is used for collimating the signal light and the idler light to make them overlap with each other on an output plane, and the 1 / 4 wave plate is used for changing the polarization states of the signal light and the idler light, wherein the signal light is changed from horizontal polarization to right circular polarization, and the idler light is changed from vertical polarization to left circular polarization; The super surface-based perfect vortex beam generation light path device comprises a perfect vortex super surface and a third lens, the optical axis direction of the perfect vortex super surface changes periodically along an angular direction, the Gaussian beam after the 1 / 4 wave plate passes through the perfect vortex super surface to obtain a spatially-varying geometric phase, the signal light is changed from the right circularly-polarized Gaussian beam to the left circularly-polarized Bessel-Gaussian beam, and the idler light is changed from the left circularly-polarized Gaussian beam to the right circularly-polarized Bessel-Gaussian beam; and the third lens is used for performing Fourier transform on the left circularly-polarized Bessel-Gaussian beam and the right circularly-polarized Bessel-Gaussian beam to obtain the coincident perfect vortex beams on a focal plane. The application discloses a super-surface-based light separation optical path device, which comprises a one-dimensional grating, a light splitting prism, a fourth lens, a fifth lens, a first band-pass filter and a second band-pass filter. The optical axis direction of the one-dimensional grating periodically changes along the axis direction; the signal light and the idle light pass through the one-dimensional grating, and respectively generate momentum differences in the positive and negative directions of the y axis in the real space So as to separate the entangled photons in the y direction, the light splitting prism divides the entangled photons after the one-dimensional grating into two paths, the fourth lens and the first band-pass filter are arranged in one path, and the fifth lens and the second band-pass filter are arranged in the other path. The entangled photon measurement light path device comprises two optical fiber couplers and single-photon detectors connected to the optical fiber couplers, and the two optical fiber couplers are arranged on the end light paths of two first band-pass filters and two second band-pass filters to record coincidence numbers.

9. The entangled light source device according to claim 8, wherein the first lens is a focusing lens, and the second lens is a collimating lens, and the first lens and the second lens constitute a 4f system; the signal light and the idler light generated through the periodically poled potassium titanyl phosphate crystal are collimated by the second lens, and are expressed as , , , , , where, and denote the signal light and the idler light, is the Jones vector representation of the light field, and are the polarization components in the directions, denotes the transpose, is the spatial distribution of the complex amplitude of the Gaussian beam, , , is the position of a point in space in three directions, is the propagation distance and k is the wave vector, is the half-width at any z position, , denotes the radius of curvature of the wavefront, denotes the Gouy phase.

10. The entangled light source device according to claim 8, wherein Perfect vortex metasurface has Phase delay, period is ; The one-dimensional grating has a phase delay of 300 mm; The first metasurface optical axis and The angle between the axis and wherein, in is a polar coordinate, is a polar radius, is a polar angle, with is a metasurface parameter; is a 0th order Bessel-Gauss beam, the complex amplitude of the 0th order Bessel-Gauss beam is: wherein is the polar radius, is the polar angle, is the l-th Bessel function is the wave vector in the direction, is the imaginary unit, is the topological charge, is ; The second metasurface optical axis is at an angle of 45 degrees to the optical axis of the lens. The angle between the axis and the optical axis of the lens is 45 degrees. wherein, is the super surface period, y is the position of a point in space in the y direction.

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