Preparation method and equipment of entangled light source based on grating and perfect vortex metasurface

Abstract

The invention relates to a preparation method of an entangled light source based on a grating and a perfect vortex metasurface, and the method comprises the steps: providing a Gaussian beam, carrying out the linear polarization of the Gaussian beam in a specific polarization direction, and generating signal light and idle light through a periodically poled potassium titanate crystal; after the photons are collimated, the polarization states of the signal light and the idle light are changed through a 1 / 4 wave plate, and the signal light and the idle light are respectively converted into right-handed circular polarization and left-handed circular polarization; the Gaussian beam passes through the perfect vortex metasurface to obtain a geometric phase changing along with space; through the one-dimensional grating, the left-handed perfect vortex light and the right-handed perfect vortex light are separated to output an overlapped part entangled state; and respectively coupling the photons in the two paths into an optical fiber, and transmitting the photons to a single-photon detector to record a coincidence number.

Description

Technical Field

[0001] This application relates to the field of quantum entanglement source emission, and particularly to a method and device for preparing an entangled light source based on a grating and a perfect vortex metasurface. Background Art

[0002] Entangled photon sources are the core resources in the fields of quantum communication, quantum computing, and quantum cryptography. Entangled photon pairs have non-classical quantum correlation properties and can enable applications such as quantum teleportation and quantum key distribution.

[0003] The realization of traditional entangled photon sources mainly relies on nonlinear crystals to achieve phase matching between the pump light and the down-converted photons. Subsequently, single-photon detectors are used to detect the signal photons and idler photons, and the entanglement characteristics of the photon pairs are verified 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: such as the low conversion efficiency of the nonlinear process of the crystal, the need for high-power light incidence, and the sensitivity to phase matching conditions, being sensitive to environmental conditions such as temperature and humidity. Also, the angular range of the entangled photons is limited, etc. Therefore, a new scheme for generating entangled light resources is urgently needed.

[0004] Based on this, this application is proposed. Summary of the Invention

[0005] Aiming at the above technical problems, the purpose of this application is to provide a method and device for preparing an entangled light source based on a grating and a perfect vortex metasurface.

[0006] In the first aspect of this application, a method for preparing an entangled light source based on a grating and a perfect vortex metasurface is provided. Under one general inventive concept, it includes:

[0007] Providing a Gaussian beam, linearly polarizing the Gaussian beam along a specific polarization direction and then generating signal light and idler light via a periodically poled potassium titanate crystal;

[0008] After collimating the photons, changing the polarization states of the signal light and the idler light through a quarter-wave plate, and respectively converting the signal light and the idler light into right-handed and left-handed circular polarizations;

[0009] The Gaussian beam passes through a perfect vortex metasurface to obtain a geometric phase that varies with space;

[0010] Then, through a one-dimensional grating, the two perfect vortex lights of right and left hands are separated to output an overlapping part of the entangled state;

[0011] Coupling the photons in the two paths into optical fibers respectively and transmitting them to a single-photon detector to record the coincidence count.

[0012] Specifically, first, a Gaussian beam is provided. The Gaussian beam is linearly polarized along a specific polarization direction and is focused by a first lens and incident on a periodically poled potassium titanate crystal to complete spontaneous parametric down-conversion under the nonlinear optical effect, generating a signal light and an idler light. The signal light and the idler light are collimated by a second lens so that they overlap with each other in the output plane. The polarization states of the signal light and the idler light are changed by a quarter-wave plate, where the signal light is changed from horizontal polarization to right-handed circular polarization, and the idler light is changed from vertical polarization to left-handed circular polarization. A perfect vortex metasurface is provided, where the optical axis direction of the perfect vortex metasurface changes periodically in the angular direction. The Gaussian beam after passing through the quarter-wave plate passes through the perfect vortex metasurface to obtain a geometric phase that varies with space. Among them, the signal light is changed from a right-handed circularly polarized Gaussian beam to a left-handed circularly polarized Bessel-Gaussian beam, and the idler light is changed from a left-handed circularly polarized Gaussian beam to a right-handed circularly polarized Bessel-Gaussian beam. The left-handed circularly polarized Bessel-Gaussian beam and the right-handed circularly polarized Bessel-Gaussian beam are Fourier-transformed by a third lens, and a coincident perfect vortex beam is obtained on the focal plane. A one-dimensional grating is provided, and the optical axis direction of the one-dimensional grating changes periodically along the y-axis direction. The signal light and the idler light pass through a second PB phase metasurface to generate momentum differences Δk in the positive and negative y-axis directions in real space respectively y To separate them along the y direction to output the prepared entangled state.

[0013] In a further solution of the present application, linearly polarizing the Gaussian beam along a specific polarization direction includes: providing a Glan polarizer and a half-wave plate to adjust the Gaussian beam to be horizontally polarized, and finely adjusting the polarization state of the Gaussian beam through the half-wave plate.

[0014] In a further solution of the present application, in step S20, the first lens is a focusing lens to focus the beam and make it incident on the center of the periodically poled potassium titanate crystal; the second lens is a collimating lens, and the first lens and the second lens form a 4f system.

[0015] After passing through the periodically poled potassium titanate crystal, the generated signal light and idler light are collimated by the second lens and are expressed as

[0016] E s1 =(E x E y ) T =E G (x,y)(1 0) T

[0017] E i1 =(E x E y ) T =E G (x,y)(0 1) T

[0018]

[0019] Among them,

[0020] is the waist width of the Gaussian beam, and z is the propagation distance.

[0021] In a further aspect of the present application, step S40 further includes: step S41, using a beam splitting prism to split the entangled photons after passing through the one-dimensional grating into two paths; step S42, arranging a fourth lens in one path and a fifth lens in the other path to collimate the entangled photons in the two paths respectively.

[0022] In a further aspect of the present application, the method for preparing the entangled light source further includes: step S51, arranging a first band-pass filter in one path to remove photons with non-measured wavelengths in the beam after passing through the fourth lens; arranging a second band-pass filter in the other path to remove photons with non-measured wavelengths in the beam after passing through the fifth lens; step S52, using an optical fiber coupler to couple the signal photons and the idler photons in the two paths into the optical fiber respectively, and transmitting them to a single-photon detector to record the coincidence count.

[0023] In a further aspect of the present application, the perfect vortex metasurface has a phase delay of π and a period of 2π; the one-dimensional grating has a phase delay of π and a period of n mm, preferably 300 mm;

[0024] The angle between the optical axis of the first metasurface and the x-axis is:

[0025]

[0026] Among them, Arg is the argument of the complex number, is the electric field distribution of the 0th-order Bessel-Gaussian beam, ρ is the radial distance, φ is the polar angle, q is the topological charge number of the metasurface, and α0 is the initial orientation angle of the optical axis of the metasurface, generally taken as 0.

[0027] is the 0th-order Bessel-Gaussian beam. The complex amplitude of the lth-order Bessel-Gaussian beam is:

[0028]

[0029] Among them, exp is the natural exponential function, J l (x) is the lth-order Bessel function of the first kind,

[0030] Among them, Γ(n) is the gamma function, and when n is a positive integer, Γ(n)=(n - 1)!, and w0 is the waist width of the Gaussian beam.

[0031] The angle between the second metasurface optical axis and the x-axis is:

[0032]

[0033] where d is the metasurface period and Ω is the phase gradient.

[0034] In a further aspect of the present application, the ideal model of the perfect vortex beam adopted is:

[0035] E(r,φ)=δ(r-r0)exp(ilφ)

[0036] where δ(r) is the Dirac function, r is the radial distance, r0 is the spot radius, i is the imaginary unit, and l is the topological charge number of the perfect vortex beam.

[0037] The left-handed circularly polarized Bessel-Gaussian beam and the right-handed circularly polarized Bessel-Gaussian beam are Fourier-transformed by the third lens, and the coincident perfect vortex beam obtained on the focal plane is expressed as:

[0038]

[0039] where r0=k ρ f / k is the radius of the annular spot, w=2f / kw0 is the waist width of the Gaussian beam on the focal plane of the lens, where f is the focal length of the lens, k is the wave number, and k ρ is the projection of the wave number k in the direction parallel to the metasurface.

[0040] The signal light and the idler light after passing through the second PB phase metasurface are:

[0041]

[0042] The finally prepared entangled state output is:

[0043]

[0044] The second aspect of the present application further provides a metasurface-based entangled light source device, including: a focusing optical 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 the half-wave plate finely tunes the polarization state of the Gaussian beam, it is focused by the first lens and incident on the periodically poled potassium titanyl phosphate crystal to complete spontaneous parametric down-conversion under the nonlinear optical effect, generating a signal light and an idler light; a collimation optical path device for the signal light and the idler light, including a second lens and a quarter-wave plate. The second lens collimates the signal light and the idler light so that they overlap with each other in the output plane. The quarter-wave plate changes the polarization states of the signal light and the idler light, where the signal light is changed from horizontal polarization to right-handed circular polarization, and the idler light is changed from vertical polarization to left-handed circular polarization; a metasurface-based perfect vortex beam generation optical path device, including a perfect vortex metasurface and a third lens. The optical axis direction of the perfect vortex metasurface changes periodically in the angular direction. The Gaussian beam after passing through the quarter-wave plate passes through the perfect vortex metasurface to obtain a geometric phase that varies with space. The signal light is changed from a right-handed circularly polarized Gaussian beam to a left-handed circularly polarized Bessel Gaussian beam, and the idler light is changed from a left-handed circularly polarized Gaussian beam to a right-handed circularly polarized Bessel Gaussian beam; the third lens is used to perform a Fourier transform on the left-handed circularly polarized Bessel Gaussian beam and the right-handed circularly polarized Bessel Gaussian beam to obtain overlapping perfect vortex beams on the focal plane; a metasurface-based light separation optical path device, including a one-dimensional grating, a beam 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 y-axis direction; the signal light and the idler light pass through the second PB phase metasurface to generate momentum differences Δk in the positive and negative y-axis directions in real space respectively y to separate them along the y direction. The beam splitting prism divides the entangled photons after passing through 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 optical path device for the entangled photons, including two fiber couplers and single-photon detectors connected to the fiber couplers. The two fiber couplers are respectively arranged on the end optical paths of the first band-pass filter and the second band-pass filter in the two paths to record the coincidence counts.

[0045] In a further embodiment of the present application, the first lens is a focusing lens to focus the beam onto the center of the periodically poled potassium titanyl phosphate crystal for incidence; the second lens is a collimating lens, and the first lens and the second lens form a 4f system;

[0046] 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

[0047] E s1 =(E x E y) T = E G (x,y)(1 0) T

[0048] E i1 = (E x E y ) T = E G (x,y)(0 1) T

[0049]

[0050] where arctan is the arctangent function, is the waist width of the Gaussian beam, and z is the propagation distance.

[0051] In a further embodiment of the present application, the perfect vortex metasurface has a phase delay of π and a period of 2π; the one-dimensional grating has a phase delay of π and a period of n mm, preferably 300 mm; the angle between the optical axis of the first metasurface and the x-axis is:

[0052]

[0053] is the 0th order Bessel-Gaussian beam. The complex amplitude of the lth order Bessel-Gaussian beam is:

[0054]

[0055] The angle between the optical axis of the second metasurface and the x-axis is:

[0056]

[0057] where d is the period of the metasurface.

[0058] In summary, the preparation method of the entangled light source based on the grating and the perfect vortex metasurface proposed by the general inventive concept of the present application has at least the following technical effects:

[0059] 1. A new type of entangled source is generated through the metasurface, replacing the BBO crystal with a periodically poled potassium titanyl phosphate crystal. At the same time, there are fewer restrictions on environmental conditions such as temperature and humidity, improving environmental robustness, achieving quasi-phase matching (QPM), and making the nonlinear process more efficient.

[0060] 2. The present application only requires low-power pump light to achieve efficient generation of entangled photon pairs; compared with the traditional method that requires high-power pump light to achieve the same efficiency, and high-power pump light may bring thermal effects and system instability, the present application can make the conversion efficiency of the nonlinear process higher.

[0061] 3. Different from the preparation of traditional entangled photon sources, this device prepares entangled states of horizontal polarization and vertical polarization states. This application can prepare entangled states of left-handed and right-handed states, which are insensitive to the rotation of the optical system. Therefore, it is more robust in free-space quantum communication, and circular polarization states can be more conveniently combined with other optical elements to achieve more complex manipulation of photon states.

[0062] 4. The precise control of photon states is achieved through the metasurface. The introduction of the metasurface makes the overall device more compact and integrated.

[0063] Other features and advantages of the embodiments of the present invention will be described in the subsequent specific implementation embodiments section. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] In order to more clearly illustrate the specific implementation manners of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific implementation manners or the prior art. Obviously, the drawings in the following description are some implementation manners of the present application. For those skilled in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0065] Figure 1 It is a schematic flowchart of the entangled light source method provided by the present application;

[0066] Figure 2 It is a schematic structural diagram of the entangled light source device provided by the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0067] For the descriptions of the terms "second direction", "first direction", "third direction", "inner", "outer", etc. indicating the orientation or positional relationship hereinafter, without special instructions, it is understood as the orientation or positional relationship based on the drawings shown, which is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, so it cannot be understood as a limitation to the present application.

[0068] In addition, for the features limited by "first" and "second" only for descriptive purposes, it cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. The features limited by "first" and "second" may explicitly or implicitly include at least one of the limited features. For the description of "a plurality", the general meaning is at least including two, such as two, three, etc., unless otherwise specifically limited.

[0069] In this application, unless otherwise clearly specified and defined, terms such as "installed", "connected", "linked", "fixed", etc. shall be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection, it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components or the interaction relationship between two components. For those skilled in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0070] In the description of this specification, terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of this application. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0071] Referring to Figures 1 to 2 As shown, this application first provides a method for preparing an entangled light source based on a grating and a perfect vortex metasurface, including:

[0072] Step S10: Provide a Gaussian beam, linearly polarize the Gaussian beam along a specific polarization direction, and focus it through a first lens and incident on a periodically poled potassium titanate crystal to complete spontaneous parametric down-conversion under the nonlinear optical effect, generating a signal light and an idler light;

[0073] The intensity distribution of the Gaussian beam is in the form of a Gaussian function, with the maximum central intensity and gradually weakening outward; the Gaussian beam provided here is the initial light source for subsequent nonlinear optical processes. Linear polarization means that the polarization direction of the light is restricted to a specific direction (such as the horizontal or vertical direction); the Gaussian beam is adjusted to linearly polarized light in a specific direction through a polarizer, such as horizontal polarization or vertical polarization.

[0074] Use the first lens to focus the linearly polarized Gaussian beam to concentrate its energy. The focused beam is incident on the periodically poled potassium titanate crystal. Due to the nonlinear optical properties of this crystal, under spontaneous parametric down-conversion, a high-energy photon (pump light) can spontaneously split into two low-energy photons, namely the signal light and the idler light.

[0075] Step S20: Collimate the signal light and the idler light through the second lens so that they overlap with each other in the output plane, and change the polarization states of the signal light and the idler light through a quarter-wave plate, where the signal light is changed from horizontal polarization to right-handed circular polarization, and the idler light is changed from vertical polarization to left-handed circular polarization;

[0076] After the signal light and the idler light emerge from the crystal, they are collimated through the second lens; the signal light and the idler light are made to overlap on the output plane, and a quarter-wave plate is used to change the polarization states of the signal light and the idler light: the signal light is changed from horizontal polarization to right-handed circular polarization; the idler light is changed from vertical polarization to left-handed circular polarization; The characteristic of circularly polarized light is that the electric field vector of the light rotates with time and is divided into two types: left-handed and right-handed.

[0077] Step S30: Provide a perfect vortex metasurface (the first PB-phase metasurface), where the optical axis direction of the perfect vortex metasurface changes periodically in the angular direction, and the Gaussian beam after passing through the quarter-wave plate passes through the perfect vortex metasurface to obtain a geometric phase that varies with space. Among them, the signal light is changed from a right-handed circularly polarized Gaussian beam to a left-handed circularly polarized Bessel-Gaussian beam, and the idler light is changed from a left-handed circularly polarized Gaussian beam to a right-handed circularly polarized Bessel-Gaussian beam; Fourier transform the left-handed circularly polarized Bessel-Gaussian beam and the right-handed circularly polarized Bessel-Gaussian beam through the third lens to obtain overlapping perfect vortex beams on the focal plane;

[0078] After the signal light and the idler light pass through the perfect vortex metasurface, their polarization states and spatial distributions change: the signal light is changed from a right-handed circularly polarized Gaussian beam to a left-handed circularly polarized Bessel-Gaussian beam; the idler light is changed from a left-handed circularly polarized Gaussian beam to a right-handed circularly polarized Bessel-Gaussian beam; Fourier transform the transformed Bessel-Gaussian beam through the third lens; on the focal plane, the signal light and the idler light overlap to form a perfect vortex beam. The perfect vortex beam has a spiral phase structure and carries orbital angular momentum (OAM).

[0079] Step S40: Provide a one-dimensional grating (the second PB-phase metasurface), where the optical axis direction of the one-dimensional grating changes periodically along the y-axis direction. The signal light and the idler light pass through the one-dimensional grating, respectively generating a momentum difference Δk along the positive and negative directions of the y-axis in real space y so that they are separated along the y-direction to output the prepared entangled state.

[0080] The optical axis direction of the one-dimensional grating changes periodically along the axial direction, which is different from the angular change of the perfect vortex metasurface. After the signal light and the idler light pass through the second PB-phase metasurface, a momentum difference along the positive and negative directions of the y-axis is generated in real space. The momentum difference causes the signal light and the idler light to be separated in space and propagate along the y-direction; finally, the signal light and the idler light are separated and output to form an entangled state photon source.

[0081] In summary, the preparation method of the entangled light source based on a grating and a perfect vortex metasurface proposed by the general inventive concept of the present application has at least the following technical effects:

[0082] 1. A new type of entangled source is generated through the metasurface, enabling the replacement of the BBO crystal with a periodically poled potassium titanyl phosphate crystal. At the same time, there are fewer restrictions on environmental conditions such as temperature and humidity, improving environmental robustness and achieving quasi-phase matching (QPM), making the nonlinear process more efficient.

[0083] 2. The present application only requires a low-power pump light to achieve efficient generation of entangled photon pairs; in contrast, traditional methods require high-power pump light to achieve the same efficiency, and high-power pump light may bring thermal effects and system instability. The present application can make the conversion efficiency of the nonlinear process higher.

[0084] 3. Different from the preparation of traditional entangled photon sources where the present device prepares entangled states of horizontal polarization and vertical polarization, the present application can prepare entangled states of left-handed and right-handed states, which are insensitive to the rotation of the optical system. Therefore, it is more robust in free-space quantum communication, and the circular polarization state can be more conveniently combined with other optical elements to achieve more complex manipulation of photon states.

[0085] In step S10, linearly polarize the Gaussian beam along a specific polarization direction, including:

[0086] Provide a Glan polarizer and a half-wave plate to adjust the Gaussian beam to horizontal polarization and finely adjust the polarization state of the Gaussian beam through the half-wave plate.

[0087] In this step, the Glan polarizer converts the Gaussian beam into horizontally polarized light. 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, adjusting the horizontally polarized light to linearly polarized light at a certain angle with the x-axis. After being adjusted by the Glan polarizer and the half-wave plate, the Gaussian beam is precisely adjusted to linearly polarized light in a specific direction; the linearly polarized light then passes through the first lens and is focused and incident into the periodically poled potassium titanyl phosphate crystal for the spontaneous parametric down-conversion (SPDC) process.

[0088] In step S20, the first lens is a focusing lens to focus the beam onto the center of the periodically poled potassium titanyl phosphate crystal for incidence; the second lens is a collimating lens, and the first lens and the second lens form a 4f system.

[0089] The first lens is a focusing lens used to focus the Gaussian beam onto 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 used to collimate the signal light and the idler light after passing through the periodically poled potassium titanyl phosphate crystal. The first lens and the second lens constitute a 4f system: the first lens focuses the Gaussian beam onto the center of the periodically poled potassium titanyl phosphate crystal (object plane), and the second lens collimates the signal light and the idler light and overlaps the signal light and the 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 rear focal plane of the system, respectively.

[0090] After passing through the periodically poled potassium titanyl phosphate crystal, the generated signal light and idler light are collimated by the second lens and represented as

[0091] E s1 =(E x E y ) T =E G (x,y)(1 0) T

[0092] E i1 =(E x E y ) T =E G (x,y)(0 1) T

[0093]

[0094] where

[0095] is the waist width of the Gaussian beam, and z is the propagation distance.

[0096] In summary, after passing through the periodically poled potassium titanyl phosphate crystal and collimated by the second lens, the electric field distributions of the signal light and the idler light can be described in the form of Gaussian beams; the collimated signal light and idler light overlap on the output plane.

[0097] In a further proposed solution, step S40 further includes:

[0098] Step S41: Use a beam splitter prism to divide the entangled photons after passing through the one-dimensional grating into two paths;

[0099] Step S42: Set a fourth lens in one path and a fifth lens in the other path to collimate the entangled photons in the two paths respectively.

[0100] In this step, the beam splitter prism divides the entangled photons after being processed by the one-dimensional grating into two paths; specifically, after the entangled photons pass through the one-dimensional grating, due to the periodic change of the optical axis direction of the metasurface along the axial direction, the signal light and the idler light generate a momentum difference in the positive and negative y-axis directions in real space, resulting in their spatial separation. The beam splitter prism further divides these two separated beams into two paths and enters different optical paths respectively. The fourth lens and the fifth lens are respectively used to collimate the two paths of entangled photons. The beam splitter prism divides the entangled photons into two paths, realizing spatial separation and independent processing; the fourth lens and the fifth lens respectively collimate the two paths of entangled photons to ensure the high-quality output of the light beam.

[0101] The method for preparing the entangled light source further includes:

[0102] Step S51: Set a first band-pass filter in one path to remove photons with non-measured wavelengths in the light beam after passing through the fourth lens; set a second band-pass filter in the other path to remove photons with non-measured wavelengths in the light beam after passing through the fifth lens;

[0103] Step S52: Use an optical fiber coupler to couple the signal photons and the idler photons in the two paths into the optical fiber respectively, and transmit them to a single-photon detector to record the coincidence number.

[0104] Set a first band-pass filter in the first path to filter out photons with non-measured wavelengths in the light beam after passing through the fourth lens, and only retain the signal photons with the target wavelength; set a second band-pass filter in the second path to filter out photons with non-measured wavelengths in the light beam after passing through the fifth lens, and only retain the idler photons with the target wavelength, improving the signal-to-noise ratio of the detected signal, reducing the influence of background noise on the experimental results, ensuring that only photons with the target wavelength enter the subsequent optical fiber coupling and detection process, and improving the accuracy of the experiment. In the first path, use an optical fiber coupler to couple the signal photons into the optical fiber, and in the second path, use an optical fiber coupler to couple the idler photons into the optical fiber; the coupled photons are transmitted to the single-photon detector through the optical fiber. 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 idler photons, the generation and characteristics of the entangled photon pairs can be verified.

[0105] In this application, the perfect vortex metasurface has a phase delay of π and a period of 2π; the one-dimensional grating has a phase delay of π and a period of n mm, preferably 300 mm; the included angle between the optical axis of the first metasurface (i.e., the perfect vortex metasurface) and the x-axis is:

[0106]

[0107] wherein, is the 0th order Bessel-Gaussian beam.

[0108] The complex amplitude of the l-order Bessel-Gaussian beam is as follows:

[0109]

[0110] Among them, the angle between the optical axis of the one-dimensional grating and the x-axis is:

[0111]

[0112] Among them, d is the period of the metasurface.

[0113] Among them, the ideal model of the perfect vortex beam adopted is:

[0114] E(r,φ) = δ(r - r0)exp(ilφ)

[0115] By performing Fourier transform on the left-handed circularly polarized Bessel-Gaussian beam and the right-handed circularly polarized Bessel-Gaussian beam through the third lens, the coincident perfect vortex beam obtained on the focal plane is expressed as:

[0116]

[0117] The signal light and the idler light after passing through the second PB phase metasurface are:

[0118]

[0119] The finally prepared entangled state output is:

[0120]

[0121] Specifically as follows:

[0122] In the ideal state, the distribution of the complex amplitude of the perfect vortex beam on the cross-section can be expressed as

[0123] E(r,φ) = δ(r - r0)exp(ilφ)

[0124] Among them, (r,φ) are the polar coordinates of a point on the cross-section of the beam, δ(r) is the Dirac function, r0 is the radius of the annular light spot, and l is the topological charge number.

[0125] This device generates a perfect vortex beam by performing Fourier transform on the high-order Bessel-Gaussian beam using the third lens.

[0126] The incident light passes through the periodically poled potassium titanyl phosphate crystal, and the generated signal light and idler light after being collimated by the lens are expressed as:

[0127] E s1 =(E x E y ) T =E G(x,y)(1 0) T

[0128] E i1 = (E x E y ) T = E G (x,y)(0 1) T

[0129]

[0130] where w0 is the waist width of the Gaussian beam, and z is the propagation distance.

[0131] The Jones matrix representation of a quarter-wave plate with its optical axis parallel to the x-axis is

[0132]

[0133] The signal light and the idler light passing through this quarter-wave plate are expressed as:

[0134]

[0135] Then, the Gaussian beam passes through a perfect vortex metasurface, and its Jones matrix representation is:

[0136]

[0137]

[0138] where (ρ, φ) are the polar coordinates of a point on the metasurface, J l is the l-th order Bessel function of the first kind, w0 is the waist width of the Gaussian beam on the metasurface, and k ρ is the projection of the wave number k in the direction parallel to the metasurface. After passing through this metasurface, the signal light and the idler light are respectively expressed as:

[0139]

[0140] The l-th order Bessel-Gaussian beam is Fourier-transformed by a lens, and the complex amplitude on the focal plane of the lens can be expressed as:

[0141]

[0142] where (r, φ) are the polar coordinates of a point on the focal plane of the lens, I l is the l-th order modified Bessel function of the first kind, w = 2f / kw0 is the waist width of the Gaussian beam on the focal plane of the lens, and r0 = k ρ f / k is the radius of the circular ring-shaped light spot. The approximate representation of the above formula is:

[0143]

[0144] The intensity distribution on the focal plane of the lens can be expressed as:

[0145]

[0146] After the Fourier transform of the lens, the signal light and the idler light are expressed as:

[0147]

[0148] 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 x-axis where d is the metasurface period of the one-dimensional grating.

[0149] Therefore, the Jones matrix of the metasurface is expressed as:

[0150]

[0151] The signal light and the idler light pass through a one-dimensional grating (i.e., a one-dimensional grating) and can be expressed as:

[0152]

[0153]

[0154] The geometric phase distributed along the space generates a momentum difference:

[0155]

[0156] where the left-handed state |L> corresponds to σ + = +1, and the right-handed state |R> corresponds to σ - = -1. There is a small angle between the two, and the circular spot is separated.

[0157] The photons at the two intersection points of the circular spot are in an entangled state of the left-handed state |L> and the right-handed state |R>:

[0158]

[0159] That is the final output of this device.

[0160] The second aspect of this application also provides an entanglement light source device 100 based on a metasurface, including:

[0161] The focusing optical path device 10 includes 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 the half-wave plate 13 finely tunes the polarization state of the Gaussian beam, it is focused and incident on the periodically poled potassium titanyl phosphate crystal 15 via the first lens 14 to complete spontaneous parametric down-conversion under the nonlinear optical effect, generating a signal light and an idler light;

[0162] The collimation optical path device 20 for the signal light and the idler light includes a second lens 21 and a quarter-wave plate 22. The second lens 21 collimates the signal light and the idler light so that they overlap with each other in the output plane. The quarter-wave plate 22 changes the polarization states of the signal light and the idler light, where the signal light is changed from horizontal polarization to right-handed circular polarization, and the idler light is changed from vertical polarization to left-handed circular polarization;

[0163] The optical path device 30 for generating a perfect vortex beam based on a metasurface includes a perfect vortex metasurface 31 and a third lens 32. The optical axis direction of the perfect vortex metasurface 31 changes periodically in the angular direction. The Gaussian beam after passing through the quarter-wave plate 22 passes through the perfect vortex metasurface 31 to obtain a geometric phase that varies with space. The signal light is changed from a right-handed circularly polarized Gaussian beam to a left-handed circularly polarized Bessel Gaussian beam, and the idler light is changed 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 transforms on the left-handed circularly polarized Bessel Gaussian beam and the right-handed circularly polarized Bessel Gaussian beam to obtain overlapping perfect vortex beams on the focal plane;

[0164] The optical path device 40 for light separation based on a metasurface includes a one-dimensional grating 41, a beam splitter prism 42, a fourth lens 43, a fifth lens 44, a first band-pass filter 45, and a second band-pass filter 46. The optical axis direction of the one-dimensional grating changes periodically along the y-axis direction; The signal light and the idler light pass through the one-dimensional grating to generate a momentum difference Δk along the positive and negative directions of the y-axis in real space respectively y to separate them along the y-direction. The beam splitter 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 for the entangled photons includes two fiber couplers 51 and a single-photon detector 52 connected to the fiber couplers. The two fiber couplers 51 are respectively arranged on the end optical paths of the first band-pass filter 45 and the second band-pass filter 46 in the two paths to record the coincidence number.

[0165] The first lens is a focusing lens to focus the beam to be incident at the center of the periodically poled potassium titanyl phosphate crystal; The second lens is a collimating lens. The first lens and the second lens form a 4f system;

[0166] After passing through the periodically poled potassium titanate crystal, the generated signal light and idler light are collimated by the second lens and expressed as

[0167] E s1 =(E x E y ) T =E G (x,y)(1 0) T

[0168] E i1 =(E x E y ) T =E G (x,y)(0 1) T

[0169]

[0170] wherein,

[0171] is the waist width of the Gaussian beam, and z is the propagation distance.

[0172] The perfect vortex metasurface has a phase delay of π and a period of 2π;

[0173] The one-dimensional grating has a phase delay of π and a period of n mm, preferably 300 mm;

[0174] The angle between the optical axis of the first metasurface and the x-axis is:

[0175]

[0176] is the 0th order Bessel-Gaussian beam. The complex amplitude of the lth order Bessel-Gaussian beam is:

[0177]

[0178] The angle between the optical axis of the second metasurface and the x-axis is:

[0179]

[0180] wherein, d is the period of the metasurface.

[0181] The technical features described above can be combined arbitrarily. Although all possible combinations of these technical features are not described, any combination of these technical features should be considered to be covered by this specification as long as such a combination does not exist in contradiction.

[0182] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still adjust the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some 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: include: Step S10, providing a Gaussian beam, linearly polarizing the Gaussian beam along a specific polarization direction, and focusing the Gaussian beam through a first lens to be incident on a periodically polarized potassium titanate crystal, so as to complete spontaneous parametric down conversion under a nonlinear optical effect to generate signal light and idle light; Step S20, collimating the signal light and the idle light through a second lens so that they overlap each other in an output plane, and changing the polarization states of the signal light and the idle light through a quarter wave plate, wherein the signal light is changed from horizontal polarization to right-handed circular polarization, and the idle light is changed from vertical polarization to left-handed circular polarization; Step S30, providing a perfect vortex metasurface, wherein the optical axis direction of the perfect vortex metasurface changes periodically along the angular direction, and the Gaussian beam after passing through the 1 / 4 wave plate passes through the perfect vortex metasurface to obtain a geometric phase that varies with space, wherein the signal light is changed from a right-handed circularly polarized Gaussian beam to a left-handed circularly polarized Bessel-Gaussian beam, and the idle light is changed from a left-handed circularly polarized Gaussian beam to a right-handed circularly polarized Bessel-Gaussian beam; Fourier transform is performed on the left-handed circularly polarized Bessel-Gaussian beam and the right-handed circularly polarized Bessel-Gaussian beam through a third lens to obtain a coincident perfect vortex beam on the focal plane; Step S40: provide a one-dimensional grating, the optical axis direction of the one-dimensional grating changes periodically along the y-axis direction, and the signal light and the idle light pass through the one-dimensional grating, respectively generating a momentum difference Δk along the positive and negative directions of the y-axis in real space. y So that they are separated along the y direction to output the prepared entangled state.

2. The method for preparing an entangled light source based on a grating and a perfect vortex metasurface according to claim 1, characterized in that: The step of linearly polarizing the Gaussian beam along a specific polarization direction comprises: A Glan polarizer and a half-wave plate are provided to adjust the Gaussian beam to be horizontally polarized, and the polarization state of the Gaussian beam is finely adjusted by the half-wave plate.

3. The method for preparing an entangled light source based on a grating and a perfect vortex metasurface according to claim 1, characterized in that: In step S20, The first lens is a focusing lens, which focuses the light beam to the center of the periodically polarized potassium titanate crystal for incidence; the second lens is a collimating lens, and the first lens and the second lens constitute a 4f system; After the periodically poled potassium titanate crystal, the generated signal light and idle light are collimated by the second lens and are expressed as AND s1 =(And x AND y ) T =And G (x,y)(1 0) T AND i1 =(And x AND y ) T =And G (x,y)(0 1) T in, R(z)=z[1+(x 2 +y 2 ) / z 2 ], is the waist width of the Gaussian beam and z is the propagation distance.

4. The method for preparing an entangled light source based on a grating and a perfect vortex metasurface according to claim 1, characterized in that: Also includes: Step S41, using a beam splitter prism to split the entangled photons after passing through the one-dimensional grating into two paths; Step S42: setting a fourth lens in one of the paths and a fifth lens in the other path, so as to collimate the entangled photons in the two paths respectively.

5. The method for preparing an entangled light source based on a grating and a perfect vortex metasurface according to claim 1, characterized in that: The method for preparing the entangled light source further includes: Step S51, setting a first bandpass filter in one of the paths to remove photons of non-measurement wavelengths in the light 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 light beam after passing through the fifth lens; Step S52: Use an optical fiber coupler to couple the signal photons and idle photons in the two paths into the optical fiber respectively, and transmit them to the single photon detector to record the coincidence number.

6. The method for preparing an entangled light source based on a grating and a perfect vortex metasurface according to claim 1, characterized in that: The perfect vortex metasurface has a phase delay of π and a period of 2π; The one-dimensional grating has a phase delay of π and a period of n mm, preferably 300 mm; The angle between the optical axis of the first metasurface and the x-axis is: is a 0th-order Bessel-Gaussian beam, and the complex amplitude of the lth-order Bessel-Gaussian beam is: The angle between the one-dimensional grating and the x-axis is: Where d is the metasurface period.

7. The method for preparing an entangled light source based on a grating and a perfect vortex metasurface according to claim 1, characterized in that: in, The ideal model of the perfect vortex beam used is: E(r,φ)=δ(r-r0)exp(ilφ) The left-handed circularly polarized Bessel-Gaussian beam and the right-handed circularly polarized Bessel-Gaussian beam are subjected to Fourier transform through the third lens, and the perfectly overlapped vortex beams obtained on the focal plane are expressed as: The signal light and the idle light are obtained after passing through the one-dimensional grating: The final entangled state output is:

8. An entangled light source device based on a grating and a perfect vortex metasurface, characterized in that: include: A focusing optical path device, comprising a laser light source, a Glan polarizer, a half-wave plate, a first lens, and a periodically polarized potassium titanate phosphate crystal, wherein the laser light source can emit a Gaussian light beam, the Glan polarizer is used to adjust the polarization state of the Gaussian light beam, the half-wave plate fine-tunes the polarization state of the Gaussian light beam, and then focuses the Gaussian light beam through the first lens and incidents on the periodically polarized potassium titanate crystal, so as to complete spontaneous parametric down-conversion under nonlinear optical effect, and generate signal light and idle light; A signal light and idle light collimating optical path device, comprising a second lens and a quarter wave plate, wherein the second lens collimates the signal light and the idle light so that they overlap each other in an output plane, and the quarter wave plate changes the polarization states of the signal light and the idle light, wherein the signal light is changed from horizontal polarization to right-handed circular polarization, and the idle light is changed from vertical polarization to left-handed circular polarization; A perfect vortex beam generating optical path device based on a metasurface comprises a perfect vortex metasurface and a third lens, wherein the optical axis direction of the perfect vortex metasurface changes periodically along the angular direction, and the Gaussian beam after passing through a 1 / 4 wave plate passes through the perfect vortex metasurface to obtain a geometric phase that changes with space, and the signal light is changed from a right-handed circularly polarized Gaussian beam to a left-handed circularly polarized Bessel-Gaussian beam, and the idle light is changed from a left-handed circularly polarized Gaussian beam to a right-handed circularly polarized Bessel-Gaussian beam; the third lens is used to perform Fourier transform on the left-handed circularly polarized Bessel-Gaussian beam and the right-handed circularly polarized Bessel-Gaussian beam, and obtain coincident perfect vortex beams on the focal plane; A light separation optical path device based on a metasurface comprises a one-dimensional grating, a beam splitter, a fourth lens, a fifth lens, a first bandpass filter and a second bandpass filter, wherein the optical axis direction of the one-dimensional grating changes periodically along the y-axis direction; the signal light and the idle light pass through the second PB phase metasurface, and respectively generate a momentum difference Δk along the positive and negative directions of the y-axis in real space y To separate them along the y direction, the beam splitter prism divides the entangled photons after passing through the one-dimensional grating into two paths, the fourth lens and the first bandpass filter are arranged in one path, and the fifth lens and the second bandpass filter are arranged in the other path; The measuring optical path device of entangled photons includes two optical fiber couplers and single photon detectors connected to the optical fiber couplers. The two optical fiber couplers are respectively arranged on the end optical paths of the first bandpass filter and the second bandpass filter of two paths to record the coincidence number.

9. The entangled light source device according to claim 8, characterized in that: The first lens is a focusing lens, which focuses the light beam to the center of the periodically polarized potassium titanate crystal for incidence; the second lens is a collimating lens, and the first lens and the second lens constitute a 4f system; After the periodically poled potassium titanate crystal, the generated signal light and idle light are collimated by the second lens and are expressed as AND s1 =(And x AND y ) T =And G (x,y)(1 0) T AND i1 =(And x AND y ) T =And G (x,y)(0 1) T Among them, R(z) = z[1 + (x 2 + y 2 ) / z 2 , is the waist width of the Gaussian beam and z is the propagation distance.

10. The entangled light source device according to claim 8, characterized in that: The perfect vortex metasurface has a phase delay of π and a period of 2π; The one-dimensional grating has a phase delay of π and a period of n mm, preferably 300 mm; The angle between the optical axis of the first metasurface and the x-axis is: is a 0th-order Bessel-Gaussian beam, and the complex amplitude of the lth-order Bessel-Gaussian beam is: The angle between the optical axis of the second metasurface and the x-axis is: Where d is the metasurface period.

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