A method and apparatus for generating entangled photon pairs based on a double-layered crystal optical cavity

Abstract

The application provides a method and device for generating entangled photon pairs based on a double-layer crystal optical cavity, the method comprising: coupling pump light into an optical resonant cavity provided with first and second nonlinear crystal layers, the polarization directions of the two layers being opposite to each other; using the resonant effect of the cavity to make the pump light multiple back-and-forth propagation in the two layers, and virtually expanding in space to form an equivalent periodic polarization structure; satisfying the quasi-phase matching condition through the equivalent structure, and generating entangled photon pairs by using spontaneous parametric down-conversion; the device comprises an optical resonant cavity and the above-mentioned double-layer crystal located in the cavity; the application replaces the traditional periodic polarization crystal with a simple double-layer structure combined with optical back-and-forth effect, greatly simplifies the preparation process and reduces the cost, can easily realize a sub-micron short polarization period, meets special application requirements such as back-propagation entangled photon pairs, and simultaneously improves the generation efficiency of photon pairs by the resonant enhancement effect, which is beneficial to the miniaturization and integration of devices.

Description

Technical Field

[0001] This invention relates to the field of quantum information technology, and specifically to a method and apparatus for generating entangled photon pairs based on a double-layer crystal optical cavity. Background Technology

[0002] Entangled photon pairs are an indispensable key resource in quantum information technology, widely used in quantum communication, quantum computing, quantum precision measurement, and quantum imaging. The mainstream technology for generating entangled photon pairs uses laser-pumped nonlinear optical crystals to convert a high-frequency pump photon into two low-frequency photons with quantum entanglement properties (usually called signal light and idler light) through a spontaneous parametric down-conversion process. To improve conversion efficiency and the brightness of the generated photon pairs, it is necessary to maintain phase matching between the pump light and the down-conversion light during crystal propagation. This is usually achieved using birefringence phase matching or quasi-phase matching techniques.

[0003] Quasi-phase matching technology is currently the most commonly used and flexible method. Its principle is to periodically reverse the spontaneous polarization direction of a nonlinear crystal (i.e., form a periodic polarization structure), introducing a reciprocal lattice vector to compensate for phase mismatch caused by material dispersion. In existing technologies, typical quasi-phase matching structures utilize periodically polarized lithium niobate or periodically polarized potassium titanate phosphate ferroelectric crystals, fabricated through an external electric field polarization process: patterned electrodes are prepared on the crystal surface, and a high-voltage pulse of several kilovolts is applied, causing the ferroelectric domains of the crystal to reverse their orientation according to a preset periodic pattern. The polarization period Λ of this periodic structure determines the quasi-phase matching condition, satisfying the momentum conservation relationship Δk=k p -k s -k i =G m =2πm / Λ, where k p k s k i The wave vectors G represent the pump light, signal light, and idle light, respectively. m It is a reciprocal lattice vector, where m is the order (usually taken as 1); the typical value of the polarization period is between a few micrometers and tens of micrometers, and the specific value depends on the selected pump wavelength and operating wavelength.

[0004] However, existing periodic polarization crystal technology has several objective drawbacks in practical applications. First, the fabrication of short polarization periods is extremely difficult. When the target polarization period enters the submicron range, traditional electric field polarization technology requires the fabrication of patterned electrodes with extremely small spacing on the crystal surface. The electrode spacing is usually in the submicron range, which not only makes the electrode fabrication process complex but also easily causes uneven electric field distribution under high voltage, making it impossible for ferroelectric domains to completely reverse according to the predetermined pattern, resulting in extremely low polarization success rate and yield. Second, the fabrication process is complex and costly, requiring multiple steps such as electrode patterning, precise control of high voltage pulse waveform and amplitude, and optimization of polarization temperature environment, which places stringent requirements on equipment and operating conditions. Third, to obtain sufficiently high conversion efficiency, the crystal length usually needs to reach several millimeters or even several centimeters, but maintaining the uniformity of short-period polarization in long crystals is very difficult, limiting further improvement in device performance. Finally, in certain special application scenarios, such as quantum interference experiments that require the generation of back-propagating entangled photon pairs, the polarization period needs to be close to or even less than 1 μm, which existing technologies can hardly meet. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method and apparatus for generating entangled photon pairs based on a double-layer crystal optical cavity.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This application provides a method for generating entangled photon pairs based on a dual-layer crystal optical cavity, including: The pump light is coupled into an optical resonant cavity, which contains a first nonlinear crystal layer and a second nonlinear crystal layer, the first nonlinear crystal layer and the second nonlinear crystal layer having opposite polarization directions. By utilizing the resonance effect of the optical resonant cavity, the pump light propagates back and forth multiple times in the first nonlinear crystal layer and the second nonlinear crystal layer, so as to virtually unfold the double-layer crystal in space and form an equivalent periodic polarization structure. By satisfying the quasi-phase matching condition through the equivalent periodic polarization structure, entangled photon pairs are generated using a spontaneous parametric downconversion process, wherein the entangled photon pairs include a signal light and an idle light.

[0007] Preferably, the thickness of the first nonlinear crystal layer and the second nonlinear crystal layer are equal, each being 1 / 4 of the target polarization period, so that the total thickness of the two crystal layers is 1 / 2 of the target polarization period.

[0008] Preferably, the optical resonator is configured to simultaneously resonate with the pump light, the signal light generated by spontaneous parametric down-conversion, and the idle light to achieve three-wave resonance enhancement; the optical resonator is configured to resonate with the pump light and selectively resonate with the signal light and / or the idle light.

[0009] Preferably, by selecting the target polarization period to achieve greater wave vector compensation and selecting the unilateral reflectivity of the target light, the back propagation of the signal light and idle light photon pairs can be achieved.

[0010] Preferably, the method further includes optimizing the generation brightness or entanglement characteristics of the entangled photon pairs by adjusting at least one of the cavity mirror reflectivity, cavity length, or coupling strength of the optical resonant cavity.

[0011] Preferably, the preparation method of the first nonlinear crystal layer and the second nonlinear crystal layer includes: growing two crystal layers with opposite polarization directions sequentially on a substrate by epitaxial growth; or bonding two crystal sheets with opposite polarization directions together by bonding; or achieving spontaneous reversal of polarization direction during the preparation process by controlling the crystal growth conditions to form a bilayer structure.

[0012] An entangled photon pair generation device based on a double-layer crystal optical cavity includes: An optical resonant cavity; A first nonlinear crystal layer and a second nonlinear crystal layer are located within the optical resonant cavity, the first nonlinear crystal layer and the second nonlinear crystal layer having opposite polarization directions; The optical resonant cavity is configured to allow the pump light to propagate back and forth multiple times in the first nonlinear crystal layer and the second nonlinear crystal layer, forming an equivalent periodic polarization structure through spatial virtual unfolding, thereby satisfying the quasi-phase matching condition and generating entangled photon pairs.

[0013] Preferably, the thickness of the first nonlinear crystal layer and the second nonlinear crystal layer are equal, each being 1 / 4 of the target polarization period, so that the total thickness of the two crystal layers is 1 / 2 of the target polarization period.

[0014] Preferably, the optical resonator is further configured to simultaneously resonate with the pump light, the signal light generated by spontaneous parametric down-conversion, and the idle light to achieve three-wave resonance enhancement; the optical resonator is configured to resonate with the pump light and selectively resonate with the signal light and / or the idle light. The optical resonant cavity is any one of a Fabry-Perot cavity, a ring cavity, or a confocal cavity; By selecting the target polarization period to achieve greater wave vector compensation and selecting the unilateral reflectivity of the target light, the back propagation of the signal light and idle light photon pairs can be achieved. It also includes a pump light input coupler and an entangled photon pair output coupler, wherein the input coupler and the output coupler are selected from any one of free space coupling, fiber coupling or waveguide coupling.

[0015] Preferably, it also includes a selectable intracavity electro-optic modulator or acousto-optic modulator for actively modulating the optical field within the optical resonant cavity to optimize the generation efficiency and entanglement characteristics of entangled photon pairs.

[0016] Compared with the prior art, this application has the following beneficial effects: This invention proposes a method and apparatus for generating entangled photon pairs based on a double-layer crystal optical cavity. It utilizes a combination of a double-layer nonlinear crystal with opposite polarization directions and an optical resonant cavity, replacing the physical multi-layer periodic polarization processing with a virtual spatial unfolding of optical round-trip propagation. This completely eliminates the patterned electrodes and multiple high-voltage pulse processes required for traditional electric field polarization, significantly simplifying the fabrication process, reducing manufacturing costs, and improving yield. Furthermore, since only two crystal layers are required, and the thickness of each layer is only 1 / 4 of the target polarization period, even if the target period is submicron, the single-layer thickness is only a few hundred nanometers, well within the achievable range of existing thin-film fabrication or crystal processing technologies, easily solving problems that traditional technologies cannot achieve. The challenge lies in achieving short polarization periods. Furthermore, the resonant effect of the optical cavity, on the one hand, effectively generates long interaction distances through multiple round trips, allowing for conversion efficiencies comparable to long crystals with extremely thin actual crystals, which is beneficial for device miniaturization and integration. On the other hand, the field enhancement effect of the resonant cavity on the pump light and downconversion light field further enhances the brightness of entangled photon pair generation, an additional gain not found in traditional traveling wave periodically polarized crystals. In addition, by adjusting parameters such as the cavity mirror reflectivity, cavity length, or coupling strength, the equivalent period number and photon pair generation characteristics can be flexibly controlled without altering the crystal structure itself, making it particularly suitable for special applications requiring extremely short polarization periods, such as back-propagating entangled photon pairs. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of a traditional periodic polarized crystal in the prior art.

[0018] Figure 2 This is a schematic diagram of the structure of the entangled photon pair generation device based on a double-layer crystal optical cavity provided in an embodiment of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Furthermore, in this invention, an element referred to as fixed to or disposed on another element may be directly disposed on the other element, or there may be an intermediate element. When an element is considered to be connected to another element, it may be directly connected to the other element, or there may be an intermediate element present simultaneously. The terms vertical, horizontal, left, right, and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0021] See Figures 1-2 This application provides a method for generating entangled photon pairs based on a double-layer crystal optical cavity, including: steps S1-S3: Step S1. The pump light is coupled into an optical resonant cavity, in which a first nonlinear crystal layer and a second nonlinear crystal layer are disposed, the first nonlinear crystal layer and the second nonlinear crystal layer having opposite polarization directions.

[0022] Specifically: the pump light is coupled into an optical resonant cavity; the optical resonant cavity contains a first nonlinear crystal layer and a second nonlinear crystal layer; the first and second nonlinear crystal layers can be made of ferroelectric crystal materials with second-order nonlinear optical effects, such as lithium niobate or potassium titanium phosphate, wherein the spontaneous polarization direction of the first nonlinear crystal layer is along the positive c-axis of its crystal, and the spontaneous polarization direction of the second nonlinear crystal layer is along the negative c-axis, that is, the polarization directions of the two layers are opposite to each other; the two end faces of the optical resonant cavity are coated with a high-reflectivity film for the working wavelength to confine the pump light and the generated downconversion photons within the cavity.

[0023] Step S2. Utilizing the resonance effect of the optical resonant cavity, the pump light propagates back and forth multiple times in the first nonlinear crystal layer and the second nonlinear crystal layer to virtually unfold the double-layer crystal in space, forming an equivalent periodic polarization structure.

[0024] Specifically, the pump light is made to travel back and forth multiple times in the first and second nonlinear crystal layers by utilizing the resonance effect of the optical resonant cavity. After entering from one end of the cavity, the pump light passes through the first and second nonlinear crystal layers in sequence, reaches the cavity mirror at the other end and is reflected, and then passes through the two crystal layers again to return. This process is repeated, and the pump light travels back and forth in the cavity multiple times. Each time it passes through the two crystal layers with opposite polarization directions, it experiences a periodic reversal of the polarization direction. During the multiple round trips, the physical two-layer crystal structure is virtually unfolded in space, forming an equivalent periodic polarization structure.

[0025] Step S3. By satisfying the quasi-phase matching condition through the equivalent periodic polarization structure, entangled photon pairs are generated using a spontaneous parametric downconversion process, wherein the entangled photon pairs include: signal light and idle light.

[0026] Specifically, by satisfying the quasi-phase-matching condition through the equivalent periodic polarization structure, the pump photon is converted into a pair of entangled photons with a lower frequency, namely the signal light and the idle light, in the first nonlinear crystal layer and the second nonlinear crystal layer by the spontaneous parametric downconversion process.

[0027] In a preferred embodiment, the thicknesses of the first nonlinear crystal layer and the second nonlinear crystal layer are equal, each being 1 / 4 of the target polarization period, such that the total thickness of the two crystal layers is 1 / 2 of the target polarization period.

[0028] In this embodiment, the thicknesses of the first and second nonlinear crystal layers are equal, both being 1 / 4 of the target polarization period, making the total thickness of the two crystal layers 1 / 2 of the target polarization period. For example, when the target polarization period Λ is designed to be 1 μm, the thickness of each crystal layer is only 0.25 μm. When light travels back and forth once in the cavity, passing through the two crystal layers with opposite polarization directions in sequence, it is equivalent to experiencing a complete polarization period Λ. By controlling the number of round trips N, it is equivalent to passing through a traditional periodic polarization crystal with a length of N·Λ. For a target polarization period in the submicron range, the thickness of a single layer is correspondingly reduced to several hundred nanometers, which can be fully realized within the existing process capabilities such as thin film epitaxial growth or precision grinding and polishing.

[0029] In a preferred embodiment, the optical resonator is configured to simultaneously resonate with the pump light, the signal light generated by spontaneous parametric down-conversion, and the idle light to achieve three-wave resonance enhancement; the optical resonator is configured to resonate with the pump light and selectively resonate with the signal light and / or the idle light.

[0030] It should be noted that the optical resonant cavity is configured to resonate simultaneously with the pump light wavelength, signal light wavelength, and idle light wavelength, thus achieving three-wave resonance. The pump light, signal light, and idle light all form standing wave fields within the cavity, significantly enhancing the optical field intensity. The high intracavity power density of the pump light increases the driving field strength of the nonlinear interaction, while the resonance of the signal light and idle light increases the effective lifetime and interaction distance of downconverted photons within the cavity. The synergistic effect of the triple resonance significantly improves the generation efficiency of entangled photon pairs. The three-wave resonance can be achieved by coating a broadband high-reflectivity film to cover the three wavelength ranges, or by designing the cavity length so that the three wavelengths simultaneously satisfy the resonance condition.

[0031] In a preferred embodiment, by selecting the target polarization period to achieve greater wave vector compensation and selecting the unilateral reflectivity of the target light, the signal light and idle light photon pairs can be made to propagate in opposite directions.

[0032] It is understandable that the target polarization period is set on the submicron scale. Since the thickness of a single layer of the bilayer crystal is only 1 / 4 of the target polarization period, when the target polarization period is less than 1 μm, the single layer thickness is less than 0.25 μm, which is entirely feasible in terms of fabrication. Under the submicron polarization period condition, according to the quasi-phase matching condition, the generated signal light and idle light will propagate in approximately opposite directions, forming a back-propagating entangled photon pair. This back-propagating photon pair has unique application value in quantum repeaters, linear optical quantum computing, and two-photon interference experiments.

[0033] In a preferred embodiment, the brightness or entanglement characteristics of the entangled photon pairs are optimized by adjusting at least one of the cavity mirror reflectivity, cavity length, or coupling strength of the optical resonant cavity.

[0034] The method also includes optimizing the brightness or entanglement characteristics of entangled photon pairs by adjusting at least one of the cavity mirror reflectivity, cavity length, or coupling strength of the optical resonant cavity. For example, adjusting the cavity length can change the number of round trips of light within the cavity, thereby controlling the equivalent number of polarization periods and affecting the effective interaction length; adjusting the cavity mirror reflectivity can change the cavity quality factor, affecting the enhancement factor of the optical field within the cavity and the generation rate of photon pairs; adjusting the input coupling strength can optimize the coupling efficiency of the pump light. The flexibility and adjustability of these parameters allow the same device to be adapted to different operating conditions and application requirements through external adjustment without changing the crystal structure itself.

[0035] In a preferred embodiment, the preparation method of the first nonlinear crystal layer and the second nonlinear crystal layer includes: growing two crystal layers with opposite polarization directions sequentially on a substrate by epitaxial growth; or bonding two crystal sheets with opposite polarization directions together by bonding; or achieving spontaneous reversal of polarization direction during the preparation process by controlling the crystal growth conditions to form a bilayer structure.

[0036] It should be noted that there are several methods for preparing the first and second nonlinear crystal layers. The first method is epitaxial growth, in which two crystal layers with opposite polarization directions are grown sequentially on a substrate. By controlling the growth time and rate, the thickness of each layer can be precisely controlled, resulting in a large-area, highly uniform bilayer film structure. The second method is bonding, in which two pre-prepared crystal sheets with opposite polarization directions are bonded together by optical bonding or direct bonding. This method is simple, fast, and suitable for prototype verification. The third method involves inducing spontaneous reversal of polarization direction at the growth interface during crystal growth by controlling melt composition, temperature gradient, or growth rate, thus forming a bilayer structure with opposite polarization directions in one step. This method can produce bilayer crystals with high interface quality and low loss.

[0037] This application provides an entangled photon pair generation device based on a double-layer crystal optical cavity, comprising: An optical resonant cavity; A first nonlinear crystal layer and a second nonlinear crystal layer are located within the optical resonant cavity, the first nonlinear crystal layer and the second nonlinear crystal layer having opposite polarization directions; The optical resonant cavity is configured to allow the pump light to propagate back and forth multiple times in the first nonlinear crystal layer and the second nonlinear crystal layer, forming an equivalent periodic polarization structure through spatial virtual unfolding, thereby satisfying the quasi-phase matching condition and generating entangled photon pairs.

[0038] Specifically: An entangled photon pair generation device based on a double-layer crystal optical cavity includes an optical resonant cavity and a first nonlinear crystal layer and a second nonlinear crystal layer located inside it.

[0039] The optical resonant cavity consists of two cavity mirrors coated with a high-reflectivity film for the operating wavelength. The optical resonant cavity is configured to allow the pump light to propagate back and forth multiple times in the first and second nonlinear crystal layers. When the pump light enters the optical resonant cavity, it is repeatedly reflected within the cavity, and each round trip passes through the two crystal layers with opposite polarization directions. Through this spatial virtual unfolding, the physical double-layer structure is equivalent to a periodic polarization structure with multiple polarization periods, thus satisfying the quasi-phase matching condition. In this equivalent structure, the pump light generates mutually entangled signal light and idler photon pairs through a spontaneous parametric downconversion process.

[0040] Interference occurs between the spontaneous parametric downconversion amplitudes generated by forward and reverse propagation. By carefully designing the cavity length, the position of the crystal layer in the cavity, and the phase dispersion characteristics of the cavity mirror, the relative phase between the forward and reverse amplitudes can be adjusted to satisfy the constructive interference condition, thereby maximizing the generation intensity of the signal light and idle light.

[0041] In a preferred embodiment, the thicknesses of the first nonlinear crystal layer and the second nonlinear crystal layer are equal, both being 1 / 4 of the target polarization period, such that the total thickness of the two crystal layers is 1 / 2 of the target polarization period.

[0042] The first and second nonlinear crystal layers have equal thicknesses, each being 1 / 4 of the target polarization period, and the total thickness of the two layers is 1 / 2 of the target polarization period. Taking a target polarization period of 1 μm as an example, the thickness of a single layer is 0.25 μm, and the total thickness of the two layers is 0.5 μm. Such a thin crystal structure greatly reduces the size of the entire device, which is beneficial for on-chip integration and system miniaturization.

[0043] In a preferred embodiment, the optical resonator is further configured to simultaneously resonate with the pump light, the signal light generated by spontaneous parametric down-conversion, and the idle light to achieve three-wave resonance enhancement; the optical resonator is configured to resonate with the pump light and selectively resonate with the signal light and / or the idle light; the optical resonator is any one of a Fabry-Perot cavity, a ring cavity, or a confocal cavity; the optical resonator may be selected as, but is not limited to, a Fabry-Perot cavity, a ring cavity, or a confocal cavity; by selecting the target polarization period to achieve larger wave vector compensation, and by selecting the unilateral reflectivity of the target light, the backpropagation of the photon pairs of the signal light and the idle light can be achieved; it also includes a pump light input coupler and an entangled photon pair output coupler, the input coupler and the output coupler being selected from any one of free-space coupling, fiber coupling, or waveguide coupling.

[0044] It should be noted that the optical resonant cavity is further configured to simultaneously achieve three-wave resonance enhancement, i.e., high resonance quality factor for pump light, signal light, and idler light. Three-wave resonance simultaneously enhances the intracavity field strength of the pump light, signal light, and idler light, significantly improving the nonlinear conversion efficiency of spontaneous parametric down-conversion, thereby increasing the brightness of entangled photon pair generation. The optical resonant cavity can adopt a Fabry-Perot cavity structure, consisting of two parallel planar or concave cavity mirrors, which is simple in structure and easy to align; it can also adopt a ring cavity, forming a closed optical path through multiple mirrors, enabling unidirectional traveling wave propagation, avoiding spatial hole burning effects, and improving output stability; or it can adopt a confocal cavity, consisting of two spherical mirrors with a radius of curvature equal to the cavity length, possessing good mode selectivity and focusing characteristics, suitable for mode matching with the pump beam; the device also includes a pump light input coupler and an entangled photon pair output coupler; the input coupler and output coupler can be selected according to the specific needs of system integration, using free-space coupling, fiber coupling, or waveguide coupling, flexibly adapting to different application scenarios.

[0045] In a preferred embodiment, the device further includes an optional intracavity electro-optic modulator or acousto-optic modulator for actively modulating the optical field within the optical resonant cavity to optimize the generation efficiency and entanglement characteristics of entangled photon pairs. The device also includes an electro-optic modulator or an acousto-optic modulator; the electro-optic modulator or acousto-optic modulator is set inside or outside the optical resonant cavity to actively modulate the optical field inside the cavity; by applying an external electrical signal, the refractive index of the crystal can be changed or periodic refractive index modulation can be introduced, thereby dynamically controlling the phase or amplitude of the optical field inside the cavity, realizing the active optimization and control of the generation efficiency, spectral characteristics or entanglement characteristics of entangled photon pairs, and expanding the application flexibility of the device.

[0046] The technical solution proposed in the following section will be further elaborated in detail with reference to specific applications: Example 1 This embodiment provides an entangled photon pair generation device based on a double-layer crystal optical cavity, the structure of which is as follows: Figure 2 As shown, the device includes an optical resonant cavity 1, which is composed of two cavity mirrors 1a and 1b, with a high-reflectivity film coated on the cavity mirrors for the working wavelength. Inside the optical resonant cavity 1, there is a first nonlinear crystal layer 2 and a second nonlinear crystal layer 3, which are tightly bonded or integrally fabricated. The materials of the first nonlinear crystal layer 2 and the second nonlinear crystal layer 3 can be ferroelectric crystals with second-order nonlinear optical effects, such as lithium niobate or potassium titanium phosphate. The spontaneous polarization direction (ferroelectric domain direction) of the first nonlinear crystal layer 2 is along the positive direction of its crystal c-axis, while the spontaneous polarization direction of the second nonlinear crystal layer 3 is along the negative direction of the c-axis, that is, the polarization directions of the two layers are opposite to each other.

[0047] The thicknesses d1 and d2 of the first nonlinear crystal layer 2 and the second nonlinear crystal layer 3 are equal and are set to 1 / 4 of the target polarization period Λ, i.e., d1=d2=Λ / 4; therefore, the total thickness of the two crystal layers is Λ / 2; for example, when it is necessary to generate back-propagating entangled photon pairs and the target polarization period Λ is designed to be 1μm, the thickness of each crystal layer is only 0.25μm, which can be easily achieved by existing processes such as thin film epitaxial growth or precision grinding and polishing.

[0048] The distance (i.e., cavity length) between the cavity mirrors 1a and 1b of the optical resonant cavity 1 is set according to the resonance condition. The reflectivity of the cavity mirrors is optimized so that the cavity has a high resonance quality factor for the pump light wavelength λp, the signal light wavelength λs, and the idle light wavelength λi, thereby realizing the three-wave resonance of the pump light and the downconversion photon pair. The pump light enters the optical resonant cavity 1 through the partially transmitted cavity mirror 1a via the input coupler (not shown separately in the figure), and the entangled photon pair generated is output from the cavity mirror 1b via the output coupler. The coupling method can be free space coupling, fiber coupling, or waveguide coupling.

[0049] During operation, the pump light enters the optical resonant cavity 1 from the left, passes through the first nonlinear crystal layer 2 and the second nonlinear crystal layer 3 to reach the right cavity mirror 1b, and after reflection, it passes through the two crystal layers again to return to the left cavity mirror 1a, and repeats this process multiple times. In each single-pass propagation, the pump light and the downconversion light pass through two crystal layers with opposite polarization directions in sequence, that is, they experience a complete polarization period Λ in space. When the light travels back and forth N times in the cavity, it is equivalent to passing through a periodic polarized crystal of length N·Λ. The entire spontaneous parametric downconversion process is completed in the virtual unfolded long-period structure, satisfying the quasi-phase matching condition.

[0050] In particular, interference occurs between the SPDC amplitudes generated by forward and reverse propagation. By carefully designing the cavity length, crystal layer position, and cavity mirror phase dispersion, the relative phase Φ between the forward and reverse amplitudes can be adjusted to satisfy the constructive interference condition (Φ = 0 or an integer multiple of 2π), thereby maximizing the generation intensity of signal light and idle light. This interference enhancement mechanism is one of the important physical foundations for the efficient output of entangled photon pairs in this invention.

[0051] This device significantly simplifies the manufacturing process: it only requires the fabrication of a bilayer structure, completely avoiding the complex patterned electrodes, alignment, and multiple high-voltage polarization steps required by traditional technologies; through epitaxial growth, two crystal layers with opposite polarization directions can be grown sequentially on a substrate, and the thickness can be precisely controlled to achieve a large-area, highly uniform bilayer film; through bonding, two ultrathin crystal sheets with opposite polarization directions can be directly bonded together to quickly construct prototype devices; and during crystal growth, the polarization direction can be spontaneously reversed by controlling conditions such as melt composition or temperature gradient to form a high-quality bilayer structure; these fabrication methods all fall within the scope of existing mature processes, are easy to implement and mass-produce.

[0052] Example 2 This embodiment provides a method for generating entangled photon pairs based on the above-described device. A pump laser with a center wavelength of 532 nm is injected into an optical resonant cavity 1 through an input coupler. The thickness of the first nonlinear crystal layer 2 and the second nonlinear crystal layer 3 in the cavity is both set to 0.5 μm, and the corresponding target polarization period Λ is 2 μm. The cavity mirror of the optical resonant cavity 1 has a reflectivity of more than 99% for the 532 nm pump light, and also has high reflectivity for the 810 nm signal light and 1550 nm idle light generated by downconversion (one end can be set to high transmission for directional output), forming a three-wave resonance. The pump light travels back and forth in the cavity multiple times, and each time it passes through the double-layer crystal, it undergoes a periodic reversal of the polarization direction, virtually unfolding into a long-distance quasi-phase-matched structure, thereby efficiently generating entangled photon pairs.

[0053] By adjusting the cavity length using electrically controlled piezoelectric ceramics, the equivalent number of periods and the relative phase of the forward and reverse SPDC amplitudes can be precisely controlled, enabling active optimization of the brightness and spectral characteristics of entangled photon pairs. Furthermore, electro-optic modulators or acousto-optic modulators can be added inside or outside the optical resonant cavity to dynamically control the phase or amplitude of the optical field within the cavity, further expanding the device's functionality.

[0054] Example 3 As a variation, the nonlinear crystal layer is not limited to two layers; for example, a four-layer structure is used, with the polarization directions of adjacent two layers being opposite, and the thickness of each layer is still Λ / 4, with a total thickness of Λ; the light undergoes a complete polarization cycle once it travels back and forth in the cavity, which can also achieve virtual unfolding; the multilayer structure can reduce the requirements of the single-layer thickness on the processing accuracy in specific designs.

[0055] The type of optical resonator can also be flexibly selected according to application requirements; except Figure 2 In addition to the Fabry-Perot cavity shown, a ring cavity or a confocal cavity can also be used; the ring cavity can avoid the spatial hole-burning effect through unidirectional propagation, while the confocal cavity can provide better mode-matching characteristics.

[0056] For designs with target polarization periods in the submicron range (e.g., 0.8 μm), the thickness of a single layer of the bilayer crystal is only 0.2 μm. The resulting signal light and idle light pairs will propagate in approximately opposite directions. Such back-propagating entangled photon pairs have unique application value in quantum repeaters and linear optical quantum computing, and this invention fills the technological gap in this field.

[0057] In summary, this invention achieves quasi-phase matching through the synergistic design of a double-layer crystal and an optical resonant cavity, using a virtual space unfolding method. It represents a significant improvement over traditional periodic polarized crystal technology in terms of simplifying the process, reducing costs, achieving short cycles, and increasing efficiency.

[0058] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention; therefore, the embodiments should be regarded as exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and thus all variations falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention; no reference numerals in the claims should be construed as limiting the scope of the claims.

[0059] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for generating entangled photon pairs based on a double-layer crystal optical cavity, characterized in that, include: The pump light is coupled into an optical resonant cavity, which contains a first nonlinear crystal layer and a second nonlinear crystal layer, the first nonlinear crystal layer and the second nonlinear crystal layer having opposite polarization directions. By utilizing the resonance effect of the optical resonant cavity, the pump light propagates back and forth multiple times in the first nonlinear crystal layer and the second nonlinear crystal layer, so as to virtually unfold the double-layer crystal in space and form an equivalent periodic polarization structure. By satisfying the quasi-phase matching condition through the equivalent periodic polarization structure, entangled photon pairs are generated using a spontaneous parametric downconversion process, wherein the entangled photon pairs include a signal light and an idle light.

2. The method according to claim 1, characterized in that, The first nonlinear crystal layer and the second nonlinear crystal layer have equal thicknesses, each being 1 / 4 of the target polarization period, so that the total thickness of the two crystal layers is 1 / 2 of the target polarization period.

3. The method according to claim 1 or 2, characterized in that, The optical resonant cavity is configured to simultaneously resonate with the pump light, the signal light generated by spontaneous parametric down-conversion, and the idle light to achieve three-wave resonance enhancement; the optical resonant cavity is configured to resonate with the pump light and selectively resonate with the signal light and / or the idle light.

4. The method according to claim 1, characterized in that, By selecting the target polarization period to achieve greater wave vector compensation and selecting the unilateral reflectivity of the target light, the back propagation of the signal light and idle light photon pairs can be achieved.

5. The method according to claim 1, characterized in that, It also includes optimizing the generation brightness or entanglement characteristics of the entangled photon pairs by adjusting at least one of the cavity mirror reflectivity, cavity length, or coupling strength of the optical resonant cavity.

6. The method according to claim 1, characterized in that, The preparation methods of the first nonlinear crystal layer and the second nonlinear crystal layer include: growing two crystal layers with opposite polarization directions sequentially on a substrate by epitaxial growth; or bonding two crystal sheets with opposite polarization directions together by bonding; or achieving spontaneous reversal of polarization direction during the preparation process by controlling the crystal growth conditions to form a bilayer structure.

7. An entangled photon pair generation device based on a double-layer crystal optical cavity, characterized in that, include: An optical resonant cavity; A first nonlinear crystal layer and a second nonlinear crystal layer are located within the optical resonant cavity, the first nonlinear crystal layer and the second nonlinear crystal layer having opposite polarization directions; The optical resonant cavity is configured to allow the pump light to propagate back and forth multiple times in the first nonlinear crystal layer and the second nonlinear crystal layer, forming an equivalent periodic polarization structure through spatial virtual unfolding, thereby satisfying the quasi-phase matching condition and generating entangled photon pairs.

8. The apparatus according to claim 7, characterized in that, The first nonlinear crystal layer and the second nonlinear crystal layer have equal thicknesses, each being 1 / 4 of the target polarization period, so that the total thickness of the two crystal layers is 1 / 2 of the target polarization period.

9. The apparatus according to claim 7 or 8, characterized in that, The optical resonant cavity is further configured to simultaneously resonate with the pump light, the signal light generated by spontaneous parametric down-conversion, and the idle light to achieve three-wave resonance enhancement; the optical resonant cavity is configured to resonate with the pump light and selectively resonate with the signal light and / or the idle light. The optical resonant cavity is any one of a Fabry-Perot cavity, a ring cavity, or a confocal cavity; By selecting the target polarization period to achieve greater wave vector compensation and selecting the unilateral reflectivity of the target light, the back propagation of the signal light and idle light photon pairs can be achieved. It also includes a pump light input coupler and an entangled photon pair output coupler, wherein the input coupler and the output coupler are selected from any one of free space coupling, fiber coupling or waveguide coupling.

10. The apparatus according to claim 7, characterized in that, It also includes a selectable intracavity electro-optic modulator or acousto-optic modulator for actively modulating the optical field within the optical resonant cavity to optimize the generation efficiency and entanglement characteristics of entangled photon pairs.

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