A polarization entangled light source

Abstract

A polarization entangled light source, comprising: an interference mechanism for obtaining a first pump laser beam and a second pump laser beam with the same polarization direction and different propagation directions from an initial laser; a resonant cavity adapted to receive the first pump laser beam and the second pump laser beam; a nonlinear optical crystal disposed in the resonant cavity, the two pump laser beams entering the resonant cavity pass through the nonlinear optical crystal from different directions, and under the action of the nonlinear optical crystal, photons of the first pump laser beam undergo spontaneous parametric down-conversion to obtain two first down-conversion photons, and photons of the second pump laser beam undergo spontaneous parametric down-conversion to obtain two second down-conversion photons; the interference mechanism is also adapted to ensure that the first target down-conversion photons and the second target down-conversion photons can produce quantum coherent superposition to obtain polarization entangled photon pairs.

Description

Technical Field

[0001] The present invention relates to the field of quantum communication, and particularly to a polarization entangled light source. Background Art

[0002] The spontaneous parametric down-conversion process is currently the most convenient way to generate entangled photon pairs and has been widely used in quantum information tasks and the verification of fundamental problems in quantum mechanics. The spontaneous parametric down-conversion describes a second-order nonlinear process. In the spontaneous parametric down-conversion process, a pump laser photon interacts with a nonlinear medium and splits into two photons with lower frequencies. The spontaneous parametric down-conversion process is the inverse process of the frequency doubling process and the sum frequency process. "Parametric" means that the light interacts with the medium without changing the quantum state of the medium. Therefore, energy conservation and momentum conservation are satisfied between the pump laser photon and the two down-converted photons. From the momentum conservation, the phase matching condition of the parametric down-conversion process can be deduced. According to the different polarizations of the obtained down-converted photons, the parametric down-conversion process can be divided into type-0 down-conversion, type-I down-conversion, and type-II down-conversion; according to the propagation directions of the down-converted photons, the parametric down-conversion can also be distinguished into collinear down-conversion and non-collinear down-conversion; the parametric down-conversion process can be divided into degenerate down-conversion and non-degenerate down-conversion according to the wavelength. The parametric down-conversion process needs to satisfy the phase matching condition, which is usually realized in a birefringent nonlinear crystal, and its refractive index depends on the wavelength and emission angle of the photon. Quasi-phase matching is a method to achieve phase matching through a periodically poled crystal, which has the advantages of adjustable parameters and can select the direction of the maximum nonlinear coefficient.

[0003] The optical cavity is composed of two or more reflecting mirrors, which can confine the light beam in a very small space and reflect the light beam multiple times to make the light beam oscillate in the optical cavity. Due to the multiple reflections in the optical cavity, the light intensity of the light beam can be enhanced. Therefore, applying the optical cavity in a laser can improve the output power of the laser. At the same time, since only the light beam with a wavelength equal to an integer multiple of the optical cavity length can oscillate in the optical cavity, the optical cavity can also modulate the frequency characteristics of the light field. In addition, in the field of quantum communication, the optical cavity also plays an important role. It can be used to generate polarization-entangled photon pairs. Specifically, the generation of polarization-entangled photon pairs mainly depends on the down-conversion process occurring in the optical cavity. The optical cavity can enhance the interaction between light and the nonlinear optical crystal during the down-conversion process. At the same time, the optical cavity can also modulate the light field of the down-conversion process and narrow the photon pairs generated by the down-conversion process, thereby generating narrowband polarization-entangled photon pairs. However, in the current methods for generating narrowband non-degenerate polarization-entangled photon pairs using an optical cavity, two nonlinear media are often used. Due to the fact that the photon pairs generated by the down-conversion process pass through the two nonlinear media multiple times, the paths and polarization directions of the photon pairs change, so the generation process of non-degenerate polarization-entangled photon pairs is unstable. Summary of the Invention

[0004] In view of this, one aspect of the present invention provides a polarization-entangled light source, which includes:

[0005] An interference mechanism for obtaining a first pump laser beam and a second pump laser beam with the same polarization direction and different propagation directions from the initial laser;

[0006] A resonant cavity suitable for receiving the first pump laser beam and the second pump laser beam;

[0007] A nonlinear optical crystal is arranged in the resonant cavity. The two pump laser beams entering the resonant cavity pass through the nonlinear optical crystal from different directions. Under the action of the nonlinear optical crystal, the photons of the first pump laser beam undergo spontaneous parametric down-conversion to obtain two first down-conversion photons, and the photons of the second pump laser beam undergo spontaneous parametric down-conversion to obtain two second down-conversion photons;

[0008] Among them, the first down-conversion photons and the first down-conversion photons oscillate in the resonant cavity and experience the same optical path. The resonant cavity selects the first down-conversion photons with the target frequency to obtain the first target down-conversion photons, and the resonant cavity selects the second down-conversion photons with the target frequency to obtain the second target down-conversion photons. The propagation directions of the first target down-conversion photons and the second target down-conversion photons are different when they exit the resonant cavity;

[0009] The interference mechanism is also configured to ensure that the first target down-converted photon and the second target down-converted photon can generate a quantum coherent superposition and obtain a pair of polarization-entangled photons.

[0010] According to an embodiment of the present invention, the interference mechanism includes:

[0011] A polarization beam splitter adapted to split the initial laser into a first pump laser beam and a second pump laser beam with perpendicular polarization directions;

[0012] A polarization conversion component adapted to change the polarization direction of the first pump laser beam or the second pump laser beam to obtain a first pump laser beam and a second pump laser beam with the same polarization direction;

[0013] A first mirror module adapted to make the first pump laser beam incident on the resonator along a first path;

[0014] A second mirror module adapted to make the second pump laser beam incident on the resonator along a second path.

[0015] According to an embodiment of the present invention, the first mirror module includes at least one first mirror located on the first path, and the at least one first mirror sequentially reflects the first pump laser beam to make the first pump laser beam incident on the resonator along the first path;

[0016] The second mirror module includes at least one second mirror located on the second path, and the at least one second mirror sequentially reflects the second pump laser beam to make the second pump laser beam incident on the resonator along the second path.

[0017] According to an embodiment of the present invention, the nonlinear optical crystal is a type-0 phase-matching crystal or a type-I phase-matching crystal, and each photon of the pump laser beam undergoes spontaneous parametric down-conversion to obtain two down-converted photons with the same polarization direction as the pump laser beam;

[0018] The first mirror module is further adapted to reflect the second target down-converted photon output from the resonator to make the second target down-converted photon travel along the first path;

[0019] The second mirror module is further adapted to reflect the first target down-converted photon output from the resonator to make the first target down-converted photon travel along the second path;

[0020] The polarization conversion component is further adapted to change the polarization direction of the first target down-converted photon or change the polarization direction of the second target down-converted photon to make the polarization direction of the first target down-converted photon perpendicular to the polarization direction of the second target down-converted photon;

[0021] The polarization beam splitter is also applicable to combining a first target down-converted photon and a second target down-converted photon with perpendicular polarization directions and generating a quantum coherent superposition to obtain a pair of polarization-entangled photons;

[0022] Wherein, the lengths of the first path and the second path are the same.

[0023] According to an embodiment of the present invention, the nonlinear optical crystal is a type-II phase-matching crystal, and two down-converted photons with perpendicular polarization directions are obtained by spontaneous parametric down-conversion of photons of each pump laser beam;

[0024] The first mirror module is also applicable to reflecting the second target down-converted photon output from the resonant cavity, so that the second target down-converted photon travels along the first path;

[0025] The second mirror module is also applicable to reflecting the first target down-converted photon output from the resonant cavity, so that the second target down-converted photon travels along the second path;

[0026] A polarization beam splitter, applicable to combining a first target down-converted photon and a second target down-converted photon and generating a quantum coherent superposition to obtain a pair of polarization-entangled photons;

[0027] Wherein, the lengths of the first path and the second path are the same.

[0028] According to an embodiment of the present invention, the first pump laser beam has a vertical polarization direction, and the second pump laser beam has a horizontal polarization direction;

[0029] The polarization conversion component is a half-wave plate, applicable to changing the polarization direction of the first pump laser beam or the second pump laser beam, so that the polarization direction of the first pump laser beam or the second pump laser beam is both horizontal polarization or vertical polarization.

[0030] According to an embodiment of the present invention, the resonant cavity includes:

[0031] A first dichroic mirror, for transmitting the first pump laser beam incident into the resonant cavity via the first path and the second pump laser beam incident into the resonant cavity via the second path;

[0032] The resonant cavity mirror module includes a plurality of resonant cavity mirrors, and the plurality of resonant cavity mirrors and the first dichroic mirror form an annular structure, and the nonlinear optical crystal is located between two adjacent resonant cavity mirrors;

[0033] Among them, the first pump laser passes through the nonlinear optical crystal to obtain the first down-converted photon, and the second pump laser passes through the nonlinear optical crystal to obtain the second down-converted photon. After the first down-converted photon and the second down-converted photon oscillate in the resonant cavity, the first target down-converted photon and the second target down-converted photon are obtained. The first target down-converted photon and the second target down-converted photon are output from the first dichroic mirror along different directions. The position of the nonlinear optical crystal in the resonant cavity is configured to ensure that the optical paths of the first down-converted photon and the second down-converted photon in the resonant cavity are the same.

[0034] According to an embodiment of the present invention, the resonant cavity mirrors located at both ends of the nonlinear optical crystal are concave mirrors.

[0035] According to an embodiment of the present invention, the polarization entanglement light source further includes:

[0036] A second dichroic mirror, which is suitable for transmitting the initial laser so that the initial laser enters the interference mechanism; and is suitable for reflecting the polarization-entangled photon pair; and

[0037] A third dichroic mirror, which is suitable for separating polarization-entangled photon pairs of different wavelengths to separate polarization-entangled photon pairs with different spatial modes.

[0038] According to an embodiment of the present invention, the polarization beam splitter is suitable for separating polarization-entangled photon pairs with different spatial modes;

[0039] The polarization entanglement light source further includes:

[0040] A second dichroic mirror, which is suitable for transmitting the initial laser so that the initial laser enters the interference mechanism; and is suitable for reflecting one of the spatial-mode polarization-entangled photon pairs output from the polarization beam splitter.

[0041] According to an embodiment of the present invention, due to the frequency selection effect of the resonant cavity, the polarization-entangled photon pairs obtained in the embodiments of the present invention are narrow-band polarization-entangled photon pairs.

[0042] According to an embodiment of the present invention, the target down-converted photons for coherent superposition have the same polarization direction in the resonant cavity, and only the propagation directions are opposite. Therefore, the target down-converted photons in the embodiments of the present invention have the characteristic of good identity. At the same time, since only one nonlinear optical crystal is provided in the resonant cavity, the polarization entanglement light source provided in the embodiments of the present invention can avoid the problems of changes in the paths and polarization directions of the down-converted photon pairs caused by using two nonlinear optical crystals, resulting in poor device stability and low fidelity of the prepared entangled state. Description of the Drawings

[0043] Through the following description of the embodiments of the present invention with reference to the drawings, the above and other objects, features, and advantages of the present invention will become clearer. In the drawings:

[0044] Figure 1 shows a schematic diagram of a conventional combined cavity for generating narrowband polarization-entangled photon pairs;

[0045] Figure 2 shows a schematic diagram of a conventional polarization-entangled photon pair generation device;

[0046] Figure 3 shows a schematic diagram of a polarization-entangled light source provided according to an embodiment of the present invention;

[0047] Figure 4 shows a schematic diagram of a polarization-entangled light source provided according to another embodiment of the present invention;

[0048] Figure 5 shows a schematic diagram of a polarization-entangled light source provided according to still another embodiment of the present invention.

[0049] Description of reference numerals

[0050] 11 - First crystal; 12 - Second crystal; 13 - First cavity mirror; 14 - Second cavity mirror; 15 - Third cavity mirror; 16 - Beam splitting module; 21 - First reflection unit; 22 - Second reflection unit; 23 - Polarization beam splitting unit; 24 - Third crystal; 25 - Half-wave plate inside the ring; 31 - Interference mechanism; 32 - Resonant cavity; 33 - Nonlinear optical crystal; 34 - Second dichroic mirror; 35 - Third dichroic mirror; 311 - Polarization beam splitter; 312 - First reflector; 313 - Second reflector; 314 - Polarization conversion component; 321 - First dichroic mirror; 322 - First cavity reflector; 323 - Second cavity reflector; 324 - Third cavity reflector. Detailed implementation manners

[0051] As Figure 1As shown in the figure, the traditional combined cavity for generating narrowband polarization-entangled photon pairs includes a first crystal 11 and a second crystal 12. The first crystal 11 and the second crystal 12 are two nonlinear optical crystals, that is, both the first crystal 11 and the second crystal 12 are type-II phase-matching crystals, and the two crystals are perpendicular to each other axially. The combined cavity includes three cavity mirrors (i.e., the first cavity mirror 13, the second cavity mirror 14, and the third cavity mirror 15) and a beam-splitting module 16. The polarization light with a polarization direction of 45 degrees has the same down-conversion probability in the two crystals, and generates non-degenerate photon pairs with opposite polarization directions in the two crystals. The non-degenerate photon pairs generated in the two crystals are superimposed to generate polarization-entangled photon pairs. The first cavity mirror 13 and the second cavity mirror 14 form the first group of cavity mirrors, and the second cavity mirror 14 and the third cavity mirror 15 form the second group of cavity mirrors. The beam-splitting module 16 (such as a dichroic mirror) can separate photons with different wavelengths, so that photons with the first wavelength oscillate in the first group of cavity mirrors, and photons with the second wavelength oscillate in the second group of cavity mirrors. Although the first group of cavity mirrors and the second group of cavity mirrors can modulate the down-converted photons with different wavelengths respectively and finally generate narrowband polarization-entangled photon pairs, because the coherently superimposed photons have different polarizations and the photons generated by down-conversion pass through the double-crystal structure (i.e., the first crystal 11 and the second crystal 12) multiple times, therefore, the paths and polarization directions of the photons generated by down-conversion will change, which makes the adjustment of the device very difficult and the stability of the device is poor.

[0052] In the process of implementing the present invention, it is found that the Sagnac loop structure is widely used to generate polarization-entangled photon pairs.

[0053] Figure 2 The schematic diagram of the traditional polarization-entangled photon pair generation device is shown.

[0054] As Figure 2As shown in the figure, the traditional device for generating polarization-entangled photon pairs adopts a triangular Sagnac loop structure, which includes a first reflection unit 21, a second reflection unit 22, and a polarization beam splitting unit 23. The third crystal 24 in the triangular Sagnac loop structure is a type-0 quasi-phase matching crystal, a type-I phase matching crystal, or a type-II quasi-phase matching crystal. Polarized light is input into the triangular Sagnac loop structure from the polarization beam splitting unit 23, and the polarization direction of this polarized light is the 45-degree direction. Therefore, when this polarized light passes through the polarization beam splitting unit 23, there is a 50% chance of transmission and a 50% chance of reflection. When this polarized light passes through the polarization beam splitting unit 23, it is divided into a first sub-polarized light that is transmitted and a second sub-polarized light that is reflected. The half-wave plate 25 inside the loop is a multi-color half-wave plate, and the half-wave plate 25 inside the loop can rotate the polarization direction of the passing sub-polarized light and the generated down-converted photons by 45 degrees. The photons of the transmitted first polarized light pass through the third crystal 24 clockwise to generate a pair of down-converted photons with horizontal polarization (H polarization), and this pair of down-converted photons with horizontal polarization turn into a pair of down-converted photons with vertical polarization (V polarization) after passing through the half-wave plate 25 inside the loop clockwise. The polarization direction of the reflected second sub-polarized light turns into horizontal polarization after passing through the half-wave plate 25 inside the loop, and then passes through the third crystal 24 counterclockwise to generate two pairs of horizontally down-converted polarized photons. The pair of down-converted photons with vertical polarization obtained from the first sub-polarized light reach the polarization beam splitting unit 23 clockwise, and the pair of down-converted photons with horizontal polarization obtained from the second sub-polarized light reach the polarization beam splitting unit 23 counterclockwise. The polarization photon pairs traveling clockwise and counterclockwise are superimposed at the polarization beam splitting unit 23 to generate polarization-entangled photon pairs. However, this traditional device for generating polarization-entangled photon pairs does not have an optical cavity structure and it is very difficult to generate narrow-band entangled photon pairs. Therefore, it is necessary to improve the traditional structure to obtain narrow-band polarization-entangled photon pairs.

[0055] To make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the following further elaborates on the present invention in detail with reference to specific embodiments and the accompanying drawings.

[0056] The terms used herein are merely for describing specific embodiments and are not intended to limit the present invention. The terms "including", "comprising", etc. used herein indicate the presence of the described features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0057] All terms used herein, including technical and scientific terms, have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0058] In the case of using expressions such as "at least one of A, B, and C, etc.", generally, it should be interpreted according to the meaning that those skilled in the art usually understand this expression. For example, "a system having at least one of A, B, and C" should include, but not be limited to, a system having only A, only B, only C, having A and B, having A and C, having B and C, and / or having A, B, and C, etc. In the case of using expressions such as "at least one of A, B, or C, etc.", generally, it should be interpreted according to the meaning that those skilled in the art usually understand this expression. For example, "a system having at least one of A, B, or C" should include, but not be limited to, a system having only A, only B, only C, having A and B, having A and C, having B and C, and / or having A, B, and C, etc.

[0059] It should also be noted that the directional terms mentioned in the embodiments, such as "up", "down", "front", "rear", "left", "right", etc., are only references to the directions in the accompanying drawings and are not used to limit the protection scope of the present invention. Throughout the accompanying drawings, the same elements are denoted by the same or similar reference numerals. When it may cause confusion in the understanding of the present invention, the conventional structures or configurations will be omitted.

[0060] Figure 3 A schematic diagram of a polarization entanglement light source provided according to an embodiment of the present invention is shown.

[0061] Figure 4 A schematic diagram of a polarization entanglement light source provided according to another embodiment of the present invention is shown.

[0062] Figure 5 A schematic diagram of a polarization entanglement light source provided according to still another embodiment of the present invention is shown

[0063] As Figures 3 to 5 shown, the polarization entanglement light source includes an interference mechanism 31, a resonant cavity 32, and a nonlinear optical crystal 33.

[0064] The interference mechanism 31 is configured to obtain a first pump laser beam and a second pump laser beam with the same polarization direction and different propagation directions based on an initial laser. The resonator 32 is adapted to receive the first pump laser beam and the second pump laser beam. The nonlinear optical crystal 33 is disposed in the resonator 32. The two pump laser beams entering the resonator 32 pass through the nonlinear optical crystal 33 from different directions. Under the action of the nonlinear optical crystal 33, photons of the first pump laser beam undergo spontaneous parametric down-conversion to obtain two first down-converted photons, and photons of the second pump laser beam undergo spontaneous parametric down-conversion to obtain two second down-converted photons. Among them, the first down-converted photons and the second down-converted photons oscillate in the resonator and experience the same optical path. The resonator 32 selects the first down-converted photons with the target frequency to obtain first target down-converted photons, and the resonator 32 selects the second down-converted photons with the target frequency to obtain second target down-converted photons. The propagation directions of the first target down-converted photons and the second target down-converted photons are different when they exit the resonator. The interference mechanism 31 is configured to ensure that the first target down-converted photons and the second target down-converted photons can generate quantum coherent superposition and obtain polarization-entangled photon pairs.

[0065] According to an embodiment of the present invention, due to the frequency selection effect of the resonator 32, the polarization-entangled photon pairs obtained in the embodiment of the present invention are narrowband polarization-entangled photon pairs.

[0066] According to an embodiment of the present invention, the target down-converted photons for coherent superposition have the same polarization direction in the resonator and only the propagation directions are opposite. Therefore, the target down-converted photons in the embodiment of the present invention have the characteristic of good indistinguishability. At the same time, since only one nonlinear optical crystal 3 is disposed in the resonator 32, the polarization-entangled light source provided in the embodiment of the present invention can avoid the problems of changes in the paths and polarization directions of the down-converted photon pairs caused by using two nonlinear optical crystals, resulting in poor device stability and low fidelity of the prepared entangled state.

[0067] According to an embodiment of the present invention, the interference mechanism 31 includes: a polarization beam splitter 311, a first mirror module, a second mirror module, and a polarization conversion component 314. The polarization beam splitter 311 is adapted to split the initial laser into a first pump laser beam and a second pump laser beam with perpendicular polarization directions. The polarization conversion component 314 is adapted to change the polarization direction of the first pump laser beam or the second pump laser beam to obtain a first pump laser beam and a second pump laser beam with the same polarization direction. The first mirror module is adapted to make the first pump laser beam incident on the resonator 32 along a first path. The second mirror module is adapted to make the second pump laser beam incident on the resonator 32 along a second path.

[0068] According to an embodiment of the present invention, the first pump laser beam, for example, has a vertical polarization direction, and the second pump laser beam, for example, has a horizontal polarization direction. The polarization conversion component 314 can be, for example, a half-wave plate, and the polarization conversion component 314 is adapted to change the polarization direction of the first pump laser beam or the second pump laser beam so that the polarization directions of the first pump laser beam or the second pump laser beam are both horizontally polarized or vertically polarized.

[0069] According to an embodiment of the present invention, the first mirror module includes at least one first mirror 312 located on the first path, and the at least one first mirror 312 reflects the first pump laser beam in sequence so that the first pump laser beam is incident on the resonator 32 along the first path. The second mirror module includes at least one second mirror 313 located on the second path, and the at least one second mirror 313 reflects the second pump laser beam in sequence so that the second pump laser beam is incident on the resonator 32 along the second path. Figures 3 to 4 The case where one first mirror 312 is provided on the first path and one second mirror 313 is provided on the second path is shown.

[0070] Continue to refer to Figures 3 to 5 , according to an embodiment of the present invention, the resonator 32 includes: a first dichroic mirror 321 and a resonator mirror module. Each part of the resonator 32 and each part of the interference mechanism 31 form a Sagnac loop.

[0071] Specifically, the first dichroic mirror 321 is used to transmit the first pump laser beam incident into the resonator 32 via the first path and the second pump laser beam incident into the resonator 32 via the second path. The resonator mirror module includes a plurality of resonator mirrors, and the plurality of resonator mirrors and the first dichroic mirror 321 form an annular structure, and the nonlinear optical crystal 33 is located between two adjacent resonator mirrors. Among them, the first pump laser beam passes through the nonlinear optical crystal 33 to obtain a first down-converted photon, and the second pump laser beam passes through the nonlinear optical crystal 33 to obtain a second down-converted photon. The first down-converted photon and the second down-converted photon are respectively reflected in sequence by each part of the resonator 32, and then oscillate in the resonator 32. During the process of the first down-converted photon and the second down-converted photon oscillating in the resonator 32, the resonator 32 respectively performs frequency selection on the first down-converted photon and the second down-converted photon to obtain a first target down-converted photon and a second target down-converted photon. The first target down-converted photon and the second target down-converted photon are output from the first dichroic mirror 32 along different directions. The position of the nonlinear optical crystal 33 in the resonator 32 is configured to ensure that the optical paths of the first down-converted photon and the second down-converted photon in the resonator 32 are the same. The resonator mirrors located at both ends of the nonlinear optical crystal 33 are concave mirrors to converge the light beam in the resonator 32.

[0072] Taking Figure 3 and Figure 5 as an example, the resonator 32 includes a first dichroic mirror 321 and two resonator mirrors. The first dichroic mirror 321 and the two resonator mirrors form a triangular resonator. The nonlinear optical crystal 33 is located between the two resonator mirrors. The two resonator mirrors are divided into a first cavity mirror 322 and a second cavity mirror 323. In Figure 3 , photons of the first pump laser enter the resonator through the first dichroic mirror 321, pass through the first cavity mirror 322, then pass through the nonlinear optical crystal 33 to obtain two first down-converted photons. The first down-converted photons oscillate in the resonator 32, and after being frequency-selected by the resonator 32, the first target down-converted photons are obtained. The first target down-converted photons are reflected by the second cavity mirror 323 to the first dichroic mirror 321 and are emitted through the first dichroic mirror 321. Photons of the second pump laser enter the resonator through the first dichroic mirror 321, pass through the second cavity mirror 323, then pass through the nonlinear optical crystal 33 to obtain two second down-converted photons. The second down-converted photons oscillate in the resonator 32, and after being frequency-selected by the resonator 32, the second target down-converted photons are obtained. The second target down-converted photons are reflected by the first cavity mirror 322 to the first dichroic mirror 321 and are emitted through the first dichroic mirror 321.

[0073] Taking Figure 4 as an example, the resonator 32 includes a first dichroic mirror 321 and three resonator mirrors. The first dichroic mirror 321 and the three resonator mirrors form a butterfly-shaped resonator. The three resonator mirrors include a first cavity mirror 322, a second cavity mirror 323, and a third cavity mirror 324. Among them, the first cavity mirror 322 and the second cavity mirror 323 are slightly concave mirrors, and the third cavity mirror 324 is a flat mirror. The nonlinear optical crystal 33 is located between the first cavity mirror 322 and the second cavity mirror 323. In Figure 4Among them, photons of the first pump laser enter the resonant cavity through the first dichroic mirror 321, are incident on the first cavity mirror 322 after passing through the third cavity mirror 324, pass through the nonlinear optical crystal 33 after being reflected by the first cavity mirror 322 to obtain two first down-converted photons. The first down-converted photons oscillate in the resonant cavity 32, and after being frequency-selected by the resonant cavity 32, the first target down-converted photons are obtained. The first target down-converted photons are reflected by the second cavity mirror 323 to the first dichroic mirror 321 and are emitted through the first dichroic mirror 321. Photons of the second pump laser enter the resonant cavity through the first dichroic mirror 321, pass through the nonlinear optical crystal 33 after passing through the second cavity mirror 323 to obtain two second down-converted photons. The second down-converted photons oscillate in the resonant cavity 32, and after being frequency-selected by the resonant cavity 32, the second target down-converted photons are obtained. The second target down-converted photons are reflected by the first cavity mirror 322 to the third cavity mirror 324, and then are reflected by the third cavity mirror 324 to the first dichroic mirror 321 and are emitted through the first dichroic mirror 321.

[0074] According to an embodiment of the present invention, the first target down-converted photons emitted from the resonant cavity 32 enter the second path and enter the polarization beam splitter 311 through the second path. The second target down-converted photons emitted from the resonant cavity 32 enter the first path and enter the polarization beam splitter 311 through the first path. The lengths of the first path and the second path are the same.

[0075] According to an embodiment of the present invention, the first pump laser is incident on the resonant cavity 32 along the first path, and after generating the first down-converted photons in the resonant cavity 32, it enters the polarization beam splitter 311 through the second path. Similarly, the second pump laser is incident on the resonant cavity 32 along the second path, and after the first pump laser generates the first down-converted photons in the resonant cavity 32, it enters the polarization beam splitter 311 through the first path. It can be seen that each part of the interference mechanism 31 and the resonant cavity 32 form a Sagnac loop. In the Sagnac loop, the position of the nonlinear optical crystal 33 in the resonant cavity 32 is configured to ensure that the optical paths of the first down-converted photons and the second down-converted photons in the resonant cavity 32 are the same. At the same time, the lengths of the first path and the second path being the same can make the sum of the optical path of the first pump laser and the optical path of the first down-converted photons generated by the first pump laser equal to the sum of the optical path of the second pump laser and the optical path of the second down-converted photons generated by the second pump laser. In this Sagnac loop, since the photons propagating along different paths experience the same optical path, the phase changes accumulated by the photons propagating along different paths in the loop are almost the same. Therefore, the Sagnac loop structure does not require an additional phase compensation mechanism to ensure that the target down-converted photons generated by different pump lights have the correct phase relationship when combined. This not only meets the conditions for generating quantum coherent superposition, but also simplifies the experimental design and reduces possible error sources.

[0076] According to a specific embodiment of the present invention, the nonlinear optical crystal 33 can be, for example, a type-0 phase-matching crystal or a type-I phase-matching crystal. At this time, under the action of the nonlinear optical crystal 33, photons of each pump laser undergo spontaneous parametric down-conversion to obtain two down-converted photons with the same polarization direction as the pump laser. The first mirror module is also adapted to reflect the second target down-converted photons output from the resonant cavity 32, so that the second target down-converted photons travel along the first path. The second mirror module is also adapted to reflect the first target down-converted photons output from the resonant cavity 32, so that the first target down-converted photons travel along the second path. The polarization conversion component 314 is also adapted to change the polarization direction of the first target down-converted photons or change the polarization direction of the second target down-converted photons, so that the polarization directions of the first target down-converted photons and the second target down-converted photons are perpendicular. The polarization beam splitter 311 is also adapted to combine the first target down-converted photons and the second target down-converted photons with perpendicular polarization directions and generate quantum coherent superposition to obtain a polarization-entangled photon pair, and the polarization-entangled photon pair is a non-degenerate polarization-entangled photon pair. Wherein, the lengths of the first path and the second path are the same.

[0077] According to another specific embodiment of the present invention, the nonlinear optical crystal 33 can also be, for example, a type-II phase-matching crystal. Photons of each pump laser undergo spontaneous parametric down-conversion to obtain two down-converted photons with perpendicular polarization directions. The first mirror module is also adapted to reflect the second target down-converted photons output from the resonant cavity, so that the second target down-converted photons travel along the first path; the second mirror module is also adapted to reflect the first target down-converted photons output from the resonant cavity, so that the second target down-converted photons travel along the second path. The polarization beam splitter 311 is adapted to combine the first target down-converted photons and the second target down-converted photons and generate quantum coherent superposition to obtain a polarization-entangled photon pair, and the polarization-entangled photon pair is a non-degenerate polarization-entangled photon pair or a degenerate polarization-entangled photon pair. Wherein, the lengths of the first path and the second path are the same.

[0078] As Figures 3 to 4As shown, when the nonlinear optical crystal 33 is a type-0 phase-matching crystal or a type-I phase-matching crystal, the polarization entangled light source further includes: a second dichroic mirror 34 and a third dichroic mirror 35. The second dichroic mirror 34 is adapted to transmit the initial laser so that the initial laser enters the interference mechanism 31, and is adapted to reflect the polarization entangled photon pairs. The third dichroic mirror 35 is adapted to separate the polarization entangled photon pairs of different wavelengths so as to separate the polarization entangled photon pairs having different spatial modes. According to an embodiment of the present invention, when the nonlinear optical crystal 33 is a type-0 phase-matching crystal or a type-I phase-matching crystal, by configuring the nonlinear optical crystal 33, it is possible to make two down-converted photons with the same polarization direction obtained by spontaneous parametric down-conversion of photons of each pump laser have different wavelengths. Therefore, by separating the polarization entangled photon pairs of different wavelengths, it is possible to separate the polarization entangled photon pairs having different spatial modes.

[0079] According to an embodiment of the present invention, when the nonlinear optical crystal 33 is a type-II phase-matching crystal, the polarization beam splitter 311 is adapted to separate the polarization entangled photon pairs having different spatial modes. As Figure 5 shown, the above polarization entangled light source further includes a second dichroic mirror 34, and the second dichroic mirror 34 is adapted to transmit the initial laser so that the initial laser enters the interference mechanism 31, and is adapted to reflect one of the spatial mode polarization entangled photon pairs output by the polarization beam splitter 311.

[0080] According to an embodiment of the present invention, when the nonlinear optical crystal 33 is a type-II phase-matching crystal, since two down-converted photons with perpendicular polarization directions are obtained by spontaneous parametric down-conversion of photons of each pump laser, therefore, the polarization beam splitter 311 can separate the polarization entangled photon pairs of different polarization directions, that is, it can separate the polarization entangled photon pairs having different spatial modes.

[0081] According to the polarization entangled light source provided by the embodiment of the present invention, by combining the resonant cavity and the Sagnac loop structure, a polarization entangled light source with good stability and capable of generating narrow-band polarization entangled photon pairs is realized by using a nonlinear optical crystal.

[0082] The above describes the embodiments of the present invention. However, these embodiments are only for illustrative purposes and not for limiting the scope of the present invention. Although the embodiments are separately described above, this does not mean that the measures in each embodiment cannot be used advantageously in combination. The scope of the present invention is defined by the appended claims and their equivalents. Without departing from the scope of the present invention, those skilled in the art can make various substitutions and modifications, and these substitutions and modifications should all fall within the scope of the present invention.

Claims

1. A polarization entangled light source, characterized in that: include: An interference mechanism, used for obtaining a first pump laser beam and a second pump laser beam having the same polarization direction and different propagation directions according to the initial laser beam; A resonant cavity, adapted to receive the first pump laser beam and the second pump laser beam; the resonant cavity comprises: a first dichroic mirror, for transmitting the first pump laser beam incident into the resonant cavity via a first path and the second pump laser beam incident into the resonant cavity via a second path; A resonant cavity reflector module, comprising a plurality of resonant cavity reflectors, wherein the plurality of resonant cavity reflectors and the first dichroic mirror form a ring structure; A nonlinear optical crystal is arranged in the resonant cavity and located between two adjacent resonant cavity reflectors. Two pump laser beams entering the resonant cavity pass through the nonlinear optical crystal from different directions. Under the action of the nonlinear optical crystal, the photons of the first pump laser beam undergo spontaneous parametric down-conversion to obtain two first down-converted photons, and the photons of the second pump laser beam undergo spontaneous parametric down-conversion to obtain two second down-converted photons. wherein the first down-converted photon and the second down-converted photon oscillate in the resonant cavity and experience the same optical path, the resonant cavity selects the first down-converted photon with a target frequency to obtain a first target down-converted photon, the resonant cavity selects the second down-converted photon with the target frequency to obtain a second target down-converted photon, and the first target down-converted photon and the second target down-converted photon have different propagation directions when emitted from the first dichroic mirror of the resonant cavity; The interference mechanism is also configured to ensure that the first target down-converted photon and the second target down-converted photon can produce quantum coherent superposition and obtain a polarization entangled photon pair.

2. The polarization entangled light source according to claim 1, characterized in that: The interference mechanism comprises: A polarization beam splitter, adapted to split the initial laser into a first pump laser beam and a second pump laser beam with perpendicular polarization directions; A polarization conversion component, adapted to change the polarization direction of the first pump laser beam or the second pump laser beam to obtain the first pump laser beam and the second pump laser beam having the same polarization direction; A first reflector module, adapted to allow the first pump laser beam to be incident on the resonant cavity along a first path; The second reflector module is adapted to allow the second pump laser beam to be incident on the resonant cavity along a second path.

3. The polarization entangled light source according to claim 2, characterized in that: The first reflector module includes at least one first reflector located on the first path, and the at least one first reflector sequentially reflects the first pump laser beam so that the first pump laser beam is incident on the resonant cavity along the first path; The second reflector module includes at least one second reflector located on the second path, and the at least one second reflector sequentially reflects the second pump laser beam so that the second pump laser beam is incident on the resonant cavity along the second path.

4. The polarization entangled light source according to claim 2, characterized in that: The nonlinear optical crystal is a type 0 phase-matching crystal or a type I phase-matching crystal, and each pump laser photon undergoes spontaneous parametric down-conversion to obtain two down-converted photons with the same polarization direction as the pump laser; The first reflector module is further adapted to reflect the second target down-converted photons output by the resonant cavity, so that the second target down-converted photons travel along the first path; The second reflector module is further adapted to reflect the first target down-converted photons outputted from the resonant cavity, so that the first target down-converted photons travel along the second path; The polarization conversion component is further adapted to change the polarization direction of the converted photons of the first target or change the polarization direction of the converted photons of the second target so that the polarization direction of the converted photons of the first target is perpendicular to the polarization direction of the converted photons of the second target; The polarization beam splitter is also suitable for combining the first target down-conversion photons and the second target down-conversion photons with perpendicular polarization directions and generating quantum coherent superposition to obtain the polarization entangled photon pair; The first path and the second path have the same length.

5. The polarization entangled light source according to claim 2, characterized in that: The nonlinear optical crystal is a type II phase-matching crystal, and each pump laser photon undergoes spontaneous parametric down-conversion to obtain two down-converted photons with perpendicular polarization directions; The first reflector module is further adapted to reflect the second target down-converted photons output by the resonant cavity, so that the second target down-converted photons travel along the first path; The second reflector module is further adapted to reflect the first target down-converted photons outputted from the resonant cavity, so that the second target down-converted photons travel along the second path; A polarization beam splitter, adapted to combine the first target down-converted photon and the second target down-converted photon and generate quantum coherent superposition to obtain the polarization entangled photon pair; The first path and the second path have the same length.

6. The polarization entangled light source according to claim 2, characterized in that: The first pump laser beam outputted through the polarization beam splitter has a vertical polarization direction, and the second pump laser beam outputted through the polarization beam splitter has a horizontal polarization direction; The polarization conversion component is a half-wave plate, which is suitable for changing the polarization direction of the first pump laser beam or the second pump laser beam so that the polarization direction of the first pump laser beam or the second pump laser beam is horizontal polarization or vertical polarization.

7. The polarization entangled light source according to claim 2, characterized in that: The resonant cavity comprises: The position of the nonlinear optical crystal in the resonant cavity is configured to ensure that the optical path lengths of the first down-converted photons and the second down-converted photons in the resonant cavity are the same.

8. The polarization entangled light source according to claim 7, characterized in that: The resonant cavity reflectors located at two ends of the nonlinear optical crystal are concave reflectors.

9. The polarization entangled light source according to claim 4, characterized in that: The polarization entangled light source also includes: A second dichroic mirror, adapted to transmit the initial laser light so as to enter the interference mechanism with the initial laser light; and adapted to reflect the polarization entangled photon pair; and The third dichroic mirror is suitable for separating polarization entangled photon pairs of different wavelengths to separate polarization entangled photon pairs with different spatial patterns.

10. The polarization entangled light source according to claim 5, characterized in that: The polarization beam splitter is suitable for separating polarization entangled photon pairs having different spatial modes; The polarization entangled light source also includes: The second dichroic mirror is adapted to transmit the initial laser so as to allow the initial laser to enter the interference mechanism; and is adapted to reflect the polarization entangled photon pair of one of the spatial modes output by the polarization beam splitter.

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