Polarization entangled light source

Abstract

A polarization entangled light source comprises an interference mechanism used for obtaining a first beam of pumping laser and a second beam of pumping laser which are the same in polarization direction and different in propagation direction according to initial laser; the resonant cavity is suitable for receiving the first beam of pumping laser and the second beam of pumping laser; the nonlinear optical crystal is arranged in the resonant cavity, the two beams of pumping laser entering the resonant cavity penetrate through the nonlinear optical crystal from different directions, under the action of the nonlinear optical crystal, photons of the first beam of pumping laser are subjected to spontaneous parametric down-conversion to obtain two first down-conversion photons, and the two first down-conversion photons are converted into two second down-conversion photons; the photons of the second beam of pump laser are subjected to spontaneous parametric down-conversion to obtain two second down-conversion photons; the interference mechanism is further suitable for ensuring that the first target down-conversion photons and the second target down-conversion photons can generate quantum coherence superposition, and polarization entangled photon pairs are obtained.

Description

Technical Field

[0001] The invention relates to the field of quantum communication, and in particular 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. It has been widely used in quantum information tasks and the verification of basic problems in quantum mechanics. Spontaneous parametric down-conversion describes a second-order nonlinear process. In the spontaneous parametric down-conversion process, a pump laser 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 frequency combining process. "Parametric" means that light interacts with the medium but does not change the quantum state of the medium. Therefore, the energy conservation and momentum conservation are satisfied between the pump laser and the two down-converted photons. The phase matching condition of the parametric down-conversion process can be deduced from the momentum conservation. According to the different polarizations of the down-converted photons obtained, 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 direction of the down-converted photons, the parametric down-conversion can also be divided into collinear down-conversion and non-collinear down-conversion; according to the wavelength, the parametric down-conversion process can be divided into degenerate down-conversion and non-degenerate down-conversion. The parametric down-conversion process needs to meet phase matching conditions, which are usually realized in birefringent nonlinear crystals, whose refractive index depends on the wavelength and emission angle of the photons. Quasi-phase matching is a method of achieving phase matching through periodically poled crystals. This method has the advantages of adjustable parameters and the ability to select the direction of the maximum nonlinear coefficient.

[0003] An optical cavity is composed of two or more mirrors that can confine a light beam to 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 of the optical cavity, the light intensity of the light beam can be enhanced. Therefore, the application of optical cavities in lasers can increase the output power of lasers. At the same time, since only light beams 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, optical cavities also play an important role. They 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 nonlinear optical crystals 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, the current method of generating narrowband non-degenerate polarization-entangled photon pairs using optical cavities often uses two nonlinear media. However, since the photon pairs generated in the down-conversion process pass through the two nonlinear media multiple times, the path and polarization direction of the photon pairs change, making the generation process of non-degenerate polarization-entangled photon pairs unstable. Summary of the invention

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

[0005] 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;

[0006] A resonant cavity, adapted to receive a first pump laser beam and a second pump laser beam;

[0007] A nonlinear optical crystal is arranged in a resonant cavity. 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.

[0008] 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 a 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 they are emitted from 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 produce quantum coherent superposition and obtain a polarization entangled photon pair.

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

[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 the first pump laser beam and the second pump laser beam having the same polarization direction;

[0013] A first reflector module, adapted to allow a first pump laser beam to be incident on the resonant cavity along a first path;

[0014] The second reflector module is adapted to allow the second pump laser beam to be incident on the resonant cavity along a second path.

[0015] According to an embodiment of the present invention, the first reflector module includes at least one first reflector located in 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;

[0016] The second reflector module includes at least one second reflector located on the second path. 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.

[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 pump laser photon undergoes spontaneous parametric down-conversion to obtain two down-converted photons with the same polarization direction as the pump laser.

[0018] The first reflector module is also 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;

[0019] The second reflector module is also 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;

[0020] The polarization conversion component is also adapted to change the polarization direction of the down-converted photons of the first target or change the polarization direction of the down-converted photons of the second target so that the polarization direction of the down-converted photons of the first target is perpendicular to the polarization direction of the down-converted photons of the second target;

[0021] The polarization beam splitter is also suitable for combining the first target down-converted photons and the second target down-converted photons with perpendicular polarization directions and generating quantum coherent superposition to obtain polarization entangled photon pairs;

[0022] The first path and the second path have the same length.

[0023] According to an embodiment of the present invention, 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;

[0024] The first reflector module is also 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;

[0025] The second reflector 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;

[0026] A polarization beam splitter is used to combine the first target down-converted photons and the second target down-converted photons and generate quantum coherent superposition to obtain polarization entangled photon pairs;

[0027] The first path and the second path have the same length.

[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, 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.

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

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

[0032] A resonant cavity reflector module, comprising a plurality of resonant cavity reflectors, wherein the plurality of resonant cavity reflectors and a first dichroic mirror form a ring structure, and a nonlinear optical crystal is located between two adjacent resonant cavity reflectors;

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

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

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

[0036] A second dichroic mirror, adapted to transmit the initial laser light to enter the interference mechanism with the initial laser light; and adapted to reflect the polarization entangled photon pair; and

[0037] 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.

[0038] According to an embodiment of the present invention, a polarization beam splitter is adapted to separate polarization entangled photon pairs having different spatial modes;

[0039] Polarization entangled light sources also include:

[0040] The second dichroic mirror is used to transmit the initial laser light so as to allow the initial laser light to enter the interference mechanism; and is used to reflect the polarization entangled photon pair of one of the spatial modes output by the polarization beam splitter.

[0041] According to the embodiment of the present invention, due to the frequency selection effect of the resonant cavity, the polarization entangled photon pair obtained in the embodiment of the present invention is a narrow-band polarization entangled photon pair.

[0042] According to an embodiment of the present invention, the target down-conversion photons used for coherent superposition have the same polarization direction in the resonant cavity, and only the propagation directions are opposite. Therefore, the target down-conversion photons of the embodiment of the present invention have the characteristic of good homogeneity. At the same time, since only one nonlinear optical crystal is arranged in the resonant cavity, the polarization entangled light source provided by the embodiment of the present invention can avoid the change of the path and polarization direction of the down-conversion photon pair caused by the use of two nonlinear optical crystals, thereby causing poor device stability and low fidelity of the prepared entangled state. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

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

[0045] Figure 2 A schematic diagram showing a conventional polarization entangled photon pair generation device;

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

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

[0048] Figure 5 A schematic diagram of a polarization entangled light source provided according to yet another embodiment of the present invention is shown.

[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-in-ring half-wave plate; 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 DESCRIPTION

[0051] like Figure 1As shown, a conventional 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, the first crystal 11 and the second crystal 12 are both type II phase-matched crystals, and the two crystals are perpendicular to each other in the axial direction. The combined cavity includes three cavity mirrors (i.e., a first cavity mirror 13, a second cavity mirror 14, and a third cavity mirror 15) and a beam splitting module 16. Polarized light with a polarization direction of 45 degrees has the same probability of down-conversion in the two crystals, and non-degenerate photon pairs with opposite polarization directions are generated 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 constitute a first group of cavity mirrors, and the second cavity mirror 14 and the third cavity mirror 15 constitute a second group of cavity mirrors. The beam splitting module 16 (e.g., a dichroic mirror) can separate photons of different wavelengths, so that photons with a first wavelength oscillate in the first group of cavity mirrors, and photons with a second wavelength oscillate in the second group of cavity mirrors. Although the first set of cavity mirrors and the second set of cavity mirrors can be used to modulate down-converted photons of different wavelengths respectively and ultimately generate narrow-band polarization entangled photon pairs, since the coherently superimposed photons have different polarizations and the photons generated by the down-conversion pass through the dual crystal structure (i.e., the first crystal 11 and the second crystal 12) many times, the path and polarization direction of the photons generated by the down-conversion will change, which makes it very difficult to adjust the device and the stability of the device is poor.

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

[0053] Figure 2 A schematic diagram showing a conventional polarization entangled photon pair generation device.

[0054] like Figure 2As shown, the conventional polarization entangled photon pair generating device adopts a triangular Sagnac ring structure, which includes a first reflection unit 21, a second reflection unit 22, and a polarization beam splitting unit 23. The third crystal 24 located in the triangular Sagnac ring 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 ring structure from the polarization beam splitting unit 23, and the polarization direction of the polarized light is 45 degrees, so when the polarized light passes through the polarization beam splitting unit 23, there is a half probability of transmission and a half probability of reflection. When the polarized light passes through the polarization beam splitting unit 23, it is divided into a transmitted first sub-polarized light and a reflected second sub-polarized light. The half-wave plate 25 inside the ring is a polychromatic half-wave plate, and the half-wave plate 25 inside the ring can rotate the polarization direction of the sub-polarized light passing through and the obtained down-converted photon by 45 degrees. The photons of the transmitted first polarized light pass through the third crystal 24 clockwise to generate a down-conversion photon pair of horizontal polarization (H polarization), and the down-conversion photon pair of horizontal polarization passes through the half-wave plate 25 in the ring clockwise to be converted into a down-conversion photon pair of vertical polarization (V polarization). The polarization direction of the reflected second sub-polarized light is converted into horizontal polarization after passing through the half-wave plate 25 in the ring, and then passes through the third crystal 24 counterclockwise to generate two horizontal down-conversion polarization photon pairs. The down-conversion photon pair of vertical polarization obtained from the first sub-polarized light reaches the polarization beam splitting unit 23 in the clockwise direction, and the down-conversion photon pair of horizontal polarization obtained from the second sub-polarized light reaches the polarization beam splitting unit 23 in the counterclockwise direction. The polarized photon pairs traveling clockwise and counterclockwise are superimposed in the polarization beam splitting unit 23 to generate polarization entangled photon pairs. However, the conventional polarization entangled photon pair generating device does not have an optical cavity structure, and it is difficult to generate narrow-band entangled photon pairs. Therefore, it is necessary to improve the conventional structure to obtain narrow-band polarization entangled photon pairs.

[0055] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0056] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and / or components, but do not exclude the existence 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] When using expressions such as "at least one of A, B, and C, etc.", it should generally be interpreted as the meaning of the expression generally understood by those skilled in the art. For example, "a system having at least one of A, B, and C" should include but not be limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc. When using expressions such as "at least one of A, B, or C, etc.", it should generally be interpreted as the meaning of the expression generally understood by those skilled in the art. For example, "a system having at least one of A, B, or C" should include but not be limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.

[0059] It should also be noted that the directional terms mentioned in the embodiments, such as "upper", "lower", "front", "back", "left", "right", etc., are only reference directions of the drawings and are not intended to limit the scope of protection of the present invention. Throughout the drawings, the same elements are represented by the same or similar reference numerals. Conventional structures or configurations will be omitted when they may cause confusion in the understanding of the present invention.

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

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

[0062] Figure 5 A schematic diagram showing a polarization entangled light source provided according to another embodiment of the present invention

[0063] like Figure 3~Figure 5 As shown, the polarization entangled light source includes an interference mechanism 31, a resonant cavity 32, and a nonlinear optical crystal 33.

[0064] The interference mechanism 31 is used to obtain a first pump laser beam and a second pump laser beam with the same polarization direction and different propagation directions according to the initial laser. The resonant cavity 32 is suitable for receiving the first pump laser beam and the second pump laser beam. The nonlinear optical crystal 33 is arranged in the resonant cavity 32. The two pump laser beams entering the resonant cavity 32 pass through the nonlinear optical crystal 33 from different directions. Under the action of the nonlinear optical crystal 33, 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. Among them, the first down-converted photons and the second down-converted photons oscillate in the resonant cavity and experience the same optical path. The resonant cavity 32 selects the first down-converted photons with the target frequency to obtain the first target down-converted photons. The resonant cavity 32 selects the second down-converted photons with the target frequency to obtain the 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 are emitted from the resonant cavity. The interference mechanism 31 is configured to ensure that the first target down-converted photon and the second target down-converted photon can generate quantum coherent superposition and obtain a polarization entangled photon pair.

[0065] According to the embodiment of the present invention, due to the frequency selection effect of the resonant cavity 32, the polarization entangled photon pair obtained in the embodiment of the present invention is a narrow-band polarization entangled photon pair.

[0066] According to an embodiment of the present invention, the target down-conversion photons used for coherent superposition have the same polarization direction in the resonant cavity, and only the propagation directions are opposite. Therefore, the target down-conversion photons of the embodiment of the present invention have the characteristic of good homogeneity. At the same time, since only one nonlinear optical crystal 3 is arranged in the resonant cavity 32, the polarization entangled light source provided by the embodiment of the present invention can avoid the change of the path and polarization direction of the down-conversion photon pair caused by the use of two nonlinear optical crystals, thereby causing 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 reflector module, a second reflector module, and a polarization conversion component 314. The polarization beam splitter 311 is suitable for splitting 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 suitable for changing 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 with the same polarization direction. The first reflector module is suitable for making the first pump laser beam incident on the resonant cavity 32 along the first path. The second reflector module is suitable for making the second pump laser beam incident on the resonant cavity 32 along the second path.

[0068] According to an embodiment of the present invention, the first pump laser beam has a vertical polarization direction, for example, and the second pump laser beam has a horizontal polarization direction, for example. The polarization conversion component 314 may 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 direction of the first pump laser beam or the second pump laser beam is horizontal polarization or vertical polarization.

[0069] According to an embodiment of the present invention, the first reflector module includes at least one first reflector 312 located on the first path, and the at least one first reflector 312 sequentially reflects the first pump laser beam so that the first pump laser beam is incident on the resonant cavity 32 along the first path. The second reflector module includes at least one second reflector 313 located on the second path, and the at least one second reflector 313 sequentially reflects the second pump laser beam so that the second pump laser beam is incident on the resonant cavity 32 along the second path. Figure 3~Figure 4 3 shows a case where a first reflector 312 is provided on the first path and a second reflector 313 is provided on the second path.

[0070] Continue to refer Figure 3~Figure 5 According to an embodiment of the present invention, the resonant cavity 32 includes: a first dichroic mirror 321 and a resonant cavity reflector module. The various parts of the resonant cavity 32 and the various parts of the interference mechanism 31 form a Sagnac ring.

[0071] Specifically, the first dichroic mirror 321 is used to transmit the first pump laser beam incident into the resonant cavity 32 via the first path and the second pump laser beam incident into the resonant cavity 32 via the second path. The resonant cavity reflector module includes a plurality of resonant cavity reflectors, and the plurality of resonant cavity reflectors and the first dichroic mirror 321 form a ring structure, and the nonlinear optical crystal 33 is located between two adjacent resonant cavity reflectors. 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 various parts of the resonant cavity 32, and then oscillate in the resonant cavity 32. During the oscillation of the first down-converted photon and the second down-converted photon in the resonant cavity 32, the resonant cavity 32 selects the first down-converted photon and the second down-converted photon respectively 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 in different directions. 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 photon and the second down-converted photon in the resonant cavity 32 are the same. The resonant cavity reflectors at both ends of the nonlinear optical crystal 33 are concave reflectors to converge the light beams in the resonant cavity 32.

[0072] by Figure 3 and Figure 5 For example, the resonant cavity 32 includes a first dichroic mirror 321 and two resonant cavity reflectors, the first dichroic mirror 321 and the two resonant cavity reflectors form a triangular resonant cavity, the nonlinear optical crystal 33 is located between the two resonant cavity reflectors, and the two resonant cavity reflectors are divided into a first cavity reflector 322 and a second cavity reflector 323. Figure 3 In the embodiment, the photons of the first pump laser enter the resonant cavity through the first dichroic mirror 321, pass through the first cavity reflector 322, and then pass through the nonlinear optical crystal 33 to obtain two first down-converted photons. The first down-converted photons oscillate in the resonant cavity 32, and are selected by the resonant cavity 32 to obtain first target down-converted photons. The first target down-converted photons are reflected to the first dichroic mirror 321 by the second cavity reflector 323, and are emitted through the first dichroic mirror 321. The photons of the second pump laser enter the resonant cavity through the first dichroic mirror 321, pass through the second cavity reflector 323, and then pass through the nonlinear optical crystal 33 to obtain two second down-converted photons. The second down-converted photons oscillate in the resonant cavity 32, and are selected by the resonant cavity 32 to obtain second target down-converted photons. The second target down-converted photons are reflected to the first dichroic mirror 321 by the first cavity reflector 322, and are emitted through the first dichroic mirror 321.

[0073] by Figure 4 For example, the resonant cavity 32 includes a first dichroic mirror 321 and three resonant cavity reflectors. The first dichroic mirror 321 and the three resonant cavity reflectors form a butterfly resonant cavity. The three resonant cavity reflectors include a first cavity reflector 322, a second cavity reflector 323 and a third cavity reflector 324. The first cavity reflector 322 and the second cavity reflector 323 are micro-concave reflectors, and the third cavity reflector 324 is a plane mirror. The nonlinear optical crystal 33 is located between the first cavity reflector 322 and the second cavity reflector 323. Figure 4In the embodiment, the photons of the first pump laser enter the resonant cavity through the first dichroic mirror 321, and are incident on the first cavity reflector 322 after passing through the third cavity reflector 324. After being reflected by the first cavity reflector 322, they pass through the nonlinear optical crystal 33 to obtain two first down-conversion photons. The first down-conversion photons oscillate in the resonant cavity 32, and are frequency-selected by the resonant cavity 32 to obtain first target down-conversion photons. The first target down-conversion photons are reflected to the first dichroic mirror 321 through the second cavity reflector 323, and are emitted through the first dichroic mirror 321. The photons of the second pump laser enter the resonant cavity through the first dichroic mirror 321, pass through the second cavity reflector 323, and pass through the nonlinear optical crystal 33 to obtain two second down-converted photons. The second down-converted photons oscillate in the resonant cavity 32, and are frequency-selected by the resonant cavity 32 to obtain second target down-converted photons. The second target down-converted photons are reflected by the first cavity reflector 322 to the third cavity reflector 324, and then reflected by the third cavity reflector 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, and the length of the first path is the same as the length of the second path.

[0075] According to an embodiment of the present invention, a first pump laser is incident on the resonant cavity 32 along a first path, and enters the polarization beam splitter 311 from a second path after generating a first down-conversion photon in the resonant cavity 32. Similarly, a second pump laser is incident on the resonant cavity 32 along a second path, and enters the polarization beam splitter 311 from a first path after generating a first down-conversion photon in the resonant cavity 32. It can be seen that the various parts of the interference mechanism 31 and the resonant cavity 32 form a Sagnac ring. In the Sagnac ring, the position of the nonlinear optical crystal 33 in the resonant cavity 32 is configured to ensure that the optical path of the first down-converted photon and the second down-converted photon in the resonant cavity 32 is the same, and the length of the first path is the same as the length of the second path, so that the sum of the optical path of the first pump laser and the first down-converted photon generated by the first pump laser is equal to the sum of the optical path of the second pump laser and the second down-converted photon generated by the second pump laser. In the Sagnac ring, since the photons propagating along different paths experience the same optical path, the accumulated phase changes of the photons propagating along different paths in the ring are almost the same. Therefore, the Sagnac ring 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, the 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 reflector module is also suitable for reflecting the second target down-converted photons output by the resonant cavity 32, so that the second target down-converted photons travel along the first path. The second reflector module is also suitable for reflecting the first target down-converted photons output by the resonant cavity 32, so that the first target down-converted photons travel along the second path. The polarization conversion component 314 is also suitable for changing the polarization direction of the first target down-converted photons or changing the polarization direction of the second target down-converted photons, so that the polarization direction of the first target down-converted photons is perpendicular to the polarization direction of the second target down-converted photons. The polarization beam splitter 311 is also suitable for combining the first target down-converted photons and the second target down-converted photons with perpendicular polarization directions and generating quantum coherent superposition to obtain a polarization entangled photon pair, which is a non-degenerate polarization entangled photon pair. The first path and the second path have the same length.

[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, and each beam of pump laser photons undergoes spontaneous parametric down-conversion to obtain two down-converted photons with perpendicular polarization directions. The first reflector module is also suitable for reflecting 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 also suitable for reflecting the first target down-converted photons output by the resonant cavity so that the second target down-converted photons travel along the second path. The polarization beam splitter 311 is suitable for combining the first target down-converted photons and the second target down-converted photons and generating quantum coherent superposition to obtain a polarization entangled photon pair, which 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] like Figure 3~Figure 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 suitable for transmitting the initial laser so as to enter the interference mechanism 31 with the initial laser, and is suitable for reflecting the polarization entangled photon pairs. The third dichroic mirror 35 is suitable for separating polarization entangled photon pairs of different wavelengths to separate polarization entangled photon pairs with 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, the two down-converted photons with the same polarization direction obtained by spontaneous parametric down-conversion of the photons of each pump laser beam can have different wavelengths. Therefore, by separating polarization entangled photon pairs of different wavelengths, the separation of polarization entangled photon pairs with different spatial modes can be achieved.

[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 suitable for separating polarization entangled photon pairs with different spatial modes. Figure 5 As shown, the above-mentioned polarization entangled light source also includes a second dichroic mirror 34, which is suitable for transmitting the initial laser so that the initial laser enters the interference mechanism 31, and is suitable for reflecting the polarization entangled photon pairs of one of the spatial modes 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 the photons of each pump laser undergo spontaneous parametric down-conversion to obtain two down-converted photons with perpendicular polarization directions, the polarization beam splitter 311 can separate polarization entangled photon pairs with different polarization directions, that is, it can separate polarization entangled photon pairs with different spatial modes.

[0081] The polarization entangled light source provided by the embodiment of the present invention combines a resonant cavity and a Sagnac ring structure, and utilizes a nonlinear optical crystal to realize a polarization entangled light source with good stability and capable of generating narrow-band polarization entangled photon pairs.

[0082] The embodiments of the present invention are described above. However, these embodiments are only for the purpose of illustration, and are not intended to limit the scope of the present invention. Although each embodiment is described above, it does not mean that the measures in each embodiment cannot be used in combination. The scope of the present invention is defined by the attached claims and their equivalents. Without departing from the scope of the present invention, those skilled in the art may make various substitutions and modifications, which 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; A nonlinear optical crystal is arranged in the resonant cavity. 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, and 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 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: a first dichroic mirror, used 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; 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, and the nonlinear optical crystal is located between two adjacent resonant cavity reflectors; The first pump laser passes through the nonlinear optical crystal to obtain the first down-conversion photon, the second pump laser passes through the nonlinear optical crystal to obtain the second down-conversion photon, the first down-conversion photon and the second down-conversion photon oscillate in the resonant cavity to obtain the first target down-conversion photon and the second target down-conversion photon, the first target down-conversion photon and the second target down-conversion photon are output from the first dichroic mirror along different directions, and the position of the nonlinear optical crystal in the resonant cavity is configured to ensure that the first down-conversion photon and the second down-conversion photon have the same optical path in the resonant cavity.

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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