A polarization-maintaining quantum frequency conversion device and wavelength conversion method

Abstract

The present application discloses a polarization-maintaining quantum frequency conversion device and a wavelength conversion method, relating to the technical field of optical modulation. The device includes: a polarization beam splitting device, an optical rotation device, a first off-axis parabolic mirror coated with a metal reflection film, a nonlinear crystal, and a second off-axis parabolic mirror coated with a metal reflection film; the polarization beam splitting device is configured to separate a pump light and a signal light into a first light beam and a second light beam; the optical rotation device is configured to flip the polarization direction of the light beam; the first off-axis parabolic mirror is configured to couple the first light beam into the nonlinear crystal, and collimate the second light beam and the output light and then input them into the polarization beam splitting device; the second off-axis parabolic mirror is configured to couple the second light beam into the nonlinear crystal, and collimate the first light beam and the output light and then input them into the polarization beam splitting device; the nonlinear crystal is configured to perform a nonlinear transformation on the first light beam or the second light beam to generate an output light.

Description

Technical Field

[0001] This application relates to the field of optical modulation technology, and particularly to a polarization-maintaining quantum frequency conversion device and a wavelength conversion method. Background Art

[0002] In the field of atomic and molecular optics, in order to conduct research on quantum networks and distributed quantum computing, it is usually necessary to achieve information transfer of quantum bits through the polarization state of photons. However, when different atomic and molecular systems absorb or emit photons, their specific energy level structures determine that they can only interact with photons of specific wavelengths, and the wavelengths of photons required by different atomic and molecular systems may vary greatly. If one wants to connect different atomic and molecular systems with photons of a fixed polarization state, it is necessary to achieve the conversion of the wavelength of photons with unchanged polarization state within a wide wavelength band, that is, to achieve the conversion of the frequency of photons with unchanged polarization state within a wide wavelength band. Summary of the Invention

[0003] Based on the above problems, this application provides a polarization-maintaining quantum frequency conversion device, which can achieve the conversion of the wavelength of photons with unchanged polarization state within a wide wavelength band.

[0004] The embodiments of this application disclose the following technical solutions:

[0005] This application discloses a polarization-maintaining quantum frequency conversion device in a first aspect. The device includes: a polarization beam splitting device, an optical rotation device, a first off-axis parabolic mirror coated with a metal reflection film, a nonlinear crystal, and a second off-axis parabolic mirror coated with a metal reflection film;

[0006] Among them, the polarization beam splitting device, the optical rotation device, the first off-axis parabolic mirror, the nonlinear crystal, and the second off-axis parabolic mirror form an annular structure; the annular structure provides an annular path with opposite propagation directions for the vertically polarized light beam and the horizontally polarized light beam;

[0007] The polarization beam splitting device is used to separate the pump light and the signal light into a first light beam and a second light beam; the first light beam is a vertically polarized light beam, and the second light beam is a horizontally polarized light beam;

[0008] The optical rotation device is used to change the polarization direction of the light beam;

[0009] The first off-axis parabolic mirror is used to couple the first light beam into the nonlinear crystal, and collimate the second light beam and the output light and then input them into the polarization beam splitting device;

[0010] The second off-axis parabolic mirror is used to couple the second light beam into the nonlinear crystal, and collimate the first light beam and the output light and then input them into the polarization beam splitting device;

[0011] The nonlinear crystal is used to perform a nonlinear transformation on the first light beam or the second light beam to generate the output light.

[0012] In an optional implementation manner, the polarization beam splitting device, the optical rotation device, the first off-axis parabolic mirror, the nonlinear crystal, and the second off-axis parabolic mirror form an annular structure, specifically:

[0013] Starting from the polarization beam splitting device, the first off-axis parabolic mirror, the nonlinear crystal, and the second off-axis parabolic mirror are sequentially arranged in the counterclockwise direction;

[0014] The first off-axis parabolic mirror and the second off-axis parabolic mirror are distributed on both sides of the polarization beam splitting device; the nonlinear crystal is placed on the opposite side of the polarization beam splitting device;

[0015] The optical rotation device is placed between the polarization beam splitting device and the first off-axis parabolic mirror, or between the polarization beam splitting device and the second off-axis parabolic mirror.

[0016] In an optional implementation manner, both the first off-axis parabolic mirror and the second off-axis parabolic mirror are off-axis parabolic mirrors with an off-axis angle of 45 degrees.

[0017] In an optional implementation manner, the polarization beam splitting device is any one of a polarization beam splitter PBS, a Glan prism, a Wollaston prism, and a polarization shifter.

[0018] In an optional implementation manner, the optical rotation device is any one of a half-wave plate, a Fresnel rhomb phase retarder with a pi phase delay, and a polarization adjuster; the polarization adjuster is composed of a quarter-wave plate and a target mirror.

[0019] In an optional implementation manner, if both the first off-axis parabolic mirror and the second off-axis parabolic mirror are off-axis parabolic mirrors with an off-axis angle of 90 degrees, the device further includes: a first mirror and a second mirror;

[0020] The polarization beam splitting device, the first mirror, the optical rotation device, the first off-axis parabolic mirror, the nonlinear crystal, the second off-axis parabolic mirror, and the second mirror form the annular structure;

[0021] The first mirror is configured to reflect the first light beam, the second light beam, and the output light;

[0022] The second mirror is configured to reflect the first light beam, the second light beam, and the output light.

[0023] In an alternative implementation, if the optical rotation device is a half-wave plate or a Fresnel rhombic phase retarder with a pi phase delay; the polarization beam splitting device, the first mirror, the optical rotation device, the first off-axis parabolic mirror, the nonlinear crystal, the second off-axis parabolic mirror, and the second mirror form the annular structure, including:

[0024] Starting from the polarization beam splitting device, the first mirror, the first off-axis parabolic mirror, the nonlinear crystal, the second off-axis parabolic mirror, and the second mirror are sequentially arranged in the counterclockwise direction;

[0025] The first off-axis parabolic mirror and the second off-axis parabolic mirror are distributed on both sides of the polarization beam splitting device;

[0026] The first mirror and the second mirror are distributed on both sides of the polarization beam splitting device;

[0027] The optical rotation device is placed between the first mirror and the first off-axis parabolic mirror, or between the second mirror and the second off-axis parabolic mirror;

[0028] The nonlinear crystal is placed on the opposite side of the polarization beam splitting device.

[0029] In an alternative implementation, if the optical rotation device is a polarization adjuster, the polarization adjuster is composed of a quarter-wave plate and a target mirror, and the polarization beam splitting device, the first mirror, the optical rotation device, the first off-axis parabolic mirror, the nonlinear crystal, the second off-axis parabolic mirror, and the second mirror form the annular structure, specifically:

[0030] Starting from the polarization beam splitting device, the first mirror, the first off-axis parabolic mirror, the nonlinear crystal, the second off-axis parabolic mirror, and the second mirror are sequentially arranged in the counterclockwise direction;

[0031] The first off-axis parabolic mirror and the second off-axis parabolic mirror are distributed on both sides of the polarization beam splitting device;

[0032] The first mirror and the second mirror are distributed on both sides of the polarization beam splitting device;

[0033] Both the nonlinear crystal and the polarization adjuster are placed on the opposite side of the polarization beam splitting device, and the polarization adjuster is between the polarization beam splitting device and the nonlinear crystal.

[0034] In an alternative implementation, the device further includes: a first dichroic mirror and a second dichroic mirror; after being processed by the first dichroic mirror and the second dichroic mirror, the pump light and the signal light are input into the polarization beam splitting device;

[0035] The dichroic mirror combination formed by the first dichroic mirror and the second dichroic mirror is further configured to split the pump light, the output light, and the signal light based on wavelength.

[0036] A second aspect of the present application discloses a wavelength conversion method, the method including:

[0037] Determine the wavelength of the pump light based on the wavelength of the signal light and the wavelength of the output light;

[0038] Input the pump light and the signal light into a polarization-maintaining quantum frequency conversion device to generate the output light; the polarization-maintaining quantum frequency conversion device is the device according to any one of the first aspect.

[0039] Compared with the prior art, the present application has the following beneficial effects:

[0040] The polarization-maintaining quantum frequency conversion device in the present application includes two off-axis parabolic mirrors coated with metal reflection films. Since the metal reflection film can achieve high-efficiency reflection of light with wavelengths ranging from ultraviolet 0.2 μm to infrared 20 μm, the off-axis parabolic mirrors coated with metal reflection films can reflect light with multiple wavelengths within a wide wavelength band, and can couple and collimate light with different wavelengths. Therefore, through these two off-axis parabolic mirrors, linearly polarized light with different wavelengths (the first beam or the second beam) can be well coupled into the nonlinear crystal, and the chromatic aberration problem existing when the light beams with different wavelengths are coupled into the nonlinear crystal can be eliminated as much as possible; it can also collimate linearly polarized light with different wavelengths (the first beam, the second beam, and the output light) and input it into the polarization beam splitting device, without significantly reducing the working performance of the polarization beam splitting device. At the same time, the polarization beam splitting device, the optical rotation device, the first off-axis parabolic mirror, the nonlinear crystal, and the second off-axis parabolic mirror in the device form an annular structure, which is equivalent to forming a Sagnac annular optical path, having the advantages of phase self-stabilization and low loss.

[0041] Therefore, the polarization-maintaining quantum frequency conversion device disclosed in the present application fully combines the advantages of the Sagnac annular optical path and the off-axis parabolic mirror coated with metal reflection film, and can achieve wavelength conversion with the polarization state remaining unchanged in the ultraviolet to infrared light range. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0043] Figure 1 FIG. [X] is a schematic structural diagram of a polarization-maintaining quantum frequency conversion device provided by an embodiment of the present application;

[0044] Figure 2 FIG. [Y] is a schematic structural diagram of another polarization-maintaining quantum frequency conversion device provided by an embodiment of the present application;

[0045] Figure 3 FIG. [Z] is a schematic structural diagram of yet another polarization-maintaining quantum frequency conversion device provided by an embodiment of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] In the field of atomic and molecular optics, in order to conduct research on quantum networks and distributed quantum computing, it is usually necessary to use the polarization state of photons to transmit qubit information. However, when different atomic and molecular systems absorb or emit photons, their specific energy level structures determine that they can only interact with photons of specific wavelengths, and the wavelengths of photons required between different atomic and molecular systems may vary greatly. If photons with a fixed polarization state are used to connect different atomic and molecular systems, it is necessary to achieve the conversion of the wavelengths of photons with a constant polarization state within a wide wavelength band.

[0047] Based on the above problems, the present application provides a polarization-maintaining quantum frequency conversion device, which can achieve the conversion of the wavelengths of photons with a constant polarization state within a wide wavelength band.

[0048] In order to enable those skilled in the art to better understand the solutions of the present application, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0049] The components that must be included in the polarization-maintaining quantum frequency conversion device in the present application are: a first dichroic mirror, a second dichroic mirror, a polarization beam splitting device, an optical rotation device, a first off-axis parabolic mirror coated with a metal reflection film, a nonlinear crystal, and a second off-axis parabolic mirror coated with a metal reflection film. Note: The [X], [Y], [Z] in the translation of FIG. [X], FIG. [Y], FIG. [Z] need to be replaced with the actual figure numbers in the original Chinese text. Since the original text does not provide specific figure numbers, they are left as placeholders here.

[0050] Among them, the device for polarization beam splitting is any one of a polarization beam splitter PBS, a Glan prism, a Wollaston prism, and a polarization shifter.

[0051] Among them, the optical rotation device is any one of a half-wave plate, a Fresnel rhomb phase retarder with a pi phase delay, and a polarization adjuster; the polarization adjuster is composed of a quarter-wave plate and a target mirror.

[0052] Among them, the first off-axis parabolic mirror and the second off-axis parabolic mirror can both be off-axis parabolic mirrors with an off-axis angle of 45 degrees; they can also both be off-axis parabolic mirrors with an off-axis angle of 90 degrees; or one can be an off-axis parabolic mirror with an off-axis angle of 45 degrees and the other can be an off-axis parabolic mirror with an off-axis angle of 90 degrees.

[0053] It can be understood that when different types of devices are selected, the structures of the polarization-maintaining quantum frequency conversion devices in this application are not exactly the same. For ease of understanding, below, in combination with specific embodiments, the specific structures of the polarization-maintaining quantum frequency conversion devices, as well as the propagation processes of lights with different wavelengths in the polarization-maintaining quantum frequency conversion devices, will be introduced in detail.

[0054] Embodiment 1:

[0055] In the polarization-maintaining quantum frequency conversion device in Embodiment 1, the device for polarization beam splitting is a polarization beam splitter, the optical rotation device is a half-wave plate, and the first off-axis parabolic mirror and the second off-axis parabolic mirror are both off-axis parabolic mirrors with an off-axis angle of 45 degrees.

[0056] Figure 1 It is a schematic structural diagram of a polarization-maintaining quantum frequency conversion device provided by an embodiment of this application. In combination with Figure 1 as shown, this polarization-maintaining quantum frequency conversion device includes: a first dichroic mirror (dichroic mirror 1), a second dichroic mirror (dichroic mirror 2), a polarization beam splitter, a half-wave plate, a first off-axis parabolic mirror coated with a metal reflective film (off-axis parabolic mirror 1), a nonlinear crystal, ( Figure 1 the periodic poled nonlinear crystal including a waveguide in

[0057] Figure 1 and a second off-axis parabolic mirror coated with a metal reflective film (off-axis parabolic mirror 2).

[0058] Starting from the polarization beam splitter, an off-axis parabolic mirror 1, a nonlinear crystal, and an off-axis parabolic mirror 2 are placed in a counterclockwise direction in sequence. The off-axis parabolic mirror 1 and the off-axis parabolic mirror 2 are distributed on both sides of the polarization beam splitting device; the nonlinear crystal is placed on the opposite side of the polarization beam splitter. The half-wave plate is placed between the polarization beam splitter and the off-axis parabolic mirror 2.

[0059] For Figure 1 the polarization beam splitter, the polarization state of the light beam reflected by the polarization beam splitter is the vertical polarization state, and the polarization state of the light beam transmitted through the polarization beam splitter is the horizontal polarization state. Figure 1 The cutting direction of the periodically poled nonlinear crystal in

[0060] determines that the polarized light coupled to the nonlinear crystal should be a light beam with a vertical polarization state. It should be noted that whether the half-wave plate is placed between the polarization beam splitter and the off-axis parabolic mirror 2 or between the polarization beam splitter and the off-axis parabolic mirror 1 depends on the cutting direction of the periodically poled nonlinear crystal. Those skilled in the art can determine the polarization states of the pump light and the signal light required for the nonlinear transformation of the nonlinear crystal according to the cutting direction of the periodically poled nonlinear crystal, and then determine whether the half-wave plate is placed between the polarization beam splitter and the off-axis parabolic mirror 2 or between the polarization beam splitter and the off-axis parabolic mirror 1.

[0061] The polarization beam splitter is an optical device that can separate or combine light beams with different polarization states. Those skilled in the art can determine whether the polarization state of the light beam transmitted through the polarization beam splitter is the horizontal polarization state or the vertical polarization state by selecting the material and structure of the polarization beam splitter.

[0062] Figure 1 The polarization-maintaining quantum frequency conversion device in

[0063] Combined with Figure 1 , taking the difference frequency conversion as an example, if it is desired to convert the wavelength of a photon from 580 nm to 1550 nm, a pump light with a wavelength of 927 nm is required. Then, the light with a wavelength of 927 nm is used as the pump light, the light with a wavelength of 580 nm is used as the signal light, and the pump light and the signal light are simultaneously input into Figure 1 the device in

[0064] Specifically, the spot sizes of the pump light with a wavelength of 927nm and the signal light with a wavelength of 580nm are adjusted so that the pump light can pass through dichroic mirror 1 and dichroic mirror 2, and be combined with the signal light reflected by dichroic mirror 2 to obtain an initial combined light beam; after the initial combined light beam is incident on the polarization beam splitter, the light beam in the vertical polarization state in the initial combined light beam is reflected to obtain a first light beam, and the light beam in the horizontal polarization state in the initial combined light beam is transmitted to obtain a second light beam.

[0065] The first light beam, i.e., the light beam in the vertical polarization state, is transmitted in the counterclockwise direction to the off-axis parabolic reflector 1, which couples the first light beam into the nonlinear crystal to generate an output light of 1550nm (difference frequency signal light); the output light transmitted in the counterclockwise direction and the first light beam are recorded as the third light beam; the third light beam is transmitted to the off-axis parabolic reflector 2, and is collimated by the off-axis parabolic reflector 2 and then input into the half-wave plate, which converts the polarization state of the third light beam from the vertical polarization state to the horizontal polarization state to obtain the fourth light beam; the fourth light beam is transmitted to the off-axis parabolic reflector 2, and the third light beam is collimated by the off-axis parabolic reflector 2 and then input into the half-wave plate, and the half-wave plate converts the polarization state of the third light beam from the vertical polarization state to the horizontal polarization state to obtain the fourth light beam; It is transmitted to dichroic mirror 2 through the polarization beam splitter. Dichroic mirror 2 reflects the signal light with a wavelength of 580nm in the fourth light beam. The remaining 927nm pump light and 1550nm output light in the fourth light beam are transmitted to dichroic mirror 1 through dichroic mirror 2. The 1550nm output light in the fourth light beam is reflected by dichroic mirror 1, and the 927nm pump light passes through dichroic mirror 1. At this point, the three different wavelengths of pump light (927nm), signal light (580nm) and output light (1550nm) are separated.

[0066] The second light beam, i.e., the light beam in the horizontal polarization state, is transmitted to the half-wave plate in the clockwise direction, and the half-wave plate transforms the polarization state of the second light beam from the horizontal polarization state to the vertical polarization state to obtain the fifth light beam; the fifth light beam is transmitted to the off-axis parabolic reflector 2, and the off-axis parabolic reflector 2 couples the fifth light beam into the nonlinear crystal to generate an output light (difference frequency signal light) of 1550nm; the output light transmitted in the clockwise direction and the fifth light beam are used as the sixth light beam; the sixth light beam is transmitted to the off-axis parabolic reflector 1, and is collimated by the off-axis parabolic reflector 1 and then input into the polarization beam splitter , is reflected by the polarization beam splitter and input into the dichroic mirror 2. The dichroic mirror 2 reflects the signal light with a wavelength of 580nm in the sixth light beam. The remaining 927nm pump light and 1550nm output light in the sixth light beam are transmitted to the dichroic mirror 1 through the dichroic mirror 2. The 1550nm output light in the sixth light beam is reflected by the dichroic mirror 1, and the pump light with a wavelength of 927nm is transmitted through the dichroic mirror 1. At this point, the three different wavelengths of pump light (927nm), signal light (580nm) and output light (1550nm) are separated.

[0067] In order to couple the 580-nm signal light and the 927-nm pump light into the nonlinear crystal simultaneously, it is necessary to adjust the spot size before entering the polarization beam splitter and maintain parallel light or nearly parallel light. The key to waveguide coupling lies in the matching between the mode field of the focused spot by the off-axis parabolic mirror and the mode field propagating in the waveguide in the nonlinear crystal. The mode field of the focused spot can be adjusted by regulating the spot size incident on the off-axis parabolic mirror.

[0068] It should be emphasized that Figure 1 the polarization beam splitter, off-axis parabolic mirror 1, nonlinear crystal, off-axis parabolic mirror 2, and half-wave plate in Figure 1 form an annular structure, similar to the optical path of the Sagnac loop. Since the Sagnac loop optical path itself has the advantages of phase self-stabilization and low loss, the

[0069] Embodiment 2:

[0070] In the polarization-maintaining quantum frequency conversion device of Embodiment 2, the polarization beam splitting device is a polarization beam splitter, the optical rotation device is a half-wave plate, and both the first off-axis parabolic mirror and the second off-axis parabolic mirror are off-axis parabolic mirrors with a 90-degree off-axis angle. When both the first off-axis parabolic mirror and the second off-axis parabolic mirror are off-axis parabolic mirrors with a 90-degree off-axis angle, Embodiment 2 further includes a first mirror and a second mirror.

[0071] Figure 2 It is a schematic structural diagram of another polarization-maintaining quantum frequency conversion device provided by an embodiment of the present application. Combining Figure 2 as shown, the polarization-maintaining quantum frequency conversion device includes: a first dichroic mirror (dichroic mirror 1), a second dichroic mirror (dichroic mirror 2), a polarization beam splitter, a first mirror (mirror 1), a first off-axis parabolic mirror with a metal reflective film (off-axis parabolic mirror 1), a nonlinear crystal ( Figure 2 the periodically poled nonlinear crystal including a waveguide in

[0072] Figure 2 ), a second off-axis parabolic mirror with a metal reflective film (off-axis parabolic mirror 2), a half-wave plate, and a second mirror (mirror 2).

[0073] The relative position relationship of the devices in the device shown in

[0074] The off-axis parabolic mirror 1 and the off-axis parabolic mirror 2 are distributed on both sides of the polarization beam splitter; the mirror 1 and the mirror 2 are distributed on both sides of the polarization beam splitter. The half-wave plate is placed between the polarization beam splitter and the off-axis parabolic mirror 2.

[0075] It should be noted that Figure 2 The off-axis parabolic mirror 1 and the off-axis parabolic mirror 2 used in [[ ]] are off-axis parabolic mirrors with an off-axis angle of 90 degrees, so two plane mirrors (mirror 1 and mirror 2) are required to change the propagation directions of the first beam, the second beam, and the output beam. The first mirror is used to reflect the first beam, the second beam, and the output light; the second mirror is used to reflect the first beam, the second beam, and the output light.

[0076] For Figure 2 the polarization beam splitter in [[ ]], the polarization state of the beam reflected by the polarization beam splitter is a vertical polarization state, and the polarization state of the beam transmitted through the polarization beam splitter is a horizontal polarization state. Figure 2 The cutting direction of the periodically poled nonlinear crystal in [[ ]] determines that the polarized light coupled to the nonlinear crystal should be a beam with a vertical polarization state.

[0077] It should be noted that whether the half-wave plate is placed between the mirror 2 and the off-axis parabolic mirror 2 or between the mirror 1 and the off-axis parabolic mirror 1 depends on the cutting direction of the periodically poled nonlinear crystal. Those skilled in the art can determine the polarization states of the pump light and the signal light required for the nonlinear transformation of the nonlinear crystal according to the cutting direction of the periodically poled nonlinear crystal, and then determine the position where the half-wave plate is placed.

[0078] Figure 2 The polarization-maintaining quantum frequency conversion device in [[ ]] can generate the photons required for the interconnection of the ion trap node and the solid-state storage node in the quantum network. Next, taking the process of converting the 370-nm photon spontaneously emitted by the ytterbium ion with a mass number of 171 to the 580-nm wavelength required for solid-state storage as an example to illustrate this embodiment. That is Figure 2 The wavelength of the signal light in [[ ]] is 370 nm, and the wavelength of the output light is 580 nm. The spin state of the ytterbium ion and the polarization state of the 370-nm photon spontaneously emitted are in an entangled state, so it is necessary to keep the polarization state unchanged or only undergo a unitary transformation during the frequency conversion process.

[0079] Combined with Figure 2 , taking the difference frequency conversion as an example, if you want to convert the light with a wavelength of 370 nm to the light with a wavelength of 580 nm, the wavelength of the required pump light is 1018 nm. Taking the light with a wavelength of 370 nm as the signal light and the light with a wavelength of 1018 nm as the pump light, input the pump light and the signal light into Figure 2The device in can convert the output light with a wavelength of 580nm.

[0080] Specifically, the spot sizes of the pump light with a wavelength of 1018nm and the signal light with a wavelength of 370nm are adjusted so that the pump light can pass through the dichroic mirror 1 and the dichroic mirror 2, and be combined with the signal light reflected by the dichroic mirror 2 to obtain an initial combined light beam; after the initial combined light beam is incident on the polarization beam splitter, the light beam with a vertical polarization state in the initial combined light beam is reflected to obtain a first light beam, and the light beam with a horizontal polarization state in the initial combined light beam is transmitted to obtain a second light beam.

[0081] The first light beam, i.e., the light beam in the vertical polarization state, is transmitted to the reflector 1 in the counterclockwise direction. The reflector 1 reflects the first light beam to the off-axis parabolic reflector 1. The off-axis parabolic reflector 1 couples the first light beam to the nonlinear crystal to generate an output light of 580nm (difference frequency signal light). The output light of 580nm transmitted in the counterclockwise direction and the first light beam are recorded as the third light beam. The third light beam is transmitted to the off-axis parabolic reflector 2, and is collimated by the off-axis parabolic reflector 2 and then input to the half-wave plate. The half-wave plate converts the polarization state of the third light beam from the vertical polarization state to the horizontal polarization state to obtain the fourth light beam. The fourth light beam is transmitted to the reflector Mirror 2, after being reflected by reflector 2, the fourth light beam is transmitted to polarization beam splitter, the fourth light beam is transmitted to dichroic mirror 2 through polarization beam splitter, dichroic mirror 2 reflects the signal light with a wavelength of 370nm in the fourth light beam, the remaining 1018nm pump light and 580nm output light in the fourth light beam are transmitted to dichroic mirror 1 through dichroic mirror 2, the 580nm output light in the fourth light beam is reflected by dichroic mirror 1, and the 1018nm pump light is transmitted through dichroic mirror 1. At this point, the three different wavelengths of pump light (1018nm), signal light (370nm) and output light (580nm) are separated.

[0082] The second beam, i.e., the beam in the horizontal polarization state, is transmitted clockwise to the mirror 2. The mirror 2 reflects the second beam to the half-wave plate. The half-wave plate transforms the polarization state of the second beam from the horizontal polarization state to the vertical polarization state, obtaining the fifth beam. The fifth beam is transmitted to the off-axis parabolic mirror 2. The off-axis parabolic mirror 2 couples the fifth beam into the nonlinear crystal, generating an output light (difference-frequency signal light) of 580 nm. The 580-nm output light transmitted along the clockwise direction and the fifth beam are used as the sixth beam. The sixth beam is transmitted to the off-axis parabolic mirror 1, collimated by the off-axis parabolic mirror 1 and then transmitted to the mirror 1. The mirror 1 reflects the sixth beam to the polarization beam splitter. After being reflected by the polarization beam splitter, the sixth beam is transmitted to the dichroic mirror 2. The dichroic mirror 2 reflects the signal light with a wavelength of 370 nm in the sixth beam. The remaining 1018-nm pump light and 580-nm output light in the sixth beam pass through the dichroic mirror 2 and are transmitted to the dichroic mirror 1. The 580-nm output light in the sixth beam is reflected by the dichroic mirror 1, and the 1018-nm pump light passes through the dichroic mirror 1. Thus, the pump light (1018 nm), the signal light (370 nm), and the output light (580 nm) with three different wavelengths are separated.

[0083] In order to simultaneously couple the 580-nm signal light and the 1018-nm pump light into the nonlinear crystal, it is necessary to adjust the spot size before entering the polarization beam splitter and maintain parallel light or near-parallel light.

[0084] It should be emphasized that Figure 2 The polarization beam splitter, the mirror 1, the off-axis parabolic mirror 1, the nonlinear crystal, the off-axis parabolic mirror 2, the half-wave plate, and the mirror 2 in Figure 2 form an annular structure, similar to the optical path of the Sagnac loop. Since the Sagnac loop optical path itself has the advantages of phase self-stabilization and low loss, the

[0085] Optical path of

[0086] also has the advantages of phase self-stabilization and low loss.

[0087] Figure 3 is a schematic structural diagram of another polarization-maintaining quantum frequency conversion device provided by the embodiments of the present application. CombiningFigure 3 As shown in Figure 3 , the polarization-maintaining quantum frequency conversion device includes: a first dichroic mirror (dichroic mirror 1), a second dichroic mirror (dichroic mirror 2), a polarization beam splitter, a first mirror (mirror 1), a first off-axis parabolic mirror (off-axis parabolic mirror 1) coated with a metal reflective film, a nonlinear crystal ( Figure 3 the periodic poled nonlinear crystal including a waveguide in Figure 3 ), a second off-axis parabolic mirror (off-axis parabolic mirror 2) coated with a metal reflective film, a second mirror, and a polarization adjuster (composed of a quarter-wave plate and mirror 3); mirror 3 is the target mirror in this application.

[0088] Figure 3 In the device shown in Figure 3 , the relative positional relationship of each component is as follows: along the transmission direction of the pump light incident on the device, the dichroic mirror 1, the dichroic mirror 2, and the polarization beam splitter are sequentially placed.

[0089] Starting from the polarization beam splitter, mirror 1, off-axis parabolic mirror 1, nonlinear crystal, off-axis parabolic mirror 2, and mirror 2 are sequentially placed in the counterclockwise direction.

[0090] Off-axis parabolic mirror 1 and off-axis parabolic mirror 2 are distributed on both sides of the polarization beam splitter; mirror 1 and mirror 2 are distributed on both sides of the polarization beam splitter.

[0091] Both the nonlinear crystal and the polarization adjuster are placed on the opposite side of the polarization beam splitter, and the polarization adjuster is between the polarization beam splitter and the nonlinear crystal.

[0092] It should be noted that Figure 3 the off-axis parabolic mirror 1 and off-axis parabolic mirror 2 used in Figure 3 are off-axis parabolic mirrors with an off-axis angle of 90 degrees, so two plane mirrors (mirror 1 and mirror 2) are required to change the propagation directions of the first beam, the second beam, and the output beam. The first mirror is used to reflect the first beam, the second beam, and the output light; the second mirror is used to reflect the first beam, the second beam, and the output light.

[0093] For Figure 3 the polarization beam splitter in Figure 3 , the polarization state of the beam reflected by the polarization beam splitter is a vertical polarization state, and the polarization state of the beam transmitted through the polarization beam splitter is a horizontal polarization state. Figure 3 The cutting direction of the periodic poled nonlinear crystal in Figure 3 determines that the polarized light coupled to the nonlinear crystal should be a beam with a vertical polarization state.

[0094] Figure 3The polarization-maintaining quantum deviation conversion device in it can generate photons required for the interconnection between the ion trap node and any communication band node in the quantum network. Next, the process of converting photons with a wavelength of 370 nm spontaneously emitted by ytterbium ions with a mass number of 171 to a wavelength of 1550 nm, which is the required wavelength for the communication wavelength, will be used to illustrate this embodiment. That is Figure 3 The wavelength of the signal light in it is 370 nm, and the wavelength of the output light is 1550 nm. The spin state of the ytterbium ions and the polarization state of the 370-nm photons spontaneously emitted are in an entangled state. Therefore, it is necessary to keep the polarization state unchanged or only undergo a unitary transformation during the frequency conversion process.

[0095] Combined with Figure 3 , taking difference frequency conversion as an example, if you want to convert light with a wavelength of 370 nm to light with a wavelength of 1550 nm, the wavelength of the pump light required is 485 nm. That is, the light with a wavelength of 370 nm is used as the signal light, the light with a wavelength of 485 nm is used as the pump light, and the pump light and the signal light are simultaneously input into Figure 3 the device in it, and the output light with a wavelength of 1550 nm can be obtained through conversion.

[0096] Specifically, the spot sizes of the pump light with a wavelength of 485 nm and the signal light with a wavelength of 370 nm are adjusted so that the pump light can pass through dichroic mirror 1 and dichroic mirror 2, and be combined with the signal light reflected by dichroic mirror 2 to obtain an initial combined beam; after the initial combined beam is incident on the polarization beam splitter, the vertically polarized beam in the initial combined beam is reflected to obtain a first beam, and the horizontally polarized beam in the initial combined beam is transmitted to obtain a second beam.

[0097] The first light beam, i.e., the light beam in the vertical polarization state, is transmitted counterclockwise to mirror 1. Mirror 1 reflects the first light beam to off-axis parabolic mirror 1. Off-axis parabolic mirror 1 couples the first light beam into a nonlinear crystal to generate output light (difference-frequency signal light) at 1550 nm. The 1550-nm output light and the first light beam that are transmitted counterclockwise are denoted as the third light beam. The third light beam is transmitted to off-axis parabolic mirror 2, collimated by off-axis parabolic mirror 2 and then transmitted to mirror 2. Mirror 2 reflects the third light beam to a polarization beam splitter. The polarization beam splitter reflects the third light beam to a polarization adjuster. Specifically, the polarization beam splitter reflects the third light beam to a quarter-wave plate in the polarization adjuster. The light beam passes through the quarter-wave plate and is then transmitted to mirror 3, reflected by mirror 3 and then transmitted to the quarter-wave plate again, and passes through the quarter-wave plate again. In this way, the polarization adjuster converts the polarization state of the third light beam from the vertical polarization state to the horizontal polarization state to obtain the fourth light beam. The fourth light beam is transmitted through the polarization beam splitter to dichroic mirror 2. Dichroic mirror 2 reflects the signal light with a wavelength of 370 nm in the fourth light beam. The remaining pump light at 485 nm and the 1550-nm output light in the fourth light beam pass through dichroic mirror 2 and are transmitted to dichroic mirror 1. The 1550-nm output light in the fourth light beam is reflected by dichroic mirror 1, and the 485-nm pump light passes through dichroic mirror 1. Thus, the pump light (485 nm), the signal light (370 nm), and the output light (1550 nm) with three different wavelengths are separated.

[0098] The second beam, i.e., the beam with horizontal polarization state, is transmitted clockwise to the polarization adjuster. After passing through the quarter-wave plate in the polarization adjuster, it is transmitted to mirror 3, and then reflected by mirror 3. Then it passes through the quarter-wave plate again, which is equivalent to passing through a half-wave plate. That is, after the light of the second beam passes through the polarization adjuster, the polarization state is converted from the horizontal polarization state to the vertical polarization state, obtaining the fifth beam; the fifth beam is reflected by the polarization beam splitter to mirror 2, and mirror 2 reflects the fifth beam to off-axis parabolic mirror 2. Off-axis parabolic mirror 2 couples the fifth beam into the nonlinear crystal to generate the output light at 1550 nm (difference-frequency signal light); the output light at 1550 nm transmitted clockwise and the fifth beam are used as the sixth beam; the sixth beam is transmitted to off-axis parabolic mirror 1, collimated by off-axis parabolic mirror 1 and then transmitted to mirror 1. Mirror 1 reflects the sixth beam to the polarization beam splitter, and the sixth beam is reflected by the polarization beam splitter and input to dichroic mirror 2. Dichroic mirror 2 reflects the signal light with a wavelength of 370 nm in the sixth beam. The remaining pump light at 485 nm and the output light at 1550 nm in the sixth beam pass through dichroic mirror 2 and are transmitted to dichroic mirror 1. The output light at 1550 nm in the sixth beam is reflected by dichroic mirror 1, and the pump light at 485 nm passes through dichroic mirror 1. Thus, the pump light (485 nm), the signal light (370 nm), and the output light (1550 nm) with three different wavelengths are separated.

[0099] It should be emphasized that Figure 3 the polarization beam splitter, mirror 1, off-axis parabolic mirror 1, nonlinear crystal, off-axis parabolic mirror 2, polarization adjuster, and mirror 2 in Figure 3 constitute an annular structure, similar to the optical path of the Sagnac loop. Since the Sagnac loop optical path itself has the advantages of phase self-stabilization and low loss, therefore, the

[0100] optical path in this application also has the advantages of phase self-stabilization and low loss.

[0101] As can be seen from the introduction of the foregoing embodiments, the wavelengths of the pump light, the signal light, and the output light differ greatly. When the light beams of each wavelength are transmitted in the polarization-maintaining quantum frequency conversion device, it involves the process of simultaneously coupling the signal light and the pump light into the nonlinear crystal, and the process of coupling the output light out of the nonlinear crystal. In the above coupling process, if an objective lens or an aspherical mirror is used for coupling, a serious chromatic aberration effect will be generated, reducing the coupling effect. Especially when ultraviolet light and infrared light are involved, the refractive index of optical glass rises sharply in the ultraviolet band, and the chromatic aberration problem is serious and it is difficult to use.

[0102] To solve the chromatic aberration problem, an off-axis parabolic mirror is used for coupling in this application, and a metal film is deposited on the surface of the off-axis parabolic mirror. The metal film can reflect light with a wide wavelength range from ultraviolet 0.2 μm to infrared 20 μm; the off-axis parabolic mirror converges light through reflection, and there is no need to consider the problem of chromatic aberration when using a lens. Therefore, the optical path in this application realizes polarization-maintaining quantum frequency conversion in a wide wavelength band through a simple optical path design.

[0103] It can be understood that the positions of the two dichroic mirrors in each of the foregoing embodiments can be swapped, as long as the pump light, the signal light, and the output light of three different wavelengths are separated. Although the function of the first dichroic mirror given in each of the foregoing embodiments is to transmit the pump light and reflect the output light, and the function of the second dichroic mirror is to transmit the pump light, transmit the output light, and reflect the signal light. However, in some alternative implementation manners, it is also possible to make the first dichroic mirror transmit the pump light and reflect the signal light, and the second dichroic mirror transmit the pump light and the signal light and reflect the output light, that is, by depositing film layers with different parameters on the two dichroic mirrors, the beam splitting of the pump light, the signal light, and the output light of three different wavelengths is realized. That is, in this application, the specific functions of each dichroic mirror in the dichroic mirror combination composed of the first dichroic mirror and the second dichroic mirror are not limited, and the specific parameters of the reflection film and the transmission film deposited on each dichroic mirror are not limited. What is limited in this application is that the dichroic mirror combination composed of the first dichroic mirror and the second dichroic mirror can realize spectral beam splitting of the pump light, the output light, and the signal light based on the wavelength.

[0104] It can be understood that the foregoing nonlinear transformation method in each embodiment is not limited to difference frequency. After determining the wavelength of the signal light, the wavelength of the output light, and the nonlinear crystal, the wavelength of the pump light can also be determined by sum frequency or frequency doubling.

[0105] It can be understood that through the polarization-maintaining quantum frequency conversion device introduced in the foregoing embodiments, those skilled in the art can obtain output light of more wavelengths. In the specific use process, as long as the appropriate parameters of each device in the polarization-maintaining quantum frequency conversion device are selected based on the wavelength of the signal light, the wavelength of the output light, and the wavelength of the pump light, this application will not elaborate on this content.

[0106] It is understandable that the polarization beam splitting device used in each of the foregoing embodiments is a three-wavelength polarization beam splitting device, which can separate or combine light beams with three different wavelengths and different polarization states.

[0107] Based on the polarization-maintaining quantum frequency conversion device disclosed in the foregoing embodiments, the present application also discloses a wavelength conversion method. The wavelength conversion method disclosed in the present application is implemented based on the polarization-maintaining quantum frequency conversion device disclosed in the foregoing embodiments. The wavelength conversion method includes:

[0108] The first step: Based on the wavelength of the signal light and the wavelength of the output light, determine the wavelength of the pump light; then based on the wavelength of the signal light, the wavelength of the output light, and the wavelength of the pump light, determine the nonlinear crystal. This process is well-known to those skilled in the art and will not be elaborated here.

[0109] The second step is to design the size of the input light spot and select the focal length of the off-axis parabolic mirror so that the light spot focused by the off-axis parabolic mirror matches the waveguide mode, thereby achieving the purpose of coupling.

[0110] The third step is to input the pump light and the signal light into the polarization-maintaining quantum frequency conversion device to generate the output light.

[0111] Based on the polarization-maintaining quantum frequency conversion device and the wavelength conversion method provided in the foregoing embodiments, correspondingly, the present application also provides a computer-readable storage medium, on which a computer program is stored. When the program is executed by a processor, it implements some or all of the steps in the wavelength conversion method mentioned above.

[0112] Based on the polarization-maintaining quantum frequency conversion device and the wavelength conversion method provided in the foregoing embodiments, the present application also provides an electronic device, including:

[0113] A memory, on which a computer program is stored;

[0114] A processor, configured to execute the computer program in the memory to implement some or all of the steps in the wavelength conversion method provided in the foregoing embodiments.

[0115] It should be noted that the various embodiments in this specification are described in a progressive manner. For the same or similar parts among the embodiments, reference can be made to each other, and the key point of each embodiment is to illustrate the differences from other embodiments. In particular, for method embodiments, since they are basically similar to method embodiments, they are described relatively simply, and for the relevant parts, reference can be made to the partial description of method embodiments. The method embodiments described above are only illustrative. The units described as separated components may or may not be physically separated, and the components indicated as units may or may not be physical units, that is, they may be located in one place or distributed to multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of this embodiment. Those of ordinary skill in the art can understand and implement it without creative efforts.

[0116] As described above, this is only a specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the present application should be covered by the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A polarization-maintaining quantum frequency conversion device, characterized in that, The device is used to generate photons required for a quantum network; the device includes: a polarization beam splitting device, an optical rotation device, a first off-axis parabolic mirror coated with a metal reflective film, a periodically poled nonlinear crystal containing a waveguide, a second off-axis parabolic mirror coated with a metal reflective film, a first mirror and a second mirror; both the first off-axis parabolic mirror and the second off-axis parabolic mirror are off-axis parabolic mirrors with an off-axis angle of 90 degrees. Wherein, the polarization beam splitting device, the first mirror, the optical rotation device, the first off-axis parabolic mirror, the periodically poled nonlinear crystal containing a waveguide, the second off-axis parabolic mirror and the second mirror form an annular structure; the annular structure provides an annular path with opposite propagation directions for the vertically polarized light beam and the horizontally polarized light beam. The polarization beam splitting device is used to separate the pump light and the signal light into a first light beam and a second light beam; the first light beam is a vertically polarized light beam, and the second light beam is a horizontally polarized light beam. The optical rotation device is used to change the polarization direction of the light beam. The first off-axis parabolic mirror is used to couple the first light beam into the periodically poled nonlinear crystal containing a waveguide, and collimate the second light beam and the output light and then input them into the polarization beam splitting device. The second off-axis parabolic mirror is used to couple the second light beam into the periodically poled nonlinear crystal containing a waveguide, and collimate the first light beam and the output light and then input them into the polarization beam splitting device. The periodically poled nonlinear crystal containing a waveguide is used to perform a nonlinear transformation on the first light beam or the second light beam to generate the output light. The first mirror is used to reflect the first light beam, the second light beam and the output light. The second mirror is used to reflect the first light beam, the second light beam and the output light. The mode field of the focused spot of the off-axis parabolic mirror matches the mode field propagating in the waveguide in the nonlinear crystal, and the mode field of the focused spot is adjusted by adjusting the size of the spot incident on the off-axis parabolic mirror; the off-axis parabolic mirror includes the first off-axis parabolic mirror and the second off-axis parabolic mirror.

2. The device according to claim 1, wherein The polarization beam splitting device is any one of a polarization beam splitter PBS, a Glan prism, a Wollaston prism and a polarization displacer.

3. The device according to claim 1, characterized in that, The optical rotation device is any one of a half-wave plate, a Fresnel rhomb phase retarder with a pi phase delay and a polarization adjuster; the polarization adjuster is composed of a quarter-wave plate and a target mirror.

4. The device according to claim 1, wherein If the optical rotation device is a half-wave plate or a Fresnel rhomb phase retarder with a pi phase delay; the polarization beam splitting device, the first mirror, the optical rotation device, the first off-axis parabolic mirror, the periodically poled nonlinear crystal containing a waveguide, the second off-axis parabolic mirror and the second mirror form the annular structure, including: Starting from the polarization beam splitting device, the first mirror, the first off-axis parabolic mirror, the periodic poled nonlinear crystal including a waveguide, the second off-axis parabolic mirror, and the second mirror are sequentially arranged in the counterclockwise direction; The first off-axis parabolic mirror and the second off-axis parabolic mirror are distributed on both sides of the polarization beam splitting device; The first mirror and the second mirror are distributed on both sides of the polarization beam splitting device; The optical rotation device is placed between the first mirror and the first off-axis parabolic mirror, or between the second mirror and the second off-axis parabolic mirror; The periodic poled nonlinear crystal including a waveguide is placed on the opposite side of the polarization beam splitting device.

5. The device according to claim 1, characterized in that, If the optical rotation device is a polarization regulator, the polarization regulator is composed of a quarter-wave plate and a target mirror, and the polarization beam splitting device, the first mirror, the optical rotation device, the first off-axis parabolic mirror, the periodic poled nonlinear crystal including a waveguide, the second off-axis parabolic mirror, and the second mirror form the ring structure. Specifically: Starting from the polarization beam splitting device, the first mirror, the first off-axis parabolic mirror, the periodic poled nonlinear crystal including a waveguide, the second off-axis parabolic mirror, and the second mirror are sequentially arranged in the counterclockwise direction; The first off-axis parabolic mirror and the second off-axis parabolic mirror are distributed on both sides of the polarization beam splitting device; The first mirror and the second mirror are distributed on both sides of the polarization beam splitting device; The periodic poled nonlinear crystal including a waveguide and the polarization regulator are both placed on the opposite side of the polarization beam splitting device, and the polarization regulator is between the polarization beam splitting device and the periodic poled nonlinear crystal including a waveguide.

6. The device according to any one of claims 1-5, characterized in that, The device further includes: a first dichroic mirror and a second dichroic mirror; the pump light and the signal light are input to the polarization beam splitting device after being processed by the first dichroic mirror and the second dichroic mirror; The dichroic mirror combination formed by the first dichroic mirror and the second dichroic mirror is used to split the pump light, the output light, and the signal light based on the wavelength.

7. A wavelength conversion method, characterized in that, The method includes: Determining the wavelength of the pump light based on the wavelength of the signal light and the wavelength of the output light; Inputting the pump light and the signal light into a polarization-maintaining quantum frequency conversion device to generate the output light; the polarization-maintaining quantum frequency conversion device is the device according to any one of claims 1-6.

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