CN114578628A - Down-conversion optical resonant cavity for generating narrow linewidth photon pair - Google Patents
Down-conversion optical resonant cavity for generating narrow linewidth photon pair Download PDFInfo
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- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
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- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
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Abstract
The invention belongs to the field of quantum information science, and particularly relates to a down-conversion optical resonant cavity for generating narrow-linewidth photon pairs. The two concave mirrors are used for generating proper cavity mode cross sections, the generation efficiency of down-conversion photons is increased, different optical cavity longitudinal modes are used as the cavity length of the locking optical locking resonant cavity, the frequency of the two concave mirrors is far away from a down-conversion optical field, and the 100% working period can be realized without introducing noise.
Description
Technical Field
The invention belongs to the field of quantum information science, and particularly relates to a down-conversion optical resonant cavity for generating a narrow-linewidth photon pair.
Background
The quantum network has wide application prospect in the aspects of quantum communication, quantum computation and the like. It consists of a large number of nodes and channels. In general, a node is a material bit with a long coherence time, such as an atom, an ion, or the like, and has the capability of receiving, sending, processing, and storing quantum information. In addition, a single photon forms a channel, which is a good information carrier, and quantum information can be encoded in polarization, angular momentum and the like for transmission. Researches on the interaction between single photons and matter particles, such as quantum logic control, quantum storage and the like, have an indispensable significance for the final realization of quantum networks.
Based on the development background, a single photon source has wide application and great potential as a typical important quantum light source. Typical methods by which a single photon source can be generated are: excitation of single quantum systems based on microstructures, such as single atoms, single molecules, single ions, single quantum dots, NV colour centers, etc.; a four-wave mixing process based on an atomic ensemble; spontaneous parametric down-conversion (SPDC). If a single photon source needs to efficiently interact with quantum nodes such as atomic ions, the frequency of the single photon source needs to be perfectly matched with the quantum nodes and the line width needs to be close to MHz magnitude. The line width of photons generated by cavity-enhanced spontaneous parametric down-conversion is 0.64 times of the line width of the cavity, and the method is an important technology for generating a narrow-band single photon source. It has the characteristics of controllable line width and frequency and relatively simple system. In addition, SPDC produces a pair of simultaneous, frequency-dependent photons. By detecting one of the photons (the idle photon), the presence of the other photon (the signal photon) can be demonstrated for some prognostic experiments.
Cavity enhanced spontaneous parametric down-conversion utilizes the second-order nonlinear polarizability of a nonlinear crystal to convert photons at frequency 2 omega to photon pairs at frequency omega. When the type of nonlinear crystal used is quasi-phase-matched of two types, the polarizations of the resulting twin photon pairs are orthogonal to each other. Pairs of photons with mutually orthogonal polarizations cause phase retardation due to the difference in refractive index in the crystal. The photon pair affects the dual resonance condition of the cavity when it is cycled many times in the cavity. Compensation for birefringence is thus required to achieve dual resonance of vertically polarized photon pairs in the cavity.
The compensation scheme for birefringence is generally to compensate for the retardation introduced by birefringence. The common methods are as follows: adding a periodically poled compensating crystal with its optical axis perpendicular to that of the periodically poled crystal, the compensating crystal having a refractive index ne→noAnd no→neThe interchange produces an opposite delay to compensate, but the compensating crystal introduces intracavity loss, and related documents such as 2009 m.scholz produce 852nm photon pairs with mutually perpendicular polarizations and a line width of 3MHz using KTP as the compensating crystal; other modes can also be compensated by using cluster effect (cluster effect) caused by birefringence in the cavity, for example, the p. -j.tsai in 2018 generates 852nm photons with 6.6MHz line width and the like by using cluster effect for compensation.
Disclosure of Invention
The invention utilizes a cavity structure of a Z-shaped resonant cavity, which comprises two Quarter Wave Plates (QWP) with optical axes at 45 degrees with the optical axis of the crystal, and performs double refraction compensation to further realize double resonance of photon pairs with mutually vertical polarization, and simultaneously uses different longitudinal modes of optical cavities as the cavity length of a locking optical locking resonant cavity, and the frequency of the cavity structure is far away from a down-conversion optical field.
In order to achieve the purpose, the invention adopts the following technical scheme:
a down-conversion optical resonant cavity for generating narrow-linewidth photon pairs comprises a Z-shaped resonant cavity part and a locking part, wherein the Z-shaped resonant cavity part comprises pump light, a first concave mirror, a nonlinear crystal, a first quarter-wave plate, a second concave mirror, a first plane mirror, a second quarter-wave plate, a second plane mirror and a first detector, and the locking part comprises a first high-reflection mirror, a second high-reflection mirror, a locking light source, a third high-reflection mirror, a fourth high-reflection mirror, a first polarizing beam splitter prism, a third quarter-wave plate, a fourth quarter-wave plate, a fifth high-reflection mirror, a second detector, a subtracter, a second polarizing beam splitter prism, a third detector and a long-pass filter;
the laser emitted by the pump is used as a pump light source for parametric down-conversion of cavity enhancement of the Z-shaped resonant cavity part, the laser passes through the first concave mirror, the nonlinear crystal, the first quarter-wave plate and the second concave mirror once and then exits the cavity, the laser passes through the nonlinear crystal, a two-photon pair generated by spontaneous parametric down-conversion passes through the first quarter-wave plate, the nonlinear crystal and the first concave mirror after passing through the nonlinear crystal, the two-photon pair reflected by the first concave mirror passes through the first plane mirror, the second quarter-wave plate and the second flat mirror after being reflected by the second concave mirror, then passes through the second quarter-wave plate, the first flat mirror, the first concave mirror and the nonlinear crystal in turn again after reaching the first quarter-wave plate, a cycle in the resonant cavity is completed, a narrow-band photon pair is output by the second flat mirror, the first concave mirror and the first flat mirror convert the light field under the small-angle reflection, the optical cavity is folded into a Z shape, the space size of the optical cavity is reduced, an observation window of an optical field is provided at the same time, and the first detector is placed behind the first plane mirror and used for recording the transmission peak of the resonant cavity;
the locking light source reaches the first polarization beam splitter prism after being reflected by the third high-reflection mirror and the fourth high-reflection mirror, the light transmitted by the first polarization beam splitter prism passes through the third quarter wave plate, then is incident on the second plane mirror through the long-pass filter, the second high-reflection mirror and the first high-reflection mirror, part of the locking light reflected by the second plane mirror enters the first polarization beam splitter prism through the first high-reflection mirror, the second high-reflection mirror, the long-pass filter and the third quarter wave plate again, then enters the second polarization beam splitter prism through the fourth quarter wave plate after being reflected, the second polarization beam splitter prism is divided into two beams of light to be emitted, one beam of transmitted light enters a third detector, the other beam of reflected light enters a second detector through the reflection of a fifth high reflecting mirror, and two paths of electric signals of the third detector and the second detector enter a subtracter to be subtracted to obtain an error signal and lock a resonant cavity;
and the narrow-band photon pair output by the second plane mirror is reflected by the first high-reflection mirror and the second high-reflection mirror, filtered by the long-pass filter and then transmitted and guided out.
Furthermore, the Z-shaped resonant cavity part also comprises piezoelectric ceramics, the piezoelectric ceramics are adhered to the second concave mirror, and the length of the resonant cavity is changed by applying sawtooth-shaped scanning voltage.
Further, the first quarter-wave plate and the second quarter-wave plate have an action wavelength of 852nm, and the placement angle of the optical axis of the first quarter-wave plate and the optical axis of the second quarter-wave plate is 45 degrees with respect to the optical axis of the crystal.
Furthermore, the nonlinear crystal is PPKTP with quasi-phase matching of two types, and both ends are plated with antireflection films of pumping light and down-conversion photon pairs at two wave bands, so that horizontally polarized pumping light can be polarized to generate photon pairs with mutually vertical polarization which are respectively horizontal H and vertical V.
Furthermore, the curvature radius of the first concave mirror and the second concave mirror is 100mm, the reflectivity is more than 99.99% in a 852nm wave band, and is less than 1% in a 426nm wave band; the reflectivity of the first plane mirror is more than 99.99% at a 852nm wave band and less than 1% at a 426nm wave band; the second plane mirror is used as an output mirror, the reflectivity is 98% at a 852nm wave band, and the reflectivity is less than 1% at a 426nm wave band.
Further, the wavelength of the locking light source is 840nm away from the down-converted light field.
Further, the long-pass filter is placed at an angle so as to have the characteristics of 852nm high transmission and 840nm high reflection, and is used for separating photon pairs and locking light.
Furthermore, the first high-reflection mirror and the second high-reflection mirror are used for leading out the generated narrow-band photon pairs and are plated with high-reflection films of 700nm-1000 nm.
Further, the third high-reflection mirror and the fourth high-reflection mirror are high-reflection mirrors with high reflectivity for 700nm-1000nm wave bands and are used for adjusting the cavity-entering light path of the locking light.
Compared with the prior art, the invention has the following advantages:
the Z-shaped resonant cavity consists of two concave mirrors, two plane mirrors, a second-class quasi-phase-matched down-conversion crystal and two quarter-wave plates, wherein the two quarter-wave plates are respectively placed on two sides of the PPKTP down-conversion crystal, the placing angle of the optical axis of the two quarter-wave plates and the optical axis of the crystal form 45 degrees, and the structure can realize double resonance of photon pairs with mutually vertical polarization generated by the second-class quasi-phase-matched crystal; the two concave mirrors are used for generating appropriate optical cavity mode cross-section size, so that the generation efficiency of down-converted photons is increased; the first concave mirror is used for converting a light field under the reflection of a small angle by transmitting and coupling a fundamental frequency light field; the first plane mirror reflects the down-conversion light field at a small angle, the second concave mirror and the second plane mirror serving as an output mirror reflect the down-conversion light field at zero angle, and finally a closed optical cavity folded into a Z shape is formed.
The loss introduced by the two quarter-wave plates is almost ignored relative to the compensating crystal, and the attenuation of the cavity fineness can not be obviously caused; in the cavity structure, the length of an optical cavity is twice that of a physical cavity, so that the line width of a single photon source can be further narrowed; the resonant cavity is locked by using light of 840nm through a Hansch-Couillaud method, the wavelength of the resonant cavity deviates from the photon pair optical field, and compared with other methods using a chopper, the working period of 100% can be realized under the condition of introducing noise as little as possible.
Drawings
FIG. 1 is a diagram of an apparatus for narrow linewidth photon pairs according to the present invention;
FIG. 2 is a graph showing the polarization change of photons with orthogonal polarizations in a Z-shaped cavity;
FIG. 3 shows the transmission peaks of two quarter-wave plates at different angles in the resonant cavity;
FIG. 4 shows the transmission peak of 852nm light at the same frequency in a Z-shaped resonant cavity;
FIG. 5 shows the error signal and transmission peak before and after the cavity is locked with 840 nm.
Detailed Description
Example 1
As shown in fig. 1, a down-conversion optical resonator for narrow linewidth photon pair generation includes a zigzag resonator part and a locking part, the zigzag resonator part includes a pump light 1, a first concave mirror 2, a nonlinear crystal 3, a first quarter-wave plate 4, a second concave mirror 5, a first plane mirror 7, a second quarter-wave plate 8, a second plane mirror 9, and a first detector 10, the locking part includes a first high-reflection mirror 11, a second high-reflection mirror 12, a locking light source 13, a third high-reflection mirror 14, a fourth high-reflection mirror 15, a first polarization beam splitter prism 16, a third quarter-wave plate 17, a fourth quarter-wave plate 18, a fifth high-reflection mirror 19, a second detector 20, a subtracter 21, a second polarization beam splitter prism 22, a third detector 23, and a long pass filter 24;
the pump light frequency is locked to the cesium atom D2 line; light of 840nm is locked to a certain vertical and horizontal through a frequency chain, and a resonant cavity is locked through a Hansch-Couillaud method, and the physical cavity length of the resonant cavity is about d-0.65 m. The laser emitted by the pump light 1 is used as a pump light source of parametric down-conversion of cavity enhancement of the Z-shaped resonant cavity part, the laser passes through the first concave mirror 2, the nonlinear crystal 3, the first quarter-wave plate 4 and the second concave mirror 5 once and then is discharged from the cavity, when the laser passes through the nonlinear crystal 3, a two-photon pair generated by spontaneous parametric down-conversion passes through the first quarter-wave plate 4, after being reflected by the second concave mirror 5, passes through the first quarter-wave plate 4, the nonlinear crystal 3 and the first concave mirror 2, after being reflected by the first quarter-wave plate 7, the second quarter-wave plate 8 and the second flat mirror 9, the two-photon pair reflected by the first concave mirror 2 reversely passes through the second quarter-wave plate 8, the first flat mirror 7, the first concave mirror 2 and the nonlinear crystal 3 in turn again, and when reaching the first quarter-wave plate 4, a primary circulation in the resonant cavity is completed, outputting a narrow-band photon pair through a second plane mirror 9, converting a light field under the reflection of the first concave mirror 2 and the first plane mirror 7 at a small angle, folding the optical cavity into a Z shape, and placing a first detector 10 behind the first plane mirror 7 for recording a transmission peak of the resonant cavity;
the locking light source 13 is reflected by a third high reflecting mirror 14 and a fourth high reflecting mirror 15 and then reaches a first polarization beam splitter 16, the light transmitted by the first polarization beam splitter 16 passes through a third quarter wave plate 17 and then enters a second plane mirror 9 through a long pass filter 24, a second high reflecting mirror 12 and a first high reflecting mirror 11, part of the locking light reflected by the second plane mirror 9 enters the first polarization beam splitter 16 through the first high reflecting mirror 11, the second high reflecting mirror 12, the long pass filter 24 and the third quarter wave plate 17 again and then enters a second polarization beam splitter 22 through a fourth quarter wave plate 18 after being reflected, the second polarization beam splitter 22 is divided into two beams of light for emergence, one beam of the transmission light enters a third detector 23, the other beam of the reflection light enters a second detector 20 through a fifth high reflecting mirror 19 and the two paths of electric signals of the third detector 23 and the second detector 20 are subtracted by the subtracter 21, obtaining an error signal and locking a resonant cavity;
the narrow-band photon pair output by the second plane mirror 9 is reflected by the first high-reflection mirror 11 and the second high-reflection mirror 12, filtered by the long-pass filter 24, and then transmitted and guided out.
The Z-shaped resonant cavity part further comprises piezoelectric ceramics 6, and the piezoelectric ceramics 6 are adhered to the second concave mirror 5. The first quarter-wave plate 4 and the second quarter-wave plate 8 have an action wavelength of 852nm, and the placement angle of the optical axis of the first quarter-wave plate and the optical axis of the second quarter-wave plate is 45 degrees. The nonlinear crystal 3 is a second-class quasi-phase-matched periodically polarized potassium titanyl phosphate crystal (PPKTP) with the length of 10mm, and the temperature can be accurately stabilized to 0.001 degrees. The curvature radius of the first concave mirror 2 and the second concave mirror 5 is 100mm, the reflectivity is more than 99.99% in a 852nm wave band, and is less than 1% in a 426nm wave band; the reflectivity of the first plane mirror 7 is more than 99.99% at a 852nm wave band and less than 1% at a 426nm wave band; the second plane mirror 9 is used as an output mirror, the reflectivity is 98% at 852nm wave band, and is less than 1% at 426nm wave band. The wavelength of the locking light source 13 is 840nm, far from the down-converted light field. The long-pass filter 24 is placed at an angle so as to have the characteristics of 852nm high transmission and 840nm high reflection. The first high-reflection mirror 11, the second high-reflection mirror 12, the third high-reflection mirror 14 and the fourth high-reflection mirror 15 are all plated with high-reflection films of 700nm-1000 nm.
To explain the polarization change of the photon pair with orthogonal polarization in the zigzag resonant cavity, the generation of photon pair with orthogonal polarization from the crystal is shown in fig. 2, taking the generated photon with horizontal polarization as an example (a in fig. 2): the horizontal polarization H polarization photon is changed into a left-handed circular polarization L after passing through the quarter-wave plate, is changed into a vertical polarization V after passing through the quarter-wave plate again after being reflected by the reflector, is changed into a right-handed polarization R after passing through the second quarter-wave plate after being transmitted for one section, and is changed into H polarization transmission after being reflected again, so that one cycle is completed. The resulting polarization change within the vertically polarized photon cavity is similar (b in fig. 2). This achieves dual resonance of two photons of orthogonal polarization, compensating for walk-off effects caused by birefringence. At this time, the free spectral range of the resonant cavity is about c/2d, c is the speed of light, and d is the physical cavity length.
To adjust the angle of the two quarter-wave plates, a bundle of 852nm of right-handed circularly polarized light R, with the same frequency as the generated photon pairs and carrying variable sidebands, is poured back from the output mirror. The angles of the second quarter-wave plate 8 and the first quarter-wave plate 4, which are included angles between the optical axes of the wave plates and the optical axis of the crystal, are sequentially adjusted by changing the value of the sideband. Only the second quarter-wave plate 8 is arranged in the cavity at the beginning, the angle of the second quarter-wave plate 8 is adjusted by combining the side bands and then the second quarter-wave plate is fixed, and the first quarter-wave plate 4 is added and the angle of the first quarter-wave plate is adjusted. Fig. 3 shows the transmission peaks of the cavity measured in different cases: in FIG. 3 (a) without sidebands, the free spectral range is about c/4d at an angle of 45 ° for the second quarter-wave plate 8; in fig. 3 (b) where the sidebands are at 112.9MHz and the angle of the second quarter-wave plate 8 is 45 deg., the sidebands coincide exactly with a certain longitudinal mode of the cavity; in fig. 3 (c) where the sidebands are 31.6MHz and the angle of the second quarter wave plate 8 is 45 °, the sidebands do not coincide with some longitudinal mode of the middle of the cavity; in FIG. 3 (d) where the sideband is 112.9MHz and the angle of the second quarter-wave plate 8 is 53 deg., the free spectral region of the cavity is not c/4d, and the sideband does not coincide with a certain longitudinal mode of the cavity; in FIG. 3 (e) the free spectral range is about c/2d with the sideband at 113.1MHz and the angle of the second quarter-wave plate 8 at 0 °, the sideband being exactly symmetrical about the two longitudinal modes of the cavity, located in the middle of the two longitudinal modes; in FIG. 3 (f) there is no sideband, and the free spectral range is about c/2d at both angles of 45, achieving dual resonance. In the figure, a small number of higher order modes exist, except for the zero-zero mode and the sidebands of the cavity.
To calibrate the linewidth of the cavity, a beam of 852nm light of the same frequency as the generated photon pair was back-flowed. The first high-reflection mirror 11 and the second high-reflection mirror 12 are adjusted to be incident on the second plane mirror 9. The transmission spectrum obtained by the first detector 10 is shown in fig. 4. The fineness of the cavity obtained by fitting is 118, and the line width of the cavity obtained by combining the free spectral region of 226MHz is about 1.9 MHz.
A840 nm locking light source 13 enters the resonant cavity from the first polarization splitting prism 16, and the reflection signal of the light source is used for locking the cavity length by the Hansch-Couillad method as shown in FIG. 5. From top to bottom in the figure are the 840nm error signal and 852nm transmission peak before and after lock, respectively: after locking, the error signal is almost locked to zero (for ease of illustration, the peak is shifted upwards by about 6V); the transmission peak locks to the peak value.
Claims (8)
1. The down-conversion optical resonant cavity for generating the narrow linewidth photon pair is characterized by comprising a Z-shaped resonant cavity part and a locking part, wherein the Z-shaped resonant cavity part comprises pump light (1), a first concave mirror (2), a nonlinear crystal (3), a first quarter-wave plate (4), a second concave mirror (5), a first plane mirror (7), a second quarter-wave plate (8), a second plane mirror (9) and a first detector (10), and the locking part comprises a first high-reflection mirror (11), a second high-reflection mirror (12), a locking light source (13), a third high-reflection mirror (14), a fourth high-reflection mirror (15), a first polarization splitting prism (16), a third quarter-wave plate (17), a fourth quarter-wave plate (18), a fifth high-reflection mirror (19), a second detector (20), a subtracter (21), A second polarization splitting prism (22), a third detector (23) and a long-pass filter (24);
the pump light source of conversion under the cavity reinforcing's of pump light (1) transmission parameter as zigzag resonant cavity part cavity, laser single is through first concave mirror (2), nonlinear crystal (3), first quarter wave plate (4), second concave mirror (5) back play chamber, the two-photon that the spontaneous parametric down-conversion produced when laser passes through nonlinear crystal (3) is to permeating first quarter wave plate (4), after second concave mirror (5) reflection, through first quarter wave plate (4), nonlinear crystal (3), first concave mirror (2), the two-photon that reflects through first concave mirror (2) is to through first level crossing (7), second quarter wave plate (8), second plane mirror (9) reflection back backward again in proper order through second quarter wave plate (8), first plane mirror (7), The first concave mirror (2) and the nonlinear crystal (3) complete one cycle in the resonant cavity when reaching the first quarter-wave plate (4), a narrow-band photon pair is output through the second plane mirror (9), the first concave mirror (2) and the first plane mirror (7) reflect down-converted light fields at small angles, so that the optical cavity is folded into a Z shape, and the first detector (10) is placed behind the first plane mirror (7) and used for recording the transmission peak of the resonant cavity;
the locking light source (13) is reflected by a third high reflecting mirror (14) and a fourth high reflecting mirror (15) and then reaches a first polarization splitting prism (16), the light transmitted by the first polarization splitting prism (16) passes through a third quarter wave plate (17), then passes through a long-pass filter (24), a second high reflecting mirror (12) and a first high reflecting mirror (11) and then is incident on a second plane mirror (9), part of the locking light reflected by the second plane mirror (9) enters the first polarization splitting prism (16) through the first high reflecting mirror (11), the second high reflecting mirror (12), the long-pass filter (24) and the third quarter wave plate (17) again and then enters a second polarization splitting prism (22) through the fourth quarter wave plate (18), the second polarization splitting prism (22) is divided into two beams of light and then exits, one beam of the transmission light enters a third detector (23), the other beam of reflected light is reflected by a fifth high-reflection mirror (19) and enters a second detector (20), two paths of electric signals of a third detector (23) and the second detector (20) enter a subtracter (21) for subtraction, an error signal is obtained, and a resonant cavity is locked;
the narrow-band photon pair output by the second plane mirror (9) is reflected by the first high-reflection mirror (11) and the second high-reflection mirror (12), filtered by the long-pass filter (24) and then transmitted and guided out.
2. A down-converting optical resonator for narrow linewidth photon pair generation according to claim 1, wherein said zigzag resonator portion further comprises a piezoceramic (6), said piezoceramic (6) being bonded to the second concave mirror (5).
3. A down-conversion optical resonator for narrow linewidth photon pair generation according to claim 1, wherein the first quarter-wave plate (4) and the second quarter-wave plate (8) have a working wavelength of 852nm and their optical axes are placed at an angle of 45 ° to the crystal optical axis.
4. A down-conversion optical resonator for narrow linewidth photon pair generation according to claim 1, wherein the nonlinear crystal (3) is a quasi-phase-matched class two PPKTP.
5. A down-converting optical resonator for narrow linewidth photon pair generation according to claim 1, wherein said first (2) and second (5) concave mirrors have a radius of curvature of 100mm, a reflectivity of greater than 99.99% at 852nm band and less than 1% at 426nm band; the reflectivity of the first plane mirror (7) is more than 99.99% in a 852nm wave band and less than 1% in a 426nm wave band; the second plane mirror (9) is used as an output mirror, the reflectivity is 98% at a 852nm wave band, and the reflectivity is less than 1% at a 426nm wave band.
6. A down-converting optical resonator for narrow linewidth photon pair generation according to claim 1, wherein the wavelength of the locking optical source (13) is 840nm away from the down-converted optical field.
7. A down-converting optical resonator for narrow linewidth photon pair generation according to claim 1, wherein said long pass filter (24) is positioned at an angle that provides 852nm high transmission and 840nm high reflection.
8. A down-converting optical resonator for narrow linewidth photon pair generation according to claim 1, wherein the first high-reflection mirror (11), the second high-reflection mirror (12), the third high-reflection mirror (14) and the fourth high-reflection mirror (15) are coated with a high-reflection film of 700nm to 1000 nm.
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