CN114578628B - 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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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 narrow-linewidth photon pairs.
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 manipulation, quantum storage and the like, have an indispensable meaning 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 (idle photons), the presence of the other photon (signal photon) can be demonstrated for some predictive 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. Thus birefringence compensation is 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 commonly used methods are: 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 n e →n o And n o →n e The 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 be suppressed to compensate by using cluster effect (cluster effect) caused by birefringence in the cavity, for example, the P.J. Tsai in 2018 generates 852nm photons with line width of 6.6MHz by using cluster effect to compensateAnd (6) peer-to-peer.
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 a narrow linewidth photon pair 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 polarization splitting prism, a third quarter wave plate, a fourth quarter wave plate, a fifth high-reflection mirror, a second detector, a subtracter, a second polarization splitting prism, a third detector and a long-pass filter plate;
the laser emitted by the pump is used as a pump light source for parametric down-conversion of cavity enhancement of a Z-shaped resonant cavity part, the laser passes through a first concave mirror, a nonlinear crystal, a first quarter-wave plate and a second concave mirror once and then exits a cavity, a two-photon pair generated by spontaneous parametric down-conversion when the laser passes through the nonlinear crystal penetrates through the first quarter-wave plate, after being reflected by the second concave mirror, the laser passes through the first quarter-wave plate, the nonlinear crystal and the first concave mirror, 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 first plane mirror, the second quarter-wave plate, the first plane mirror, the first concave mirror and the second flat mirror pass through the second quarter-wave plate, the first plane mirror, the first concave mirror and the nonlinear crystal in turn again after being reflected by the first plane mirror, the second quarter-wave plate, the first plane mirror, the second concave mirror and the first plane mirror output a narrow-band photon pair, the first concave mirror and the first plane mirror convert-down-conversion under reflection at a small angle so that the optical cavity is folded into a Z-shaped optical field, and an observation window of the optical field is provided while the spatial dimension of the optical cavity is reduced, and the first plane detector is placed behind the first plane mirror for recording the transmission peak of the transmission of the resonant cavity;
the locking light source is reflected by a third high reflecting mirror and a fourth high reflecting mirror and then reaches a first polarization beam splitter prism, light transmitted by the first polarization beam splitter prism passes through a third quarter wave plate, then enters a second plane mirror through a long pass filter, a second high reflecting mirror and the first high reflecting mirror, part of locking light reflected by the second plane mirror enters the first polarization beam splitter prism through the first high reflecting mirror, the second high reflecting mirror, the long pass filter and the third quarter wave plate again for reflection, then enters the second polarization beam splitter prism through the fourth quarter wave plate, the second polarization beam splitter prism is divided into two beams for emergence, one beam of transmission light enters a third detector, the other beam of reflection light enters a second detector through the fifth high reflecting mirror for reflection, and two paths of electric signals of the third detector and the second detector enter a subtracter for subtraction to obtain an error signal and lock the 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 a two-class quasi-phase-matched PPKTP, and two ends of the nonlinear crystal are plated with antireflection films of pump light and down-conversion photon pairs at two wave bands, so that horizontally polarized pump 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, and the generation efficiency of down-converted photons is improved; 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-converted light field at a small angle, the second concave mirror and the second plane mirror serving as an output mirror reflect the down-converted light field at zero degree, 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 a Hansch-Couillaud method by using light with the wavelength of 840nm, and the wavelength of the resonant cavity deviates from a photon pair light field, so that the 100% working period can be realized under the condition of introducing no noise as much as possible compared with other methods using a chopper.
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 840nm.
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 frequency of the pump light is locked to the cesium atom D2 line; light at 840nm is locked to a certain aspect by a frequency chain, and the physical cavity length of the cavity is about d =0.65m by using a Hansch-Couillaud method to lock the cavity. The laser emitted by the pump light 1 is used as a pump light source of parametric down-conversion of cavity enhancement of the zigzag 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, when the laser passes through the nonlinear crystal 3, a two-photon pair generated by spontaneous parametric down-conversion penetrates through the first quarter-wave plate 4, after being reflected by the second concave mirror 5, the laser passes through the first quarter-wave plate 4, the nonlinear crystal 3 and the first concave mirror 2, the two-photon pair reflected by the first concave mirror 2 passes through the first flat mirror 7, the second quarter-wave plate 8 and the second flat mirror 9, after being reflected by the first quarter-wave plate 8, the first flat mirror 7, the first concave mirror 2 and the nonlinear crystal 3 in turn, when reaching the first quarter-wave plate 4, a primary cycle in the resonant cavity is completed, a narrow-band photon pair is output by the second flat mirror 9, the first concave mirror 2 and the first flat mirror 7 are down-converted by reflection, so that the optical field cavity is folded into a zigzag, the first concave mirror 10 is placed behind the first concave mirror 7 for recording a transmission peak of the resonant cavity, and a small-peak of the resonant cavity 7;
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 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 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 a fourth quarter wave plate 18, the second polarization splitting prism 22 is divided into two beams of light to be emitted, one beam of the transmission light enters a third detector 23, the other beam of the reflection light is reflected by the fifth high reflecting mirror 19 and enters a second detector 20, and two paths of electric signals of the third detector 23 and the second detector 20 enter a subtracter 21 for resonance cavity, so as to obtain an error signal and lock the error signal;
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 the 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 are 45 degrees. The nonlinear crystal 3 is a quasi-phase-matched second-class periodically polarized potassium titanyl phosphate crystal (PPKTP) with the length of 10mm, and the temperature can be accurately stabilized to 0.001 degree. 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% 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 is less than 1% at a 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 horizontally polarized H polarized photons are polarized into a left-handed circular polarization L after passing through the quarter-wave plate, the horizontally polarized H polarized photons are polarized into a vertical polarization V after passing through the quarter-wave plate again after being reflected by the reflector, the V polarized photons are converted into a right-handed polarization R after passing through the second quarter-wave plate after being transmitted for a period of time, and the horizontally polarized H photons are reflected into H polarized H to be transmitted for one cycle. 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 band and then is fixed, and the first quarter-wave plate 4 is added and the angle is adjusted. Fig. 3 shows the transmission peaks of the cavity measured in different cases: in FIG. 3 (a) there is no sideband and the free spectral range of the second quarter-wave plate 8 is about c/4d at an angle of 45 °; 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 are no sidebands and the free spectral range is about c/2d at 45 for both angles, 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 through fitting is 118, and the line width of the cavity obtained through combining the free spectral region of 226MHz is about 1.9MHz.
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 up by about 6V); the transmission peak locks to the peak value.
Claims (8)
1. A down-conversion optical resonator for generating narrow-linewidth photon pairs is characterized by comprising a Z-shaped resonator part and a locking part, wherein the Z-shaped resonator 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 plate (24);
the laser emitted by the pump light (1) is used as a pump light source of cavity-enhanced parametric down-conversion of a Z-shaped resonant cavity part, the laser passes through a first concave mirror (2), a nonlinear crystal (3), a first quarter-wave plate (4) and a second concave mirror (5) for a single time to form a cavity, when passing through the nonlinear crystal (3), a two-photon pair generated by spontaneous parametric down-conversion penetrates through the first quarter-wave plate (4), is reflected by the second concave mirror (5), then passes through the first quarter-wave plate (4), the nonlinear crystal (3) and the first concave mirror (2), after being reflected by the first concave mirror (2), the two-photon pair passes through the first plane mirror (7), the second quarter-wave plate (8) and the second plane mirror (9), reversely and sequentially passes through the second quarter-wave plate (8), the first plane mirror (7), the first concave mirror (2) and the nonlinear crystal (3), when reaching the first quarter-wave plate (4), primary circulation in the cavity is completed, the first plane mirror (9) outputs a light field, the first plane mirror (2) outputs a narrow-band-shaped photon field, the first concave mirror (7) and the first concave mirror (7) is folded into a narrow-peak-reflected optical detector, so that the first plane mirror (7) is used for being folded into a narrow-peak-reflected by the first plane resonant cavity, and then the first concave mirror (10, and the narrow-shaped resonant cavity is used for recording a small-peak;
the locking light source (13) is reflected by a third high-reflection mirror (14) and a fourth high-reflection mirror (15) and then reaches a first polarization beam splitter prism (16), light transmitted by the first polarization beam splitter prism (16) passes through a third quarter-wave plate (17), then passes through a long-pass filter plate (24), a second high-reflection mirror (12) and a first high-reflection mirror (11) and then enters a second plane mirror (9), part of locking light reflected by the second plane mirror (9) enters the first polarization beam splitter prism (16) for reflection and then enters a second polarization beam splitter prism (22) through a fourth quarter-wave plate (18), the second polarization beam splitter prism (22) is divided into two beams of light for emergence, one beam of the transmitting light enters a third detector (23), the other beam of reflecting light enters a second detector (20) through a fifth high-reflection mirror (19), the third polarization beam splitter prism (23) and the second polarization beam splitter prism (20) subtract signals to obtain two paths of locking signals, and the signals enter a resonant cavity for locking and the error is obtained;
narrow-band photon pairs output by the second plane mirror (9) are 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 placing angle of the optical axes of the first quarter-wave plate (4) and the second quarter-wave plate (8) is 45 degrees with the crystal optical axis, and the wavelength of the locking light source (13) is far away from the down-conversion light field.
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 an effective wavelength of 852nm.
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% 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 a 852nm wave band, and 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-conversion optical resonator for narrow linewidth photon pair generation according to claim 1, wherein the long pass filter (24) is angled to exhibit 852nm high transmission and 840nm high reflection.
8. A down-conversion 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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