CN112068380A - BBO crystal-based multimode receiving miniaturized entanglement source system - Google Patents
BBO crystal-based multimode receiving miniaturized entanglement source system Download PDFInfo
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Abstract
The invention discloses a BBO crystal-based multimode receiving miniaturized entanglement source system, which comprises a pump light source 1, a pump light transmission module, an entanglement device 5, a light separation device 7, a first collection device 8-1 and a second collection device 8-2. Wherein the first collecting device 8-1 and the second collecting device 8-2 are structurally asymmetric. The first collecting device 8-1 comprises a first multimode optical fiber, a temporal filtering unit 9 and a spatial filtering unit 11, and the temporal filtering unit 9 is located in front of the spatial filtering unit 11. The second collection device 8-2 includes a spatial filter unit 12 and a second multimode optical fiber. By means of the present invention, entangled light of high polarization contrast, high brightness and quality can be stably provided with a simplified structure.
Description
Technical Field
The invention relates to the technical field of quantum information, in particular to a BBO crystal-based multimode receiving miniaturized entanglement source system.
Background
The concept of quantum entanglement was first proposed by schrodinger and the famous EPR paradox, and is a research hotspot of recent disciplines such as physics and information communication. Due to the special property of quantum entanglement, the quantum entanglement has obvious application value in the aspects of quantum computation, quantum secret communication, quantum invisible state transfer and the like. So far, entangled states can be generated in cavity QED, ion trap, quantum dot, etc. systems, but the entangled states based on optical systems are most easily realized at high speed and high brightness, and thus are widely used and studied.
To date, generation of entangled photon pairs using a spontaneous parametric down-conversion (SPDC) process in nonlinear crystals is the most mature method, see for example chinese patent documents CN201721027813.5 and CN 201110170177.2. The SPDC is generated by parametric oscillation of strong pump light and spontaneous radiation caused by quantum vacuum noise in a nonlinear medium, namely: a high frequency pump photon spontaneously splits with some probability into a pair of low frequency down-converted photons-signal photons, idler photons. Subsequently, a scheme of generating entangled photon pairs by using a periodically polarized nonlinear crystal such as PPKTP (periodically polarized KTiOPO4) or PPLN (periodically polarized LiNbO3) and a quasi-phase matching technique has been proposed, for example, refer to chinese patent document CN 201810955748.5. The quasi-phase matching (QPM) technique is a technique for compensating for phase mismatch between optical waves due to refractive index dispersion by periodically modulating the nonlinear polarizability of an optical crystal.
The BBO crystal is a typical type nonlinear crystal that produces signal photons and idler photons with orthogonal polarizations, and the polarization of the signal photons coincides with the polarization of the pump light. The BBO crystal has obvious comprehensive advantages and good performance in a nonlinear optical crystal, has an extremely wide light transmission range, an extremely low absorption coefficient and a weaker piezoelectric ringing effect, has a higher extinction ratio, a larger phase matching angle, a higher light damage resistance threshold, broadband temperature matching and excellent optical uniformity compared with other electro-optical modulation crystals, and is favorable for improving the stability of laser output power. Compared with other periodically polarized nonlinear crystals, the BBO crystal is more economical and practical and is more suitable for teaching and experiment systems.
Therefore, the prior art successively proposes some miniaturized entanglement source systems based on BBO crystals.
For example, a micro-integrated small BBO polarization entanglement source system is disclosed in chinese patent No. CN 201921874226.9. As shown in fig. 1, the polarization entanglement source system includes: a collimating device realized by an optical fiber collimator OCA; a focusing device realized by a focusing lens LensA; an incident light polarization state adjusting means realized by two half-wave plates HWPA1, HWPA2 and a polarization beam splitter PBS; an entanglement device realized by BBO crystal; a light reflecting means realized by two mirrors RR1, RR 2; a first collecting means realized by a lens LensB1, a filter IF1 and a fiber collimator OCB 1; a second collection means realized by lens LensB2, filter IF2 and fiber collimator OCB 2. In the polarization entanglement source system, the collecting devices on both sides have a completely symmetrical device structure, and the filter plates arranged therein are only used for filtering out stray light to reduce noise. Also, as shown in fig. 2, an entanglement source teaching system is disclosed in chinese patent No. CN201721027813.5, and has an entanglement source optical circuit structure similar to that of chinese patent No. CN201921874226.9, and the collecting devices on both sides thereof have completely symmetrical device structures, and the filters IF1 and IF2 provided in the collecting devices are only used for filtering stray light to reduce noise. A small quantum entanglement source teaching system is also disclosed in chinese patent No. CN201120214023.4, and as shown in fig. 3, the same measuring device composed of a half-wave plate and a beam splitter is also used on both sides.
By analyzing the prior art solutions, it can be found that the existing polarization entanglement source system has disadvantages in terms of output optical performance (such as polarization contrast and brightness), performance stability, and economy.
For example, a single-mode fiber is generally used for receiving entangled light in the existing entanglement source system, but the single-mode fiber has relatively high cost, consumes much power, is afraid of bending, has high requirements on welding, is easy to generate additional loss and has high requirements on cleaning; in addition, in the prior art, a fully symmetrical device structure is adopted in the collecting devices on the two sides, so that the number of devices in the system is large, and resource waste is caused to a certain extent. The above problems can lead to instability of the output optical performance of the system and economic problems, particularly in experimental and teaching environments.
In the aspect of output optical performance, the conventional quantum entanglement source system usually adopts a symmetrical structure, and filters for filtering out stray light are only arranged in the collecting devices on two sides, so that the polarization contrast is low, and the high brightness and the high quality cannot be guaranteed. The prior art has also proposed that spatial filtering and pattern matching techniques can be used to increase the brightness of BBO crystal-based entanglement sources to 1000 cps/mw. However, the quantum entanglement source system adopting the scheme still cannot obtain satisfactory polarization contrast and brightness.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a BBO crystal-based multimode receiving miniaturized entanglement source system, wherein the concept of receiving entanglement light by adopting multimode optical fibers is provided, and for the first time, an entanglement light processing scheme realized by organically combining a temporal filtering technology and a spatial filtering technology is introduced into a single-side collecting device of the entanglement source system, and an asymmetric device structure is formed in the entanglement source system, so that the polarization contrast, the brightness, the quality, the stability and the economy of the entanglement source are further improved while the multimode receiving entanglement source is realized.
Specifically, the BBO crystal-based multimode receiving miniaturized entanglement source system of the present invention may include a pump light source 1, a pump light transmission module, an entanglement device 5, a light separation device 7, a first collection device 8-1, and a second collection device 8-2.
The light output by the pumping light source 1 acts through the pumping light transmission module and is input to the entanglement device 5 as pumping light;
the entanglement unit 5 comprises a BBO crystal and is arranged to generate, with the pump light, a polarization-entangled photon pair comprising a signal light and an idler light having polarization directions orthogonal to each other;
the optical separation device 7 is configured to separate the signal light and the idler light for transmission in different directions;
the first collecting means 8-1 is arranged to receive one of the signal light and the idler light;
the second collecting means 8-2 is arranged to receive the other of the signal light and the idler light;
characterized in that said first collection device 8-1 and said second collection device 8-2 are structurally asymmetric, wherein:
a temporal filtering unit 9 and a spatial filtering unit 11 are provided in the first collecting means 8-1 only at the same time, and the temporal filtering unit 9 is located in front of the spatial filtering unit 11; a spatial filtering unit 12 is arranged in the second collecting device 8-2; and the first collection device 8-1 further comprises a first multimode optical fiber and the second collection device 8-2 further comprises a second multimode optical fiber.
Preferably, the temporal filtering unit 9 is a narrow-band filter, and/or the spatial filtering units 11,12 are diaphragms.
Further, the first collecting device 8-1 further includes a collimating unit 13 for collimating the signal light or the idler light; and/or the like, and/or,
the second collecting means 8-2 further comprises a collimating unit 14 for collimating the signal light or idler light; and/or the like, and/or,
the second collecting device 8-2 further includes a stray light filtering unit 10 for filtering stray light other than the signal light or the idler frequency light.
Preferably, the first and/or second multimode optical fiber is 105 multimode optical fiber; and/or; the narrow-band filter has a central wavelength of 810nm and a full width at half maximum of 5 nm; and/or the diaphragm has an aperture of 1.5 mm.
Further, the pump light transmission module comprises a collimating device 2, a first light reflecting device 3 and a focusing device 4; and/or the like, and/or,
the optical separating means 7 comprises second optical reflecting means for separating the signal light and the idler light in the polarization-entangled photon pair.
Preferably, the wavelength of the pump light is 405 nm; and/or the like, and/or,
the first light reflecting means 3 comprises a first violet reflector; and/or the like, and/or,
the second light reflecting device comprises a second purple light reflector; and/or the like, and/or,
the collimating means 2 comprises a collimating lens; and/or the like, and/or,
the focusing means 4 comprise a focusing lens.
Further, the entanglement source system of the present invention may further comprise a compensation device 6, said compensation device 6 being arranged for compensating for a walk-off effect of said polarization-entangled photon pair. Wherein the compensating means 6 may comprise a half-wave plate 6-1 and a BBO crystal 6-2.
Further, the entanglement source system of the present invention may further comprise a measuring device provided between the compensating device 6 and the collecting device 8-1, 8-2. Wherein the measuring means may comprise a half-wave plate and a polarizer.
Drawings
The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 shows a miniaturized quantum entanglement source system of the prior art;
FIG. 2 illustrates another miniaturized quantum entanglement source system of the prior art;
FIG. 3 illustrates yet another miniaturized quantum entanglement source system of the prior art;
fig. 4 shows a schematic structural view of one example of the entanglement source system of the present invention.
Detailed Description
Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided by way of illustration in order to fully convey the spirit of the invention to those skilled in the art to which the invention pertains. Accordingly, the present invention is not limited to the embodiments disclosed herein.
Fig. 4 shows a schematic structural view of an example of the entanglement source system of the present invention.
As shown in the figure, the entanglement source system may include a pump light source 1, a pump light transmission module, an entanglement device 5, a light separation device 7, a first collection device 8-1, and a second collection device 8-2.
The pump light source 1 can be arranged to output polarized light, which can be realized, for example, by means of a pump laser. In one example, the pump light source 1 may include a pump laser having a wavelength of 405 nm.
The polarized light output from the pump light source 1 is input to the entanglement device 5 via the pump light transmission module, and is used as the pump light of the entanglement device 5.
In the embodiment shown in fig. 4, the pump light transmission module may include a collimating means 2, a first light reflecting means 3, and a focusing means 4.
In this embodiment, the collimating means 2 is used to collimate the polarized light output by the pump light source 1, the focusing means 4 is used to focus the polarized light, and the first light reflecting means 3 is used to reflect the polarized light.
As an example, the focusing means 4 may comprise a focusing lens 4. The collimating means 2 may comprise a collimating lens 2. The first light reflecting means 3 may comprise a first violet reflector, for example an E03 reflector.
In this embodiment, the first light reflecting means 3 may be arranged between the collimating means 2 and the focusing means 4. Alternatively, the exit center of the pump light source 1, the center of the first light reflecting device 3, the center of the collimating device 2, and the focal point of the focusing device 4 may be arranged on a straight line. It will be understood by those skilled in the art that the pump light source 1, the first light reflecting means 3, the collimating means 2 and the focusing means 4 may have other relative positional relationships.
The entanglement device 5 comprises a BBO crystal 5, and the input pump light is converted under spontaneous parameters in the BBO crystal to generate polarization entangled photon pairs. As known to those skilled in the art, the polarization-entangled photon pair includes a signal light (s-light) and an idler light (i-light), the polarization directions of the s-light and the i-light being orthogonal.
The optical separation device 7 is used for separating the signal light and the idler light in the entangled photon pair, so that the signal light and the idler light are respectively transmitted along different directions.
In the embodiment shown in fig. 4, the light separating means 7 may comprise a second light reflecting means 7 for reflecting the entangled photon pair, thereby separating the signal light from the idler light.
As an example, the second light reflecting means 7 may comprise a second violet reflector, such as an EO3 reflector.
The first collecting means 8-1 is for collecting one of the signal light and the idler light, and the second collecting means 8-2 is for collecting the other of the signal light and the idler light.
According to the invention, the first collecting means 8-1 and the second collecting means 8-2 will have an asymmetric device structure.
The first collecting means 8-1 may comprise a temporal filtering unit 9, a spatial filtering unit 11 and a first multimode optical fibre. The second collection device 8-2 may include a spatial filtering unit 12, a parasitic light filtering unit 10, and a second multimode optical fiber.
The temporal filtering unit 9 is configured to perform temporal mode selection processing on the entangled light. As a preferred example, the temporal filtering unit 9 may be implemented using a narrow-band filter 9.
The spatial filtering unit 11/12 is used to perform spatial mode selection processing on the entangled light. As a preferred example, the spatial filtering unit 11/12 may be implemented using a diaphragm 11/12.
The stray light filtering unit 10 is used for filtering stray light except for entangled light to provide a noise reduction function. As an example, the stray light filtering unit 10 may include a long pass filter 10.
According to the present invention, in order to improve the polarization contrast of the system while realizing multimode reception, in the first collection device 8-1, the temporal filtering unit 9 must be disposed in front of the spatial filtering unit 11, as shown in fig. 4. Therefore, the single-side collecting device carries out time and space filtering processing on the entangled light in a time-space mode, and therefore the multimode receiving of entangled photon pairs with high polarization contrast, high brightness and high quality can be achieved. In addition, because the invention only sets the time and space filtering units in the single-side (first) collecting device, and can not set the time filtering unit (such as the narrow-band filter 9) and the stray light filtering unit (such as the long-pass filter 10) in the same-side collecting device, compared with the completely symmetrical device structure in the prior art, the invention can greatly simplify the structure of the entanglement source system, save resources and easily obtain high-quality entanglement sources in the experimental teaching environment.
Further, in the collecting apparatus of the present invention, the aperture size of the diaphragm 11/12 is related to the diaphragm placement position, and the closer the diaphragm is to the receiving end, the larger the aperture of the diaphragm needs to be. Thus, in the collecting device of the present invention, the diaphragm may be disposed near the receiving end.
Experiments prove that in the collecting device of the entanglement source system, if the narrow-band filter is not introduced to carry out time filtering on the entanglement light, the polarization contrast of the entanglement source system is basically unchanged only by adjusting the aperture size and the distance degree of the diaphragm. In short, the entangled photons collected by the first collecting device 8-1 are subjected to time mode selection through the narrow-band filter 9 and then subjected to space mode selection through the diaphragm 11, so that the polarization contrast of the entanglement source system can be improved remarkably finally.
In a preferred embodiment, the pump light may have a wavelength of 405 nm. The multimode fiber may be 105 multimode fiber. The narrow band filter may have a center wavelength of 810nm and a full width at half maximum of 5 nm. The diaphragm may have an aperture of 1.5 mm. As described above, those skilled in the art can understand that the aperture of the diaphragm can be appropriately adjusted according to the placement position thereof. Optionally, a diaphragm positioning device may be further included in the entanglement source system of the present invention, for realizing the positioning of the diaphragm. For example, in one embodiment of the diaphragm positioning device, the single-mode fiber may be used to collect entangled photons, and then the single-mode fiber may be used to turn red light, so as to facilitate positioning of the diaphragm.
In one embodiment, the number of stops in the collection device may be 2 or more.
Further, collimating units 13, 14 may also be provided in the collecting device of the invention for collimating the entangled light for reception by the multimode optical fiber, as shown in fig. 4.
In an embodiment, the collimating units 13, 14 may comprise collimating lenses.
Further, as shown in fig. 4, the entanglement source system may further include a compensation device 6 disposed behind the entanglement device 5 for compensating for the walk-off effect of the entangled photon pairs.
In one embodiment, the compensation device 6 may include a half-wave plate 6-1 and a BBO crystal 6-2 for compensation.
Those skilled in the art will appreciate that the entanglement source system of the present invention may be applied to student laboratory course experiments (preparation/analysis of different entangled states, examination of the Bell inequality, quantum state tomography measurements, quantum key distribution), scientific experiments (quantum optics, quantum communication, quantum information), and also to commercial applications (encryption, metrology, optical sensing).
For example, after the multi-mode fiber is used to collect entangled photons, coincidence counting and collection can be performed by a time-correlated single photon detection technique, and polarization contrast is measured, thereby obtaining single photon counting, coincidence counting, and polarization contrast.
Optionally, especially when the entanglement source system is designed for teaching or scientific research scenes, a measuring device can be further arranged in the entanglement source system according to experimental requirements, so as to measure the entanglement photon pair, realize the selection of the polarization state, perform subsequent experimental operations, and realize other experiments such as Bell inequality inspection.
In one embodiment, the measurement device may include a half-wave plate and a polarizer. The measuring means may be arranged between the compensating means 6 and the collecting means 8-1, 8-2.
In particular, in the embodiment of the entanglement source system for teaching, any of the narrow-band filters, long-pass filters and diaphragms may be arranged in the first and/or second collecting device for adjustment for comparative experiments with other entanglement source systems, in order to understand the improvement of the entanglement performance due to the mutual synergy of spatial filtering and temporal filtering, for example.
Based on the above, in the entanglement source system of the present invention, by providing the time-space-first double filtering process for the entangled light only in the single-sided collection device, the polarization contrast can be significantly improved at the time of multimode reception. Therefore, the multimode receiving of the high-quality entanglement source system can be realized in a mode of saving resources by using the least devices, the whole structure is simple, portable and economic, the output optical performance is stable, the optical quality is high, and the multimode receiving device has good teaching and scientific practical values.
Furthermore, due to the fact that multimode receiving can be allowed, multimode optical fibers can be used in an entanglement source system to receive entangled light, the problems caused by single-mode optical fibers in the prior art are solved, and the optical fiber entanglement source system is portable, economical and particularly suitable for teaching and scientific research application scenes. Meanwhile, different from the current situation that a fully symmetrical device structure and low polarization contrast ratio are generally adopted in a small and medium entanglement source system in the prior art, the single-side collection device is only provided with the space and time filtering unit to form an asymmetric device layout, so that the entanglement source system is further simplified in structure, optical resources are saved to the maximum extent, the polarization contrast ratio and the brightness are improved, and the single-side collection device has good teaching and scientific practical values.
Although the present invention has been described in connection with the embodiments illustrated in the accompanying drawings, it will be understood by those skilled in the art that the embodiments described above are merely exemplary for illustrating the principles of the present invention and are not intended to limit the scope of the present invention, and that various combinations, modifications and equivalents of the above-described embodiments may be made by those skilled in the art without departing from the spirit and scope of the present invention.
Claims (10)
1. A BBO crystal-based multimode receiving miniaturized entanglement source system comprises a pump light source (1), a pump light transmission module, an entanglement device (5), a light separation device (7), a first collection device (8-1) and a second collection device (8-2);
the light output by the pumping light source (1) acts through the pumping light transmission module and is input to the entanglement device (5) as pumping light;
the entanglement device (5) comprises a BBO crystal and is arranged to generate a polarization-entangled photon pair by using the pump light, the polarization-entangled photon pair comprising a signal light and an idler light having polarization directions orthogonal to each other;
the optical separation device (7) is arranged for separating the signal light and the idler light for transmission in different directions;
the first collecting means (8-1) is arranged to receive one of the signal light and the idler light;
the second collecting means (8-2) is arranged to receive the other of the signal light and the idler light;
it is characterized in that the preparation method is characterized in that,
the first collection device (8-1) and the second collection device (8-2) are structurally asymmetric, wherein:
-a temporal filtering unit (9) and a spatial filtering unit (11) are provided simultaneously only in the first collecting device (8-1), the temporal filtering unit (9) being located in front of the spatial filtering unit (11); a spatial filtering unit (12) is arranged in the second collecting device (8-2); and the number of the first and second groups,
said first collection device (8-1) further comprises a first multimode optical fiber;
the second collection device (8-2) further comprises a second multimode optical fiber.
2. The entanglement source system according to claim 1, wherein the temporal filtering unit (9) is a narrow-band filter and/or the spatial filtering unit (11,12) is a diaphragm.
3. The entanglement source system of claim 1, wherein:
the first collecting means (8-1) further comprises a collimating unit (13) for collimating the signal light or idler light; and/or the like, and/or,
the second collecting means (8-2) further comprises a collimating unit (14) for collimating the signal light or idler light; and/or the like, and/or,
the second collecting device (8-2) further comprises a stray light filtering unit (10) for filtering stray light except the signal light or the idle frequency light.
4. The entanglement source system of claim 2, wherein:
the first and/or second multimode optical fiber is 105 multimode optical fiber; and/or the like, and/or,
the narrow-band filter has a central wavelength of 810nm and a full width at half maximum of 5 nm; and/or the like, and/or,
the diaphragm has an aperture of 1.5 mm.
5. The entanglement source system of claim 1, wherein:
the pump light transmission module comprises a collimating device (2), a first light reflecting device (3) and a focusing device (4); and/or the like, and/or,
the optical separating means (7) comprises second optical reflecting means for separating the signal light and the idler light of the polarization-entangled photon pair.
6. The entanglement source system of claim 5, wherein:
the wavelength of the pump light is 405 nm; and/or the like, and/or,
the first light reflecting means (3) comprises a first violet reflector; and/or the like, and/or,
the second light reflecting device comprises a second purple light reflector; and/or the like, and/or,
the collimating means (2) comprises a collimating lens; and/or the like, and/or,
the focusing means (4) comprises a focusing lens.
7. The entanglement source system according to claim 1, further comprising a compensation device (6), the compensation device (6) being arranged for compensating for walk-off effects of the polarization entangled-photon pairs.
8. The entanglement source system according to claim 7, wherein the compensation device (6) comprises a half-wave plate (6-1) and a BBO crystal (6-2).
9. The entanglement source system of claim 7, further comprising a measuring device disposed between the compensating device (6) and the collecting device (8-1, 8-2).
10. The entanglement source system of claim 9, wherein the measurement device includes a half-wave plate and a polarizer.
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