All-fiber entanglement source based on PPKTP crystal and device coupling and packaging method thereof

Abstract

The present invention provides an all-fiber entanglement source based on a PPKTP crystal, as well as a coupling and packaging method for a parametric down-conversion unit in the all-fiber entanglement source. Among them, by means of fiber optic devices and fiber optic transmission channels, the optical path structure of the entanglement source is realized, and the devices for realizing the parametric down-conversion process are arranged in the optical path structure in an integrated packaging manner, which can achieve better system stability and greatly reduce the debugging difficulty. Moreover, the entanglement source of the present invention allows the use of fewer optical devices, has a simple and compact overall structure, and can be integrated into the QKD system as a light source, thereby enabling higher integration. In addition, the coupling and packaging method of the present invention can ensure good packaging and optical coupling effects on the parametric down-conversion unit.

Description

Technical Field

[0001] The present invention relates to the field of quantum technologies, and in particular to an all-fiber entanglement source implemented based on a PPKTP crystal, and a coupling and packaging method for a parametric down-conversion unit in the all-fiber entanglement source. Background Art

[0002] Quantum entanglement is one of the most important topics in the field of quantum information. There are many methods for its preparation in experiments, and the most commonly used method is the parametric down-conversion method using a nonlinear crystal.

[0003] Figure 1 Shows an entanglement source structure in the prior art. As Figure 1 shown, a pump light with a wavelength of 518 nm is focused by a focusing lens 113 onto a dichroic mirror 114, and then enters a polarization beam splitter 115 and is divided into a horizontal polarization component and a vertical polarization component. Among them, the horizontal polarization component is transmitted and output to a first mirror 116, and is reflected by the first mirror 116 and enters a PPKTP crystal 117. The vertical polarization component passes through a half-wave plate 118, and its polarization direction changes from vertical polarization to horizontal polarization, and then is reflected by a second mirror 119 and enters the PPKTP crystal 117. Through a nonlinear process, correlated photon pairs with wavelengths of 780 nm and 1550 nm are generated in the clockwise and counterclockwise directions respectively. The two pairs of correlated photon pairs generated in the two directions then simultaneously return to the polarization beam splitter 115. Finally, photons with a wavelength of 780 nm are emitted from the polarization beam splitter 115, and photons with a wavelength of 1550 nm are emitted from the dichroic mirror 114, and the two photons are in an entangled relationship.

[0004] However, the prior art is built with spatial optical devices, which is difficult to debug, has low system stability, and is not conducive to system integration. Summary of the Invention

[0005] In view of the above problems existing in the prior art, the present invention proposes an all-fiber entanglement source implemented based on a PPKTP crystal, and a coupling and packaging method for a parametric down-conversion unit in the all-fiber entanglement source. Among them, by means of fiber devices and fiber transmission channels, the optical path structure of the entanglement source is realized, and the devices for realizing the parametric down-conversion process are arranged in the optical path structure in an integrated packaging manner, which can achieve better system stability and greatly reduce the debugging difficulty. Moreover, the entanglement source of the present invention allows the use of fewer optical devices, has a simple and compact overall structure, and can be integrated into a QKD system as a light source, thereby enabling higher integration. In addition, the coupling and packaging method of the present invention can ensure good packaging and optical coupling effects on the parametric down-conversion unit.

[0006] Specifically, the first aspect of the present invention relates to a coupling and packaging method for a parametric down-conversion unit of an all-fiber entanglement source, which successively includes a basic parameter determination step, a coupling collimator arrangement step, a PPKTP crystal arrangement step, and a curing step;

[0007] In the basic parameter determination step, the working distance between the first coupling collimator and the second coupling collimator is determined, and the collimated beam diameter of the first coupling collimator and the second coupling collimator is determined according to the cross-sectional area of the PPKTP crystal, wherein the working distance is greater than the length of the PPKTP crystal;

[0008] In the coupling collimator arrangement step, the first coupling collimator and the second coupling collimator are placed in an installation structural member, and the orientation of the first coupling collimator and / or the second coupling collimator is adjusted by means of a multi-dimensional adjustment mechanism to optimize the light collection efficiency;

[0009] In the PPKTP crystal arrangement step, the PPKTP crystal is placed in the installation structural member, and the orientation of the PPKTP crystal is adjusted to optimize the light collection efficiency, wherein the first coupling collimator and the second coupling collimator are located on both sides of the PPKTP crystal;

[0010] In the curing step, the positions of the first coupling collimator, the second coupling collimator, and the PPKTP crystal are fixed.

[0011] Further, the coupling collimator arrangement step further includes the steps of connecting the pigtail of the first coupling collimator to a debugging light source, and connecting the pigtail of the second coupling collimator to an optical power meter.

[0012] Preferably, the debugging light source has the same wavelength as the pump light, or the same wavelength as the parametric down-converted light.

[0013] Further, in the curing step, the first coupling collimator, the second coupling collimator, and the PPKTP crystal are fixed by means of dispensing.

[0014] Further, the multi-dimensional adjustment mechanism at least has X, Y, and Z-axis position adjustment functions and θ X and θ Y angle adjustment functions, where the θ X and θ Y are the angles with respect to the X and Y axes respectively, and the X axis is the longitudinal axis of the PPKTP crystal.

[0015] The second aspect of the present invention relates to an all-fiber entanglement source based on a PPKTP crystal, which includes a pump light source, an optical transmission device, a polarization beam splitter, a polarization rotation unit, and a parametric down-conversion unit;

[0016] The optical transmission device has first, second, and third ports. Among them, the light input from the first port is output through the second port, and the light input from the second port is output through the third port;

[0017] The pump light source is used to generate pump light in a first wavelength band and is arranged to be connected to the first port of the optical transmission device through a first optical fiber;

[0018] The polarization beam splitter has first, second, third, and fourth ports and is arranged such that: the first port is connected to the second port of the optical transmission device through a second optical fiber to split the pump light into first and second pump light components and output them through the third and fourth ports respectively; and, the third and fourth ports are connected through a third optical fiber to form a Sagnac loop;

[0019] The polarization rotation unit and the parametric down - conversion unit are arranged in the Sagnac loop. The polarization rotation unit is arranged to rotate the polarization state of light by 90 degrees, and the parametric down - conversion unit is arranged to perform spontaneous parametric down - conversion on the pump light components to generate parametric down - converted light in a second wavelength band, where the first wavelength band is different from the second wavelength band.

[0020] Furthermore, the optical transmission device is a circulator; or, the optical transmission device is a wavelength division multiplexer, where the first port only allows the transmission of light in the first wavelength band, the third port only allows the transmission of light in the second wavelength band, and the second port allows the transmission of light in both the first and second wavelength bands.

[0021] Furthermore, the polarization beam splitter is a fiber polarization beam splitter.

[0022] Furthermore, the parametric down - conversion unit includes a PPKTP crystal, and first and second coupling collimators respectively arranged on both sides of the PPKTP crystal. Preferably, the parametric down - conversion unit can be formed by the above - mentioned coupling and packaging method.

[0023] Furthermore, the first optical fiber is a single - mode polarization - maintaining optical fiber, the second optical fiber is a full - wavelength polarization - maintaining optical fiber, and the third optical fiber is a full - wavelength polarization - maintaining optical fiber.

[0024] Even further, the polarization rotation unit is a polarization controller or is realized by rotational alignment of the third optical fiber with the polarization beam splitter. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The following further describes in detail the specific embodiments of the present invention with reference to the accompanying drawings.

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0027] Figure 1 Schematically shows a kind of entanglement source structure in the prior art;

[0028] Figure 2 Schematically shows an embodiment of an all-fiber entanglement source implemented based on a PPKTP crystal according to the present invention;

[0029] Figure 3 Schematically shows another embodiment of an all-fiber entanglement source implemented based on a PPKTP crystal according to the present invention. Detailed implementation manners

[0030] In the following, the exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided by way of example to fully convey the spirit of the present invention to those skilled in the art to which the present invention pertains. Therefore, the present invention is not limited to the embodiments disclosed herein.

[0031] Figure 2 Schematically shows an embodiment of an all-fiber entanglement source implemented based on a PPKTP crystal according to the present invention.

[0032] As Figure 2 shown, the all-fiber entanglement source includes a pump light source 1, an optical transmission device 2, a polarization beam splitter 3, a polarization rotation unit 4, and a parametric down-conversion unit.

[0033] The pump light source 1 is used to generate pump light, which can be realized by means of a laser, such as a pulsed laser or a continuous light laser.

[0034] In the present invention, the pump light may have a wavelength λ in a first wavelength band p , for example, 518 nm.

[0035] The pump light source 1 is connected to the first port P1 of the optical transmission device 2 through a first optical fiber to allow the pump light to be output through the second port P2 of the optical transmission device 2. Among them, the first optical fiber may be a single-mode polarization-maintaining optical fiber.

[0036] As an example, the optical transmission device 2 may be a circulator, as Figure 2 shown.

[0037] The second port P2 of the optical transmission device 2 is connected to the first port P1 of the fiber optic polarization beam splitter 3 through a second optical fiber, allowing pump light to enter the fiber optic polarization beam splitter 3 from the first port P1 and be split into first and second pump light components. For example, the first pump light component can be the horizontally polarized component transmitted and output from the third port P3 of the fiber optic polarization beam splitter 3, and the second pump light component can be the vertically polarized component reflected and output from the fourth port P4 of the fiber optic polarization beam splitter 3. Among them, the second optical fiber can be a full-wavelength polarization-maintaining optical fiber.

[0038] The third port P3 and the fourth port P4 of the fiber optic polarization beam splitter 3 are connected through a third optical fiber, thus forming a Sagnac loop. Among them, the third optical fiber can be a full-wavelength polarization-maintaining optical fiber.

[0039] The first pump light component will enter the Sagnac loop from the third port P3 of the fiber optic polarization beam splitter 3 and start to transmit in the loop in the clockwise direction, while the second pump light component will enter the Sagnac loop from the fourth port P4 of the fiber optic polarization beam splitter 3 and start to transmit in the loop in the counterclockwise direction.

[0040] The parametric down-conversion unit is arranged in the Sagnac loop (i.e., the third optical fiber) and is used to cause the first pump light component to undergo a spontaneous parametric down-conversion process in the PPKTP crystal to generate first parametric down-converted light, and to cause the second pump light component to undergo a spontaneous parametric down-conversion process in the PPKTP crystal to generate second parametric down-converted light. Among them, the parametric down-converted light will include a pair of signal photons and idler photons, the signal photons have a wavelength of λ s , the idler photons have a wavelength of λi, and the wavelength (λ S , λ i ) of the parametric down-converted light in the second wavelength band will be different from the first wavelength band where the pump light is located. For example, when the wavelength of the pump light is 518 nm, wavelengths of 780 nm and 1550 nm will exist in the parametric down-converted light.

[0041] Continue to refer to Figure 2 , in the entanglement source of the present invention, the parametric down-conversion unit includes a first coupling collimator 5, a second coupling collimator 7, and a PPKTP crystal 6 integrally packaged in a mounting structure.

[0042] In this parametric down-conversion unit, the PPKTP crystal 6 is located between the first coupling collimator 5 and the second coupling collimator 7, where: the first coupling collimator 5 forms an optical coupling with the PPKTP crystal 6, and its pigtail is used as a connection end of the parametric down-conversion unit to be connected to the third optical fiber to receive the first pump light component, for example; the second coupling collimator 7 forms an optical coupling with the PPKTP crystal 6, and its pigtail is used as another connection end of the parametric down-conversion unit to be connected to the third optical fiber to receive the second pump light component, for example.

[0043] In the present invention, in order to ensure good encapsulation and optical coupling effects on the parametric down-conversion unit, the coupling and encapsulation method described below can be adopted.

[0044] The coupling and encapsulation method according to the present invention may include a basic parameter determination step, a coupling collimator arrangement step, a PPKTP crystal arrangement step, and a curing step.

[0045] In the basic parameter determination step, it is necessary to determine the working distance between the first coupling collimator 5 and the second coupling collimator 7 according to the length of the PPKTP crystal 6, so that the working distance is slightly greater than the length of the PPKTP crystal 6, facilitating the placement and debugging of the PPKTP crystal 6 in the optical path. After determining this working distance, it is also necessary to determine the collimated beam diameters of the first coupling collimator 5 and the second coupling collimator 7 according to the cross-sectional area of the PPKTP crystal 6, so that, for example, the debugging light can enter the pigtail of the coupling collimator with high efficiency. Among them, it is preferable to select the debugging light source such that its wavelength is the same as that of the pump light or the same as that of the parametric down-converted light.

[0046] After determining the above parameters, the structure and dimensions of the mounting structure member can be further designed and determined according to the structural dimensions of the first coupling collimator 5, the PPKTP crystal 6, and the second coupling collimator 7.

[0047] Thereafter, in the coupling collimator arrangement step, the first coupling collimator 5 and the second coupling collimator 7 can be placed at the corresponding positions of the mounting structure member. At the same time, the first coupling collimator 5 and the second coupling collimator 7 are installed on an adjustment mechanism with multi-dimensional adjustment functions, so as to allow adjustment of the orientation of each coupling collimator.

[0048] In a preferred example, the adjustment mechanism may have adjustment dimensions such as θ X , θ Y , X, Y, and Z, where X, Y, and Z are the three coordinate axes of a rectangular coordinate system. For example, the X-axis may be the longitudinal axis of the PPKTP crystal, and θ X and θ Y are the angles with respect to the X and Y axes respectively.

[0049] Therefore, for example, the pigtail of the first coupling collimator 5 can be connected to the debugging light source, and the pigtail of the second coupling collimator 7 can be connected to an optical power meter. Turn on the debugging light source, and adjust the orientations of the two coupling collimators with the aid of the adjustment mechanism until the light receiving efficiency reaches the optimal value. Thus, the arrangement work of the coupling collimators in the mounting structure is basically achieved.

[0050] In the PPKTP crystal arrangement step, the PPKTP crystal can be placed at the corresponding position of the mounting structure member, and the orientation of the PPKTP crystal can be finely adjusted until the light receiving efficiency reaches the optimal value.

[0051] After completing the arrangement steps of the coupling collimator and the PPKTP crystal, the first coupling collimator 5, the second coupling collimator 7, and the PPKTP crystal 6 can be glued and cured, thereby completing the encapsulation steps of the parametric down-conversion unit.

[0052] Continue to refer to Figure 2 , in the present invention, the polarization rotation unit 4 is used to rotate the polarization state of light by 90 degrees.

[0053] As an example, the polarization rotation unit 4 can be implemented by means of a polarization controller, as Figure 2 shown.

[0054] As another example, the polarization rotation unit 4 can also be implemented by rotating and aligning the optical axis of the third optical fiber, which is a polarization-maintaining optical fiber, with the fiber polarization beam splitter 3.

[0055] In Figure 2 the embodiment of, the first pump light component traveling in the clockwise direction in the Sagnac loop will enter the parametric down-conversion unit through the pigtail of the first coupling collimator 5.

[0056] As described above, the first pump light component has a horizontal polarization direction, and it undergoes a spontaneous parametric down-conversion process in the PPKTP crystal 6 to generate the first parametric down-converted light with a certain probability, which includes the signal light |H s > and the idler light |V i .

[0057] The first parametric down-converted light output from the parametric down-conversion unit will further reach the polarization rotation unit 4 and undergo a 90-degree polarization state rotation. At this time, the first parametric down-converted light includes the signal light |V s > and the idler light |H i , and it will enter the fiber polarization beam splitter 3 from the port P4 of the fiber polarization beam splitter 3.

[0058] The second pump light component traveling in the counterclockwise direction in the Sagnac loop will reach the polarization rotation unit 4 first before reaching the parametric down-conversion unit and undergo a 90-degree polarization state rotation, that is, it changes from vertical polarization to horizontal polarization. Therefore, the horizontally polarized second pump light component will then enter the parametric down-conversion unit through the pigtail of the second coupling collimator 7.

[0059] The horizontally polarized second pump light component undergoes a spontaneous parametric down-conversion process in the PPKTP crystal 6 to generate the second parametric down-converted light with a certain probability, which includes the signal light |H s > and the idler light |V i , and after being output from the parametric down-conversion unit, it enters the fiber polarization beam splitter 3 from the port P3 of the fiber polarization beam splitter 3.

[0060] It can be seen that there is signal light |H at the third port P3 of the fiber optic polarization beam splitter 3 s > and idler light |V i >, and at the same time there is signal light |V at the fourth port P4 of the fiber optic polarization beam splitter 3 s > and idler light |H i >. Therefore, signal light |V will be output at the first port P1 of the fiber optic polarization beam splitter 3 s > and |H s >, which will be input into the second port P2 of the input circulator 2 and output from the third port P3; the idler light |H will be directly output at the second port P2 of the fiber optic polarization beam splitter 3 i > and |V i . Due to the indistinguishability of photons, when photons are detected simultaneously at the third port P3 of the circulator 2 and the second port P2 of the fiber optic polarization beam splitter 3, it is impossible to distinguish which path the down-converted photons come from, and at this time the two photons are in an entangled state.

[0061] Figure 3 Another embodiment of the all-fiber entanglement source based on the PPKTP crystal according to the present invention is schematically shown, which is different from the Figure 2 shown embodiment in that the optical transmission device 2 is implemented by a wavelength division multiplexer instead of a circulator.

[0062] In this embodiment, the wavelength division multiplexer 2 can be configured such that: the first port P1 only allows the transmission of light in the first wavelength band, the third port P3 only allows the transmission of light in the second wavelength band, and the second port P2 allows the transmission of light in both the first wavelength band and the second wavelength band. Therefore, in the entanglement source of the present invention, the pump light output by the laser 1 can be input through the first port P1 of the wavelength division multiplexer 2 and output through the second port P2, and the parametric down-converted light output by the polarization beam splitter 3 can be input through the second port P2 of the wavelength division multiplexer and output through the third port P3.

[0063] Based on the above description, the present invention proposes an entanglement source based on a PPKTP crystal implemented by an all-fiber structure, in which the device for realizing the parametric down-conversion process is arranged in the optical path structure in an integrated package manner. Therefore, compared with the existing entanglement source scheme based on spatial devices, better system stability can be achieved and the debugging difficulty can be greatly reduced. Moreover, in the entanglement source of the present invention, fewer optical devices are allowed, the overall structure is simple and compact, and it can be integrated into the QKD system as a light source, thereby allowing for higher integration. The present invention further proposes a coupling and packaging method for the device for realizing the parametric down-conversion process to ensure good packaging and optical coupling effects on the parametric down-conversion unit.

[0064] Although the present invention has been described through specific embodiments in conjunction with the accompanying drawings, those skilled in the art can easily recognize that the above embodiments are merely exemplary and are used to illustrate the principle of the present invention, which will not limit the scope of the present invention. Those skilled in the art can make various combinations, modifications, and equivalent substitutions to the above embodiments without departing from the spirit and scope of the present invention.

Claims

1. A coupling and packaging method for a parametric down-conversion unit of an all-fiber entanglement source, which sequentially includes a basic parameter determination step, a coupling collimator arrangement step, a PPKTP crystal arrangement step, and a curing step; In the basic parameter determination step, the working distance between the first coupling collimator and the second coupling collimator is determined, and the collimated beam diameters of the first coupling collimator and the second coupling collimator are determined according to the cross-sectional area of the PPKTP crystal, where the working distance is greater than the length of the PPKTP crystal; In the coupling collimator arrangement step, the first coupling collimator and the second coupling collimator are placed in a mounting structural member, and the orientations of the first coupling collimator and / or the second coupling collimator are adjusted by means of a multi-dimensional adjustment mechanism to optimize the light collection efficiency; In the PPKTP crystal arrangement step, the PPKTP crystal is placed in the mounting structural member, and the orientation of the PPKTP crystal is adjusted to optimize the light collection efficiency, where the first coupling collimator and the second coupling collimator are located on both sides of the PPKTP crystal; In the curing step, the positions of the first coupling collimator, the second coupling collimator, and the PPKTP crystal are fixed.

2. The coupling and encapsulation method according to claim 1, wherein, The coupling collimator arrangement step further includes the steps of connecting the pigtail of the first coupling collimator to a debugging light source, and connecting the pigtail of the second coupling collimator to an optical power meter.

3. The coupling and encapsulation method according to claim 2, wherein, The debugging light source has the same wavelength as the pump light or the same wavelength as the parametric down-conversion light.

4. The coupling and encapsulation method according to claim 1, wherein In the curing step, the first coupling collimator, the second coupling collimator, and the PPKTP crystal are fixed by means of dispensing.

5. The coupling and encapsulation method according to claim 1, wherein, The multi-dimensional adjustment mechanism has at least the functions of adjusting the positions of the X, Y, and Z axes and the X and Y angle adjustment functions, where the X and Y are the included angles with respect to the X and Y axes respectively, and the X axis is the longitudinal axis of the PPKTP crystal.

6. An all-fiber entanglement source based on a PPKTP crystal, which includes a pump light source, an optical transmission device, a polarization beam splitter, a polarization rotation unit, and a parametric down-conversion unit; The optical transmission device has first, second, and third ports, wherein, The light input from the first port is output from the second port, and the light input from the second port is output from the third port; The pump light source is used to generate pump light in a first wavelength band and is arranged to connect the first port of the optical transmission device through a first optical fiber; The polarization beam splitter has first, second, third, and fourth ports and is arranged such that: the first port is connected to the second port of the optical transmission device through a second optical fiber to split the pump light into first and second pump light components and output them through the third and fourth ports respectively; and the third and fourth ports are connected through a third optical fiber to form a Sagnac loop: The polarization rotation unit and the parametric down-conversion unit are provided in the Sagnac loop. The polarization rotation unit is arranged to rotate the polarization state of light by 90 degrees, and the parametric down-conversion unit is arranged to cause the pump light components to undergo spontaneous parametric down-conversion to generate parametric down-conversion light in a second wavelength band, where the first wavelength band is different from the second wavelength band.

7. The all-fiber entanglement source according to claim 6, wherein The optical transmission device is a circulator; or, The optical transmission device is a wavelength division multiplexer, wherein the first port only allows the transmission of light in the first wavelength band, the third port only allows the transmission of light in the second wavelength band, and the second port allows the transmission of light in both the first and second wavelength bands.

8. The all-fiber entanglement source according to claim 6, wherein, The polarization beam splitter is an optical fiber polarization beam splitter.

9. The all-fiber entanglement source according to claim 6, wherein The parametric down-conversion unit includes a PPKTP crystal, and a first and a second coupling collimator respectively disposed on both sides of the PPKTP crystal.

10. The all-fiber entanglement source according to claim 9, wherein, The parametric down-conversion unit is formed by the coupling and encapsulation method according to any one of claims 1-5.

11. The all-fiber entanglement source according to claim 6, wherein, The first optical fiber is a single-mode polarization-maintaining fiber, the second optical fiber is a full-wavelength polarization-maintaining fiber, and the third optical fiber is a full-wavelength polarization-maintaining fiber.

12. The all-fiber entanglement source according to claim 11, wherein, The polarization rotation unit is a polarization controller, or is achieved by the rotational alignment of the third optical fiber and the polarization beam splitter.

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