Multi-photon source based on periodically poled lithium niobate and preparation method thereof

Abstract

The invention provides a multi-photon source based on periodically poled lithium niobate and a preparation method thereof, and relates to the technical field of quantum information, the multi-photon source comprises a laser suitable for generating femtosecond pulse laser; the polarization regulator is suitable for carrying out polarization regulation on the femtosecond pulse laser and outputting pump pulse laser; the waveguide is suitable for receiving the pumping pulse laser, the waveguide comprises a convex structure, the convex structure is made of periodically poled lithium niobate, the convex structure has a trapezoidal cross section in the direction perpendicular to the transmission direction of the pumping pulse laser, and the waveguide is used for receiving the pumping pulse laser in the spontaneous parametric down-conversion process of the pumping pulse laser. Pump photons of the pump pulse laser are limited in the protruding structures, so that two pump photons in each pulse of the pump pulse laser are converted into two signal photons and two idler frequency photons, and a four-photon-state light beam is generated.

Description

Technical Field

[0001] The present disclosure relates to the field of quantum information technology, and more specifically to a multi-photon source based on periodically polarized lithium niobate and a preparation method thereof. Background Art

[0002] The working principle of preparing four-photon states is mainly the spontaneous parametric down-conversion process of pump light. In related technologies, the preparation of four-photon states is achieved through a complex spatial optical path. After the pump light undergoes a spontaneous parametric down-conversion process in a nonlinear crystal, a four-photon state beam is generated. After passing through a complex optical path, the four-photon state beam is divided into four single-photon state beams. Summary of the invention

[0003] In view of this, the present disclosure provides a multi-photon source based on periodically poled lithium niobate and a preparation method thereof, which can be used as an easily integrated two-photon source and four-photon source.

[0004] One aspect of the present disclosure provides a multi-photon source based on periodically poled lithium niobate, the multi-photon source comprising: a laser, adapted to generate femtosecond pulse laser; a polarization regulator, adapted to perform polarization adjustment on the femtosecond pulse laser and output pump pulse laser; and a waveguide, adapted to receive the pump pulse laser, the waveguide comprising a protruding structure, the protruding structure being made of periodically poled lithium niobate, the protruding structure having a trapezoidal cross-section in a direction perpendicular to the transmission direction of the pump pulse laser, the waveguide being used to confine the pump photons of the pump pulse laser in the protruding structure during the spontaneous parametric down-conversion of the pump pulse laser, so as to convert the two pump photons in each pulse of the pump pulse laser into two signal photons and two idler photons, thereby generating a four-photon beam.

[0005] According to an embodiment of the present disclosure, the waveguide is further adapted to confine the signal photons and the idler photons in the protruding structure during the spontaneous parametric down-conversion process of the pump pulse laser.

[0006] According to an embodiment of the present disclosure, the waveguide includes: a substrate; an energy conversion layer disposed on the substrate, the energy conversion layer including a planar structure and the protruding structure disposed on the planar structure; and a protective layer disposed on the energy conversion layer.

[0007] According to an embodiment of the present disclosure, the waveguide extends in a direction perpendicular to the transmission direction of the pump pulse laser; and the protruding structure is located at the center of the planar structure.

[0008] According to an embodiment of the present disclosure, the waveguide is also suitable for compensating for the phase mismatch in the spontaneous parametric down-conversion process of the pump pulse laser by periodically changing the polarization direction.

[0009] According to an embodiment of the present disclosure, the polarization adjuster includes: a polarization beam splitter, which is suitable for decomposing the femtosecond pulse laser to obtain two orthogonal light beams; and a 1 / 2 wave plate, which is suitable for adjusting the polarization angle of any one of the two orthogonal light beams to obtain the pump pulse laser, so that the pump pulse laser is incident on the waveguide at a predetermined polarization angle.

[0010] According to an embodiment of the present disclosure, the output light beam of the waveguide also includes residual pump pulse laser, which is the pump pulse laser that has not undergone the spontaneous parametric down-conversion process; the multi-photon source also includes a filtering unit, which is suitable for receiving the output light beam of the waveguide and filtering out the residual pump pulse laser.

[0011] According to an embodiment of the present disclosure, when the above-mentioned multi-photon source is used as a two-photon source, the above-mentioned two-photon source also includes: a first beam splitter, suitable for receiving the above-mentioned four-photon state light beam, splitting the above-mentioned four-photon state light beam with equal probability, and obtaining a first two-photon state light beam and a second two-photon state light beam; a first filter, suitable for filtering the above-mentioned first two-photon state light beam to obtain a first single-photon state light beam; and a second filter, suitable for filtering the above-mentioned second two-photon state light beam to obtain a second single-photon state light beam; the dual output interfaces of the above-mentioned two-photon source respectively output the above-mentioned first single-photon state light beam and the above-mentioned second single-photon state light beam.

[0012] According to an embodiment of the present disclosure, when the above-mentioned multi-photon source is used as a four-photon source, the above-mentioned four-photon source also includes: a second beam splitter, which is suitable for receiving the above-mentioned four-photon state beam, and performing equal probability beam splitting on the above-mentioned four-photon state beam to obtain a third two-photon state beam and a fourth two-photon state beam; a third beam splitter, which is suitable for performing equal probability beam splitting on the above-mentioned third two-photon state beam to obtain a third single-photon beam and a fourth single-photon beam; and a fourth beam splitter, which is suitable for performing equal probability beam splitting on the above-mentioned fourth two-photon state beam to obtain a fifth single-photon beam and a sixth single-photon beam; the four output interfaces of the above-mentioned four-photon source respectively output the above-mentioned third single-photon state beam, the above-mentioned fourth single-photon beam, the above-mentioned fifth single-photon beam and the above-mentioned sixth single-photon beam.

[0013] Another aspect of the present disclosure provides a method for preparing the above-mentioned multi-photon source based on periodically poled lithium niobate.

[0014] According to the embodiments of the present disclosure, the probability of generating multi-photon states can be enhanced by using a femtosecond pulse laser with high peak power. The polarization state of the femtosecond pulse laser can be precisely controlled by a polarization regulator to output a pump pulse laser. The spontaneous parametric down-conversion process of the pump pulse laser is realized by periodically polarizing the nonlinear optical material of lithium niobate. The pump photons are confined to a trapezoidal cross section by the convex structure of the waveguide, which improves the energy conversion efficiency in the spontaneous parametric down-conversion process, and can realize the preparation of a four-photon state beam through a waveguide, simplifying the spatial optical path and making it easier to integrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The above and other objects, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings.

[0016] Figure 1 The schematic diagram of the construction of a multi-photon source based on periodically poled lithium niobate according to an embodiment of the present disclosure is schematically shown.

[0017] Figure 2 The schematic diagram of the construction of a waveguide of a multi-photon source based on periodically poled lithium niobate according to an embodiment of the present disclosure is schematically shown.

[0018] Figure 3 A schematic diagram of a polarization adjuster based on a multi-photon source of periodically poled lithium niobate according to an embodiment of the present disclosure is schematically shown.

[0019] Figure 4 A schematic diagram of a filtering unit based on a multi-photon source of periodically poled lithium niobate according to an embodiment of the present disclosure is schematically shown.

[0020] Figure 5 The figure schematically shows a schematic diagram of a multi-photon source based on periodically poled lithium niobate used as a two-photon source according to an embodiment of the present disclosure.

[0021] Figure 6 The figure schematically shows a schematic diagram of a multi-photon source based on periodically poled lithium niobate used as a four-photon source according to an embodiment of the present disclosure.

[0022] Figure 7 The result of the test experiment of the frequency doubling conversion efficiency of the multi-photon source based on periodically poled lithium niobate according to the embodiment of the present disclosure is schematically shown.

[0023] Figure 8 A schematic diagram schematically shows a spectrum distribution diagram of two pump lights when a multi-photon source based on periodically poled lithium niobate is used as a two-photon source according to an embodiment of the present disclosure.

[0024] Fig. 9The schematic diagram schematically shows the experimental results of two pump lights used as a two-photon source using a multi-photon source based on periodically poled lithium niobate according to an embodiment of the present disclosure.

[0025] Fig.10 The figure schematically shows the experimental results of using a multi-photon source based on periodically poled lithium niobate as a four-photon source according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0026] In order to make the objectives, technical solutions and advantages of the present disclosure more clearly understood, the present disclosure is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0027] The terms used herein are only for describing specific embodiments and are not intended to limit the present disclosure. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and / or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

[0028] All terms used herein, including technical and scientific terms, have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification, and should not be interpreted in an idealized or overly rigid manner.

[0029] When using expressions such as "at least one of A, B, and C, etc.", it should generally be interpreted as the meaning of the expression generally understood by those skilled in the art. For example, "a system having at least one of A, B, and C" should include but not be limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc. When using expressions such as "at least one of A, B, or C, etc.", it should generally be interpreted as the meaning of the expression generally understood by those skilled in the art. For example, "a system having at least one of A, B, or C" should include but not be limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.

[0030] It should also be noted that the directional terms mentioned in the embodiments, such as "upper", "lower", "front", "back", "left", "right", etc., are only reference directions of the drawings and are not intended to limit the scope of protection of the present disclosure. Throughout the drawings, the same elements are represented by the same or similar reference numerals. Conventional structures or configurations will be omitted when they may cause confusion in the understanding of the present disclosure.

[0031] In related technologies, the preparation of four-photon states through complex spatial optical paths relies on a series of complex optical elements to achieve the separation, coupling and regulation of single photons, such as lenses, beam splitters, polarizers, etc. As a result, the stability and repeatability of the system are often limited, especially in terms of multi-photon generation efficiency and photon mode matching. On the other hand, optical systems are developing towards miniaturization and integration, and complex spatial optical paths cannot be directly integrated with existing optical chips.

[0032] Nonlinear frequency conversion refers to the phenomenon that when the pump light wave interacts with the nonlinear optical material, the frequency of the light wave changes due to the nonlinear optical effect. The frequency conversion efficiency of nonlinear frequency conversion can be used to characterize the energy conversion efficiency of the spontaneous parametric down-conversion process.

[0033] The spontaneous parametric downconversion process is a commonly used process for preparing two-photons. In the process of preparing two-photons, a total of three photons are involved and obey the law of conservation of energy and momentum:

[0034] (1)

[0035] (2)

[0036] in is the pump photon frequency, and are the frequencies of the signal photon and the idler photon, respectively; is the wave vector of the pump photon, signal photon and idler photon.

[0037] Normally, the nonlinearity of all crystal media is relatively weak. In order to achieve the maximum nonlinear effect and the highest frequency conversion efficiency, the wave vector must meet certain conditions, namely the phase matching condition.

[0038] In the related technology, birefringence phase matching technology is used to achieve phase matching of pump light during spontaneous parametric down-conversion, which requires the multi-photon light beam to be emitted from the crystal at a specific angle. For example, the polarizations of signal photons and idler photons are orthogonal and propagate in different directions. A slight deviation will lead to a significant drop in conversion efficiency. Therefore, it is difficult to build a system with a complex spatial optical path.

[0039] The present disclosure provides a multi-photon source based on periodically poled lithium niobate and a preparation method thereof, in order to solve at least one of the above technical problems.

[0040] Figure 1 The schematic diagram of the construction of a multi-photon source based on periodically poled lithium niobate according to an embodiment of the present disclosure is schematically shown.

[0041] like Figure 1As shown, the multi-photon source based on periodically poled lithium niobate includes a laser 1, a polarization regulator 2 and a waveguide 3.

[0042] According to an embodiment of the present disclosure, the laser 1 is suitable for generating femtosecond pulse laser. The polarization regulator 2 is suitable for adjusting the polarization of the femtosecond pulse laser and outputting a pump pulse laser. The waveguide 3 is suitable for receiving the pump pulse laser, and the waveguide includes a protrusion structure, which is made of periodically polarized lithium niobate, and the protrusion structure has a trapezoidal cross-section in a direction perpendicular to the transmission direction of the pump pulse laser. The waveguide 3 is used to confine the pump photons of the pump pulse laser in the protrusion structure during the spontaneous parametric down-conversion of the pump pulse laser, so as to convert the two pump photons in each pulse of the pump pulse laser into two signal photons and two idler photons, thereby generating a four-photon state beam.

[0043] According to an embodiment of the present disclosure, the laser 1 may be a femtosecond pulse laser used as a pump source. As a pump light, the femtosecond pulse laser has a higher peak power and enhances the generation probability of a multi-photon state.

[0044] According to the embodiments of the present disclosure, periodically poled lithium niobate has unique nonlinear optical properties and can achieve efficient nonlinear frequency conversion.

[0045] According to an embodiment of the present disclosure, in the spontaneous parametric down-conversion process of the pump pulse laser, two pump photons in each pulse of the pump pulse laser are annihilated to generate two signal photons and two idler photons.

[0046] According to the embodiments of the present disclosure, the probability of generating multi-photon states can be enhanced by using a femtosecond pulse laser with high peak power. The polarization state of the femtosecond pulse laser can be precisely controlled by a polarization regulator to output a pump pulse laser. The spontaneous parametric down-conversion process of the pump pulse laser is realized by periodically polarizing the nonlinear optical material of lithium niobate. The pump photons are confined to a trapezoidal cross section by the convex structure of the waveguide, which improves the energy conversion efficiency in the spontaneous parametric down-conversion process, and can realize the preparation of a four-photon state beam through a waveguide, simplifying the spatial optical path and making it easier to integrate.

[0047] According to an embodiment of the present disclosure, the waveguide 3 is also suitable for confining signal photons and idler photons in the protruding structure during the spontaneous parametric down-conversion process of the pump pulse laser.

[0048] According to the embodiments of the present disclosure, the waveguide 3 can confine the pump photons, signal photons and idler photons in the protruding structure, so that the energy distributions of the pump photons, signal photons and idler photons overlap and are concentrated in the trapezoidal cross-section of the protruding structure, thereby improving the energy conversion efficiency and the probability of the occurrence of the spontaneous parametric down-conversion process, and being able to realize two spontaneous parametric down-conversion processes in the pump pulse laser at the same time, that is, one pump photon in each pulse of the pump pulse laser is converted into one signal photon and one idler photon, and the two pump photons in each pulse are converted into two photon pairs including a signal photon and an idler photon.

[0049] Figure 2 The schematic diagram of the construction of a waveguide of a multi-photon source based on periodically poled lithium niobate according to an embodiment of the present disclosure is schematically shown.

[0050] like Figure 2 As shown, the waveguide 3 includes a substrate 31 , an energy conversion layer 32 and a protective layer 33 .

[0051] According to an embodiment of the present disclosure, the material of the substrate 31 may be silicon, silicon carbide, gallium arsenide, or the like.

[0052] According to an embodiment of the present disclosure, the energy conversion layer 32 is disposed on the substrate 31, and the energy conversion layer 32 includes a planar structure 321 and a protruding structure 322 disposed on the planar structure. The material of the energy conversion layer 32 is lithium niobate.

[0053] According to an embodiment of the present disclosure, the protection layer 33 is disposed on the energy conversion layer 32. The material of the protection layer 33 may be silicon nitride, silicon dioxide, or the like.

[0054] According to an embodiment of the present disclosure, the waveguide 3 extends in a direction perpendicular to the transmission direction of the pump pulse laser. The protruding structure 322 is located at the center of the planar structure 321 .

[0055] According to an embodiment of the present disclosure, the waveguide 3 is also adapted to compensate for phase mismatch during spontaneous parametric down-conversion of the pump pulse laser by periodically changing the polarization direction.

[0056] According to the embodiment of the present disclosure, waveguide 3 uses quasi-phase matching technology to compensate for the phase mismatch accumulated between interacting light waves due to dispersion by introducing periodic modulation of nonlinear polarizability in nonlinear optical materials. Waveguide 3 introduces a new phase mismatch , so that the phase matching condition becomes:

[0057] (3)

[0058] in, It represents the phase mismatch of the nonlinear process. Phase matching allows the polarization of pump photons, signal photons and idler photons to be the same during the spontaneous parametric down-conversion of the pump pulse laser, thereby realizing energy conversion by utilizing the maximum nonlinear coefficient of periodically poled lithium niobate.

[0059] Figure 3 A schematic diagram of a polarization adjuster based on a multi-photon source of periodically poled lithium niobate according to an embodiment of the present disclosure is schematically shown.

[0060] like Figure 3 As shown, the polarization adjuster 2 includes: a polarization beam splitter 21 , a 1 / 2 wave plate 22 and a first coupling lens 23 .

[0061] According to an embodiment of the present disclosure, the polarization beam splitter 21 is suitable for decomposing a femtosecond pulse laser to obtain two orthogonal light beams. The 1 / 2 wave plate 22 is suitable for adjusting the polarization angle of any one of the two orthogonal light beams to obtain a pump pulse laser, so that the pump pulse laser enters the waveguide at a predetermined polarization angle. The first coupling lens 23 is configured to connect one end of the waveguide close to the polarization regulator, and is suitable for introducing the pump pulse laser into the waveguide. For example, the connection method can be an optical fiber connection, and the first coupling lens 23 introduces the pump pulse laser from the spatial light path into the optical fiber. For example, the optical fiber includes a fiber lens, and the fiber lens is used to couple with the waveguide, and the first coupling lens 23 couples the pump pulse laser from the spatial light path into the fiber lens.

[0062] According to the embodiments of the present disclosure, the polarization adjuster can accurately adjust the polarization state of the pump pulse laser through the polarization beam splitter and the 1 / 2 wave plate. The pump pulse laser with a predetermined polarization angle can utilize the maximum nonlinear coefficient of periodically polarized lithium niobate to achieve the maximum nonlinear conversion efficiency, thereby improving the energy conversion efficiency.

[0063] According to an embodiment of the present disclosure, the output light beam of the waveguide also includes residual pump pulse laser light, and the residual pump pulse laser light is a pump pulse laser light that has not undergone a spontaneous parametric down-conversion process.

[0064] Figure 4 A schematic diagram of a filtering unit based on a multi-photon source of periodically poled lithium niobate according to an embodiment of the present disclosure is schematically shown.

[0065] like Figure 4 As shown, the multi-photon source based on periodically poled lithium niobate further includes a filtering unit 4. The filtering unit 4 is suitable for receiving the output light beam of the waveguide and filtering out the residual pump pulse laser.

[0066] In one example, if Figure 4As shown, the filtering unit 4 includes a second coupling lens 41, a dichroic mirror 42, a filter 43 and a third coupling lens 44 in sequence. The second coupling lens 41 receives the output light beam of the waveguide 3 and sends the output light beam of the waveguide 3 to the dichroic mirror 42, and the dichroic mirror 42 can preliminarily filter out the residual pump pulse laser. The filter 43 can be a long-wave pass filter, which can filter out the residual pump pulse laser and other noise to avoid affecting the use of the subsequently connected integrated device. The third coupling lens 44 can output only a four-photon state beam after filtering out the residual pump pulse laser.

[0067] Figure 5 The figure schematically shows a schematic diagram of a multi-photon source based on periodically poled lithium niobate used as a two-photon source according to an embodiment of the present disclosure.

[0068] like Figure 5 As shown, when the multi-photon source based on periodically poled lithium niobate is used as a two-photon source, the two-photon source further includes a first beam splitter 51 , a first filter 52 and a second filter 53 .

[0069] According to an embodiment of the present disclosure, the first beam splitter 51 is adapted to receive a four-photon state beam, and split the four-photon state beam with equal probability to obtain a first two-photon state beam and a second two-photon state beam. The first filter 52 is adapted to filter the first two-photon state beam to obtain a first single-photon state beam. The second filter 53 is adapted to filter the second two-photon state beam to obtain a second single-photon state beam. The dual output interfaces of the two-photon source output the first single-photon state beam and the second single-photon state beam respectively.

[0070] In one example, the first beam splitter 51 may be a 50 / 50 fiber beam splitter. The first filter 52 may separate the two photons of the first two-photon state light beam through a wavelength division multiplexer with two symmetrical channels. The second filter 53 may separate the two photons of the second two-photon state light beam through a wavelength division multiplexer with two symmetrical channels.

[0071] In one example, the two-photon source may be detected and counted by a first counting device. Figure 5 As shown, the first counting device includes a counter 61, a single photon detector 62 and a single photon detector 63. The single photon detector 62 is used to detect single photons of the first single photon state light beam. The single photon detector 63 is used to detect single photons of the second single photon state light beam. The counter 61 is connected to the single photon detector 62 and the single photon detector 63 to count the two photons generated by the two-photon source.

[0072] Figure 6 The figure schematically shows a schematic diagram of a multi-photon source based on periodically poled lithium niobate used as a four-photon source according to an embodiment of the present disclosure.

[0073] like Figure 6 As shown, when the multi-photon source based on periodically poled lithium niobate is used as a four-photon source, the four-photon source further includes a second beam splitter 71 , a third beam splitter 72 and a fourth beam splitter 73 .

[0074] According to an embodiment of the present disclosure, the second beam splitter 71 is suitable for receiving a four-photon state beam, and performs equal probability beam splitting on the four-photon state beam to obtain a third two-photon state beam and a fourth two-photon state beam. The third beam splitter 72 is suitable for performing equal probability beam splitting on the third two-photon state beam to obtain a third single-photon beam and a fourth single-photon beam. The fourth beam splitter 73 is suitable for performing equal probability beam splitting on the fourth two-photon state beam to obtain a fifth single-photon beam and a sixth single-photon beam. The four output interfaces of the four-photon source output the third single-photon state beam, the fourth single-photon beam, the fifth single-photon beam, and the sixth single-photon beam, respectively.

[0075] In one example, the second beam splitter 71 may be a 50 / 50 fiber beam splitter. The third beam splitter 72 may be a 50 / 50 fiber beam splitter. The fourth beam splitter 73 may be a 50 / 50 fiber beam splitter.

[0076] In one example, the four photons generated by the four-photon source can be detected and counted by a second counting device. Figure 6 As shown, the second counting device includes a counter 81, a single photon detector 82, a single photon detector 83, a single photon detector 84 and a single photon detector 85. The single photon detector 82 is used to detect single photons of the third single-photon state light beam. The single photon detector 83 is used to detect single photons of the fourth single-photon state light beam. The single photon detector 84 is used to detect single photons of the fifth single-photon state light beam. The single photon detector 85 is used to detect single photons of the sixth single-photon state light beam. The counter 81 is connected to the single photon detector 82, the single photon detector 83, the single photon detector 84 and the single photon detector 85 to count the four photons generated by the four-photon source.

[0077] Another aspect of the present disclosure provides a method for preparing a multi-photon source based on periodically poled lithium niobate.

[0078] According to an embodiment of the present disclosure, the preparation method includes: using a laser to generate a femtosecond pulse laser; using a polarization regulator to adjust the polarization of the femtosecond pulse laser and output a pump pulse laser; receiving the pump pulse laser through a waveguide, the waveguide including a protrusion structure, the protrusion structure is made of periodically polarized lithium niobate, the protrusion structure has a trapezoidal cross-section in a direction perpendicular to the transmission direction of the pump pulse laser, and during the spontaneous parametric down-conversion of the pump pulse laser through the waveguide, the pump photons of the pump pulse laser are confined in the protrusion structure to convert the two pump photons in each pulse of the pump pulse laser into two signal photons and two idler photons, thereby generating a four-photon state light beam.

[0079] According to the embodiments of the present disclosure, the energy conversion efficiency can be effectively improved by setting reasonable process parameters of the waveguide.

[0080] In one example, the waveguide is made of a straight waveguide using periodically polarized thin film lithium niobate. The polarization period of the waveguide determines the wavelength of the pump light corresponding to the maximum frequency doubling conversion efficiency. The cross-sectional shape of the waveguide protrusion structure will affect the energy mode distribution of the pump photons, signal photons, and idler photons. The more the energy mode distribution of the pump photons and signal photons overlap, the higher the energy conversion efficiency. The longer the polarization length of the waveguide, the greater the frequency doubling conversion efficiency.

[0081] For example, the polarization period of the energy conversion layer is 4.3μm±0.5μm, the polarization length of the energy conversion layer is 6mm±0.5mm, the side angle of the protruding structure of the energy conversion layer is 60 degrees±5 degrees, the top width of the protruding structure of the energy conversion layer is 1.075μm±0.05μm, the total thickness of the energy conversion layer is 600 nm±30nm, and the height of the protruding structure of the energy conversion layer, that is, the etching depth is 250nm±20 nm.

[0082] According to the embodiments of the present disclosure, the energy conversion efficiency is improved by selecting a pump pulse laser of a suitable wavelength band as pump light.

[0083] In one example, the frequency doubling conversion efficiency of a waveguide can characterize the energy conversion efficiency of the spontaneous parametric down-conversion process. The frequency doubling conversion efficiency of a waveguide indicates the frequency conversion efficiency of the nonlinear frequency conversion of the pump light in the waveguide. By testing the frequency doubling conversion efficiency of a waveguide, the wavelength range of the pump light corresponding to the higher frequency doubling conversion efficiency of the waveguide can be obtained.

[0084] Figure 7 The result of the test experiment of the frequency doubling conversion efficiency of the multi-photon source based on periodically poled lithium niobate according to the embodiment of the present disclosure is schematically shown.

[0085] like Figure 7As shown in Figure 2, when the wavelength of the pump light in the nonlinear frequency conversion process reaches 1536.2nm, the signal intensity of the frequency-doubled light reaches the maximum, and the conversion efficiency at this time is as high as 3840% / W·cm 2 . Conversion efficiency is 3000% / W·cm 2 The wavelength of the above pump light includes 1534nm~1538nm, so a pump pulse laser with a wavelength of 767nm~769nm can be selected as the pump light for the spontaneous parametric down-conversion process.

[0086] According to the embodiments of the present disclosure, high energy conversion efficiency is achieved by selecting a suitable type of laser and generating a suitable type of pump light.

[0087] In one example, in a performance test experiment of a two-photon source, two visible light band lasers were selected: a femtosecond pulse tunable laser and a continuously tunable laser, which output pulse laser and continuous laser respectively, and the output of both lasers was adjusted to 768nm.

[0088] In one example, a dense wavelength division multiplexer can measure the spectral distribution of two-photons. The 768nm pump light undergoes spontaneous parametric down-conversion to generate a two-photon state beam with a central wavelength of 1536nm.

[0089] In one example, two single photon detectors are connected to the dual output ports of the dual photon source, and a single photon counter is connected to the rear ends of the two single photon detectors. The single photon counter can be a time-correlated single photon counter that records the absolute time of arrival of photons to perform single photon counting.

[0090] Figure 8 A schematic diagram schematically shows a spectrum distribution diagram of two pump lights when a multi-photon source based on periodically poled lithium niobate is used as a two-photon source according to an embodiment of the present disclosure.

[0091] like Figure 8 As shown, Figure 8 There are two spectrum distribution diagrams in the figure. The horizontal axis of the spectrum distribution diagram represents the wavelength of the detected single photon. The spectrum distribution diagram only shows the spectrum with a wavelength band of 1525nm~1565nm. Since the frequencies of signal photons and idler photons are symmetrical in the non-intermittent case, the spectrum distribution of the two-photon source is actually greater than 60nm. The vertical axis of the spectrum distribution diagram represents the coincidence count of the single photon counter. The multiple bars of the spectrum distribution diagram represent the coincidence counts of single photons corresponding to different wavelengths detected. Figure 8 (a) in the figure shows the spectrum distribution diagram of a two-photon source using continuous laser as pump light. Figure 8 (b) in the figure shows the spectrum distribution diagram of a two-photon source using pulsed laser as pump light.

[0092] Fig. 9The schematic diagram schematically shows the experimental results of two pump lights used as a two-photon source using a multi-photon source based on periodically poled lithium niobate according to an embodiment of the present disclosure.

[0093] like Fig. 9 As shown, Fig. 9 (a) is a schematic diagram showing the relationship between the pump power of a continuous laser as pump light and the photon pair generation rate. Fig. 9 (b) shows the relationship between the pump power of continuous laser as pump light and the accidental coincidence count ratio. Fig. 9 (c) in the figure shows the relationship between the pump power of the pulsed laser as pump light and the photon pair generation rate. Fig. 9 (d) is a schematic diagram showing the relationship between the pump power of the pulsed laser as pump light and the accidental coincidence count ratio.

[0094] like Fig. 9 As shown in the figure, when continuous laser is used as pump light, the slope of the photon pair generation rate is 7.5MHz / μW, and the accidental coincidence count ratio can reach up to 2991. Fig. 9 As shown in the figure, when the pulse laser is used as pump light, the slope of the photon pair generation rate is 2.7MHz / μW, and the accidental coincidence count ratio can reach up to 15734. When the photon pair generation rate is very high, the accidental coincidence count ratios of the two pump lights can be maintained at around 100, indicating that the light sources of the two pump lights have a good signal-to-noise ratio.

[0095] According to the embodiments of the present disclosure, the conversion efficiency of a two-photon source using a continuous laser as pump light and a two-photon source using a pulsed laser as pump light can both meet the requirements of a two-photon source.

[0096] The following is an experimental verification of a specific embodiment to further illustrate the situation where a multi-photon source based on periodically poled lithium niobate is used as a four-photon source.

[0097] According to the embodiments of the present disclosure, in the performance test experiment of the four-photon source, a periodically polarized thin-film lithium niobate straight waveguide with a trapezoidal cross section is selected, a femtosecond pulse laser is selected to generate a femtosecond pulse laser with a wavelength of 768nm, and a pump pulse laser is obtained through a polarization regulator. Four single-photon detectors are connected to the four output ports of the four-photon source respectively, and the four single-photon detectors are connected to a single-photon counter. The single-photon counter can be a time-correlated single-photon counter that records the absolute time of arrival of the photon to perform single-photon counting.

[0098] Fig.10 The figure schematically shows the experimental results of using a multi-photon source based on periodically poled lithium niobate as a four-photon source according to an embodiment of the present disclosure.

[0099] In one example, if Fig.10As shown in the figure, the horizontal axis represents the pump power of the femtosecond pulse laser, and the vertical axis represents the coincidence count of the single photon counter. As the pump power increases, the coincidence count of the four-photon state beam increases significantly, and the coincidence count and pump power show a typical quadratic relationship. Fig.10 The error bars in the figure represent the statistical values ​​obtained through repeated measurements, which provide information on the precision of the experimental results.

[0100] It should be noted that in the performance test experiment of the four-photon source, the geometric shape, manufacturing error and connection unevenness of the connecting optical fiber lead to inevitable time delays between the channels. The time delay causes the signal peaks to appear at different delay positions, which is manifested as an asynchronous phenomenon, which will significantly affect the measurement at nanosecond time resolution. To solve this problem, an electrical delay adjustment mechanism is introduced in time-correlated single-photon counting. By accurately adjusting the electrical delay of the input port, the time deviation caused by the optical path difference is compensated, so that the signal peaks of each channel coincide accurately, improving the measurement accuracy and consistency. The experimental system of the performance test experiment of the four-photon source has high stability, and effective compensation for electrical delay is also performed. After multiple measurements, the test results of each test are stable without obvious fluctuations. In addition, the performance test experimental device of the four-photon source needs to strictly shield the external light source, and can be carried out in a dark room environment to reduce the noise interference of ambient light and reduce measurement fluctuations. The performance test experiment of the four-photon source achieves the preparation of four-photon states with a maximum generation rate of 3.46Hz / μW.

[0101] According to the embodiments of the present disclosure, the preparation of a multi-photon source based on periodically polarized lithium niobate is achieved through an integrated device, which has good stability and scalability, and can achieve a complexity that is difficult to achieve in space optics. By using a femtosecond pulse laser with a pump power of hundreds of milliwatts, the preparation of a four-photon state beam is achieved, which is much lower than the use of nonlinear crystals in space optics to achieve multi-photon state preparation. In space optics, a pump power of hundreds of milliwatts is generally required to achieve multi-photon states. Therefore, the multi-photon source based on periodically polarized lithium niobate disclosed in the present disclosure achieves the preparation of a four-photon state beam on a waveguide through a single femtosecond pulse laser, which reduces the technical difficulty and experimental requirements, and is easier to apply to integrated circuits, and is used in quantum communications, quantum computing and other fields.

[0102] The embodiments of the present disclosure are described above. However, these embodiments are only for illustrative purposes and are not intended to limit the scope of the present disclosure. Although the embodiments are described above separately, this does not mean that the measures in the various embodiments cannot be used in combination to advantage. The scope of the present disclosure is defined by the attached claims and their equivalents. Without departing from the scope of the present disclosure, those skilled in the art may make a variety of substitutions and modifications, which should all fall within the scope of the present disclosure.

Claims

1. A multi-photon source based on periodically polarized lithium niobate, characterized in that: The multi-photon source comprises: A laser suitable for generating femtosecond pulse lasers; A polarization regulator, adapted to adjust the polarization of the femtosecond pulse laser and output a pump pulse laser; and A waveguide is suitable for receiving the pump pulse laser, the waveguide includes a protrusion structure, the protrusion structure is made of periodically poled lithium niobate, the protrusion structure has a trapezoidal cross-section in a direction perpendicular to the transmission direction of the pump pulse laser, and the waveguide is used to confine the pump photons of the pump pulse laser in the protrusion structure during the spontaneous parametric down-conversion process of the pump pulse laser, so as to convert the two pump photons in each pulse of the pump pulse laser into two signal photons and two idler photons, thereby generating a four-photon state beam.

2. The multi-photon source according to claim 1, characterized in that: The waveguide is also suitable for confining the signal photons and the idler photons in the protruding structure during the spontaneous parametric down-conversion process of the pump pulse laser.

3. The multi-photon source according to claim 1, characterized in that: The waveguide comprises: substrate; an energy conversion layer, disposed on the substrate, the energy conversion layer comprising a planar structure and the protruding structure disposed on the planar structure; and The protective layer is arranged on the energy conversion layer.

4. The multi-photon source according to claim 3, characterized in that: The waveguide extends in a direction perpendicular to the transmission direction of the pump pulse laser; and the protruding structure is located at the center of the planar structure.

5. The multi-photon source according to claim 1, characterized in that: The waveguide is also suitable for compensating for phase mismatch during spontaneous parametric down-conversion of the pump pulse laser by periodically changing the polarization direction.

6. The multi-photon source according to claim 1, characterized in that: The polarization adjuster comprises: a polarization beam splitter, adapted to decompose the femtosecond pulse laser into two orthogonal beams; and The 1 / 2 wave plate is suitable for adjusting the polarization angle of any one of the two orthogonal light beams to obtain the pump pulse laser, so that the pump pulse laser enters the waveguide at a predetermined polarization angle.

7. The multi-photon source according to claim 1, characterized in that: The output light beam of the waveguide also includes residual pump pulse laser, and the residual pump pulse laser is the pump pulse laser that has not undergone the spontaneous parametric down-conversion process; the multi-photon source also includes a filtering unit, and the filtering unit is suitable for receiving the output light beam of the waveguide and filtering out the residual pump pulse laser.

8. The multi-photon source according to claim 1, characterized in that: In the case where the multi-photon source is used as a two-photon source, the two-photon source further comprises: A first beam splitter is adapted to receive the four-photon state light beam and perform equal-probability beam splitting on the four-photon state light beam to obtain a first two-photon state light beam and a second two-photon state light beam; A first filter is used to filter the first two-photon state light beam to obtain a first single-photon state light beam; and A second filter is used to filter the second two-photon state light beam to obtain a second single-photon state light beam; The dual output interfaces of the dual-photon source output the first single-photon state light beam and the second single-photon state light beam respectively.

9. The multi-photon source according to claim 1, characterized in that: In the case where the multi-photon source is used as a four-photon source, the four-photon source further comprises: A second beam splitter is adapted to receive the four-photon state light beam and perform equal-probability beam splitting on the four-photon state light beam to obtain a third two-photon state light beam and a fourth two-photon state light beam; A third beam splitter is adapted to perform equal-probability beam splitting on the third two-photon state beam to obtain a third single-photon beam and a fourth single-photon beam; and a fourth beam splitter, adapted to perform equal-probability beam splitting on the fourth two-photon state light beam to obtain a fifth single-photon light beam and a sixth single-photon light beam; The four output interfaces of the four-photon source respectively output the third single-photon state light beam, the fourth single-photon light beam, the fifth single-photon light beam and the sixth single-photon light beam.

10. A method for preparing the multi-photon source according to any one of claims 1 to 9.

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