A method for generating an ultra-wideband energy-time entangled two-photon state

Abstract

The present invention discloses a method for generating an ultra-wideband energy-time entangled two-photon state. By preparing a straight waveguide structure, and then the pump light is incident perpendicularly into the straight waveguide from the convex part of the trapezoidal cross-section of the straight waveguide structure for propagation. The mismatch amount in the spontaneous parametric down-conversion process in the straight waveguide, after the spontaneous parametric down-conversion process occurs in the straight waveguide structure, a pump light photon with a frequency of w P is converted by the nonlinear photonic crystal into a signal light photon with a frequency of w S and an idler light photon with a frequency of w I . The cross-section of the nonlinear straight waveguide provided by the present invention is trapezoidal. This cross-section structure can adjust the dispersion relation of the straight waveguide, change the spatial mode matching of the pump light, the signal light and the idler light, and realize a wider-band spectrum of the signal light and the idler light.

Description

Technical Field

[0001] The present invention relates to the field of quantum information technology, and particularly to a method for generating an ultra-wideband energy-time entangled two-photon state. Background Art

[0002] Quantum entanglement describes a correlation property between particles or groups of particles. A quantum entanglement source is an essential part of a quantum communication system. Based on the effect of the second-order nonlinear polarization coefficient of a nonlinear material, the process of spontaneous parametric down-conversion (SPDC) is an effective scheme for generating energy-time entanglement. Spontaneous parametric down-conversion refers to a nonlinear parametric process in which a pump light (frequency w P ) interacts with a nonlinear crystal in a second-order nonlinear crystal to generate a signal light (frequency w S ) and an idler light (frequency w I ). This process needs to satisfy the energy conservation condition (w P = w S + w I ), while the momentum conservation condition generally is not satisfied. For a LiNbO3 straight waveguide, the quasi-phase matching technique is usually adopted to compensate for the wave vector mismatch in the nonlinear process

[0003] However, the quasi-phase matching technique usually can only match one wave vector mismatch Therefore, the generated signal light and idler light have a narrow spectrum, and it is difficult to generate ultra-wideband entangled photon pairs. Thus, the prior art needs to be further improved. Summary of the Invention

[0004] In view of the above deficiencies in the prior art, the present invention provides a method for generating an ultra-wideband energy-time entangled two-photon state.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for generating an ultra-wideband energy-time entangled two-photon state, the method comprising the following steps:

[0006] Step 1: Prepare a straight waveguide structure, generate a LiNbO3 crystal thin film layer on the upper surface of a substrate, and a convex portion with a trapezoidal cross-section is provided on the LiNbO3 crystal thin film layer;

[0007] Step 2: The pump light is normally incident from the convex portion with a trapezoidal cross-section of the straight waveguide structure into the straight waveguide for propagation, and a spontaneous parametric down-conversion process occurs during the propagation;

[0008] Step 3: The second-order nonlinear polarization coefficient χ of the LiNbO3 crystal thin film layer(2) Periodically reverse along the propagation direction of the pump light to achieve the second-order nonlinear polarization coefficient χ (2) domains with = +1 and the second-order nonlinear polarization coefficient χ (2) domains with = -1 appear alternately regularly to compensate for the mismatch in multiple spontaneous parametric down-conversion processes

[0009] Step 4: After the spontaneous parametric down-conversion process occurs in the straight waveguide structure, a pump light photon with a frequency of w P is converted by the nonlinear photonic crystal into a signal light photon with a frequency of w S and an idler light photon with a frequency of w I .

[0010] The LiNbO3 crystal thin film layer is disposed on the upper surface of the substrate layer, and the thickness d2 of the LiNbO3 crystal thin film layer is 180 - 220 nm;

[0011] The protrusion with a trapezoidal cross-section is disposed in the middle of the LiNbO3 crystal thin film layer. The cross-section of the protrusion with a trapezoidal cross-section is an isosceles trapezoid. The height d1 of the isosceles trapezoid is 380 - 420 nm, the length W of the upper base is 1500 - 1700 nm, and the angle θ between the waist and the lower base is 65 - 80 degrees.

[0012] Preferably, the LiNbO3 crystal thin film layer is doped with 4% - 6% MgO.

[0013] Preferably, the LiNbO3 crystal thin film layer is doped with 5% MgO.

[0014] Preferably, the material of the substrate layer is SiO2, the length L of the substrate is 6.5 - 10 mm, and the height D is 1.8 - 2.2 μm.

[0015] Preferably, the length L of the substrate is 8 mm and the height D is 2 μm.

[0016] Preferably, the thickness of the d2 is 200 nm.

[0017] Preferably, the height d1 of the isosceles trapezoid is 400 nm, the length W of the upper base is 1600 nm, and the angle θ between the waist and the lower base is 75 degrees.

[0018] Preferably, the wave vectors of the pump light, the signal light, and the idler light are respectively denoted as and Then the mismatch can be compensated by χ (2) changing with a period of Λ, where the period Λ is expressed as: Λ = 2π / Δk.

[0019] Preferably, the wave vector mismatch Satisfy the law of conservation of momentum:

[0020] Advantageous technical effects of the present invention:

[0021] 1. The cross-section of the non-linear straight waveguide provided by the present invention is trapezoidal. This cross-sectional structure can adjust the dispersion relationship of the straight waveguide, change the spatial mode matching of the pump light, signal light and idler light, and realize a wider band of signal light and idler light spectra.

[0022] 2. The present invention adopts the quasi-phase matching technology to compensate the wave vector mismatch in the non-linear process. By changing the second-order non-linear coefficient χ of the lithium niobate crystal (2) , it is possible to compensate the phase mismatch in multiple spontaneous parametric down-conversion processes simultaneously.

[0023] 3. The present invention can generate ultra-wideband signal light and idler light spectra, which can be used to prepare energy-time entangled two-photon pairs. Brief Description of the Drawings

[0024] Figure 1 It is a step flow chart of a method for generating an ultra-wideband energy-time entangled two-photon state of the present invention.

[0025] Figure 2 It is a schematic diagram of the overall structure of the straight waveguide structure in the present invention.

[0026] Figure 3 It is a schematic diagram of the cross-section of the straight waveguide structure in the present invention.

[0027] Figure 4 It is a structural dispersion waveform diagram of the straight waveguide structure in the present invention.

[0028] Figure 5 It is a schematic diagram of the quasi-periodic lattice structure in the present invention.

[0029] Figure 6 It is a schematic diagram of the polarization length corresponding to each quasi-period in the present invention. Detailed Embodiments

[0030] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments. However, the scope of protection required by the present invention is not limited to the specific embodiments described below.

[0031] As Figure 1 shown, a method for generating an ultra-wideband energy-time entangled two-photon state, the method comprising the following steps:

[0032] Step 1: Prepare a straight waveguide structure: Generate a LiNbO3 crystal thin film layer on a substrate at an ambient temperature of 18 - 28 °C to form a straight waveguide structure. A protrusion with a trapezoidal cross-section is provided on the LiNbO3 crystal thin film layer;

[0033] Step 2: The pump light is incident normally into the straight waveguide from the protruding part of the trapezoidal cross-section of the straight waveguide structure and propagates therein, and a spontaneous parametric down-conversion process occurs during the propagation;

[0034] Step 3: The second-order nonlinear polarization coefficient χ of the LiNbO3 crystal thin film layer (2) periodically reverses along the propagation direction of the pump light, realizing the regular alternation of domains with the second-order nonlinear polarization coefficient χ (2) = +1 and domains with the second-order nonlinear polarization coefficient χ (2) = -1, compensating for the mismatch of multiple spontaneous parametric down-conversion processes

[0035] Step 4: After the spontaneous parametric down-conversion process occurs in the straight waveguide structure, a pump light photon with a frequency of w P is converted by the nonlinear photonic crystal into a signal light photon with a frequency of w S and an idler light photon with a frequency of w I .

[0036] As Figures 2 - 6 described, preferably, in this embodiment, the generation temperature of the straight waveguide structure is 20 - 25 degrees. The straight waveguide structure includes a substrate layer 1, and a LiNbO3 crystal thin film layer 2 is provided on the substrate 1 layer. The LiNbO3 crystal thin film layer 2 is doped with 4% - 6% MgO. Preferably, the ratio of MgO in this embodiment is 5%. The LiNbO3 thin film is easily damaged under strong laser light. Doping 5% MgO can improve the laser damage threshold of the LiNbO3 thin film. The nonlinear straight waveguide is prepared from a 5% MgO-doped LiNbO3 crystal thin film at room temperature (20 °C to 25 °C).

[0037] The substrate 1 uses a SiO2 substrate. The length L of the substrate 1 is 6.5 - 10 mm, and the height D is 1.8 - 2.2 μm. In this embodiment, the preferred value is L = 8 mm and the thickness D is 2 μm.

[0038] The thickness d2 of the LiNbO3 crystal thin film layer 2 is 180 - 220 nm. A protrusion 3 of the LiNbO3 crystal thin film layer is provided in the middle of the LiNbO3 crystal thin film layer. The cross-section of the LiNbO3 crystal thin film layer protrusion 3 is an isosceles trapezoid. The height d1 of the isosceles trapezoid is 380 - 420 nm, the length W of the upper base is 1500 - 1700 nm, and the angle θ between the waist and the lower base is 65 - 80 degrees. In this embodiment, the preferred value of each parameter is that the thickness of the d2 is 200 nm. The height d1 of the isosceles trapezoid is 400 nm, the length W of the upper base is 1600 nm, and the angle θ between the waist and the lower base is 75 degrees.

[0039] The structural dispersion of the present invention is as Figure 4 shown. By optimizing the design of the cross-sectional structure, the structural dispersion of the straight waveguide can be changed. According to different structural dispersions, the wave vector mismatch in the spontaneous parametric down-conversion process can be adjusted, and the nonlinear conversion efficiency in the spontaneous parametric down-conversion process can be improved.

[0040] By applying an external voltage, the positive and negative coefficients of the second-order nonlinear coefficient χ (2) of the LiNbO3 crystal are changed. The second-order nonlinear coefficients of the region where the LiNbO3 crystal is applied with voltage and the region without voltage are χ (2) =-1 and χ (2) =+1 respectively, corresponding to Figure 4 the white and black regions in

[0041] They appear alternately. The width Λ of each white and black region changes with the position of the straight waveguide, that is, the quasi-periodic polarization structure. The quasi-periodic polarization structure can compensate for the wave vector mismatch in the spontaneous parametric down-conversion process and achieve higher nonlinear conversion efficiency.

[0041] The length of the nonlinear straight waveguide is 8 mm. The pump light wavelength is preferably 775 nm. The pump light is incident perpendicularly into the straight waveguide from the trapezoidal part of the waveguide cross-section. Due to the second-order nonlinear coefficient of the LiNbO3 crystal, the spontaneous parametric down-conversion process occurs in the straight waveguide structure. A pump light photon with a frequency of w P is converted by the nonlinear photonic crystal into a signal light photon with a frequency of w S and an idler light photon with a frequency of w I . The wavelengths of the generated signal light and idler light are on both sides of 1550 nm. This process satisfies the energy matching condition but does not satisfy the momentum conservation condition

[0042] Denote the wave vectors of the pump light, signal light and idler light as and respectively. Then the mismatch can be compensated by χ (2) changing with a period of Λ. The wave vector mismatch satisfies the momentum conservation: To achieve the momentum conservation condition A quasi-periodic polarization structure is adopted to compensate for the wave vector mismatch in the nonlinear process. By changing the second-order nonlinear coefficient χ of the LiNbO3 crystal (2) , the mismatch in multiple spontaneous parametric down-conversion processes can be compensated simultaneously. The change period of the second-order nonlinear coefficient varies with the coordinates of the crystal to compensate for multiple spontaneous parametric down-conversion processes simultaneously. We adopt a chirped form to change the flipping period of χ (2) . The flipping period of χ (2) can be expressed as: Λ = 2π / Δk.

[0043] Using the effective refractive index of the LiNbO3 thin film structure, selecting the spectral widths of the signal light and the idler light to be 10 THz, and the length of the nonlinear straight waveguide to be 8 mm, the spatial distribution of χ (2) can be designed with the above parameters as shown in Figure 5 . This figure shows the χ (2) distribution of some waveguides, where black represents χ (2) = +1 and white represents χ (2) = -1. The width of each quasi-period is as shown in Figure 6 . Based on the nonlinear straight waveguide of this embodiment, an ultra-wideband energy-time two-photon entanglement with a bandwidth of up to 10 THz can be generated. By optimizing the parameters and the quasi-periodic structure, a larger bandwidth can be achieved.

[0044] According to the disclosure and teachings of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the invention should also fall within the protection scope of the claims of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the invention.

Claims

1. A method for generating an ultra-wideband energy-time entangled two-photon state, characterized in that, The method comprises the following steps: Step 1: Prepare a straight waveguide structure, generate a LiNbO3 crystal thin film layer on the upper surface of a substrate, and a protrusion with a trapezoidal cross-section is arranged on the LiNbO3 crystal thin film layer; The substrate is a SiO2 substrate, the length L of the substrate is 6.5 - 10 mm, and the height D is 1.8 - 2.2 μm; The protrusion with a trapezoidal cross-section is arranged in the middle of the LiNbO3 crystal thin film layer, the cross-section of the protrusion with a trapezoidal cross-section is an isosceles trapezoid, the height d1 of the isosceles trapezoid is 380 - 420 nm, the length W of the upper base is 1500 - 1700 nm, and the angle θ between the waist and the lower base is 71 - 80 degrees; Step 2: The pump light is normally incident into the straight waveguide from the protrusion part with a trapezoidal cross-section of the straight waveguide structure for propagation, and the mismatch amount in the process of spontaneous parametric down-conversion in the straight waveguide: The second-order nonlinear polarization coefficient χ of the LiNbO3 crystal thin film layer (2) is periodically reversed along the propagation direction of the pump light, so that the domains with the second-order nonlinear polarization coefficient χ (2) = +1 and the domains with the second-order nonlinear polarization coefficient χ (2) = -1 appear alternately regularly, thereby compensating the wave vector mismatch in multiple spontaneous parametric down-conversion processes Denote the wave vectors of the pump light, signal light, and idler light as and Then the wave vector mismatch can be compensated by χ that varies with a period of Λ (2) where the period Λ is Λ = 2π / Δk; Step 3: After the parametric down-conversion process occurs in the straight waveguide structure, a pump light photon with a frequency of w P is converted by the nonlinear photonic crystal into a signal light photon with a frequency of w S and an idler light photon with a frequency of w I .

2. The method for generating an ultra-wideband energy-time entangled two-photon state according to claim 1, characterized in that The thickness d2 of the LiNbO3 crystal thin film layer is 180 - 220 nm.

3. The method for generating an ultra-wideband energy-time entangled two-photon state according to claim 1, characterized in that, The LiNbO3 crystal thin film layer is doped with 4% - 6% MgO.

4. The method for generating an ultra-wideband energy-time entangled two-photon state according to claim 3, wherein, The LiNbO3 crystal thin film layer is doped with 5% MgO.

5. The method for generating an ultra-wideband energy-time entangled two-photon state according to claim 4, characterized in that, The length L of the substrate is 8 mm, and the height D is 2 μm.

6. The method for generating an ultra-wideband energy-time entangled two-photon state according to claim 2, wherein, The thickness of the d2 is 200 nm.

7. The method for generating an ultra-wideband energy-time entangled two-photon state according to claim 2, wherein The height d1 of the isosceles trapezoid is 400 nm, the length W of the upper base is 1600 nm, and the angle θ between the waist and the lower base is 75 degrees.

8. The method for generating an ultra-wideband energy-time entangled two-photon state according to claim 7, characterized in that, Wave vector mismatch Satisfy momentum conservation:

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