Photon pair sources without spectral correlation

Abstract

A photon pair source without spectral correlation includes: a laser light source and a periodically polarized potassium titanyl phosphate crystal. The periodically polarized potassium titanyl phosphate crystal is a type II phase-matched nonlinear crystal. The periodically polarized potassium titanyl phosphate crystal is used to generate photon pairs without spectral correlation according to the parametric down-conversion process of the laser emitted by the laser light source. Among them, the polarization region of the type II phase-matched periodically polarized potassium titanyl phosphate crystal is divided into a central region and two edge regions along the laser incident direction. By reasonably designing the duty cycle of the central region and the crystal length, the photon yield of the photon pair source is Gaussian distributed, which solves the problems of time distinguishability and spectral distinguishability, and realizes the independence of the photon pair spectrum while ensuring the original brightness of the photon. The photon pair source is easy to adjust in practical applications, has a high degree of integration, and is easy to expand.

Description

Technical Field

[0001] The present disclosure relates to the technical field of quantum information processing, and in particular to a spectrally-correlated photon pair source based on periodically polarized potassium titanyl phosphate crystals. Background Art

[0002] Entangled states are the core resource in the field of quantum information. It is called "the essence of quantum mechanics". It demonstrates the unique properties of quantum theory such as coherence, probabilistic and spatial non-locality, and has been widely used in the booming quantum information fields including quantum cryptography, quantum state teleportation, quantum entanglement exchange, quantum storage and quantum computing.

[0003] In the field of traditional linear optical quantum information, there are many ways to generate entangled photon pairs. The most commonly used method is the spontaneous parametric down-conversion process (SPDC) using nonlinear optical crystals such as β-barium borate crystals (BBO) or periodically polarized potassium titanyl phosphate crystals (PPKTP). Under appropriate conditions, SPDC sources can be used to prepare two-photon quantum entangled sources with good indistinguishability. In order to solve the problem of spectral correlation of photon pairs, the most common method is to use narrow-band filters, but this method will significantly reduce the brightness of photons. Another way to remove spectral correlation is to select a set of operating wavelengths with zero group velocity mismatch under the so-called extended phase matching conditions. Based on this method, under pulse pumping with matching bandwidth, the joint spectral density of the excitation is almost circularly symmetric, thus generating a two-photon state with low spectral correlation, thereby providing a high level of spectral purity for the predicted single photon. A method can also remove spectral correlation. In this case, the spectral widths of the two SPDC photons are highly asymmetric, and their joint spectral density is similar to a highly eccentric ellipse. The above two methods, both related to group velocity matching, have been proven to be able to produce photon pairs consisting of two photons that are spectrally indistinguishable. However, under the symmetric method, the spectral distinguishability between the generated photons is limited by the side lobes generated in the standard phase matching function, and narrowband filters are also required to achieve spectral indifference. Summary of the invention

[0004] 1. Technical issues to be resolved

[0005] In response to the above technical problems, the present disclosure proposes a spectrally uncorrelated photon pair source based on periodically poled potassium titanyl phosphate crystals, which is used to at least solve the problems that the photon yield distribution of the existing general standard periodically polarized crystal (PPKTP) is limited by the side lobes of the standard phase matching function, the spectral distinguishability between the generated photons, the need to use narrow-band filters to improve the photon purity and entanglement fidelity but reduce the output brightness of the photon source itself, and the actual operation is relatively complicated.

[0006] (II) Technical solution

[0007] The present disclosure provides a spectrally uncorrelated photon pair source, comprising: a laser light source and a periodically poled potassium titanyl phosphate crystal, wherein the periodically poled potassium titanyl phosphate crystal is a type II phase-matched nonlinear crystal, and the periodically poled potassium titanyl phosphate crystal is used to generate spectrally uncorrelated photon pairs through a parametric down-conversion process according to laser light emitted by the laser light source.

[0008] Optionally, the polarization region of the periodically poled potassium titanyl phosphate crystal with type II phase matching is divided into a central region and two edge regions along the laser incident direction, and the central region is located between the two edge regions; the duty cycle range of the central region is 0.2-0.8.

[0009] Optionally, the length of each edge region is 1.25 times the length of a basic polarization region calculated according to the wavelength of the laser.

[0010] Optionally, the duty cycle of the center position of the periodically polarized potassium titanyl phosphate crystal is 0.5.

[0011] Optionally, the laser light source is a pump laser light source, and the wavelength range of the generated laser is 770-810 nm.

[0012] Optionally, the pump laser source generates the laser in a pulsed manner, and the half-width of the laser is in the range of 0.3-10 nm.

[0013] Optionally, the photon pair source further includes: a focusing lens, disposed between the laser light source and the periodically poled potassium titanyl phosphate crystal, for focusing the laser on the periodically poled potassium titanyl phosphate crystal.

[0014] Optionally, the formula for calculating the length of the basic polarization region according to the wavelength of the laser is:

[0015] Λ=-633.91+1002.98λ-563.46λ 2 +103.86λ 3

[0016] Wherein, Λ is the length of the basic polarization region, and λ is the wavelength of the laser.

[0017] Optionally, the length of the periodically poled potassium titanyl phosphate crystal is at least greater than the effective length of the triple nonlinear crystal when the frequency correlation is released, wherein the effective length is calculated based on the half-height width.

[0018] Optionally, the pump laser source is a femtosecond pulse laser.

[0019] (III) Beneficial effects

[0020] The present disclosure proposes a spectrally uncorrelated photon pair source based on periodically polarized potassium titanyl phosphate crystals, which has the following beneficial effects:

[0021] 1. The periodic polarization potassium titanyl phosphate crystal of the photon pair source adopts a type II phase-matched nonlinear crystal, and its polarization region is divided into a central region and two edge regions along the laser incident direction. By reasonably designing the duty cycle of the central region and the crystal length, the photon yield of the photon pair source is Gaussian distributed, which solves the problems of time distinguishability and spectral distinguishability, realizes the independence of the photon pair spectrum, and ensures the original brightness of the photon.

[0022] 2. Since the photon pair source has a simple structure, it is easy to adjust in practical applications, has a high degree of integration, and is easy to expand. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] For a more complete understanding of the present disclosure and its advantages, reference will now be made to the following description in conjunction with the accompanying drawings, which are incorporated into and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the description are used to explain the principles of the present disclosure. Among them:

[0024] Figure 1 The structure of a device for preparing a photon pair source without spectral correlation according to an exemplary embodiment of the present disclosure is schematically shown;

[0025] Figure 2 Schematically shows a principle diagram of a periodically poled nonlinear crystal PPKTP according to an exemplary embodiment of the present disclosure;

[0026] Figure 3 shows the distribution of the cumulative yield function with respect to the crystal position under the condition that the photon yield is Gaussian distributed according to an exemplary embodiment of the present disclosure;

[0027] Figure 4 The structure of a device for preparing a photon pair source without spectral correlation according to another exemplary embodiment of the present disclosure is schematically shown. DETAILED DESCRIPTION

[0028] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present disclosure. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present disclosure. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present disclosure.

[0029] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. 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.

[0030] The present disclosure provides a spectrally uncorrelated photon pair source based on a periodically poled potassium titanyl phosphate crystal, the photon pair source comprising: a laser light source and a periodically poled potassium titanyl phosphate crystal, the periodically poled potassium titanyl phosphate crystal being a type II phase-matched nonlinear crystal, the periodically poled potassium titanyl phosphate crystal being used to generate spectrally uncorrelated photon pairs in a parametric down-conversion process according to laser light emitted from the laser light source. A specific embodiment is described below.

[0031] Figure 1 The structure of the photon pair source without spectral correlation according to an embodiment of the present disclosure is schematically shown. Figure 1 As shown, the photon pair source may include, for example, a laser light source 1 and a periodically poled potassium titanyl phosphate crystal 2 .

[0032] The laser light source 1 is used to generate laser light. The laser light source 1 may be, for example, a pump laser source, and the generated laser light may be an initial pump light with a wavelength range of 770-810 nm. The pump laser source generates laser light in a pulsed manner, and the half-width of the laser light may be, for example, in the range of 0.3-10 nm. In a feasible manner of the first embodiment, a femtosecond pulse laser with a Gaussian waveform is selected as the pump laser source, such as a titanium sapphire femtosecond pulse laser.

[0033] The periodically poled potassium titanyl phosphate crystal 2 adopts a type II phase-matched nonlinear crystal. By reasonably designing the length of the type II phase-matched nonlinear crystal and the duty cycle of the polarization interval, the periodically poled potassium titanyl phosphate crystal can generate photon pairs without spectral correlation according to the parametric down-conversion process of the laser emitted by the laser light source.

[0034] Specifically, Figure 2 As shown, the polarization region of the type II phase-matched periodically poled potassium titanyl phosphate crystal is divided into a central region and two edge regions along the laser incident direction, and the central region is located between the two edge regions.

[0035] In a feasible manner of the first embodiment, the duty cycle range of the central area of ​​the polarization region is 0.2-0.8, and the preferred duty cycle of the central position of the periodically polarized potassium titanyl phosphate crystal is 0.5. At this time, the nonlinear effect of the central position of the periodically polarized potassium titanyl phosphate crystal is the highest.

[0036] In a feasible manner of the first embodiment, the length of each edge region is 1.25 times the length of the basic polarization region calculated according to the wavelength of the laser. The formula for calculating the length of the basic polarization region according to the wavelength of the laser is:

[0037] Λ=-633.91+1002.98λ-563.46λ 2 +103.86λ 3

[0038] Wherein, Λ is the length of the basic polarization region, and λ is the wavelength of the laser light generated by the laser light source. This embodiment calculates the length of the basic polarization interval when the laser wavelength is 790nm, which is the length required for the parametric light and the pump light to accumulate a 2π phase.

[0039] In a feasible manner of the first embodiment of the present invention, the effective length of the corresponding nonlinear crystal when the frequency is released is calculated:

[0040]

[0041] Wherein, D is the effective length when the nonlinear crystal is frequency-free, and Δλ is the half-height width of the laser. In this embodiment, in order to achieve a Gaussian distribution of the photon yield, a periodically polarized potassium titanyl phosphate crystal having a length at least three times greater than the effective length is used, and the yield at different positions of the periodically polarized potassium titanyl phosphate crystal is regulated so that the half-height width of the yield is approximately 0.731 times the effective length.

[0042] Therefore, based on the above design, the length of the polarization interval in the central area is obtained according to the previous calculation, and the duty cycle is adjusted, and the range of the duty cycle is adjusted to control the yield of photons at different positions. For the remaining two edge areas, a quarter of the polarization interval (1.25 times) is added to the outside of each polarization interval, and finally these areas are combined. In this embodiment, the length ratio of the three polarization areas is 2:6:2. The periodically polarized potassium titanyl phosphate crystal structure designed above makes the photon yield of the photon pair source present a Gaussian distribution.

[0043] In this embodiment, the selected crystal length L=10 mm, the effective length corresponding to the 4 nm laser half-width is 2.90 mm, and the cumulative distribution function of the down-converted light under ideal conditions is first calculated:

[0044]

[0045] Where L is the length of the periodically poled potassium titanyl phosphate crystal, t is the photon yield increment, Erf is the Gaussian error function, and σ is the effective length of the nonlinear region.

[0046] Bring in the parameter drawing again. Specifically, in the central area, two strategies are alternated:

[0047] A. Each small block is a positive polarization and a negative polarization. The length of the small block is Λ. When the yield increment of this interval is t, the length of the negative polarization interval is

[0048] B. Each small block is a positive polarization and a negative polarization. The length of the small block is Λ. When the yield increment of this interval is t, the length of the positive polarization interval is

[0049] In the marginal area, two strategies are alternated:

[0050] A. Each small block is a positive polarization and a negative polarization, and the length of the small block is 2Λ. When the yield increment of this interval is t, the length of the negative polarization interval is

[0051] B. Each small block is a positive polarization and a negative polarization, and the length of the small block is 2Λ. When the yield increment of this interval is t, the length of the positive polarization interval is

[0052] Therefore, the distribution of the cumulative yield function with the crystal position under the condition of Gaussian distribution of photon yield is obtained, such as Figure 3 As shown, it can be seen that there is no spectral correlation between the photon pairs.

[0053] In the present embodiment, the periodically polarized potassium titanyl phosphate crystal of the photon pair source adopts a type II phase-matched nonlinear crystal, and its polarization region is divided into a central region and two edge regions along the laser incident direction. By reasonably designing the duty cycle of the central region and the crystal length, the photon yield of the photon pair source is Gaussian distributed, which solves the problems of time distinguishability and spectral distinguishability, realizes the independence of the photon pair spectrum, and ensures the original brightness of the photon.

[0054] In another embodiment of the present invention, Figure 4 As shown, the photon pair source also includes: a focusing lens 3, which is arranged between the laser light source 1 and the periodically polarized potassium titanyl phosphate crystal 2, and is used to focus the laser on the periodically polarized potassium titanyl phosphate crystal. The function of the focusing lens 3 is to reduce the size of the spot of the pump laser incident on the crystal, which is conducive to the collection of the subsequent photon pairs. For details not provided in this embodiment, please refer to the above embodiment, and no further description will be given here.

[0055] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present disclosure. It should be understood that the above description is only a specific embodiment of the present disclosure and is not intended to limit the present disclosure. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present disclosure should be included in the protection scope of the present disclosure.

Claims

1. A spectrally uncorrelated photon pair source, characterized in that: include: A laser light source, a periodically poled potassium titanyl phosphate crystal and a focusing lens, wherein the periodically poled potassium titanyl phosphate crystal is a type II phase-matched nonlinear crystal, and the periodically poled potassium titanyl phosphate crystal is used to generate a photon pair without spectral correlation according to a parametric down-conversion process of the laser emitted by the laser light source; the focusing lens is arranged between the laser light source and the periodically poled potassium titanyl phosphate crystal, and is used to focus the laser on the periodically poled potassium titanyl phosphate crystal; The polarization region of the periodically polarized potassium titanyl phosphate crystal with type II phase matching is divided into a central region and two edge regions along the laser incident direction, and the central region is located between the two edge regions; the duty cycle range of the central region is 0.2-0.

8.

2. The photon pair source according to claim 1, characterized in that: The length of each of the edge regions is 1.25 times the length of the basic polarization region calculated according to the wavelength of the laser.

3. The photon pair source according to claim 1, characterized in that: The duty cycle of the central position of the periodically polarized potassium titanyl phosphate crystal is 0.

5.

4. The photon pair source according to claim 1, characterized in that: The laser light source is a pump laser light source, and the wavelength range of the generated laser is 770-810nm.

5. The photon pair source according to claim 4, characterized in that: The pump laser source generates the laser in a pulsed manner, and the half-width of the laser is in the range of 0.3-10 nm.

6. The photon pair source according to claim 2, characterized in that: The formula for calculating the length of the basic polarization region according to the wavelength of the laser is: in, is the length of the basic polarization region, is the wavelength of the laser.

7. The photon pair source according to claim 5, characterized in that: The length of the periodically poled potassium titanyl phosphate crystal is at least greater than the effective length of the triple nonlinear crystal when the frequency correlation is released, wherein the effective length is calculated based on the half-height width.

8. The photon pair source according to claim 4, characterized in that: The pump laser source is a femtosecond pulse laser.

Images