Quantum device

Abstract

A quantum device according to the present invention comprises a first diamond crystal layer that has a first center point and is shaped to extend in four mutually orthogonal directions from the first center point, a second diamond crystal layer that has a second center point and is shaped to extend in four mutually orthogonal directions from the second center point, and a color center that is on one of the first diamond crystal layer and the second diamond crystal layer. The first diamond crystal layer and the second diamond crystal layer comprise a quantum bit element that is layered such that at least a portion of the first center point and the second center point overlap, a first optical waveguide that is connected to each of four first end parts of the first diamond crystal layer, and a second optical waveguide that is connected to each of four second end parts of the second diamond crystal layer.　

Description

quantum devices

[0001] The present invention relates to quantum devices.

[0002] Diamond spin quantum computers have been proposed that use the electron spins of color centers, which are complex defects of impurity atoms and vacancies in diamond crystals, as quantum bits (see, for example, Patent Documents 1 to 4).

[0003] International Publication No. WO 2022 / 070341 International Publication No. WO 2022 / 259484 U.S. Patent Publication No. 2022 / 206361 U.S. Patent Publication No. 2021 / 117845

[0004] It is desirable to provide multiple quantum bit elements each having a diamond crystal layer containing color centers, and to enable the formation of quantum entanglement between adjacent quantum bit elements among the multiple quantum bit elements.

[0005] One aspect aims to enable the formation of quantum entanglement between multiple adjacent quantum bit elements.

[0006] In one aspect, the quantum device comprises: a first diamond crystal layer having a first central point and extending in four directions from the first central point so that each direction is perpendicular to one another; a second diamond crystal layer having a second central point and extending in four directions from the second central point so that each direction is perpendicular to one another; and a color center present in either the first diamond crystal layer or the second diamond crystal layer, wherein the first diamond crystal layer and the second diamond crystal layer are stacked so that at least a part of the first central point and the second central point overlap; a quantum bit element; first optical waveguides connected to four first ends of the first diamond crystal layer, respectively; and second optical waveguides connected to four second ends of the second diamond crystal layer, respectively.

[0007] In one aspect, quantum entanglement can be formed between adjacent qubit elements.

[0008] FIG. 1(a) is a plan view of a quantum bit element in Comparative Example 1, and FIG. 1(b) is a plan view of a quantum device according to Comparative Example 1. FIG. 2(a) is a plan view of a quantum bit element in Comparative Example 2, and FIG. 2(b) is a plan view of a quantum device according to Comparative Example 2. FIG. 3 is a tentative plan view of a quantum bit element in an example. FIG. 4 is a plan view of a quantum bit element in an example. FIGS. 5(a) to 5(c) are exploded plan views of FIG. 4. FIG. 6(a) is a cross-sectional view taken along line A-A in FIG. 4, and FIG. 6(b) is a cross-sectional view taken along line B-B in FIG. 4. FIGS. 7(a) to 7(c) are diagrams (part 1) illustrating a method for manufacturing a quantum device according to an example. FIGS. 8(a) and 8(b) are diagrams (part 2) illustrating a method for manufacturing a quantum device according to an example. FIGS. 9(a) to 9(c) are diagrams (part 3) illustrating a method for manufacturing a quantum device according to an example. 10(a) and 10(b) are cross-sectional views showing other examples of the location of a color center in an embodiment. FIGS. 11(a) and 11(b) are cross-sectional views of the vicinity of a color center in an embodiment. FIG. 12 is a plan view showing transmission of an optical pulse and a photon pulse in an embodiment. FIG. 13 is a plan view of a main portion of a quantum device according to an embodiment. FIG. 14 is a block diagram of a quantum device according to an embodiment. FIGS. 15(a) and 15(b) are cross-sectional views of a quantum bit element in a first modification of an embodiment. FIGS. 16(a) and 16(b) are cross-sectional views of a quantum bit element in a second modification of an embodiment. FIGS. 17(a) and 17(b) are cross-sectional views of a quantum bit element in a third modification of an embodiment.

[0009] First, in order to clarify the problem to be solved by the present invention, a quantum device according to a comparative example will be described.

[0010] Comparative Example 1 FIG. 1( a) is a plan view of a quantum bit device 500 in Comparative Example 1. As shown in FIG. 1( a), the quantum bit device 500 in Comparative Example 1 includes a diamond crystal layer 510 including a color center 516. An input optical waveguide 512 is connected to one end 522a of the diamond crystal layer 510, and an output optical waveguide 514 is connected to the other end 522b. The color center 516 is, for example, a nitrogen-vacancy center (NV center) composed of nitrogen and a vacancy. The input optical waveguide 512 transmits an optical pulse 518 for resonant excitation or initialization introduced into the color center 516. The output optical waveguide 514 transmits a photon pulse 520 of a single photon emitted from the color center 516 in response to the introduction of the optical pulse 518.

[0011] FIG. 1B is a plan view of a quantum device 550 according to Comparative Example 1. As shown in FIG. 1B, the quantum device 550 according to Comparative Example 1 has a plurality of quantum bit elements 500 arranged side by side in the X direction. An optical switch 530 is connected to an output optical waveguide 514 connected to each of the plurality of quantum bit elements 500. The optical switch 530 is, for example, a MEMS (Micro Electro Mechanical Systems) switch. An optical waveguide 532 and an optical waveguide 534 are connected to the optical switch 530. The optical waveguide 532 to which one of adjacent quantum bit elements 500 is connected via the optical switch 530 and the optical waveguide 534 to which the other is connected via the optical switch 530 are connected to a common beam splitter 536. Photodetectors 538a and 538b, which are single-photon photodetectors, are connected to the beam splitter 536.

[0012] A photon pulse 520 emitted by one of the adjacent quantum bit elements 500 into optical waveguide 532 and a photon pulse 520 emitted by the other of the adjacent quantum bit elements 500 into optical waveguide 534 are introduced into a common beam splitter 536 and detected by photodetector 538 a or photodetector 538 b. This allows quantum entanglement to be formed between the adjacent quantum bit elements 500.

[0013] As described above, quantum device 550 according to Comparative Example 1 includes optical switch 530 downstream of quantum bit element 500. Photon pulse 520, a single photon emitted by quantum bit element 500, is a minute signal and may be attenuated or eliminated due to a slight deviation in the switching operation of optical switch 530. Therefore, in a configuration in which optical switch 530 switches the path of photon pulse 520, photon pulse 520 may be attenuated or eliminated, making it impossible for photodetectors 538a and 538b to detect photon pulse 520.

[0014] [Comparative Example 2] Figure 2(a) is a plan view of a quantum bit device 600 in Comparative Example 2. As shown in Figure 2(a), in the quantum bit device 600 in Comparative Example 2, the diamond crystal layer 610 has a cross shape and four ends 622a, 622b, 622c, and 622d. An input optical waveguide 612a is connected to end 622a, and an input optical waveguide 612b is connected to end 622b. An output optical waveguide 614a is connected to end 622c, and an output optical waveguide 614b is connected to end 622d. In this way, while the quantum bit device 500 in Comparative Example 1 had a one-input, one-output structure, the quantum bit device 600 in Comparative Example 2 has a two-input, two-output structure.

[0015] The diamond crystal layer 610 includes a color center 616. The color center 616 is, for example, an NV center, and is located at the center of the diamond crystal layer 610. The input optical waveguides 612a and 612b transmit an optical pulse 618 introduced into the color center 616. When the optical pulse 618 is transmitted from either the input optical waveguide 612a or the input optical waveguide 612b toward the color center 616, the optical pulse 618 is not transmitted from the other input optical waveguide toward the color center 616. FIG. 2( a) illustrates the case where the optical pulse 618 is transmitted through the input optical waveguide 612a. The output optical waveguide 614a transmits a photon pulse 620 of a single photon emitted from the color center 616 in response to the introduction of the optical pulse 618 from the input optical waveguide 612a. The output optical waveguide 614b transmits a photon pulse 620 of a single photon emitted from the color center 616 in response to the introduction of the optical pulse 618 from the input optical waveguide 612b.

[0016] 2B is a plan view of a quantum device 650 according to Comparative Example 2. As shown in FIG. 2B, the quantum device 650 has a plurality of quantum bit elements 600 arranged side by side in the X direction. An output optical waveguide 614a connected to one of adjacent quantum bit elements 600 and an output optical waveguide 614b connected to the other are connected to a common beam splitter 630. Photodetectors 638a and 638b, which are single-photon photodetectors, are connected to the beam splitter 630.

[0017] A photon pulse 620 emitted by one of the adjacent quantum bit elements 600 into output optical waveguide 614 a and a photon pulse 620 emitted by the other of the adjacent quantum bit elements 600 into output optical waveguide 614 b are introduced into a common beam splitter 630 and detected by photodetector 638 a or photodetector 638 b, thereby forming quantum entanglement between the adjacent quantum bit elements 600.

[0018] As described above, because the quantum bit elements 600 in Comparative Example 2 have two inputs and two outputs, quantum entanglement can be formed between quantum bit elements 600 adjacent to each other in the X direction without using an optical switch as in Comparative Example 1. However, when multiple quantum bit elements 600 are arranged side by side in the Y direction in addition to the X direction, quantum entanglement cannot be formed between quantum bit elements 600 adjacent to each other in the Y direction.

[0019] Fig. 3 is a plan view (tentative) of a quantum bit device in an embodiment. Fig. 4 is a plan view of a quantum bit device 100 in an embodiment. The inventors of the present application considered giving the quantum bit device a four-input, four-output structure in order to further increase the number of entanglements.

[0020] 3, diamond crystal layer 710 has eight ends 722a, 722b, 722c, 722d, 722e, 722f, 722g, and 722h. Input optical waveguide 712a is connected to end 722b, input optical waveguide 712b is connected to end 722e, input optical waveguide 712c is connected to end 722g, and input optical waveguide 712d is connected to end 722h. Output optical waveguide 714a is connected to end 722a, output optical waveguide 714b is connected to end 722c, output optical waveguide 714c is connected to end 722d, and output optical waveguide 714d is connected to end 722f.

[0021] The diamond crystal layer 710 includes a color center 716. The color center 716 is, for example, an NV center, and is located at the center of the diamond crystal layer 710. The input optical waveguides 712a, 712b, 712c, and 712d transmit an optical pulse 718 introduced into the color center 716. The output optical waveguide 714a transmits a photon pulse 720 emitted from the color center 716 in response to the introduction of the optical pulse 718 from the input optical waveguide 712b. The output optical waveguide 714b transmits a photon pulse 720 emitted from the color center 716 in response to the introduction of the optical pulse 718 from the input optical waveguide 712c. The output optical waveguide 714c transmits a photon pulse 720 emitted from the color center 716 in response to the introduction of the optical pulse 718 from the input optical waveguide 712d. The output optical waveguide 714d transmits a photon pulse 720 emitted from the color center 716 in response to the introduction of an optical pulse 718 from the input optical waveguide 712a. As in Comparative Example 2, when the optical pulse 718 is transmitted through one of the input optical waveguides 712a, 712b, 712c, and 712d, it is not transmitted through the remaining input optical waveguides.

[0022] As shown in Figure 3, in a diamond crystal layer 710 having a shape extending in eight directions from a central point, the width W of the optical waveguide increases near the center. If the width W increases, higher-order modes may occur that deviate from the conditions for the fundamental mode, resulting in increased optical transmission loss. As a result, quantum entanglement may not be formed between adjacent quantum bit elements 700. For this reason, the structure of a quantum bit element 100 shown in Figure 4 is disclosed.

[0023] Figure 4 is a plan view of a quantum bit element 100 in an embodiment. Figures 5(a) to 5(c) are exploded plan views of Figure 4. Figure 6(a) is a cross-sectional view taken along line A-A in Figure 4, and Figure 6(b) is a cross-sectional view taken along line B-B in Figure 4. As shown in Figure 4, the quantum bit element 100 in an embodiment comprises a first diamond crystal layer 10, a second diamond crystal layer 20, and a color center 50.

[0024] As shown in FIG. 5B , the first diamond crystal layer 10 has a cross shape in plan view, extending from a first center point 11 in four mutually orthogonal directions. That is, the first diamond crystal layer 10 has a first central portion 12 and first extension portions 13 extending from the first central portion 12 in four mutually orthogonal directions. Here, "orthogonal" does not necessarily mean that the angle α between the first extension portions 13 is 90°, but rather can be in the range of 90°±10°. The first central portion 12 is, for example, a square-shaped region in plan view. The first center point 11 is, for example, a circle defined with a certain extent and including the center of the first central portion 12 in plan view. For example, the first center point 11 is a circle with a diameter of at least 1 / 10 and at most 1 / 2 of the length of one side of the first central portion 12. Note that the first center point 11 is not limited to a circular shape, and may be other shapes such as a rectangular shape. The four first extending portions 13 are congruent with one another in a plan view.

[0025] As shown in FIG. 5A , the second diamond crystal layer 20 also has a cross-shaped configuration in plan view, extending from a second center point 21 in four mutually orthogonal directions. That is, the second diamond crystal layer 20 has a second central portion 22 and second extension portions 23 extending from the second central portion 22 in four mutually orthogonal directions. Here, "orthogonal" does not necessarily mean that the angle β between the second extension portions 23 is 90°, but rather can be in the range of 90°±10°. The second central portion 22 is, for example, a square-shaped region in plan view. The second center point 21 is, for example, a circle defined with a certain extent including the center of the second central portion 22 in plan view, and is the same size as the first center point 11 in plan view. For example, the second center point 21 is a circle with a diameter of at least 1 / 10 and at most 1 / 2 of the length of one side of the second central portion 22. The second center point 21 is not limited to a circular shape and may be a rectangular shape or other shape. The four second extension portions 23 are congruent with one another in a plan view. Furthermore, the four second extension portions 23 are congruent with the four first extension portions 13 in a plan view.

[0026] As shown in Figures 4, 6(a) and 6(b), a first optical waveguide 30 is connected to each of the four first ends 14 of the first diamond crystal layer 10. A second optical waveguide 40 is connected to each of the four second ends 24 of the second diamond crystal layer 20. The first optical waveguide 30 and the second optical waveguide 40 are formed from, for example, sapphire.

[0027] 4, 5(a), 5(b), 5(c), 6(a) and 6(b), the first diamond crystal layer 10, the second diamond crystal layer 20, the first optical waveguide 30 and the second optical waveguide 40 are provided on a substrate 52. The substrate 52 is formed from a material having a smaller refractive index than the first optical waveguide 30 and the second optical waveguide 40, and is, for example, a silicon substrate with an oxide film when the first optical waveguide 30 and the second optical waveguide 40 are formed from sapphire.

[0028] The first optical waveguide 30 and the lower optical waveguide 41 constituting the second optical waveguide 40 are provided in contact with the upper surface of the substrate 52. The first diamond crystal layer 10 is provided on the first optical waveguide 30 with its first end 14 in contact with the upper surface of the third end 34 of the first optical waveguide 30. A gap 54 is formed between the first diamond crystal layer 10 and the substrate 52.

[0029] The upper optical waveguide 42 that constitutes the second optical waveguide 40 is provided on the lower optical waveguide 41. The second diamond crystal layer 20 is provided on the upper optical waveguide 42. One fifth end 45 of the upper optical waveguide 42 contacts the top surface of the sixth end 44 of the lower optical waveguide 41, and the other fourth end 46 contacts the bottom surface of the second end 24 of the second diamond crystal layer 20.

[0030] The second diamond crystal layer 20 is stacked on the first diamond crystal layer 10 so that the second central point 21 overlaps with at least a part of the first central point 11 of the first diamond crystal layer 10. The second diamond crystal layer 20 is in contact with the first diamond crystal layer 10. In a plan view, the second extension portion 23 of the second diamond crystal layer 20 is located between the first extension portions 13 of the first diamond crystal layer 10. The angle γ between the first extension portion 13 of the first diamond crystal layer 10 and the second extension portion 23 of the second diamond crystal layer 20 is 45°±5°.

[0031] The first end 14 of the first diamond crystal layer 10 has a tapered shape with its width gradually narrowing towards its tip. Similarly, the second end 24 of the second diamond crystal layer 20 has a tapered shape with its width gradually narrowing towards its tip. The third end 34 of the first optical waveguide 30 also has a tapered shape with its width gradually narrowing towards its tip. The sixth end 44 of the lower-layer optical waveguide 41 that constitutes the second optical waveguide 40 also has a tapered shape with its width gradually narrowing towards its tip. The fifth end 45 and fourth end 46 of the upper-layer optical waveguide 42 also have a tapered shape with their width gradually narrowing towards their tips.

[0032] The first diamond crystal layer 10 and the first optical waveguide 30 are in contact with each other with the tapered portions of the first end 14 and the third end 34 overlapping each other. This allows light to be transmitted between the first diamond crystal layer 10 and the first optical waveguide 30 with reduced transmission loss.

[0033] The second diamond crystal layer 20 and the upper-layer optical waveguide 42 constituting the second optical waveguide 40 are in contact with each other at the tapered portions of their second end 24 and fourth end 46. This allows light to be transmitted between the second diamond crystal layer 20 and the upper-layer optical waveguide 42 with reduced transmission loss. The upper-layer optical waveguide 42 and the lower-layer optical waveguide 41 are in contact with each other at the tapered portions of their fifth end 45 and sixth end 44. This allows light to be transmitted between the upper-layer optical waveguide 42 and the lower-layer optical waveguide 41 with reduced transmission loss.

[0034] The thickness T1 of the first diamond crystal layer 10 and the thickness T2 of the second diamond crystal layer 20 are, for example, 100 nm to 300 nm, and one example is 150 nm. The width W1 of the first extension 13 and the width W2 of the second extension 23 are, for example, 250 nm to 400 nm, and one example is 300 nm. The length of the tapered portion of the first end 14 of the first diamond crystal layer 10 and the length of the tapered portion of the second end 24 of the second diamond crystal layer 20 are, for example, 7 μm or more, and one example is 10 μm.

[0035] The thickness T3 of the first optical waveguide 30 and the thickness T4 of the lower optical waveguide 41 that constitutes the second optical waveguide 40 are, for example, 100 nm to 300 nm, and for example, 200 nm. The thickness T5 of the upper optical waveguide 42 that constitutes the second optical waveguide 40 are, for example, 100 nm to 300 nm, and for example, 150 nm. The width W3 of the first optical waveguide 30 and the width W4 of the lower optical waveguide 41 that constitutes the second optical waveguide 40 are, for example, 500 nm to 900 nm, and for example, 700 nm. The lengths of the tapered portions of the first optical waveguide 30 and the tapered portions of the lower optical waveguide 41 and upper optical waveguide 42 that constitute the second optical waveguide 40 are, for example, 7 μm or more, and for example, 10 μm.

[0036] The first diamond crystal layer 10 contains one color center 50. The second diamond crystal layer 20 does not contain a color center. In this way, there is only one color center 50. If multiple color centers exist nearby, the light emitted from one color center will be absorbed by another color center, so there is only one color center 50.

[0037] The color center 50 is present in the first diamond crystal layer 10 near the interface between the first diamond crystal layer 10 and the second diamond crystal layer 20. The color center 50 is a type of complex defect formed by an impurity atom in the diamond single crystal and a vacancy adjacent to the impurity atom. The impurity atom is at least one of nitrogen (N), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), and boron (B). The color center 50 is formed so as to overlap the first center point 11 of the first diamond crystal layer 10 and the second center point 21 of the second diamond crystal layer 20 in plan view.

[0038] The structure of the quantum bit device 100 disclosed in FIGS. 4 to 6B allows quantum entanglement to be formed between a plurality of adjacent quantum bits, and also makes it possible to suppress the transmission loss of light.

[0039] [Manufacturing Method] Figures 7(a) to 9(c) are diagrams illustrating a manufacturing method for a quantum device according to an embodiment. As shown in Figure 7(a), at least one of nitrogen (N), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), and boron (B) is ion-implanted into a diamond substrate 60. For example, the ion implantation is performed using a focused ion beam method. As a result, multiple color centers 50 are formed in the diamond substrate 60. The depth to which the color centers 50 are formed can be controlled by the implantation energy during ion implantation. Note that a thinning process is performed before or after the ion implantation to achieve a desired thickness of the diamond substrate 60.

[0040] 7(b), a number of color centers 50 with good luminescence properties are selected from the plurality of color centers 50, and a mask pattern 61 corresponding to the shape of the first diamond crystal layer 10 is formed on the diamond substrate 60. The mask pattern 61 is formed from, for example, resist.

[0041] 7(c), the diamond substrate 60 is etched using the mask pattern 61 as a mask. For example, the diamond substrate 60 is etched using inductively coupled plasma etching. As a result, a plurality of first diamond crystal layers 10 containing color centers 50 are formed.

[0042] 8(a), a mask pattern 64 corresponding to the shape of the second diamond crystal layer 20 is formed on another diamond substrate 63 that has been thinned to a desired thickness. The mask pattern 64 is formed from, for example, a resist.

[0043] 8(b), the diamond substrate 63 is etched using the mask pattern 64 as a mask. For example, the diamond substrate 63 is etched using inductively coupled plasma etching. As a result, a plurality of second diamond crystal layers 20 are formed.

[0044] 9A, after the sapphire layer formed on the substrate 52 is thinned to a desired thickness, the sapphire layer is etched using a mask pattern (not shown) formed on the sapphire layer as a mask, thereby forming the first optical waveguide 30 and the lower-layer optical waveguide 41 constituting the second optical waveguide 40 on the substrate 52.

[0045] As shown in Figure 9(b), the first diamond crystal layer 10 produced in Figures 7(a) to 7(c) is placed on the first optical waveguide 30. At this time, the first diamond crystal layer 10 is placed on the first optical waveguide 30 with the surface adjacent to the color center 50 facing up. The first diamond crystal layer 10 is placed on the first optical waveguide 30 so that its first end 14 overlaps and contacts the sixth end 44 of the first optical waveguide 30. In addition, an upper-layer optical waveguide 42, which has been prepared in advance by etching a sapphire substrate thinned to the desired thickness, is placed on the lower-layer optical waveguide 41. The upper-layer optical waveguide 42 is placed on the lower-layer optical waveguide 41 so that its fifth end 45 overlaps and contacts the sixth end 44 of the lower-layer optical waveguide 41. The first optical waveguide 30 and the first diamond crystal layer 10, and the lower optical waveguide 41 and the upper optical waveguide 42 are fixed in place by van der Waals forces generated by their contact with each other.

[0046] As shown in Figure 9(c), the second diamond crystal layer 20 produced in Figures 8(a) and 8(b) is placed on the first diamond crystal layer 10 and on the upper optical waveguide 42. The second diamond crystal layer 20 is placed on the first diamond crystal layer 10 so that the second centre point 21 overlaps and contacts at least a part of the first centre point 11 of the first diamond crystal layer 10. The second diamond crystal layer 20 is also placed on the upper optical waveguide 42 so that the second end 24 overlaps and contacts the fourth end 46 of the upper optical waveguide 42. The first diamond crystal layer 10 and the second diamond crystal layer 20, and the upper optical waveguide 42 and the second diamond crystal layer 20 are fixed together by van der Waals forces generated by their contact with each other.

[0047] Figures 10(a) and 10(b) are cross-sectional views showing other examples of the locations of color centers 50 in the examples. As shown in Figures 10(a) and 10(b), the first diamond crystal layer 10 does not contain a color center, and the second diamond crystal layer 20 contains only one color center 50. The color center 50 is present in the second diamond crystal layer 20 near the interface between the first diamond crystal layer 10 and the second diamond crystal layer 20.

[0048] A quantum device in which the first diamond crystal layer 10 does not contain a color center and the second diamond crystal layer 20 contains a color center 50 is manufactured by the following manufacturing method. First, a second diamond crystal layer 20 containing a color center 50 is formed by a method similar to that shown in Figures 7(a) to 7(c). A first diamond crystal layer 10 not containing a color center 50 is formed by a method similar to that shown in Figures 8(a) and 8(b). Next, as in Figure 9(b), the first diamond crystal layer 10 is placed on the first optical waveguide 30, and an upper optical waveguide 42 is placed on the lower optical waveguide 41. The difference from Figure 9(b) is that the first diamond crystal layer 10 does not contain a color center 50. Next, as in Figure 9(c), a second diamond crystal layer 20 is placed on the first diamond crystal layer 10 and the upper optical waveguide 42. The difference from Figure 9(c) is that the second diamond crystal layer 20 includes a color center 50 and is placed on the first diamond crystal layer 10 with the face adjacent to the color center 50 facing towards the first diamond crystal layer 10.

[0049] Figures 11(a) and 11(b) are cross-sectional views of the vicinity of a color center 50 in an example. Figure 11(a) shows a case where the color center 50 is present in the first diamond crystal layer 10. In this case, the distance L between the color center 50 and the interface between the first diamond crystal layer 10 and the second diamond crystal layer 20 is 30 nm or less. Figure 11(b) shows a case where the color center 50 is present in the second diamond crystal layer 20. In this case, the distance L between the color center 50 and the interface between the first diamond crystal layer 10 and the second diamond crystal layer 20 is also 30 nm or less.

[0050] Figure 12 is a plan view showing the transmission of an optical pulse 70 and a photon pulse 72 in this example. As shown in Figure 12, the four first optical waveguides connected to the four first ends 14 of the first diamond crystal layer 10 are referred to as first optical waveguides 30a, 30b, 30c and 30d. The four second optical waveguides connected to the four second ends 24 of the second diamond crystal layer 20 are referred to as second optical waveguides 40a, 40b, 40c and 40d.

[0051] The first optical waveguide 30 a, the second optical waveguide 40 c, the second optical waveguide 40 d, and the first optical waveguide 30 d are input optical waveguides through which the optical pulse 70 for resonance excitation or initialization introduced into the color center 50 propagates.

[0052] The second optical waveguide 40a is an output optical waveguide that transmits a photon pulse 72 of a single photon emitted from the color center 50 in response to the introduction of the optical pulse 70 from the second optical waveguide 40c. The second optical waveguide 40b is an output optical waveguide that transmits a photon pulse 72 of a single photon emitted from the color center 50 in response to the introduction of the optical pulse 70 from the second optical waveguide 40d. The first optical waveguide 30b is an output optical waveguide that transmits a photon pulse 72 of a single photon emitted from the color center 50 in response to the introduction of the optical pulse 70 from the first optical waveguide 30d. The first optical waveguide 30c is an output optical waveguide that transmits a photon pulse 72 of a single photon emitted from the color center 50 in response to the introduction of the optical pulse 70 from the first optical waveguide 30a. As in Comparative Examples 2 and 3, when an optical pulse 70 is propagating through any one of the first optical waveguide 30 a, the second optical waveguide 40 c, the second optical waveguide 40 d, and the first optical waveguide 30 d, it is not propagating through the remaining optical waveguides. However, for convenience of explanation, in Fig. 12, the optical pulse 70 is illustrated in all of the first optical waveguide 30 a, the second optical waveguide 40 c, the second optical waveguide 40 d, and the first optical waveguide 30 d. Similarly, in Fig. 12, a photon pulse 72 is illustrated in all of the second optical waveguide 40 a, the second optical waveguide 40 b, the first optical waveguide 30 b, and the first optical waveguide 30 c.

[0053] FIG. 13 is a plan view of a main portion of a quantum device 200 according to an embodiment. As shown in FIG. 13 , the quantum device 200 according to the embodiment has a plurality of quantum bit elements 100 arranged in a lattice pattern in the X and Y directions. The second optical waveguide 40b of one of the quantum bit elements 100 adjacent in the X direction and the first optical waveguide 30c of the other quantum bit element 100 are connected to a common beam splitter 74. The beam splitter 74 is an example of a branching element. Photodetectors 76a and 76b are connected to the beam splitter 74. The photodetectors 76a and 76b are single-photon photodetectors, such as a SPAD (Single Photon Avalanche Photo Detector) or a SNSPD (Superconducting Nanowire Single Photon Detector).

[0054] A photon pulse 72 emitted by one of the quantum bit elements 100 adjacent in the X direction to the second optical waveguide 40b and a photon pulse 72 emitted by the other of the quantum bit elements 100 to the first optical waveguide 30c are introduced into a common beam splitter 74. The beam splitter 74 splits the photon pulse 72 emitted by one of the quantum bit elements 100 adjacent in the X direction to the second optical waveguide 40b and the photon pulse 72 emitted by the other of the quantum bit elements 100 to the first optical waveguide 30c into a first direction or a second direction. The first direction and the second direction are orthogonal to each other in the beam splitter 74, and the probability of splitting into the first direction and the second direction is 50% for each. The beam splitter 74 is, for example, a half mirror that divides the amount of transmitted light and the amount of reflected light at approximately 1:1. The photon pulse 72 emitted by one of the adjacent quantum bit elements 100 hits one surface of the half mirror and is reflected or transmitted. The photon pulse 72 emitted by the other of the adjacent quantum bit elements 100 strikes the other surface of the half mirror and is reflected or transmitted. The photodetector 76a detects the photon pulse 72 that is incident on one surface of the half mirror and reflected, or the photon pulse 72 that is incident on the other surface and transmitted. The photodetector 76b detects the photon pulse 72 that is incident on the other surface of the half mirror and reflected, or the photon pulse 72 that is incident on one surface and transmitted. The photon pulse 72 emitted by one of the adjacent quantum bit elements 100 and the photon pulse 72 emitted by the other are combined at the beam splitter 74 to form a two-photon correlation. This allows quantum entanglement to be formed between quantum bit elements 100 that are adjacent in the X direction.

[0055] The first optical waveguide 30b of one of the quantum bit elements 100 adjacent in the Y direction and the second optical waveguide 40a of the other quantum bit element 100 are connected to a common beam splitter 74. Therefore, a photon pulse 72 emitted into the first optical waveguide 30b by one of the quantum bit elements 100 adjacent in the Y direction and a photon pulse 72 emitted into the second optical waveguide 40a by the other quantum bit element 100 adjacent in the Y direction are introduced into the common beam splitter 74 and detected by the photodetector 76a or the photodetector 76b. This makes it possible to form quantum entanglement between the quantum bit elements 100, 110 adjacent in the Y direction.

[0056] 14 is a block diagram of a quantum device 200 according to an embodiment. For clarity, the beam splitter 74 and the photodetectors 76 a and 76 b are not shown in FIG. 14. The beam splitter 74 and the photodetectors 76 a and 76 b are provided near the quantum bit element 100.

[0057] 14 , quantum device 200 includes light source 80, optical fiber 82, optical switches 84 a, 84 b, 84 c, and 84 d, a plurality of quantum bit elements 100, and, although not shown, beam splitter 74 and photodetectors 76 a and 76 b. Light source 80 is, for example, a laser light source, and generates optical pulses 70 for resonance excitation or initialization. Optical pulses 70 are introduced via optical fiber 82 to optical switches 84 a, 84 b, 84 c, and 84 d.

[0058] The optical switches 84a, 84b, 84c, and 84d are, for example, MEMS switches. The optical switches 84a, 84b, 84c, and 84d introduce optical pulses 70 into two adjacent quantum bit elements 100 among the plurality of quantum bit elements 100 arranged in a lattice pattern. The optical pulses 70 are introduced into the color center 50 from the first optical waveguide 30a, the second optical waveguide 40c, the second optical waveguide 40d, or the first optical waveguide 30d (see FIG. 12 ) of the two adjacent quantum bit elements 100. This allows quantum entanglement to be formed between the plurality of adjacent quantum bit elements 100, as shown in FIG. 13 .

[0059] According to the embodiment, as shown in Fig. 4, the quantum bit device 100 comprises a first diamond crystal layer 10, a second diamond crystal layer 20, and a color center 50. As shown in Fig. 5(b), the first diamond crystal layer 10 has a first center point 11 and extends in four directions from the first center point 11 so that each direction is perpendicular to each other. As shown in Fig. 5(a), the second diamond crystal layer 20 has a second center point 21 and extends in four directions from the second center point 21 so that each direction is perpendicular to each other. The first diamond crystal layer 10 and the second diamond crystal layer 20 are stacked so that at least a portion of the first center point 11 and the second center point 21 overlap. Then, a first optical waveguide 30 is connected to each of the four first ends 14 of the first diamond crystal layer 10, and a second optical waveguide 40 is connected to each of the four second ends 24 of the second diamond crystal layer 20. As a result, the quantum bit element 100 has a four-input / four-output structure, and quantum entanglement can be formed between quantum bit elements 100 adjacent in the X and Y directions, as shown in FIG. 13 . The first diamond crystal layer 10 extends in four directions perpendicular to each other from the first center point 11, and the second diamond crystal layer 20 extends in four directions perpendicular to each other from the second center point 21. This prevents the width of the optical waveguide near the center of the first and second diamond crystal layers 10, 20 from increasing. This prevents the generation of higher-order modes, thereby suppressing the attenuation or extinction of the photon pulse 72, which is a single photon emitted by the color center 50. It is preferable that the first diamond crystal layer 10 and the second diamond crystal layer 20 are stacked so that at least half of the first center point 11 and the second center point 21 overlap. It is more preferable that they are stacked so that at least three-quarters of the layers overlap. It is even more preferable that they are stacked so that at least four-fifths of the layers overlap. It is most preferable that they are all stacked together.

[0060] 4 , in the example, the color center 50 is located so as to overlap, in plan view, the first center point 11 of the first diamond crystal layer 10 and the second center point 21 of the second diamond crystal layer 20. This makes it easier for the light pulse 70 transmitted through the first optical waveguide 30 and the second optical waveguide 40 to be introduced into the color center 50, and also makes it easier for the photon pulse 72 emitted by the color center 50 to be transmitted to the first optical waveguide 30 and the second optical waveguide 40.

[0061] 6(a) and 6(b), the first diamond crystal layer 10 is provided on the first optical waveguide 30 with its first end 14 in contact with the upper surface of the third end 34 of the first optical waveguide 30. The second diamond crystal layer 20 is provided on the second optical waveguide 40 with its second end 24 in contact with the upper surface of the fourth end 46 of the upper layer optical waveguide 42 that constitutes the second optical waveguide 40. This makes it easier for light to be transmitted between the first diamond crystal layer 10 and the first optical waveguide 30, and also makes it easier for light to be transmitted between the second diamond crystal layer 20 and the second optical waveguide 40.

[0062] 6(a) and 6(b), in the embodiment, the second diamond crystal layer 20 is laminated on the first diamond crystal layer 10. The first optical waveguide 30 is provided in a layer below the first diamond crystal layer 10. The second optical waveguide 40 has a lower layer optical waveguide 41 provided in the same layer as the first optical waveguide 30, and an upper layer optical waveguide 42 provided in the same layer as the first diamond crystal layer 10. This provides a configuration in which light is easily transmitted between the first diamond crystal layer 10 and the first optical waveguide 30, and between the second diamond crystal layer 20 and the second optical waveguide 40.

[0063] 5(a) to 5(c), 6(a) and 6(b), the first diamond crystal layer 10 is provided on the first optical waveguide 30 with its first end 14, which tapers towards its tip, in contact with the third end 34, which tapers towards the tip of the first optical waveguide 30. The upper optical waveguide 42 constituting the second diamond crystal layer 20 is provided on the lower optical waveguide 41 with its fifth end 45, which tapers towards its tip, in contact with the sixth end 44, which tapers towards the tip of the lower optical waveguide 41. The second diamond crystal layer 20 is provided on the upper optical waveguide 42 with its second end 24, which tapers towards its tip, in contact with the fourth end 46, which tapers towards the tip of the upper optical waveguide 42. This results in low loss in the transmission of light between the first optical waveguide 30 and the first diamond crystal layer 10, and low loss in the transmission of light between the second optical waveguide 40 and the second diamond crystal layer 20.

[0064] 12, of the first optical waveguides 30a-30d connected to the first end 14 of the first diamond crystal layer 10, the first optical waveguides 30a and 30d are input optical waveguides through which an optical pulse 70 introduced into the color center 50 is transmitted. The first optical waveguides 30b and 30c are output optical waveguides through which a photon pulse 72 emitted by the color center 50 is transmitted. Of the second optical waveguides 40a-40d connected to the second end 24 of the second diamond crystal layer 20, the second optical waveguides 40c and 40d are input optical waveguides through which an optical pulse 70 introduced into the color center 50 is transmitted. The second optical waveguides 40a and 40b are output optical waveguides through which a photon pulse 72 emitted by the color center 50 is transmitted. This makes it possible to form quantum entanglement between quantum bit elements 100 adjacent in the X and Y directions, as shown in FIG.

[0065] 11(a) and 11(b), the color center 50 is located at a position within a distance L of 30 nm from the interface between the first diamond crystal layer 10 and the second diamond crystal layer 20. This makes it easier for both the optical pulse 70 transmitted from the first optical waveguide 30 to the first diamond crystal layer 10 and the optical pulse 70 transmitted from the second optical waveguide 40 to the second diamond crystal layer 20 to be introduced into the color center 50. Furthermore, the photon pulse 72 emitted from the color center 50 is easier to be transmitted to both the first optical waveguide 30 connected to the first diamond crystal layer 10 and the second optical waveguide 40 connected to the second diamond crystal layer 20.

[0066] 4, 5(a) and 5(b), in the example, the first extensions 13 extending in four directions from the first central point 11 in the first diamond crystal layer 10 are located between the second extensions 23 extending in four directions from the second central point 21 in the second diamond crystal layer 20. This allows quantum entanglement to be formed between quantum bit elements 100 adjacent in the X and Y directions, as shown in FIG.

[0067] 13 , a plurality of quantum bit elements 100 are arranged in a lattice pattern. The second optical waveguide 40 b or the first optical waveguide 30 b connected to one of the adjacent quantum bit elements 100, and the first optical waveguide 30 c or the second optical waveguide 40 a connected to the other of the adjacent quantum bit elements 100, are connected to a common beam splitter 74. The beam splitter 74 splits the photon pulse 72 emitted by one of the adjacent quantum bit elements 100 and the photon pulse 72 emitted by the other of the adjacent quantum bit elements 100 in a first direction or a second direction. This allows quantum entanglement to be formed between the adjacent quantum bit elements 100.

[0068] In this embodiment, as shown in FIG. 13, the beam splitter 74 is provided with photodetectors 76a and 76b for detecting the photon pulse 72 split in the first direction or the photon pulse 72 split in the second direction.

[0069] In the embodiment, the first optical waveguide 30 and the second optical waveguide 40 are formed of sapphire, but they may be formed of other materials as long as they are capable of transmitting the optical pulse 70 and the photon pulse 72. For example, the first optical waveguide 30 and the second optical waveguide 40 may be formed of silicon oxide, silicon nitride, or silicon carbide. Although the substrate 52 is a silicon substrate with an oxide film, the substrate may be formed of other materials as long as they have a smaller refractive index than the first optical waveguide 30 and the second optical waveguide 40.

[0070] [Modifications] Figures 15(a) and 15(b) are cross-sectional views of a quantum bit element 110 in Modification 1 of the embodiment. As shown in Figures 15(a) and 15(b), in the quantum bit element 110 in Modification 1 of the embodiment, a recess 26 is formed in the second diamond crystal layer 20 in the region where the first diamond crystal layer 10 and the second diamond crystal layer 20 overlap. The recess 26 has, for example, a rectangular shape in cross section. The depth of the recess 26 is, for example, not less than ¼ and not more than ¾ of the thickness of the second diamond crystal layer 20. The recess 26 is, for example, provided in the entire region where the first diamond crystal layer 10 and the second diamond crystal layer 20 overlap. The other configurations are the same as those of the embodiment, so description thereof will be omitted.

[0071] Figures 16(a) and 16(b) are cross-sectional views of a quantum bit element 120 in modified example 2 of the embodiment. As shown in Figures 16(a) and 16(b), in the quantum bit element 120 in modified example 2 of the embodiment, a recess 26a is formed in the first diamond crystal layer 10 in the region where the first diamond crystal layer 10 and the second diamond crystal layer 20 overlap. No recess 26 is formed in the second diamond crystal layer 20. The recess 26a has, for example, a rectangular shape in cross section. The depth of the recess 26a is, for example, not less than 1 / 4 and not more than 3 / 4 of the thickness of the first diamond crystal layer 10. The recess 26a is, for example, provided in the entire region where the first diamond crystal layer 10 and the second diamond crystal layer 20 overlap. The other configurations are the same as those in the embodiment, so description thereof will be omitted.

[0072] Figures 17(a) and 17(b) are cross-sectional views of a quantum bit element 130 in modified example 3 of the embodiment. As shown in Figures 17(a) and 17(b), in the quantum bit element 130 in modified example 3 of the embodiment, in the region where the first diamond crystal layer 10 and the second diamond crystal layer 20 overlap, a recess 26a is formed in the first diamond crystal layer 10, and a recess 26 is formed in the second diamond crystal layer 20. As the other configurations are the same as those in the embodiment, explanations will be omitted.

[0073] In modified examples 1 to 3 of the embodiment, at least one of the first diamond crystal layer 10 and the second diamond crystal layer 20 has recesses 26, 26a in the region where the first diamond crystal layer 10 and the second diamond crystal layer 20 overlap. This prevents the thickness in the region where the first diamond crystal layer 10 and the second diamond crystal layer 20 overlap from increasing, and prevents the effective refractive index from increasing. Therefore, it is possible to prevent the generation of higher-order modes in the region where the first diamond crystal layer 10 and the second diamond crystal layer 20 overlap, and to prevent an increase in optical transmission loss.

[0074] Furthermore, in the third modified example of the embodiment, recesses 26a are formed in the first diamond crystal layer 10 and recesses 26 are formed in the second diamond crystal layer 20. This makes it possible to make the recesses 26a in the first diamond crystal layer 10 and the recesses 26 in the second diamond crystal layer 20 shallower, thereby ensuring the optical transmission paths of the first and second diamond crystal layers 10, 20, while reducing the thickness in the overlapping region.

[0075] In the first and second modified examples of the embodiment, from the viewpoint of ensuring an optical transmission path while reducing the thickness in the overlapping region of the first and second diamond crystal layers 10, 20, the depth of the recesses 26, 26a is preferably approximately 1 / 2 of the thickness of the first and second diamond crystal layers 10, 20. That is, the depth of the recesses 26, 26a is preferably not less than 1 / 3 and not more than 2 / 3 of the thickness of the first and second diamond crystal layers 10, 20, and more preferably not less than 5 / 12 and not more than 7 / 12. In the third modified example of the embodiment, since both recesses 26, 26a are provided, the depth of each of the recesses 26, 26a is preferably not less than 1 / 6 and not more than 1 / 3 of the thickness of the first and second diamond crystal layers 10, 20, and more preferably not less than 5 / 24 and not more than 7 / 24.

[0076] In the first to third modified examples of the embodiment, recesses 26, 26a are shown as having a rectangular shape in cross section, but they may also have other shapes such as a semicircular, semi-elliptical or triangular shape. From the viewpoint of preventing the effective refractive index from becoming too high, recesses 26, 26a are preferably provided over the entire region where first and second diamond crystal layers 10, 20 overlap, but they may also be provided over one-quarter or more, one-half or more, or three-quarters or more of the region.

[0077] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims.

[0078] DESCRIPTION OF SYMBOLS 10... First diamond crystal layer, 11... First center point, 12... First central part, 13... First extending part, 14... First end part, 20... Second diamond crystal layer, 21... Second central point, 22... Second central part, 23... Second extending part , 24... second end, 26, 26a... recess, 30, 30a-30d... first optical waveguide, 34... third end, 40, 40a-40d... second optical waveguide, 41... lower layer optical waveguide, 42... upper layer optical waveguide, 44... sixth end, 4 5...fifth end, 46...fourth end, 50...color center, 52...substrate, 60...diamond substrate, 61...mask pattern, 63...diamond substrate, 64...mask pattern, 70...light pulse, 72...photon pulse, 74...beam splitter, 76a, 76b...photodetector, 80...light source, 82...optical fiber, 84a to 84d...optical switch, 100, 110, 120, 130...qubit element, 200...quantum device

Claims

1. A quantum bit device comprising: a first diamond crystal layer having a first center point and having a shape extending in four directions so as to be orthogonal to each other from the first center point; a second diamond crystal layer having a second center point and having a shape extending in four directions so as to be orthogonal to each other from the second center point; and a color center existing in either the first diamond crystal layer or the second diamond crystal layer, wherein the first diamond crystal layer and the second diamond crystal layer are laminated such that at least a part of the first center point and the second center point overlap; a first optical waveguide connected to each of four first ends of the first diamond crystal layer; and a second optical waveguide connected to each of four second ends of the second diamond crystal layer.

2. The quantum device according to claim 1, wherein the color center exists overlapping the first center point and the second center point in a plan view.

3. The quantum device according to claim 1 or 2, wherein the first diamond crystal layer is provided on the first optical waveguide with the first end in contact with the upper surface of the third end of the first optical waveguide, and the second diamond crystal layer is provided on the second optical waveguide with the second end in contact with the upper surface of the fourth end of the second optical waveguide.

4. The quantum device according to claim 3, wherein the second diamond crystal layer is laminated on the first diamond crystal layer, the first optical waveguide is provided in a lower layer than the first diamond crystal layer, and the second optical waveguide includes a lower optical waveguide provided in the same layer as the first optical waveguide and an upper optical waveguide provided in the same layer as the first diamond crystal layer.

5. The first diamond crystal layer is provided on the first optical waveguide such that the first end portion, which narrows in width toward the tip, contacts the third end portion, which also narrows in width toward the tip of the first optical waveguide. The upper optical waveguide is provided on the lower optical waveguide such that the fifth end portion, which narrows in width toward the tip, contacts the sixth end portion, which also narrows in width toward the tip of the lower optical waveguide. The second diamond crystal layer is provided on the upper optical waveguide such that the second end portion, which narrows in width toward the tip, contacts the fourth end portion, which also narrows in width toward the tip of the upper optical waveguide. The quantum device according to claim 4, characterized in that it is so configured.

6. Two of the four first optical waveguides connected to the four first end portions of the first diamond crystal layer and two of the four second optical waveguides connected to the four second end portions of the second diamond crystal layer are input optical waveguides through which light introduced into the color center is transmitted. The remaining two of the four first optical waveguides and the remaining two of the four second optical waveguides are output optical waveguides through which photons emitted by the color center are transmitted. The quantum device according to claim 1 or 2, characterized in that it is so configured.

7. The color center is present within 30 nm from the interface between the first diamond crystal layer and the second diamond crystal layer. The quantum device according to claim 1 or 2, characterized in that it is so configured.

8. At least one of the first diamond crystal layer and the second diamond crystal layer has a recess in a region where the first diamond crystal layer and the second diamond crystal layer overlap. The quantum device according to claim 1 or 2, characterized in that it is so configured.

9. The first extending portions extending in the four directions from the first center point in the first diamond crystal layer are located between the second extending portions extending in the four directions from the second center point in the second diamond crystal layer. The quantum device according to claim 1 or 2, characterized in that it is so configured.

10. The quantum device according to claim 1 or 2, further comprising a branching element connected to the first optical waveguide or the second optical waveguide connected to one of the adjacent quantum bit elements and the first optical waveguide or the second optical waveguide connected to the other quantum bit element, the branching element configured to branch a first photon emitted by the one quantum bit element and a second photon emitted by the other quantum bit element in a first direction or a second direction.

11. The quantum device according to claim 10, wherein the branching element includes a first photodetector and a second photodetector configured to detect the first photon or the second photon branched in the first direction or the second direction.

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