Quantum device and method for controlling quantum device

Abstract

This quantum device comprises: a plurality of quantum bit elements each including a diamond crystal layer, which has a central part and extension parts connected to the central part and extending in 2n directions (n is an integer of 2 or more) from the central part, and a color center formed in the central part; input optical waveguides which are respectively connected to n end sections of the extension parts and transmit light that enters the color center; output optical waveguides which are respectively connected to the n end sections of the extension parts and transmit photons emitted by the color center; and a splitter connected to an output waveguide of a first quantum bit element among the plurality of quantum bit elements and an output waveguide of a second quantum bit element among the plurality of quantum bit elements.　

Description

Quantum device and method for controlling quantum device

[0001] The present invention relates to a quantum device and a method for controlling a quantum device.

[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. 2007 / 0252081 U.S. Patent Publication No. 2020 / 0287631

[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] In one aspect, the object is to enable the formation of quantum entanglement between adjacent qubit elements.

[0006] In one aspect, the quantum device comprises a plurality of quantum bit elements, each comprising a diamond crystal layer having a central portion and an extension portion connected to the central portion and extending in a 2n direction (n is an integer of 2 or more) from the central portion, and a color center formed in the central portion; input optical waveguides connected to n ends of the extension portion, respectively, for transmitting light introduced to the color center; output optical waveguides connected to n ends of the extension portion, respectively, for transmitting photons emitted by the color center; and branching elements connected to the output waveguide of a first quantum bit element of the plurality of quantum bit elements and the output waveguide of a second quantum bit element of the plurality of quantum bit elements.

[0007] In one aspect, a method for controlling a quantum device includes a plurality of quantum bit elements, each of which includes a diamond crystal layer having a central portion and an extension portion connected to the central portion and extending in a 2n direction (n is an integer of 2 or more) from the central portion, and a color center formed in the central portion; input optical waveguides connected to n ends of the extension portion, respectively, for transmitting light introduced to the color center; output optical waveguides connected to n ends of the extension portion, respectively, for transmitting photons emitted from the color center; and branching elements connected to the output waveguide of a first quantum bit element of the plurality of quantum bit elements and the output waveguide of a second quantum bit element of the plurality of quantum bit elements, wherein the method introduces light into the first quantum bit element and the second quantum bit element, and causes quantum entanglement to be formed between a first photon emitted from the first quantum bit and a second photon emitted from the second quantum bit by the branching element.

[0008] In one aspect, quantum entanglement can be formed between adjacent qubit elements.

[0009] FIG. 1 is a plan view of a quantum device according to a comparative example. FIG. 2 is a plan view of a quantum bit element according to a first example. FIG. 3 is a plan view of a quantum device according to the first example. FIG. 4 is a plan view of a quantum device according to a second example. FIG. 5 is a cross-sectional view of a location where a common optical waveguide and an output optical waveguide intersect in the second example. FIGS. 6( a) and 6(b) are cross-sectional views illustrating transmission of an optical pulse from a common optical waveguide to an input optical waveguide in the second example. FIG. 7 is a plan view illustrating an example of control by a control unit in the second example. FIG. 8 is a flowchart illustrating an example of control by a control unit in the second example. FIG. 9 is a plan view of a quantum bit element according to a third example. FIG. 10 is a plan view of a quantum device according to the third example. FIG. 11 is a plan view of a quantum device according to a fourth example. FIGS. 12(a) to 12(c) are cross-sectional views of a location where a common optical waveguide and an output optical waveguide intersect in the fourth example. 13(a) and 13(b) are cross-sectional views illustrating transmission of an optical pulse from a common optical waveguide to an input optical waveguide in Example 4. FIG. 14 is a plan view illustrating an example of control by a control unit in Example 4. FIG. 15 is a flowchart showing an example of control by a control unit in Example 4. FIG. 16 is a plan view of a quantum device according to Example 5. FIG. 17 is a plan view of a quantum device according to Example 6.

[0010] 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.

[0011] Comparative Example FIG. 1 is a plan view of a quantum device 1000 according to a comparative example. As shown in FIG. 1, the quantum device 1000 according to the comparative example has multiple quantum bit elements 900 arranged side by side in one direction. The quantum bit element 900 includes a diamond crystal layer 912 including a color center 914. An input optical waveguide 916 is connected to one end of the diamond crystal layer 912, and an output optical waveguide 918 is connected to the other end. The color center 914 is, for example, a nitrogen-vacancy center (NV center) formed from nitrogen and a vacancy. The input optical waveguide 916 transmits an optical pulse 920 introduced into the color center 914. The output optical waveguide 918 transmits a single-photon photon pulse 922 emitted from the color center 914 in response to the introduction of the optical pulse 920.

[0012] An optical switch 930 is connected to the output optical waveguide 918 of each of the multiple quantum bit elements 900. The optical switch 930 is, for example, a MEMS (Micro Electro Mechanical Systems) switch. An optical waveguide 932 and an optical waveguide 934 are connected to the optical switch 930. The optical waveguide 932 to which one of adjacent quantum bit elements 900 is connected via the optical switch 930 and the optical waveguide 934 to which the other is connected via the optical switch 930 are connected to a common beam splitter 936. Photodetectors 938a and 938b, which are single-photon photodetectors, are connected to the beam splitter 936.

[0013] A photon pulse 922 emitted by one of adjacent quantum bit elements 900 into optical waveguide 932 via optical switch 930, and a photon pulse 922 emitted by the other of adjacent quantum bit elements 900 into optical waveguide 934 via optical switch 930, are introduced into a common beam splitter 936 and detected by photodetector 938a or photodetector 938b. This makes it possible to form quantum entanglement between adjacent quantum bit elements 900. The provision of optical switch 930 makes it possible to form quantum entanglement between adjacent quantum bit elements 900 on both sides.

[0014] As described above, in the comparative example, quantum entanglement can be formed by changing the combination of quantum bit elements 900 by switching optical switch 930. However, since single-photon photon pulse 922 emitted by quantum bit element 900 is a minute signal with low energy, it may be attenuated or extinguished due to a slight deviation in the switching operation of optical switch 930. Therefore, in a configuration in which the path of photon pulse 922 is switched by optical switch 930, photon pulse 922 may be attenuated or extinguished, and photon pulse 922 may not be detected by photodetectors 938a and 938b.

[0015] Therefore, an embodiment will be described below in which quantum entanglement can be formed between adjacent quantum bit elements without passing photons emitted from the quantum bit elements through an optical switch.

[0016] FIG. 2 is a plan view of the quantum bit device 10 according to the first embodiment. As shown in FIG. 2, the quantum bit device 10 according to the first embodiment includes a diamond crystal layer 12 and a color center 14. The diamond crystal layer 12 has a cross shape in plan view, extending in four directions perpendicular to one another from a center point 16. That is, the diamond crystal layer 12 has a central portion 18 and extension portions 20a, 20b, 20c, and 20d extending in four directions perpendicular to one another from the central portion 18. Here, "orthogonal" does not necessarily mean that the angle α formed by the extension portions 20a to 20d is 90°, but rather can be in the range of 90°±10°. The central portion 18 is, for example, a square-shaped region in plan view. The central point 16 is, for example, a circle defined with a certain extent and including the center of the central portion 18 in plan view. For example, the central point 16 is a circle having a diameter greater than or equal to 1 / 10 and less than or equal to 1 / 4 of the length of one side of the central portion 18. The center point 16 is not limited to a circular shape, but may be a rectangular shape or other shape. The four extension portions 20a to 20d are congruent with one another in a plan view.

[0017] The color center 14 is formed so as to overlap the center point 16 in plan view. The color center 14 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).

[0018] An optical waveguide 30 is connected to each of the four ends 21a, 21b, 21c, and 21d of the diamond crystal layer 12. The optical waveguide 30 is formed of, for example, sapphire. The optical waveguides 30 connected to the ends 21a and 21b are, for example, input optical waveguides 32a and 32b. The optical waveguides 30 connected to the ends 21c and 21d are, for example, output optical waveguides 34a and 34b. In this way, while the quantum bit device 900 in the comparative example has a one-input, one-output structure, the quantum bit device 10 in Example 1 has a two-input, two-output structure.

[0019] The input optical waveguides 32a and 32b transmit an optical pulse 40 for resonance excitation or initialization introduced into the color center 14. A photon pulse 42 of a single photon emitted from the color center 14 in response to the optical pulse 40 introduced from the input optical waveguide 32a is transmitted through the output optical waveguide 34a. A photon pulse 42 of a single photon emitted from the color center 14 in response to the optical pulse 40 introduced from the input optical waveguide 32b is transmitted through the output optical waveguide 34b. When the optical pulse 40 is introduced toward the color center 14 from one of the input optical waveguides 32a and 32b, it is not introduced toward the color center 14 from the other. However, for convenience of explanation, in FIG. 2, optical pulses 40 are illustrated in both the input optical waveguides 32a and 32b, and photon pulses 42 are illustrated in both the output optical waveguides 34a and 34b (the same applies to similar figures below).

[0020] The diamond crystal layer 12 has a thickness of, for example, 100 nm to 300 nm. The extensions 20a to 20d have a width of, for example, 250 nm to 400 nm. The optical waveguide 30 has a thickness of, for example, 100 nm to 300 nm. The optical waveguide 30 has a width of, for example, 200 nm to 700 nm.

[0021] FIG. 3 is a plan view of a quantum device 100 according to a first embodiment. As shown in FIG. 3 , the quantum device 100 according to the first embodiment has a plurality of quantum bit elements 10 arranged side by side in the X direction. A common beam splitter 50 is connected to an output optical waveguide 34 a connected to one of adjacent quantum bit elements 10 and an output optical waveguide 34 b connected to the other adjacent quantum bit element 10. The beam splitter 50 is an example of a branching element. Photodetectors 52 a and 52 b are connected to the beam splitter 50. The photodetectors 52 a and 52 b are single-photon photodetectors, such as a SPAD (Single Photon Avalanche Photo Detector) or a SNSPD (Superconducting Nanowire Single Photon Detector).

[0022] A photon pulse 42 emitted by one of adjacent quantum bit elements 10 to the output optical waveguide 34a and a photon pulse 42 emitted by the other of adjacent quantum bit elements 10 to the output optical waveguide 34b are introduced into a common beam splitter 50. The beam splitter 50 splits the photon pulse 42 emitted by one of adjacent quantum bit elements 10 to the output optical waveguide 34a and the photon pulse 42 emitted by the other of adjacent quantum bit elements 10 to the output optical waveguide 34b into a first direction or a second direction. In the beam splitter 50, for example, the first direction and the second direction are orthogonal, and the probability of splitting into the first direction and the second direction is 50% for each. The beam splitter 50 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 42 emitted by one of adjacent quantum bit elements 10 hits one surface of the half mirror and is reflected or transmitted. The photon pulse 42 emitted by the other quantum bit element 10 strikes the other surface of the half mirror and is reflected or transmitted. The photodetector 52a detects the photon pulse 42 that is incident on one surface of the half mirror and transmitted therethrough, or the photon pulse 42 that is incident on the other surface and reflected therethrough. The photodetector 52b detects the photon pulse 42 that is incident on one surface of the half mirror and reflected therethrough, or the photon pulse 42 that is incident on the other surface and transmitted therethrough. The photon pulse 42 emitted by one quantum bit element 10 and the photon pulse 42 emitted by the other quantum bit element 10 merge at the beam splitter 50, thereby forming a two-photon correlation. This allows quantum entanglement to be formed between the adjacent quantum bit elements 10.

[0023] According to Example 1, as shown in Figure 2, the quantum bit device 10 comprises a diamond crystal layer 12 and a color center 14 present in the diamond crystal layer 12. The diamond crystal layer 12 has a central portion 18 and extension portions 20a-20d connected to the central portion 18 and extending in four directions from the central portion 18. The color center 14 is formed in the central portion 18 of the diamond crystal layer 12. Input optical waveguides 32a and 32b, through which an optical pulse 40 introduced into the color center 14 propagates, are connected to ends 21a and 21b of the two extension portions 20a and 20b of the diamond crystal layer 12. Output optical waveguides 34a and 34b, through which a photon pulse 42 emitted by the color center 14 propagates, are connected to ends 21c and 21d of the remaining two extension portions 20c and 20d of the diamond crystal layer 12. In this way, because the quantum bit element 10 has a two-input, two-output configuration, quantum entanglement can be formed between adjacent quantum bit elements 10 on both sides in the X direction, as described in FIG. 3 , without using an optical switch as in the comparative example.

[0024] 2, in Example 1, color center 14 exists so as to overlap, in plan view, with center point 16 of diamond crystal layer 12. This makes it possible for light pulses 40 introduced from either input optical waveguide 32a or 32b to be easily introduced into color center 14, and for photon pulses 42 emitted by color center 14 to be easily transmitted to either output optical waveguide 34a or 34b.

[0025] 3, in the first embodiment, a beam splitter 50 is connected to an output optical waveguide 34a connected to one of adjacent quantum bit elements 10 in the X direction and an output optical waveguide 34b connected to the other of adjacent quantum bit elements 10. The photon pulse 42 split by the beam splitter 50 is detected by a photodetector 52a or 52b. Therefore, quantum entanglement can be formed between adjacent quantum bit elements 10 on both sides in the X direction.

[0026] FIG. 4 is a plan view of a quantum device 200 according to a second embodiment. As shown in FIG. 4 , the quantum device 200 according to the second embodiment has a plurality of quantum bit elements 10 arranged in a lattice pattern in the X and Y directions. The input optical waveguide 32 a of each of the quantum bit elements 10 arranged in the Y direction is connected to a common optical waveguide 60 a via an adjustment element 62 a. The adjustment element 62 a is an element that adjusts the ratio of the optical pulse 40 transmitted to the common optical waveguide 60 a and the input optical waveguide 32 a, and in the second embodiment, is an optical switch that switches so that only one of the common optical waveguide 60 a or the input optical waveguide 32 a is transmitted. Similarly, the input optical waveguide 32 b of each of the quantum bit elements 10 arranged in the Y direction is connected to the common optical waveguide 60 b via an adjustment element 62 b. The adjustment element 62b is an element that adjusts the ratio of the optical pulse 40 transmitted to the common optical waveguide 60b and the input optical waveguide 32b, and in the second embodiment is an optical switch that switches so that the optical pulse 40 is transmitted through only one of the common optical waveguide 60b and the input optical waveguide 32b.

[0027] The quantum device 200 includes a plurality of light sources 70. The light sources 70 are, for example, laser light sources, and generate optical pulses 40 for resonance excitation or initialization. A common optical waveguide 60a to which the input optical waveguides 32a of one quantum bit element 10 adjacent in the X direction are connected, and a common optical waveguide 60b to which the input optical waveguides 32b of the other quantum bit element 10 are connected, are connected to a single common light source 70. The optical pulses 40 output by the light sources 70 are introduced into the common optical waveguides 60a and 60b via an optical fiber 72 and an optical connector 74.

[0028] The light source 70 is electrically connected to the control unit 64 and outputs the optical pulse 40 based on instructions from the control unit 64. The adjustment elements 62a and 62b are also electrically connected to the control unit 64 and switch the path between the common optical waveguide and the input optical waveguide based on instructions from the control unit 64. The control unit 64 may be, for example, a processor such as a CPU (Central Processing Unit) that performs processing in cooperation with a program. The control unit 64 may be a general-purpose circuit or a circuit designed specifically for that purpose. The other configuration is the same as that shown in FIG. 3 of the first embodiment, and therefore description thereof will be omitted.

[0029] Figure 5 is a cross-sectional view of the intersection of the common optical waveguides 60a, 60b and the output optical waveguides 34a, 34b in Example 2. As shown in Figure 5, the common optical waveguides 60a, 60b are provided on a substrate 80. At least the portions of the substrate 80 that contact the common optical waveguides 60a, 60b are formed of a material with a smaller refractive index than the common optical waveguides 60a, 60b. For example, if the common optical waveguides 60a, 60b are made of sapphire, the substrate 80 is a silicon substrate with an oxide film. A film 82 is provided on the substrate 80 to cover the common optical waveguides 60a, 60b, and the output optical waveguides 34a, 34b are provided on the film 82. The film 82 is formed of a material having a smaller refractive index than the common optical waveguides 60 a, 60 b and the output optical waveguides 34 a, 34 b, and is a silicon oxide film when the common optical waveguides 60 a, 60 b and the output optical waveguides 34 a, 34 b are made of sapphire, for example. In this way, the common optical waveguides 60 a, 60 b and the output optical waveguides 34 a, 34 b are provided in different layers and intersect without contacting each other.

[0030] 6(a) and 6(b) are cross-sectional views illustrating the transmission of optical pulses 40 from common optical waveguides 60a and 60b to input optical waveguides 32a and 32b in Example 2. As shown in FIG. 6(a), the common optical waveguides 60a and 60b provided on a substrate 80 extend toward and contact the input optical waveguides 32a and 32b provided on a film 82. By appropriately adjusting the contact length between the common optical waveguides 60a and 60b and the input optical waveguides 32a and 32b, light is transferred from the common optical waveguides 60a and 60b to the input optical waveguides 32a and 32b. In this manner, optical pulses 40 may be transmitted from the common optical waveguides 60a and 60b to the input optical waveguides 32a and 32b.

[0031] 6(b), common optical waveguides 60a, 60b provided on a substrate 80 and input optical waveguides 32a, 32b provided on a film 82 are spaced apart and arranged parallel to each other at a fixed distance, sandwiching the film 82. By appropriately adjusting the spacing and parallel lengths between the common optical waveguides 60a, 60b and the input optical waveguides 32a, 32b, light is transferred from the common optical waveguides 60a, 60b to the input optical waveguides 32a, 32b. In this manner, optical pulses 40 may be transmitted from the common optical waveguides 60a, 60b to the input optical waveguides 32a, 32b.

[0032] [Control Method] An example of control for forming quantum entanglement between quantum bit elements 10 adjacent in the X direction will be described. FIG. 7 is a plan view illustrating an example of control by the control unit 64 in Example 2. FIG. 8 is a flowchart illustrating an example of control by the control unit 64 in Example 2. As shown in FIG. 7 , the multiple quantum bit elements 10 are quantum bit elements 10a, 10b, 10c, 10d, 10e, and 10f. A light source 70 connected to a common optical waveguide 60a connected to the input optical waveguides 32a of the quantum bit elements 10a and 10d and a common optical waveguide 60b connected to the input optical waveguides 32b of the quantum bit elements 10b and 10e is referred to as light source 70a. A light source 70 connected to a common optical waveguide 60a connected to the input optical waveguides 32a of the quantum bit elements 10b and 10e and a common optical waveguide 60b connected to the input optical waveguides 32b of the quantum bit elements 10c and 10f is referred to as light source 70b. 7 and 8, a case where quantum entanglement is formed between quantum bit devices 10a and 10b, and between quantum bit devices 10e and 10f will be described.

[0033] 7 and 8 , the control unit 64 switches the adjustment element 62a connected to the quantum bit element 10a to connect it to the input optical waveguide 32a (step S10). Next, the control unit 64 switches the adjustment element 62b connected to the quantum bit element 10b to connect it to the input optical waveguide 32b (step S12). Next, the control unit 64 causes the light source 70a to emit a light pulse 40 (step S14). This introduces the light pulse 40 into the quantum bit elements 10a and 10b, and the photon pulses 42 emitted by each of them are incident on the common beam splitter 50. Thus, quantum entanglement can be formed between the quantum bit elements 10a and 10b. Note that steps S10 and S12 may be performed in reverse order, or simultaneously.

[0034] The control unit 64 switches the adjustment element 62a connected to the quantum bit device 10b to the common optical waveguide 60a (step S16). Next, the control unit 64 switches the adjustment element 62b connected to the quantum bit device 10c to the common optical waveguide 60b (step S18). Next, the control unit 64 switches the adjustment element 62a connected to the quantum bit device 10e to the input optical waveguide 32a (step S20). Next, the control unit 64 switches the adjustment element 62b connected to the quantum bit device 10f to the input optical waveguide 32b (step S22). Next, the control unit 64 causes the light source 70b to emit an optical pulse 40 (step S24). This introduces the optical pulse 40 into the quantum bit devices 10e and 10f, and the photon pulses 42 emitted by each of them are incident on the common beam splitter 50. Thus, quantum entanglement can be formed between the quantum bit devices 10e and 10f. The order of steps S16 to S22 may be reversed, or steps S16 to S22 may be performed simultaneously.

[0035] Note that the order of steps S10 to S14 and steps S16 to S24 may be reversed, so that quantum entanglement is formed between quantum bit devices 10e and 10f and then between quantum bit devices 10a and 10b. Furthermore, quantum entanglement may be formed between quantum bit devices 10a and 10b and between quantum bit devices 10e and 10f simultaneously.

[0036] According to the second embodiment, as shown in Fig. 4 , common optical waveguides 60a, 60b are connected to the input optical waveguides 32a, 32b of two or more quantum bit elements 10 arranged in the Y direction. Adjustment elements 62a, 62b, which are optical switches that switch the transmission direction of the optical pulse 40, are provided between the common optical waveguides 60a, 60b and the input optical waveguides 32a, 32b. This makes it possible to form quantum entanglement not only between quantum bit elements adjacent in the X direction in the first stage in the Y direction, but also between quantum bit elements adjacent in the X direction in the second stage and thereafter, as described in Figs. 7 and 8 .

[0037] 5, in the second embodiment, the output optical waveguides 34a, 34b and the common optical waveguides 60a, 60b are provided in different layers, and a film 82 is provided between them. This makes it possible to easily obtain a configuration in which the common optical waveguides 60a, 60b do not come into contact with the output optical waveguides 34a, 34b, even when they are provided.

[0038] FIG. 9 is a plan view of the quantum bit element 11 of Example 3. As shown in FIG. 9, the quantum bit element 11 of Example 3 includes a diamond crystal layer 12a extending in eight directions from a center point 16a at equiangular angles. That is, the diamond crystal layer 12a has a central portion 18a and extension portions 20a, 20b, 20c, 20d, 20e, 20f, 20g, and 20h extending in eight directions from the central portion 18a at equiangular angles. Here, the equiangular angle is not limited to when the angle β formed by the extension portions 20a to 20h is 45°, but can be in the range of 45°±5°. The central portion 18a is, for example, a region having a regular octagonal shape in a planar view. The central point 16a is, for example, a circle defined with a certain degree of extent and including the center of the central portion 18a in a planar view. For example, the center point 16a is a circle having a diameter equal to or greater than 1 / 5 and equal to or less than 1 / 2 of the length of one side of the central portion 18a. The color center 14 is located on the center point 16a in a planar view. The center point 16a is not limited to a circular shape and may be a rectangular or other shape. The eight extension portions 20a to 20h are congruent with one another in a planar view.

[0039] An optical waveguide 30 is connected to each of the eight ends 21a, 21b, 21c, 21d, 21f, 21d, 21g, and 21h of the diamond crystal layer 12a. The optical waveguides 30 connected to the ends 21b, 21e, 21g, and 21h are, for example, input optical waveguides 32a, 32b, 32c, and 32d. The optical waveguides 30 connected to the ends 21a, 21c, 21d, and 21f are, for example, output optical waveguides 34a, 34b, 34c, and 34d. In this way, the quantum bit device 11 in Example 2 has a four-input, four-output structure.

[0040] The input optical waveguides 32a to 32d transmit an optical pulse 40 introduced into the color center 14. A photon pulse 42 of a single photon emitted from the color center 14 in response to the optical pulse 40 introduced from the input optical waveguide 32a is transmitted through the output optical waveguide 34d. A photon pulse 42 of a single photon emitted from the color center 14 in response to the optical pulse 40 introduced from the input optical waveguide 32b is transmitted through the output optical waveguide 34a. A photon pulse 42 of a single photon emitted from the color center 14 in response to the optical pulse 40 introduced from the input optical waveguide 32c is transmitted through the output optical waveguide 34b. A photon pulse 42 of a single photon emitted from the color center 14 in response to the optical pulse 40 introduced from the input optical waveguide 32d is transmitted through the output optical waveguide 34c.

[0041] The diamond crystal layer 12a has a thickness of, for example, 100 nm to 300 nm. The extensions 20a to 20h have a width of, for example, 250 nm to 400 nm. The optical waveguide 30 has a thickness of, for example, 100 nm to 300 nm. The optical waveguide 30 has a width of, for example, 200 nm to 700 nm.

[0042] FIG. 10 is a plan view of a quantum device 300 according to a third embodiment. As shown in FIG. 10 , the quantum device 300 according to the third embodiment has a plurality of quantum bit elements 11 arranged in a lattice pattern in the X and Y directions. A common beam splitter 50 is connected to an output optical waveguide 34 b connected to one of the quantum bit elements 11 adjacent to each other in the X direction and an output optical waveguide 34 d connected to the other of the quantum bit elements 11. Photodetectors 52 a and 52 b are connected to the beam splitter 50. As a result, photon pulses 42 emitted by two quantum bit elements 11 adjacent to each other in the X direction are introduced into the common beam splitter 50 and detected by the photodetector 52 a or 52 b. Therefore, quantum entanglement can be formed between the quantum bit elements 11 adjacent to each other in the X direction.

[0043] A common beam splitter 50a is connected to the output optical waveguide 34c connected to one of the quantum bit elements 11 adjacent in the Y direction, and the output optical waveguide 34a connected to the other. Photodetectors 52c and 52d are connected to the beam splitter 50a. As a result, photon pulses 42 emitted by two quantum bit elements 11 adjacent in the Y direction are introduced into the common beam splitter 50a and detected by the photodetector 52c or 52d. Therefore, quantum entanglement can be formed between the quantum bit elements 11 adjacent in the Y direction.

[0044] According to Example 3, as shown in Figure 9, the quantum bit element 11 comprises a diamond crystal layer 12a and a color center 14 present in the diamond crystal layer 12a. The diamond crystal layer 12a has a center point 16a and extends in eight directions from the center point 16a at equal angles to one another. Each of the eight ends 21a-21h of the diamond crystal layer 12a is connected to an optical waveguide 30, which is either an input optical waveguide 32a-32d through which an optical pulse 40 introduced into the color center 14 is transmitted, or an output optical waveguide 34a-34d through which a photon pulse 42 emitted from the color center 14 is transmitted. Because the quantum bit element 11 has a four-input / four-output configuration, quantum entanglement can be formed between adjacent quantum bit elements 11 on both sides in the X and Y directions, as described in Figure 10.

[0045] 9, in Example 3, color center 14 is located so as to overlap center point 16a of diamond crystal layer 12a in plan view. This makes it possible for light pulses 40 introduced from any of input optical waveguides 32a to 32d to be easily introduced into color center 14, and for photon pulses 42 emitted by color center 14 to be easily transmitted to any of output optical waveguides 34a to 34d.

[0046] 10 , in Example 3, a beam splitter 50 is connected to output optical waveguide 34b connected to one quantum bit element 11 adjacent in the X direction and output optical waveguide 34d connected to the other quantum bit element 11. Photon pulse 42 split by beam splitter 50 is detected by photodetector 52a or 52b. Beam splitter 50a is connected to output optical waveguide 34c connected to one quantum bit element 11 adjacent in the Y direction and output optical waveguide 34a connected to the other quantum bit element 11. Photon pulse 42 split by beam splitter 50a is detected by photodetector 52c or 52d. With this configuration, quantum entanglement can be formed between quantum bit elements 11 adjacent on both sides in the X direction and the Y direction.

[0047] FIG. 11 is a plan view of a quantum device 400 according to a fourth embodiment. As shown in FIG. 11 , the quantum device 400 according to the fourth embodiment has a plurality of quantum bit elements 11 arranged in a lattice pattern in the X and Y directions. The input optical waveguide 32c of each of the quantum bit elements 11 arranged in the Y direction is connected to a common optical waveguide 60a via an adjustment element 62a. Similarly, the input optical waveguide 32a of each of the quantum bit elements 11 arranged in the Y direction is connected to a common optical waveguide 60b via an adjustment element 62b. Furthermore, the input optical waveguide 32b of each of the quantum bit elements 11 arranged in the X direction is connected to the common optical waveguide 60c via an adjustment element 62c, and each of the input optical waveguides 32d is connected to the common optical waveguide 60d via an adjustment element 62d. The adjustment elements 62a, 62b, 62c, and 62d are optical switches, as in the second embodiment, but are simplified for clarity.

[0048] The quantum device 400 includes a plurality of light sources 70. A common optical waveguide 60a to which the input optical waveguide 32c of one quantum bit element 11 adjacent in the X direction is connected, and a common optical waveguide 60b to which the input optical waveguide 32a of the other quantum bit element 11 is connected, are connected to a single common light source 70. Optical pulses 40 output from the light sources 70 are introduced into the common optical waveguides 60a and 60b via optical fibers 72 and optical connectors 74. Similarly, in the Y direction, a common optical waveguide 60c to which the input optical waveguide 32b of one adjacent quantum bit element 11 is connected, and a common optical waveguide 60d to which the input optical waveguide 32d of the other quantum bit element 11 is connected, are connected to a single common light source 70. Optical pulses 40 output from the light sources 70 are introduced into the common optical waveguides 60c and 60d via optical fibers 72 and optical connectors 74.

[0049] The light source 70, like in the second embodiment, is electrically connected to the control unit 64 and outputs an optical pulse 40 based on an instruction from the control unit 64. The adjustment elements 62a to 62d, like in the second embodiment, are also electrically connected to the control unit 64 and switch the paths of the common optical waveguide and the input optical waveguide based on an instruction from the control unit 64. The other configurations are the same as those in FIG. 10 of the third embodiment, and therefore description thereof will be omitted.

[0050] 12(a) to 12(c) are cross-sectional views of the intersections of the common optical waveguides 60a and 60b, the common optical waveguides 60c and 60d, and the output optical waveguides 34a to 34d in the fourth embodiment. As shown in FIGS. 12(a) to 12(c), the common optical waveguides 60c and 60d, the common optical waveguides 60a and 60b, and the output optical waveguides 34a to 34d are provided in this order on different layers. That is, the common optical waveguides 60c and 60d are provided on a substrate 80. A film 82 is provided on the substrate 80, covering the common optical waveguides 60c and 60d, and the common optical waveguides 60a and 60b are provided on the film 82. A film 84 is provided on the film 82, covering the common optical waveguides 60a and 60b. The output optical waveguides 34a to 34d are provided on the film 84. As a result, the common optical waveguides 60c and 60d, the common optical waveguides 60a and 60b, and the output optical waveguides 34a to 34d intersect without contacting each other.

[0051] 13( a) and 13(b) are cross-sectional views illustrating the transmission of optical pulses 40 from the common optical waveguides 60c and 60d to the input optical waveguides 32b and 32d in Example 4. As shown in FIG. 13( a), an intermediate optical waveguide 66 is provided in a film 84, which is in contact with the common optical waveguides 60c and 60d and the input optical waveguides 32b and 32d. By appropriately adjusting the contact length between the common optical waveguides 60c and 60d and the intermediate optical waveguide 66, light is transferred from the common optical waveguides 60c and 60d to the intermediate optical waveguide 66. Similarly, by appropriately adjusting the contact length between the input optical waveguides 32b and 32d and the intermediate optical waveguide 66, light is transferred from the intermediate optical waveguide 66 to the input optical waveguides 32b and 32d. In this manner, optical pulses 40 may be transmitted from common optical waveguides 60c, 60d through intermediate optical waveguide 66 to input optical waveguides 32b, 32d.

[0052] As shown in Figure 13(b), an intermediate optical waveguide 66a is provided in the film 84, spaced apart and parallel to the common optical waveguides 60c, 60d and the input optical waveguides 32b, 32d with films 82, 84 interposed therebetween and spaced a certain distance apart. By appropriately adjusting the distance and parallel length between the common optical waveguides 60c, 60d and the intermediate optical waveguide 66a, light can be transferred from the common optical waveguides 60c, 60d to the intermediate optical waveguide 66a. Similarly, by appropriately adjusting the distance and parallel length between the input optical waveguides 32b, 32d and the intermediate optical waveguide 66a, light can be transferred from the intermediate optical waveguide 66a to the input optical waveguides 32b, 32d. In this manner, optical pulses 40 may be transmitted from the common optical waveguides 60c, 60d to the input optical waveguides 32b, 32d via the intermediate optical waveguide 66a.

[0053] The transmission of the optical pulse 40 from the common optical waveguides 60a, 60b provided on the film 82 to the input optical waveguides 32a, 32c provided on the film 84 is similar to the case of Figures 6(a) and 6(b), so a description thereof will be omitted here.

[0054] [Control Method] An example of control for forming quantum entanglement between quantum bit elements 11 adjacent in the X direction and the Y direction will be described. FIG. 14 is a plan view illustrating an example of control by the control unit 64 in Example 4. FIG. 15 is a flowchart illustrating an example of control by the control unit 64 in Example 4. As shown in FIG. 14 , the multiple quantum bit elements 11 are quantum bit elements 11a, 11b, 11c, and 11d. A light source 70 connected to a common optical waveguide 60a connected to the input optical waveguides 32c of the quantum bit elements 11a and 11c and a common optical waveguide 60b connected to the input optical waveguides 32a of the quantum bit elements 11b and 11d is defined as light source 70a. A light source 70 connected to a common optical waveguide 60d connected to the input optical waveguides 32d of the quantum bit elements 10a and 10b and a common optical waveguide 60c connected to the input optical waveguides 32b of the quantum bit elements 11c and 11d is defined as light source 70b. 14 and 15, a case will be described in which quantum entanglement is formed between quantum bit devices 11a and 11b adjacent to each other in the X direction, and between quantum bit devices 11a and 11c adjacent to each other in the Y direction.

[0055] 14 and 15 , the control unit 64 switches the adjustment element 62a connected to the quantum bit element 11a to the input optical waveguide 32c (step S30). Next, the control unit 64 switches the adjustment element 62b connected to the quantum bit element 11b to the input optical waveguide 32a (step S32). Next, the control unit 64 causes the light source 70a to emit a light pulse 40 (step S34). This introduces the light pulse 40 into the quantum bit elements 11a and 11b, and the photon pulses 42 emitted by each of the quantum bit elements 11a and 11b are incident on the common beam splitter 50. Thus, quantum entanglement can be formed between the quantum bit elements 11a and 11b, which are adjacent in the X direction. Note that steps S30 and S32 may be performed in reverse order, or may be performed simultaneously.

[0056] The control unit 64 switches the adjustment element 62d connected to the quantum bit element 11a to connect it to the input optical waveguide 32d (step S36). Next, the control unit 64 switches the adjustment element 62c connected to the quantum bit element 11c to connect it to the input optical waveguide 32b (step S38). Next, the control unit 64 causes the light source 70b to emit a light pulse 40 (step S40). This introduces the light pulse 40 into the quantum bit elements 11a and 11c, and the photon pulses 42 emitted by each of them are incident on the common beam splitter 50a. Thus, quantum entanglement can be formed between the quantum bit elements 11a and 11c, which are adjacent in the Y direction. Note that steps S16 to S22 may be performed in reverse order or simultaneously.

[0057] It is also possible to reverse the order of steps S30 to S34 and steps S36 to S40, and form quantum entanglement between quantum bit elements 11a and 11b after forming quantum entanglement between quantum bit elements 11a and 11c. It is not desirable to simultaneously introduce optical pulses 40 from both light sources 70a and 70b into quantum bit element 11a, so it is not desirable to simultaneously form quantum entanglement between quantum bit elements 11a and 11b and quantum entanglement between quantum bit elements 11a and 11c. In FIGS. 14 and 15, an example is shown in which quantum entanglement is formed between adjacent quantum bit elements in the first row in the X and Y directions. To form quantum entanglement between adjacent quantum bit elements in the second and subsequent rows, a method similar to that shown in steps S16 to S24 of FIG. 8 can be performed.

[0058] According to the fourth embodiment, as shown in FIG. 11 , the input optical waveguides 32 b and 32 d of two or more quantum bit elements 11 arranged in the X direction are connected to common optical waveguides 60 c and 60 d. Adjusting elements 62 c and 62 d, which are optical switches that switch the transmission direction of the optical pulse 40, are provided between the input optical waveguides 32 b and 32 d and the common optical waveguides 60 c and 60 d. The input optical waveguides 32 c and 32 a of two or more quantum bit elements 11 arranged in the Y direction are connected to common optical waveguides 60 a and 60 b. Adjusting elements 62 a and 62 b, which are optical switches that switch the transmission direction of the optical pulse 40, are provided between the input optical waveguides 32 c and 32 a and the common optical waveguides 60 a and 60 b. This makes it possible to form quantum entanglement not only between adjacent quantum bit elements in the first stage in the X and Y directions, but also between adjacent quantum bits in the second stage.

[0059] 12(a) to 12(c), the common optical waveguides 60c and 60d, the common optical waveguides 60a and 60b, and the output optical waveguides 34a to 34d are provided in this order on different layers. A film 82 is provided between the common optical waveguides 60c and 60d and the common optical waveguides 60a and 60b, and a film 84 is provided between the common optical waveguides 60a and 60b and the output optical waveguides 34b and 34d. This makes it easy to obtain a configuration in which the output optical waveguides 34a to 34d, the common optical waveguides 60a and 60b, and the common optical waveguides 60c and 60d do not come into contact with each other, even when the common optical waveguides 60a to 60d are provided.

[0060] 13(a), an intermediate optical waveguide 66 that is in contact with the common optical waveguides 60c, 60d and the input optical waveguides 32b, 32d is provided in the film 84. Alternatively, as shown in Fig. 13(b), an intermediate optical waveguide 66a that is spaced apart from the common optical waveguides 60c, 60d and the input optical waveguides 32b, 32d and that is parallel to the common optical waveguides 60c, 60d and the input optical waveguides 32b, 32d is provided in the film 84. With this configuration, the optical pulses 40 may be transmitted from the common optical waveguides 60c, 60d to the input optical waveguides 32b, 32d via the intermediate optical waveguides 66, 66b.

[0061] Fig. 16 is a plan view of a quantum device 500 according to a fifth embodiment. As shown in Fig. 16, in the quantum device 500 according to the fifth embodiment, an adjusting element 63a, which is a beam splitter, is connected between the input optical waveguide 32a and the common optical waveguide 60a. An adjusting element 63b, which is a beam splitter, is connected between the input optical waveguide 32b and the common optical waveguide 60b. The other configurations are the same as those in Fig. 4 of the second embodiment, and therefore description thereof will be omitted.

[0062] In the fifth embodiment, adjustment elements 63a and 63b, which are beam splitters, are used. This allows the optical pulse 40 to be transmitted to both the common optical waveguides 60a and 60b and the input optical waveguides 32a and 32b while changing the ratio of transmission to both. This allows quantum entanglement to be formed between adjacent quantum bit elements 10 simultaneously in multiple stages in the X and Y directions. Note that beam splitters and optical switches may be mixed and used as multiple adjustment elements.

[0063] Fig. 17 is a plan view of a quantum device 600 according to a sixth embodiment. As shown in Fig. 17, in the quantum device 600 according to the sixth embodiment, an adjusting element 63a, which is a beam splitter, is connected between the input optical waveguide 32c and the common optical waveguide 60a. An adjusting element 63b, which is a beam splitter, is connected between the input optical waveguide 32a and the common optical waveguide 60b. An adjusting element 63c, which is a beam splitter, is connected between the input optical waveguide 32b and the common optical waveguide 60c. An adjusting element 63d, which is a beam splitter, is connected between the input optical waveguide 32d and the common optical waveguide 60d. The other configurations are the same as those in Fig. 11 of the fourth embodiment, and therefore description thereof will be omitted.

[0064] In Example 6, adjustment elements 63a to 63d that are beam splitters are used. As a result, similar to Example 5, it is possible to transmit the optical pulse 40 to both the common optical waveguide and the input optical waveguide while changing the ratio at which the optical pulse 40 is transmitted to both, or to transmit the optical pulse 40 to only one of them. Note that beam splitters and optical switches may be mixed and used as multiple adjustment elements.

[0065] Although the first to sixth embodiments have been described with reference to an example in which the optical waveguide 30 is made of sapphire, the optical waveguide 30 may be made of other materials as long as they are capable of transmitting the optical pulse 40 and the photon pulse 42. For example, the optical waveguide 30 may be made of silicon oxide, silicon nitride, or silicon carbide. Although the substrate 80 is a silicon substrate with an oxide film as an example, the substrate may be made of other materials as long as they have a smaller refractive index than the optical waveguide 30.

[0066] Furthermore, in Examples 1 to 6, examples were shown in which the diamond crystal layer has extensions extending in four or eight directions from the center, but this is not limited to this case, and it is acceptable as long as the extensions extend in 2n directions (n ​​is an integer of 2 or more) from the center.

[0067] 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.

[0068] 10, 10a, 10b, 10c, 10d, 10e. 10f... Qubit element, 11, 11a, 11b, 11c, 11d... Qubit element, 12, 12a... Diamond crystal layer, 14... Color center, 16, 16a... Center point, 18, 18a... Center part, 20a, 20b, 20c, 20d, 20e , 20f, 20g, 20h...extension part, 21a, 21b, 21c, 21d, 21e, 21f, 21g, 21h...end part, 30...optical waveguide, 32a, 32b, 32c, 32d...input optical waveguide, 34a, 34b, 34c, 34d...output optical waveguide, 40 ...optical pulse, 42...photon pulse, 50, 50a...beam splitter, 52a, 52b, 52c, 52d...photodetector, 60a, 60b, 60c, 60d...common optical waveguide, 62a, 62b, 62c, 62d...adjusting element, 63a, 63b, 63c, 63d...adjusting element, 64...controller, 66, 66a...intermediate optical waveguide, 70, 70a, 70b...light source, 72...optical fiber, 74...optical connector, 80...substrate, 82...film, 84...film, 100, 200, 300, 400, 500, 600...quantum device

Claims

1. A plurality of qubit elements each comprising a diamond crystal layer having a central portion and an extended portion connected to the central portion and extending from the central portion in the direction of 2n (where n is an integer of 2 or more), and a color center formed in the central portion, An input optical waveguide connected to each of the n ends of the extension portion and through which light introduced into the color center is transmitted, and an output optical waveguide connected to each of the n ends of the extension portion and through which photons emitted by the color center are transmitted, The invention is characterized by comprising a branching element connected to the output optical waveguide of a first qubit element among the plurality of qubit elements and the output optical waveguide of a second qubit element among the plurality of qubit elements. A quantum device.

2. The above n is 2, The branching element is connected to an output optical waveguide connected to one of the plurality of qubit elements adjacent in the first direction and to an output optical waveguide connected to the other qubit element, thereby branching the first photon emitted by the one qubit element and the second photon emitted by the other qubit element. The quantum device according to claim 1, further comprising a first photodetector and a second photodetector for detecting the first or second photon that has been branched by the branching element.

3. A common optical waveguide to which the input optical waveguides of two or more qubit elements aligned in the second direction among the plurality of qubit elements are connected, The quantum device according to claim 2, further comprising an adjustment element provided between the common optical waveguide and the input optical waveguide for adjusting the distribution ratio of the light.

4. The quantum device according to claim 3, further comprising a film provided in a layer different from the input optical waveguide, the output optical waveguide and the common optical waveguide, and provided between the input optical waveguide, the output optical waveguide and the common optical waveguide.

5. The quantum device according to claim 3 or 4, characterized in that the adjustment element includes an optical switch.

6. The quantum device according to claim 3 or 4, characterized in that the adjustment element includes a beam splitter.

7. The above n is 4, The branching element includes a first branching element and a second branching element. The first branching element is connected to an output optical waveguide connected to one of the plurality of qubit elements adjacent in a first direction and to an output optical waveguide connected to the other qubit element, and branches the first photon emitted by the one qubit element and the second photon emitted by the other qubit element. The second branching element is connected to the output optical waveguide connected to one of the plurality of qubit elements adjacent in the second direction and to the output optical waveguide connected to the other qubit element, and branches the third photon emitted by the one qubit element and the fourth photon emitted by the other qubit element. The quantum device according to claim 1, further comprising: a first photodetector and a second photodetector for detecting the first or second photon branched by the first branching element; and a third photodetector and a fourth photodetector for detecting the third or fourth photon branched by the second branching element.

8. A first common optical waveguide to which the input optical waveguides of each of the plurality of qubit elements, which are arranged in the first direction, are connected, A first adjustment element is provided between the first common optical waveguide and the input optical waveguide to adjust the distribution ratio of the light, A second common optical waveguide to which the input optical waveguides of each of the two or more qubit elements among the plurality of qubit elements aligned in the second direction are connected, The quantum device according to claim 7, further comprising a second adjusting element provided between the second common optical waveguide and the input optical waveguide for adjusting the distribution ratio of the light.

9. A method for controlling quantum devices, The aforementioned quantum device is A plurality of qubit elements each comprising a diamond crystal layer having a central portion and an extended portion connected to the central portion and extending from the central portion in the direction of 2n (where n is an integer of 2 or more), and a color center formed in the central portion, An input optical waveguide connected to each of the n ends of the extension portion and transmitting light introduced into the color center, and an output optical waveguide connected to each of the n ends of the extension portion and transmitting photons emitted from the color center, A branching element connected to the output optical waveguide of the first qubit element among the plurality of qubit elements and the output optical waveguide of the second qubit element among the plurality of qubit elements, Equipped with, By introducing light into the first qubit element and the second qubit element, A method for controlling a quantum device, characterized by forming quantum entanglement between a first photon emitted from the first qubit element and a second photon emitted from the second qubit element using the branching element.