WO2025134908A1 - Photon count detector - Google Patents

Abstract

A photon count detector according to the present disclosure comprises: a detection unit having at least one light detection element capable of single photon detection; and a branch unit that causes inputted light to be transmitted stochastically and branches the light into a first path toward the light detection element and a second path different from the first path.

Description

Photon Number Detector

This disclosure relates to a photon number detector.

In recent years, technology that counts the number of photons in a light pulse (hereinafter also referred to as "light") has been attracting attention (see, for example, Non-Patent Document 1). For example, technology has been provided that counts the number of photons using a photon number discriminator called a superconducting transition edge sensor (TES: Transition Edge Sensor).

JP 2023-061076 A

However, there is room for improvement in the above conventional technology. For example, in the above conventional technology, in order to count the number of photons, it takes a long time after detecting one photon until the next photon can be detected, and there are limitations on the installation environment due to strict cooling requirements, etc. For this reason, it is desirable to make it possible to realize a flexible configuration for detecting the number of photons.

Therefore, this disclosure proposes a photon number detector that can realize a flexible configuration for photon number detection.

In order to solve the above problems, one form of photon number detector according to the present disclosure includes a detection section having at least one photodetection element capable of single photon detection, and a branching section that probabilistically transmits input light and branches it into a first path toward the photodetection element and a second path different from the first path.

FIG. 2 is a diagram illustrating an example of a first configuration of a photon number detector according to the embodiment. FIG. 13 is a diagram showing an example of a route branched by a branching portion. 1 is a flow chart illustrating a method of introduction; FIG. 13 is a diagram illustrating an example of a second configuration of a photon number detector according to the embodiment. FIG. 13 is a diagram illustrating an example of a third configuration of a photon number detector according to an embodiment. FIG. 13 is a diagram illustrating an example of a fourth configuration of a photon number detector according to an embodiment. FIG. 13 is a diagram illustrating an example of a fifth configuration of a photon number detector according to an embodiment. 13 is a flow chart illustrating a method for a waveguide section. FIG. 13 is a diagram illustrating an example of a sixth configuration of a photon number detector according to an embodiment. FIG. 13 is a diagram illustrating an example of a seventh configuration of a photon number detector according to an embodiment. 1 is a flow chart illustrating a photon splitter method. FIG. 13 is a diagram illustrating an example of the reflectance of a branching portion. FIG. 13 is a diagram illustrating an example of the reflectance of a branching portion. 13 is a flow chart illustrating a method of the photon detector. FIG. 13 is a diagram showing an example of simulation comparison conditions. FIG. 13 is a diagram showing an example of an experimental result. FIG. 13 is a diagram showing an example of an experimental result. FIG. 13 is a diagram showing an example of an experimental result. FIG. 13 is a diagram showing an example of an experimental result. FIG. 13 is a diagram showing an example of an experimental result. FIG. 13 is a conceptual diagram showing the probability that the numbers of input and output photons match. FIG. 13 is a conceptual diagram showing the maximum measurement rate at which the numbers of input and output photons match.

Below, an embodiment of the present disclosure is described in detail with reference to the drawings.

One or more of the embodiments (including examples and variations) described below can be implemented independently. However, at least a portion of the embodiments described below may be implemented in appropriate combination with at least a portion of another embodiment. These embodiments may include novel features that are different from one another. Thus, these embodiments may contribute to solving different purposes or problems and may provide different effects.

The present disclosure will be described in the following order.
1. Embodiment 1-1. Overview of this embodiment 1-2. First configuration 1-3. Method for realizing the introduction section 1-4. Second configuration 1-5. Third configuration 1-6. Fourth configuration 1-7. Fifth configuration 1-8. Method for realizing the waveguide section 1-9. Sixth configuration 1-10. Seventh configuration 1-11. Method for realizing the photon branching section 1-12. Reflectance of the branching section 1-12-1. First example (constant reflectance)
1-12-2. Second example (reflectance is inconsistent)
1-13. Realization method for photon detection unit 1-14. Experimental results (simulation results)
1-15. Others 2. Effects of the present disclosure

1. Embodiment
<1-1. Overview of this embodiment>
First, an overview of the premise of the present disclosure will be described, and then a configuration of the present disclosure will be described. In recent years, the performance of elements capable of detecting single photons (also called "single photon detectors") such as SPADs (Single-Photon Avalanche Diodes) and SSPDs (Super Conducting Single Photon Detectors) has improved and their prices have decreased, leading to the application of high-sensitivity measurement of light in various fields, and various applications using these have been commercialized.

On the other hand, these single-photon detectors can only detect one photon of the multiple photons that arrive, which has emerged as a factor limiting the development of various applications. For example, if it becomes possible to measure multiple photons that arrive at the same time (multiple photon count state), it is expected that the functionality of various applications will be expanded and performance will be improved by improving the measurement S/N (signal/noise) ratio and detecting physical phenomena expressed as photon counts.

The ability to measure such multiple photon number states is expected to expand functionality and improve performance in applications such as those shown in Table 1 below. Table 1 shows examples of applications for photon detectors. For each application, Table 1 shows an overview, expectations for photon number detection, and important detector performance indicators. Table 1 shows quantum key distribution, quantum communication, biosensing, high energy physics, and LiDAR (Light Detection And Ranging) as examples of applications, but is not limited to the above as long as the application is applicable to photon detectors.

In terms of expanding applications, important performance indicators for photon number detectors include the probability that the input and output photon numbers match, the maximum measurement rate at which the input and output photon numbers match, and the operating temperature.

For example, the probability that the number of input and output photons will match is calculated by the following formula (1) when the number of input photons is n and the number of output photons is k, as shown in Figure 21. Figure 21 is a conceptual diagram showing the probability that the number of input and output photons will match.

For example, the maximum measurement rate at which the numbers of input and output photons match is calculated by the following formula (2) when the number of input photons is n, the number of output photons is k, and the maximum measurement rate of the element is _Rmax , as shown in Fig. 22. Fig. 22 is a conceptual diagram showing the maximum measurement rate at which the numbers of input and output photons match.

This is because accurate detection of physical phenomena expressed as the number of photons is required, and the more photons that can be detected at one time, the better the signal-to-noise ratio of the measurement.

From the above perspective, conventional photon number detectors have issues. One example of a conventional photon number detector is an array detector that receives diffuse light, but it has issues such as a low probability that the numbers of input and output photons will match, and low efficiency in measuring multiple photons.

Another example of a conventional photon number detector is the Transition Edge Sensor (TES), but it has issues such as a low measurement rate (maximum of about 1 MHz), strict cooling requirements (about 100 mK), and sensitivity to radiation noise, which limits the installation environment. As such, although there are already several examples of conventional photon number detectors, each has its own issues, and a system that satisfies the highly-important performance indexes has not yet been provided.

As mentioned above, conventional photon number detectors require the use of diffuse light, have strict cooling requirements, and are subject to restrictions on the installation environment, which places constraints on the configuration of the photon number detector, making it difficult to realize the photon number detector.

Therefore, this disclosure proposes a photon number detector that can realize a flexible configuration for photon number detection.

<1-2. First configuration>
A first configuration of a photon number detector will be described with reference to Fig. 1. Fig. 1 is a diagram showing an example of the first configuration of a photon number detector according to an embodiment. Fig. 1 is a schematic diagram showing the arrangement relationship of each component in a photon number detector 1. For example, Fig. 1 shows an example of a configuration in which light is introduced from an optical fiber.

The photon number detector 1 has an introduction section 2, a waveguide section 3, branch sections 4a to 4j, and detection sections 5a and 5b. When there is no particular distinction between the branch sections 4a to 4j, they are referred to as branch section 4. When there is no particular distinction between the detection sections 5a and 5b, they are referred to as detection section 5. Among the components of the photon number detector 1, the waveguide section 3, branch section 4, and detection section 5 may be collectively referred to as the detection unit 20. For example, in the photon number detector 1, light from the introduction section 2 is introduced into the detection unit 20. At this time, a member or structure for appropriately guiding the light may be provided on the way from the introduction section 2 to the branch section 4.

The introduction section 2 introduces light in a photon number state to be measured into the waveguide section 3. The introduction section 2 introduces light in a photon number state arriving from the outside into the waveguide section 3. For example, the introduction section 2 introduces the detection target TG, which is input light from an optical fiber, into the waveguide section 3. For example, a connector is used for the introduction section 2. In FIG. 1, for example, input light from an optical fiber (such as cable CB in FIG. 2) is introduced into the waveguide section 3 by the introduction section 2, which is a connector with the outside (such as an optical fiber connector). In this way, when introducing light from an optical fiber, various connectors that connect to an optical fiber can be used as the introduction section 2, which will be described later.

The waveguide section 3 guides the light towards the light detection element 10, which will be described later. In FIG. 1, for example, the waveguide section 3 is made of a transparent resin or the like that fills the space between the detection sections 5a and 5b, but details of the waveguide section 3 will be described later. The image of the light guided by the waveguide section 3 is shown by the dotted arrow line within the waveguide section 3. That is, in FIG. 1, the light within the waveguide section 3 is guided from the left side to the right side.

In FIG. 1, the branching units 4a to 4j are provided corresponding to each of the light detection elements 10 of the detection unit 5. For example, a reflective film or the like is used for the branching unit 4. The branching unit 4 may also be a beam splitter.

In FIG. 1, the detection unit 5a is the detection unit 5 having the light detection elements 10a-10e and arranged below the waveguide unit 3. The detection unit 5a is used as a first detection unit arranged on one side (the lower side in FIG. 1) in a direction (the up-down direction in FIG. 1) that intersects with one direction (the left-right direction in FIG. 1) that corresponds to the guiding of light by the waveguide unit 3.

In FIG. 1, the detection unit 5b is the detection unit 5 that has the light detection elements 10f-10j and is arranged on the upper side of the waveguide unit 3. In this way, the detection unit 5b is used as a second detection unit that is arranged on the other side (the upper side in FIG. 1) of the direction that intersects with the direction corresponding to the guiding of light by the waveguide unit 3.

When there is no particular distinction between the photodetection elements 10a to 10j, they will be referred to as photodetection elements 10. The photodetection elements 10 are elements capable of single photon detection. For example, the photodetection elements 10 are pixels. For example, single photon detectors such as SPADs and SSPDs are used as the photodetection elements 10.

As shown in FIG. 1, the detection unit 5a has a plurality of photodetection elements 10a-10e arranged along one direction corresponding to the guiding of light by the waveguide unit 3. The detection unit 5b has a plurality of photodetection elements 10f-10j arranged along one direction corresponding to the guiding of light by the waveguide unit 3.

In FIG. 1, branch 4a is arranged corresponding to light detection element 10a, branch 4b is arranged corresponding to light detection element 10b, branch 4c is arranged corresponding to light detection element 10c, branch 4d is arranged corresponding to light detection element 10d, and branch 4e is arranged corresponding to light detection element 10e. Also, branch 4f is arranged corresponding to light detection element 10f, branch 4g is arranged corresponding to light detection element 10g, branch 4h is arranged corresponding to light detection element 10h, branch 4i is arranged corresponding to light detection element 10i, and branch 4j is arranged corresponding to light detection element 10j. In this way, multiple branch 4a to 4j are provided corresponding to each of the multiple light detection elements 10a to 10j.

For example, branch 4a is arranged along the surface (top surface in FIG. 1) where light from light detection element 10a is input. Also, each of branch 4b to 4e is arranged along the surface where light from corresponding light detection elements 10b to 10e is input. For example, branch 4f is arranged along the surface (bottom surface in FIG. 1) where light from light detection element 10f is input. Also, each of branch 4g to 4j is arranged along the surface where light from corresponding light detection elements 10g to 10j is input.

In this way, each branch 4 is arranged along the surface where the light of the corresponding light detection element 10 is input. That is, each branch 4 is arranged between the waveguide section 3 and the detection section 5. In FIG. 1, each of the branches 4a to 4e is arranged between the waveguide section 3 and the detection section 5a, and each of the branches 4f to 4j is arranged between the waveguide section 3 and the detection section 5b.

With the above-described configuration, the photodetector element 10a detects light transmitted through the branch 4a. Additionally, each of the photodetector elements 10b to 10e detects light transmitted through the corresponding branch 4b to 4e. Additionally, the photodetector element 10f detects light transmitted through the branch 4f. Additionally, each of the photodetector elements 10g to 10j detects light transmitted through the corresponding branch 4g to 4j. In this way, each photodetector element 10 detects light transmitted through the corresponding branch 4.

The branching section 4 probabilistically transmits the light input from the waveguide section 3 and branches it into a first path toward the corresponding photodetector element 10 and a second path that reflects the light back to the waveguide section 3.

This point will be explained using Figure 2. Figure 2 is a diagram showing an example of a branching path by a branching section. Of the photodetection element 10 and branching section 4, Figure 2 shows only one photodetection element 10a of the detection section 5a and the branching section 4a corresponding to that photodetection element 10a. In Figure 2, the branching section 4a stochastically transmits the optical IP input from the waveguide section 3, and branches it into a first path FP toward the photodetection element 10a and a second path SP different from the first path FP.

Continuing the explanation, let's return to Figure 1. The waveguide section 3 guides the light introduced from the introduction section 2 toward each of the multiple branch sections 4a to 4j. In Figure 1, the waveguide section 3 guides the light branched to the second path by branch section 4a (second path SP in Figure 2) toward branch section 4f. The waveguide section 3 also guides the light branched to the second path by branch section 4f toward branch section 4b. The waveguide section 3 also guides the light branched to the second path by branch section 4b toward branch section 4g.

The waveguide section 3 also guides the light branched to the second path by branch section 4g toward branch section 4c. The waveguide section 3 also guides the light branched to the second path by branch section 4c toward branch section 4h. The waveguide section 3 also guides the light branched to the second path by branch section 4h toward branch section 4d. The waveguide section 3 also guides the light branched to the second path by branch section 4d toward branch section 4i.

The waveguide unit 3 also guides the light branched to the second path by branch 4i toward branch 4e. The waveguide unit 3 also guides the light branched to the second path by branch 4e toward branch 4j. The waveguide unit 3 also guides the light branched to the second path by branch 4j toward the destination. For example, if the photon number detector 1 has more than 10 photodetection elements 10, the waveguide unit 3 guides the light branched to the second path by branch 4j toward the photodetection element 10 provided further ahead (to the right in FIG. 1).

As described above, the waveguide section 3 guides the light from the introduction section 2 toward the individual light detection elements 10 (detectors) that make up the detection section 5. The light reflected from the branching section 4 is again input to the branching section 4 by the waveguide section 3. The branching section 4 stochastically transmits the light input from the waveguide section 3 and inputs it to the light detection elements 10 (detectors). Light that does not transmit through the branching section 4 is reflected toward the waveguide section 3 and input again to the branching section 4. The detection section 5 is composed of light detection elements 10 (detectors) that can detect multiple independent single photons. The detection section 5 stochastically detects the transmitted light that has transmitted through the branching section 4.

In this way, the photon number detector 1 has a repeating structure of the waveguide section 3, the branch section 4, and the detection section 5. In the photon number detector 1, the light reflected from the branch section 4 is guided toward the detection section 5 until the light has been guided toward all of the photon detection elements 10 (detectors). For example, the photon number detector 1 can easily realize a configuration that can detect the number of photons by arranging multiple sets of branch sections 4 and photodetection elements 10 along the waveguide section 3. For example, the photon number detector 1 can easily arrange a desired number of sets of branch sections 4 and photodetection elements 10 depending on the number of photons to be detected, and any configuration can be easily realized. Therefore, the photon number detector 1 can realize a flexible configuration for photon number detection.

As described above, the photon number detector 1 can stochastically create multiple beams of light in a single-photon state by splitting a photon in a multiple-photon number state multiple times with high reflectivity, and these multiple single photons can be measured by a single-photon detection element.

The photon number detector 1 is expected to provide the following improvements in important performance indicators. For example, the photon number detector 1 can improve the probability of matching the input and output photon numbers by suppressing the variation in the number of measured photons by not diffusing light. Also, for example, the photon number detector 1 can improve the maximum measurement rate at which the input and output photon numbers match by using a single photon detector (SPAD, SSPD, etc.) with a high measurement rate as the photodetector. Also, for example, the photon number detector 1 can use a SPAD that can operate at room temperature as the photodetector, which can remove the cooling requirement required for TES and reduce the operating temperature limit.

The above-described configuration allows the photon number detector 1 to reduce the possibility of not being able to properly count the number of photons when multiple photons enter the introduction section at the same time (as one event). In addition, the photon number detector 1 can reduce the possibility of missing photons that arrive within the dead time when photons enter the introduction section within a short period of time (as different events).

The photon number detector 1 may have a minimum configuration as shown in FIG. 2. For example, when the photon number detector 1 detects one photon, it may have a configuration as shown in FIG. 1. For example, the photon number detector 1 may be configured by arranging multiple sets of one branching portion 4 and one photodetecting element 10, such as the branching portion 4a and photodetecting element 10a shown in FIG. 2.

<1-3. How to implement the introduction>
Here, a method for implementing the introduction part will be described with reference to Fig. 3. Fig. 3 is a flowchart showing the method for the introduction part. For example, the introduction part has elements as shown in blocks S11 and S12 in Fig. 3. Each block in the flowchart will be described below.

(External connector: corresponds to block S11)
The introduction path for light from the outside may be an optical fiber or a free section, etc. The following can be adopted as a means for realizing a connector with the outside for connecting these to the photon number detector 1, etc.

For example, when introducing light from an optical fiber, various connectors can be used to connect to the optical fiber. In this case, for example, FC connectors, SC connectors, ST connectors, etc. are used.

For example, when introducing light from free space, an optical focusing mechanism can be used. In this case, for example, a lens system, various focusing mirrors, etc. are used.

(Beam splitter: corresponds to block S12)
The light input from the connector may be split by a beam splitter and optical detection may be performed using multiple independent paths. The number of times the light may be split is arbitrary. Also, the introduction section may not have a beam splitter. In other words, the introduction section may not have to split the light.

<1-4. Second Configuration>
The photon number detector 1 described above is merely an example, and any configuration can be adopted, not limited to the photon number detector 1. For example, an element other than a connector may be used for the introduction section 2. A second configuration, which is an example of this point, will be described with reference to FIG. 4. FIG. 4 is a diagram showing an example of the second configuration of the photon number detector according to the embodiment. For example, FIG. 4 shows an example of the configuration when light is introduced from free space. Note that the description of the same points as those described above in the first configuration and the like will be omitted as appropriate.

As shown in FIG. 4, photon number detector 1A, which is an example of the second configuration, has introduction section 2A instead of introduction section 2. Introduction section 2A is a lens system for introducing the detection target TG, which is input light, into waveguide section 3. In FIG. 4, it is a lens system in which input light from free space functions as a connector with the outside. Introduction section 2A, which is a lens system, has a lens and focuses the input light from free space and introduces it into waveguide section 3.

Introduction section 2A introduces light in a photon number state to be measured into waveguide section 3. Introduction section 2A introduces light in a photon number state arriving from the outside into waveguide section 3. For example, introduction section 2A introduces the detection target TG, which is input light from an optical fiber, into waveguide section 3. In this way, when introducing light from free space, an optical focusing mechanism such as introduction section 2A can be used. Note that introduction section 2A is not limited to a lens system, and various elements can be used as long as it is possible to introduce light from free space.

<1-5. Third Configuration>
Furthermore, for example, the input light to be detected TG may be split into multiple beams before being introduced into the waveguide section 3. In this case, multiple detection units 20 may be used. A third configuration, which is an example of this point, will be described with reference to FIG. 5. FIG. 5 is a diagram showing an example of the third configuration of the photon number detector according to the embodiment. For example, FIG. 5 shows an example of a configuration in which a beam splitter is used as a component related to the introduction section. Note that the description of the same points as those described above in the first and second configurations will be omitted as appropriate.

As shown in FIG. 5, photon number detector 1B, which is an example of the third configuration, has multiple beam splitters 6a-6c that split the input light, and multiple detection units 20a-20d. Note that when there is no particular distinction between beam splitters 6a-6c, etc., they will be referred to as beam splitter 6. For example, beam splitter 6 is an optical component that splits (divides) the input light into multiple (e.g., two) beams.

Furthermore, when there is no particular distinction between the detection units 20a to 20d, they are referred to as detection units 20. In FIG. 5, the components of the detection unit 20 are the same as those of the detection unit 20 shown in FIG. 1, so only some of them are labeled with reference numerals. Each detection unit 20 includes a waveguide section 3, a branch section 4, and a detection section 5. The detection unit 20a includes one waveguide section 3, ten branch sections 4 (corresponding to the branches 4a to 4j in FIG. 1), and two detection sections 5 (corresponding to the detection sections 5a and 5b in FIG. 1), each of which has five light detection elements 10 (corresponding to the light detection elements 10a to 10j in FIG. 1). Note that the components of each of the detection units 20a to 20d are the same as those of the detection unit 20 shown in FIG. 1, so detailed explanations will be omitted.

In FIG. 5, a beam splitter 6 is placed between the introduction section 2, which is a connector to the outside, and the detection unit 20. Note that the arrangement shown in FIG. 5 is merely an example, and any arrangement can be adopted. For example, the beam splitter 6 may be placed before the introduction section 2, such as a connector. In this case, the photon number detector 1B may have four connectors corresponding to each of the detection units 20a to 20d. Note that the beam splitter 6 may be included as a component of the introduction section 2. In this case, the introduction section 2 of the photon number detector 1B may be configured to include a connector and a beam splitter.

In FIG. 5, beam splitter 6a is disposed in the path of light introduced from the connector with the outside, which is introduction unit 2. For example, light introduced from the connector with the outside, which is introduction unit 2, is split into two paths by beam splitter 6a. Beam splitter 6b and beam splitter 6c are disposed in each of the two paths split by beam splitter 6a.

In FIG. 5, beam splitter 6b is disposed on one of the two paths split by beam splitter 6a (the upper path in FIG. 5). For example, the light on one of the two paths split by beam splitter 6a is split into two paths by beam splitter 6b.

In FIG. 5, the light from one of the two paths split by the beam splitter 6b (the upper path in FIG. 5) is introduced into the detection unit 20a. As a result, the waveguide section 3 of the detection unit 20a guides the light introduced from the beam splitter 6b toward the detection section 5 (photodetection element 10) of the detection unit 20a.

In FIG. 5, the light from the other of the two paths split by the beam splitter 6b (the lower path in FIG. 5) is introduced into the detection unit 20b. As a result, the waveguide section 3 of the detection unit 20b guides the light introduced from the beam splitter 6b toward the detection section 5 (photodetection element 10) of the detection unit 20b.

In FIG. 5, beam splitter 6c is disposed on the other of the two paths split by beam splitter 6a (the lower path in FIG. 5). For example, the light on the other of the two paths split by beam splitter 6a is split into two paths by beam splitter 6c.

In FIG. 5, the light from one of the two paths split by the beam splitter 6c (the upper path in FIG. 5) is introduced into the detection unit 20c. As a result, the waveguide section 3 of the detection unit 20c guides the light introduced from the beam splitter 6c toward the detection section 5 (photodetection element 10) of the detection unit 20a.

In FIG. 5, the light from the other of the two paths split by the beam splitter 6c (the lower path in FIG. 5) is introduced into the detection unit 20d. As a result, the waveguide section 3 of the detection unit 20d guides the light introduced from the beam splitter 6c toward the detection section 5 (photodetection element 10) of the detection unit 20b.

In this way, the photon number detector 1B includes a beam splitter 6 that splits the light before it is introduced into the waveguide section 3. The photon number detector 1B has a plurality of detection units 20. A plurality of waveguide sections 3 are provided corresponding to the paths branched by the beam splitter 6. A plurality of detection sections 5 are provided corresponding to the plurality of waveguide sections 3.

With the above-mentioned configuration, the photon number detector 1B may perform light detection using multiple paths by splitting the light input from the connector (introduction section 2, etc.) with the beam splitter 6. FIG. 5 shows an example in which light is detected using four paths by splitting twice with the beam splitters 6a to 6c, but the number of paths is not limited to four, and may be more or less than four. The branched paths may be independent, or may be realized within the same detector array.

<1-6. Fourth Configuration>
Furthermore, the component that splits the light before being introduced into the detection unit 20 is not limited to the beam splitter 6, and various other components may be used. A fourth configuration, which is an example of this point, will be described with reference to FIG. 6. FIG. 6 is a diagram showing an example of the fourth configuration of the photon number detector according to the embodiment. For example, FIG. 6 shows an example of a configuration in which a wavelength-dependent beam splitter is used as a component related to the introduction section. Note that the description of the same points as those described above in the first to third configurations will be omitted as appropriate.

As shown in FIG. 6, in the fourth configuration, the photon number detector 1C has a plurality of wavelength-dependent beam splitters 7a-7c that split the input light according to the wavelength, and a plurality of detection units 20a-20d. Note that when there is no particular distinction between the wavelength-dependent beam splitters 7a-7c, etc., they are referred to as wavelength-dependent beam splitter 7. For example, the wavelength-dependent beam splitter 7 is an optical element that splits (divides) the input light into light of different wavelength bands. Also, the optical element for splitting the wavelength is not limited to a beam splitter.

In FIG. 6, the different wavelengths of light introduced into each detection unit 20 are shown diagrammatically by different types of lines. Detection units 20a to 20d in FIG. 6 are similar to detection units 20a to 20d in FIG. 5, so a detailed description is omitted.

In FIG. 6, a wavelength-dependent beam splitter 7 is placed between the introduction unit 2, which is a connector (lens system) to the outside, and the detection unit 20. Note that the arrangement shown in FIG. 6 is merely an example, and any arrangement can be adopted. For example, the wavelength-dependent beam splitter 7 may be placed before the connector. In this case, in the photon number detector 1C, four connectors corresponding to each of the detection units 20a to 20d may be placed. Note that the wavelength-dependent beam splitter 7 may be included as a component of the introduction unit 2. In this case, the introduction unit 2 of the photon number detector 1C may be configured to include a connector and a beam splitter. In FIG. 6, input light of multiple colors or a mixture of multiple colors is introduced by the lens system, which is the introduction unit 2.

In FIG. 6, wavelength-dependent beam splitter 7a is disposed on the path of light introduced from the connector with the outside, which is introduction unit 2. For example, light introduced from the connector with the outside, which is introduction unit 2, is split into two paths by wavelength-dependent beam splitter 7a. For example, light introduced from the connector with the outside, which is introduction unit 2, is split by wavelength-dependent beam splitter 7a into a path corresponding to the long wavelength side and a path corresponding to the short wavelength side. Wavelength-dependent beam splitter 7b and wavelength-dependent beam splitter 7c are disposed on each of the two paths split by wavelength-dependent beam splitter 7a.

In FIG. 6, wavelength-dependent beam splitter 7b is disposed on the path corresponding to the longer wavelength of the two paths split by wavelength-dependent beam splitter 7a (the upper path in FIG. 6). For example, of the two paths split by wavelength-dependent beam splitter 7a, the light on the path corresponding to the longer wavelength is split into two paths by wavelength-dependent beam splitter 7b.

In FIG. 6, of the two paths split by the wavelength-dependent beam splitter 7b, the light of the path corresponding to the longer wavelength side (the upper path in FIG. 6) is introduced into the detection unit 20a. For example, of the introduced light, light of a wavelength in the range corresponding to red is introduced into the detection unit 20a. As a result, the waveguide section 3 of the detection unit 20a guides the light introduced from the wavelength-dependent beam splitter 7b toward the detection section 5 (photodetection element 10) of the detection unit 20a.

In FIG. 6, of the two paths split by wavelength-dependent beam splitter 7b, the light of the path corresponding to the short wavelength side (the lower path in FIG. 6) is introduced into detection unit 20b. For example, of the introduced light, light of a wavelength in the range corresponding to green is introduced into detection unit 20a. As a result, the waveguide section 3 of detection unit 20b guides the light introduced from wavelength-dependent beam splitter 7b toward the detection section 5 (photodetection element 10) of detection unit 20b.

In FIG. 6, wavelength-dependent beam splitter 7c is disposed on the path corresponding to the shorter wavelength side of the two paths split by wavelength-dependent beam splitter 7a (the lower path in FIG. 6). For example, of the two paths split by wavelength-dependent beam splitter 7a, the light on the path corresponding to the shorter wavelength side is split into two paths by wavelength-dependent beam splitter 7c.

In FIG. 6, of the two paths split by the wavelength-dependent beam splitter 7c, the light of the path corresponding to the longer wavelength side (the upper path in FIG. 6) is introduced into the detection unit 20c. For example, of the introduced light, the light with a wavelength in the range corresponding to blue is introduced into the detection unit 20a. As a result, the waveguide section 3 of the detection unit 20c guides the light introduced from the wavelength-dependent beam splitter 7c toward the detection section 5 (photodetection element 10) of the detection unit 20a.

In FIG. 6, of the two paths split by the wavelength-dependent beam splitter 7c, the light of the path corresponding to the short wavelength side (the lower path in FIG. 6) is introduced into the detection unit 20d. For example, of the introduced light, light with a wavelength in the range corresponding to purple is introduced into the detection unit 20a. As a result, the waveguide section 3 of the detection unit 20d guides the light introduced from the wavelength-dependent beam splitter 7c toward the detection section 5 (photodetection element 10) of the detection unit 20b.

In this way, the photon number detector 1C includes a wavelength-dependent beam splitter 7 that splits the light before it is introduced into the waveguide section 3. The photon number detector 1C has a plurality of detection units 20. A plurality of waveguide sections 3 are provided corresponding to the paths branched by the wavelength-dependent beam splitter 7. A plurality of detection sections 5 are provided corresponding to the plurality of waveguide sections 3.

With the above-mentioned configuration, the photon number detector 1C is provided with a wavelength-dependent beam splitter 7c, which is a beam splitter with wavelength selectivity, so that the input light can be separated into individual wavelength components and received. The photon number detector 1C can be used, for example, for multicolor imaging (photon count imaging) in sensing, and for wavelength division multiplexing in communications.

<1-7. Fifth Configuration>
Furthermore, components other than the beam splitter 6, the wavelength-dependent beam splitter 7, etc. may be used as components that split the light before it is introduced into the detection unit 20. A fifth configuration, which is an example of this point, will be described with reference to FIG. 7. FIG. 7 is a diagram showing an example of the fifth configuration of the photon number detector according to the embodiment. For example, FIG. 7 shows an example of a configuration in which an optical switch is used as a component related to the introduction section. Note that the description of the same points as those described above in the first to fourth configurations, etc. will be omitted as appropriate.

As shown in FIG. 7, in the fifth configuration, the photon number detector 1D has multiple optical switches 8a-8c that branch the input light, and multiple detection units 20a-20d. When the optical switches 8a-8c are not particularly distinguished from one another, they are referred to as the optical switch 8. For example, the optical switch 8 is a component that branches (splits) the input light into multiple (e.g., two) beams by switching the path. For example, the optical switch 8 branches the path of the input light into two paths by switching the path depending on time.

The detection units 20a to 20d in FIG. 7 are similar to the detection units 20a to 20d in FIG. 5, so a detailed description will be omitted.

In FIG. 7, an optical switch 8 is placed between the introduction section 2, which is a connector to the outside, and the detection unit 20. Note that the arrangement shown in FIG. 7 is merely an example, and any arrangement can be adopted. For example, the optical switch 8 may be placed before the connector. In this case, the photon number detector 1D may have four connectors corresponding to each of the detection units 20a to 20d. Note that the optical switch 8 may be included as a component of the introduction section 2. In this case, the introduction section 2 of the photon number detector 1D may be configured to include a connector and an optical switch.

In FIG. 7, optical switch 8a is placed on the path of light introduced from the connector with the outside, which is introduction unit 2. For example, light introduced from the connector with the outside, which is introduction unit 2, is branched into two paths by optical switch 8a. Optical switch 8b and optical switch 8c are placed on each of the two paths branched by optical switch 8a.

In FIG. 7, optical switch 8b is disposed on one of the two paths branched by optical switch 8a (the upper path in FIG. 7). For example, the light on one of the two paths branched by optical switch 8a is branched into two paths by optical switch 8b.

In FIG. 7, the light from one of the two paths branched by the optical switch 8b (the upper path in FIG. 7) is introduced into the detection unit 20a. As a result, the waveguide section 3 of the detection unit 20a guides the light introduced from the optical switch 8b toward the detection section 5 (photodetection element 10) of the detection unit 20a.

In FIG. 7, the light from the other of the two paths branched by the optical switch 8b (the lower path in FIG. 7) is introduced into the detection unit 20b. As a result, the waveguide section 3 of the detection unit 20b guides the light introduced from the optical switch 8b toward the detection section 5 (photodetection element 10) of the detection unit 20b.

In FIG. 7, optical switch 8c is disposed on the other of the two paths branched by optical switch 8a (the lower path in FIG. 7). For example, the light on the other of the two paths branched by optical switch 8a is branched into two paths by optical switch 8c.

In FIG. 7, the light from one of the two paths branched by the optical switch 8c (the upper path in FIG. 7) is introduced into the detection unit 20c. As a result, the waveguide section 3 of the detection unit 20c guides the light introduced from the optical switch 8c toward the detection section 5 (photodetection element 10) of the detection unit 20a.

In FIG. 7, the light from the other of the two paths branched by the optical switch 8c (the lower path in FIG. 7) is introduced into the detection unit 20d. As a result, the waveguide section 3 of the detection unit 20d guides the light introduced from the optical switch 8c toward the detection section 5 (photodetection element 10) of the detection unit 20b.

In this way, the photon number detector 1D includes an optical switch 8 that branches the light before it is introduced into the waveguide section 3. The photon number detector 1D has a plurality of detection units 20. A plurality of waveguide sections 3 are provided corresponding to the paths branched by the optical switch 8. A plurality of detection sections 5 are provided corresponding to the plurality of waveguide sections 3.

With the above-mentioned configuration, the photon number detector 1D detects light by switching the path of the light input from the connector depending on time using the optical switch 8. The number of branchings is arbitrary. In FIG. 7, an example is shown in which the light is branched twice using the optical switches 8a to 8c to perform light detection on four paths, but the number of paths is not limited to four, and may be more or less than four. The branched paths may be independent, or may be realized within the same detector array.

<1-8. Realization method of the waveguide section>
Fig. 8 is a flow chart showing a method for a waveguide section. Fig. 8 is a flow chart showing a method for a waveguide section. For example, a waveguide section (also called a "waveguide") has elements as shown in blocks S21 and S22 in Fig. 8. Each block of the flow chart will be described below.

(Waveguide: corresponds to block S21)
The waveguide guides the light toward the individual photodetector elements and introduces the light directly into the branching section.

For example, the waveguide section (waveguide) uses optical fiber. For example, the waveguide section (waveguide) may use various materials that transmit light. For example, the waveguide section (waveguide) may use transparent resin, glass, etc. The waveguide section (waveguide) may also be a vacuum, air, etc. Furthermore, when realizing a photon number detector 1 or the like on an optical circuit, a waveguide chip or the like is used for the waveguide section (waveguide).

If it is desired to introduce light in a specific direction that is wavelength-dependent, a holographic waveguide or a photonic crystal waveguide is used in the waveguide section (waveguide). If it is desired to dynamically change the waveguide path, a liquid crystal waveguide is used. If it is desired to realize special light manipulation using a waveguide, a metamaterial-based waveguide is used in the waveguide section (waveguide).

(Mirror: corresponds to block S22)
A mirror that totally reflects light may be provided at any position in the waveguide. In this case, the following embodiment is given.

For example, when changing the refractive index locally within a waveguide, a change in the refractive index within the waveguide may be formed. Also, when forming a mirror that reflects light of a specific wavelength, this may be achieved by introducing a periodic change in the refractive index into the waveguide. In this case, a technology such as Bragg Gratings may be used.

For example, metamaterials can be used to form a mirror that achieves total reflection at a specific angle within the waveguide. For example, a metallic reflective mirror can be used, which is realized by placing a thin metal layer (e.g., aluminum or gold) at a specific position within the waveguide.

<1-9. Sixth Configuration>
In the above-mentioned first to fifth configurations, the configuration in which the opposed detection units 5a, 5b are connected by the waveguide unit 3 has been described as an example, but the detection units 5 do not have to be arranged to face each other. A sixth configuration, which is an example of this point, will be described with reference to FIG. 9. FIG. 9 is a diagram showing an example of the sixth configuration of the photon number detector according to the embodiment. For example, FIG. 9 shows an example of a configuration in which a mirror is opposed to a detector and connected by a waveguide. Note that the description of the same points as those described above in the first to fifth configurations etc. will be omitted as appropriate.

As shown in FIG. 9, in the sixth configuration, the photon number detector 1E has an introduction section 2, a waveguide section 3, branch sections 4a-4e, a detection section 5a, and a reflector 9. Among the components of the photon number detector 1E, the waveguide section 3, branch section 4, detection section 5, and reflector 9 may be referred to as a detection unit 21. For example, in the photon number detector 1E, light from the introduction section 2 is introduced into the detection unit 21.

The introduction section 2 of the photon number detector 1E is similar to the introduction section 2 of the photon number detector 1, so a detailed description will be omitted. In FIG. 9, for example, the waveguide section 3 is made of a transparent resin or the like that fills the space between the detection section 5a and the reflection section 9, and the light in the waveguide section 3 is guided from the left side to the right side.

The branches 4a to 4e of the photon number detector 1E are the same as the branches 4a to 4e of the photon number detector 1. In FIG. 9, the detector 5a is the detector 5 having the light detection elements 10a to 10e and arranged below the waveguide 3. The detector 5a is arranged on one side (the lower side in FIG. 9) in a direction (the up-down direction in FIG. 9) that intersects with a direction (the left-right direction in FIG. 9) that corresponds to the guiding of light by the waveguide 3. The detector 5a is arranged in a position facing the reflector 9 across the waveguide 3. Note that the detector 5a of the photon number detector 1E is the same as the detector 5a of the photon number detector 1, so a detailed description will be omitted.

The reflecting unit 9 is disposed on the other side (upper side in FIG. 9) of the direction intersecting with the direction corresponding to the guiding of light by the waveguide unit 3. The reflecting unit 9 reflects light toward the detecting unit 5. A mirror is used for the reflecting unit 9. For example, the reflecting unit 9 is a member that totally reflects the input (incident) light. The reflecting unit 9 is disposed in a position facing the detecting unit 5a across the waveguide unit 3.

The waveguide section 3 of the photon number detector 1E guides the light introduced from the introduction section 2 toward each of the multiple branch sections 4a to 4e. In FIG. 9, the waveguide section 3 guides the light branched to the second path (second path SP in FIG. 2) by the branch section 4a toward the reflecting section 9. The waveguide section 3 also guides the light reflected by the reflecting section 9 toward the branch section 4b. The waveguide section 3 also guides the light branched to the second path by the branch section 4b toward the reflecting section 9.

The waveguide section 3 also guides the light reflected by the reflector 9 toward the branching section 4c. The waveguide section 3 also guides the light branched to the second path by the branching section 4c toward the reflector 9. The waveguide section 3 also guides the light reflected by the reflector 9 toward the branching section 4d. The waveguide section 3 also guides the light branched to the second path by the branching section 4d toward the reflector 9.

The waveguide section 3 also guides the light reflected by the reflector 9 towards the branch section 4e. The waveguide section 3 also guides the light branched to the second path by the branch section 4e towards the reflector 9. The waveguide section 3 also guides the light reflected by the reflector 9 further ahead. For example, if the photon number detector 1E has more than five light detection elements 10, the waveguide section 3 guides the light reflected by the reflector 9 towards the light detection element 10 provided further ahead of the branch section 4e (on the right side in FIG. 9).

As described above, in photon number detector 1E, light reflected from branching section 4 and then reflected by reflecting section 9 is input again to branching section 4 by waveguide section 3. Branching section 4 stochastically transmits the light input from waveguide section 3 and inputs it to photodetection element 10 (detector). Light that does not transmit through branching section 4 is reflected towards waveguide section 3, reflected by reflecting section 9, and input again to branching section 4. Detection section 5 is composed of photodetection element 10 (detector) capable of detecting multiple independent single photons. Detection section 5 stochastically detects the transmitted light that has transmitted through branching section 4.

In this way, the photon number detector 1E has a repeating structure of the waveguide section 3, the branch section 4, the detection section 5, and the reflection section 9. In the photon number detector 1E, the light reflected from the branch section 4 and the reflection section 9 is guided toward the detection section 5 until the light is guided toward all the photodetection elements 10 (detectors). For example, the photon number detector 1E can easily realize a configuration that can detect the number of photons by arranging multiple sets of the branch section 4 and the photodetection elements 10 along the waveguide section 3. For example, the photon number detector 1E can easily arrange a desired number of sets of the branch section 4 and the photodetection elements 10 according to the number of photons to be detected, and can easily realize any configuration. Therefore, the photon number detector 1E can realize a flexible configuration for photon number detection. In addition, these structures may have a structure in which the set of the reflection section 9, the branch section 4, and the detection section 5 is upside down in the middle, or a structure in which the set of the branch section 4 and the detection section 5 is arranged instead of the reflection section 9.

<1-10. Seventh Configuration>
Also, for example, the light detection elements 10 may be arranged two-dimensionally (planarly). A seventh configuration, which is an example of this point, will be described with reference to FIG. 10. FIG. 10 is a diagram showing an example of the seventh configuration of the photon number detector according to the embodiment. For example, FIG. 10 shows an example of a configuration in which light is introduced to each element of the array detector using a mirror. Note that the description of the same points as those described above in the first to sixth configurations will be omitted as appropriate.

As shown in FIG. 10, in the seventh configuration, the photon number detector 1F has an introduction section 2, a waveguide section 30, a plurality of branch sections 4, light guide sections 41 and 42, a detection section 50, and a reflection section 90.

The introduction section 2 of the photon number detector 1F is similar to the introduction section 2 of the photon number detector 1, so a detailed description will be omitted. In FIG. 9, for example, the waveguide section 30 is made of a transparent resin or the like that fills the space between the detection section 50 and the reflection section 90, and the light in the waveguide section 30 is guided within the waveguide section 30 so as to be input to the branch section 4 corresponding to each photodetection element 10 of the detection section 50.

The detection unit 50 has a plurality of photodetection elements 10 arranged two-dimensionally along a surface. In FIG. 10, the detection unit 50 has 25 photodetection elements 10. The plurality of photodetection elements 10 are arranged two-dimensionally (in a plane) along one side of the waveguide section 30 (the lower side in FIG. 10).

In the photon number detector 1F, light guides 41, 42 are arranged at both ends of the multiple light detection elements 10 in one direction along the surface (left and right direction in FIG. 10). The light guides 41, 42 shift the light in the other direction along the surface and reflect it in one direction. Mirrors are used for the light guides 41, 42. For example, the light guides 41, 42 are members that totally reflect the input (incident) light. The light guides 41, 42 are arranged in opposing positions across the waveguide section 30.

The multiple branching sections 4 are provided corresponding to each of the light detection elements 10 of the detection section 50. In FIG. 10, 25 branching sections 4 are provided corresponding to the 25 light detection elements 10. Each branching section 4 is disposed along the surface where the light of the corresponding light detection element 10 is input. In other words, each branching section 4 is disposed between the waveguide section 30 and the detection section 50.

With the above-mentioned configuration, in photon number detector 1F, light from introduction section 2 is guided toward each of the photodetection elements 10 (detectors) that make up detection section 5. For example, in photon number detector 1F, light from introduction section 2 is guided from left to right toward the row of five photodetection elements 10 (detectors) in the first row (the first (frontmost) from the front in FIG. 10). After that, the light is shifted by one row in the other direction along the surface (depth direction in FIG. 10) by light guiding section 41 located at the right end, and guided from right to left toward the row in the second row (the second from the front in FIG. 10).

Then, the light is shifted by one row in the other direction along the surface (depth direction in FIG. 10) by the light guiding section 42 located at the left end, and is guided from left to right toward the third row (third from the front in FIG. 10). The light is then shifted by one row in the other direction along the surface (depth direction in FIG. 10) by the light guiding section 41 located at the right end, and is guided from right to left toward the fourth row (fourth from the front in FIG. 10). The light is then shifted by one row in the other direction along the surface (depth direction in FIG. 10) by the light guiding section 42 located at the left end, and is guided from left to right toward the fifth row (fifth from the front in FIG. 10).

In this way, the photon number detector 1F has a repeating structure of the waveguide section 30, the branch section 4, the light guiding sections 41, 42, the detection section 50, and the reflection section 90. In the photon number detector 1F, the light reflected from the branch section 4, the light guiding sections 41, 42, and the reflection section 90 is guided toward the detection section 50 until the light is guided toward all the light detection elements 10 (detectors). For example, the photon number detector 1F can easily realize a configuration that can detect the number of photons by arranging multiple sets of the branch section 4 and the light detection elements 10 along the waveguide section 30. For example, the photon number detector 1F can easily arrange a desired number of sets of the branch section 4 and the light detection elements 10 depending on the number of photons to be detected, and can easily realize any configuration. Therefore, the photon number detector 1F can realize a flexible configuration for photon number detection.

<1-11. Realization method of photon splitting unit>
Here, a method for realizing the photon splitting unit will be described with reference to Fig. 11. Fig. 11 is a flowchart showing a method of the photon splitting unit. For example, the photon splitting unit (also called a "splitting unit") has elements as shown in block S31 in Fig. 11. Each block in the flowchart will be described below.

(Beam splitter: corresponds to block S31)
A beam splitter, which is an example of a branching section, stochastically transmits light input from the waveguide section and inputs it to the detection section. Light that does not transmit is reflected toward the waveguide section and is input from the waveguide section back to the branching section. The reflectance of the beam splitter may be changed in correspondence with each photodetection element. Examples of the branching section include the following.

For example, the branching section may be formed integrally with the waveguide structure. The branching section may be configured with a highly reflective wall surface required for forming the waveguide as a beam splitter. In this case, the reflectance of the branching section may be adjusted by adjusting the refractive index of the wall surface.

For example, the branching section may be a component that reflects light by utilizing the periodic changes in refractive index of Bragg gratings. The branching section may be a Multi-Mode Interference (MMI) or the like. For example, MMI uses a wide waveguide section to excite multiple modes, which interfere with each other to obtain a specific branching ratio. By changing the shape and size of the interference region, a branching section (such as a beam splitter) with a different reflectance can be formed.

For example, the branching section may use liquid crystal adjustment or the like. In this case, the photon number detector 1 or the like can electrically adjust the refractive index of a specific region in the waveguide using liquid crystal. This allows the photon number detector 1 or the like to dynamically control branching sections (beam splitters, etc.) with different reflectances in the same waveguide.

For example, a microring resonator or the like may be used as the branching section. In this case, the photon number detector 1 or the like can couple light at a specific wavelength by arranging a tiny ring structure near the waveguide. This allows the photon number detector 1 or the like to form branching sections (beam splitters, etc.) with different reflectivities by using microring resonators with different sizes and coupling strengths.

<1-12. Reflectance of branching part>
The reflectance of the branching portion 4 can be set arbitrarily. Below, several examples of the reflectance of the branching portion 4 are shown. Note that, of the above-mentioned configurations, the photon number detector 1E having the sixth configuration is shown below as an example, but the setting of the reflectance of the branching portion 4 may be similarly applied to the photon number detectors 1, 1A-D, 1F, etc.

<1-12-1. First example (constant reflectance)>
For example, the reflectance of the branching portion 4 may be constant. One example of this point will be described as a first example with reference to FIG. 12. FIG. 12 is a diagram showing one example of the reflectance of the branching portion. As a specific example, FIG. 12 shows an example in which the reflectance of the branching portion is constant. The percentage values shown superimposed on each of the branching portions 4a to 4e in FIG. 12 indicate the reflectance of each of the branching portions 4a to 4e.

The photon number detector 1E in FIG. 12 shows a case where the branching section 4 is configured with a beam splitter or the like with a constant reflectance. In the photon number detector 1E in FIG. 12, the multiple branching sections 4 have the same reflectance. FIG. 12 shows a case where the reflectance of all five branching sections 4a to 4e is 99%. For example, beam splitters with a reflectance of 99% are used for branching sections 4a to 4e. In this way, the reflectance of the branching sections 4 may be set to be the same.

<1-12-2. Second example (reflectance is indefinite)>
The reflectance of the branching portion 4 may be different. An example of this point will be described as a second example with reference to FIG. 13. FIG. 13 is a diagram showing an example of the reflectance of the branching portion. As a specific example, FIG. 13 shows an example where the reflectance of the branching portion is indefinite. The percentage values shown superimposed on each of the branching portions 4a to 4e in FIG. 13 indicate the reflectance of each of the branching portions 4a to 4e.

The photon number detector 1E in FIG. 13 shows a case where the branching section 4 is configured with a beam splitter or the like having a different reflectance for each light detection element 10 (detector). In the photon number detector 1E in FIG. 13, at least some of the multiple branching sections 4 have different reflectances.

In FIG. 13, the branching portion 4a is set to 99%. For example, a beam splitter with a reflectance of 99% is used for the branching portion 4a. The branching portion 4b is set to 80%. For example, a beam splitter with a reflectance of 80% is used for the branching portion 4b.

In FIG. 13, the branching portion 4c is set to 60%. For example, a beam splitter with a reflectance of 60% is used for the branching portion 4c. Furthermore, the branching portion 4d is set to 30%. For example, a beam splitter with a reflectance of 10% is used for the branching portion 4d. Furthermore, the branching portion 4d is set to 30%. For example, a beam splitter with a reflectance of 10% is used for the branching portion 4d.

In the photon number detector 1E of FIG. 13, each of the multiple branching sections 4 has a reflectance equal to or lower than the reflectance of the branching section 4 that reflects the input light. In the photon number detector 1E of FIG. 13, each of the multiple branching sections 4 has a reflectance that attenuates with increasing distance from the introduction section 2. In this way, the reflectance of the branching sections 4 may be set to different values.

<1-13. Realization method of photon detection unit>
Here, a method for implementing the photon detection unit will be described with reference to Fig. 14. Fig. 14 is a flowchart showing a method of the photon detection unit. For example, the photon detection unit (also called a "photodetection unit") has elements as shown in block S41 in Fig. 14. Each block in the flowchart will be described below.

(Photodetector: corresponds to block S41)
The detection section (light detection section) is composed of a plurality of independent light detection elements, and detects light transmitted through the branching section (light branching section). The light detection elements may be independent as detectors for each element, or may form an array of elements.

The photodetector element used is capable of detecting single photons. The following are examples of the photodetector element. For example, a SPAD or the like may be used as the photodetector element. In this case, the photodetector element detects a single photon by taking advantage of the electron avalanche that occurs when the reverse bias voltage is high.

For example, photomultiplier tubes (PMTs) or the like may be used as the photodetector element. In this case, the photodetector element utilizes the phenomenon that thermal electrons are released when a photon hits an electrode (photocathode). These electrons collide with the electrode repeatedly in a series, amplifying the number of electrons released, and ultimately becoming a signal pulse that can be considered as light detection.

For example, the photodetector may be an SSPD or the like. In this case, the photodetector uses a superconducting nanowire to detect the localized destruction of the superconducting state caused by the absorption of a photon.

For example, a photon number detector or the like may be used as the light detection element. In this case, the light detection element itself may be a light detector having photon number detection capability (TES, MKID (Microwave Kinetic Inductance Detector), VLPC (Visible Light Photon Counters), etc.). In this case, the detection unit may have one light detection element having photon number detection capability.

<1-14. Experimental results (simulation results)>
From here, experimental results (simulation results) using the above-mentioned configuration will be shown. For example, the results of verifying the effect of using the configuration of the photon number detector 1E by simulation will be shown. First, the conditions on which the simulation is based will be described. FIG. 15 is a diagram showing an example of simulation comparison conditions. The present method in the second row of the table in FIG. 15 corresponds to the case where the configuration of the photon number detector 1E is used. Also, as shown in FIG. 15, as reference conditions corresponding to the present method, an array detector that receives diffused light in the third row of the table in FIG. 15 and a TES (superconducting transition edge sensor) in the fourth row of the table in FIG. 15 were used as comparison objects.

Next, the experimental results will be explained using Figures 16 to 20. Figures 16 to 20 are diagrams showing an example of the experimental results. First, the results of Figures 16 and 17 will be explained. Figure 16 shows the probability (correct rate) that the number of input and output photons matches. Result RS1 in Figure 16 is a graph with the vertical axis representing probability and the horizontal axis representing the number of input and output photons. Line L11 in result RS1 shows the probability in the case of this method. Line L12 in result RS1 shows the probability in the case of an array detector. Line L13 in result RS1 shows the probability in the case of TES.

Figure 17 shows the maximum rate of measurement events where the number of input and output photons match. Result RS2 in Figure 17 is a graph with the measurement rate on the vertical axis and the number of input and output photons on the horizontal axis. Line L21 in result RS2 shows the measurement rate for this method. Line L22 in result RS2 shows the measurement rate for an array detector. Line L23 in result RS2 shows the measurement rate for TES. As shown in Figures 16 and 17, in both indicators, this method is expected to have the best performance.

Next, the experimental results (simulation results) of Figures 18 to 20 will be described. First, Figure 18 shows the beam splitter reflectance coupled to each element of the photodetector. Setting RS3 in Figure 18 is a graph with the vertical axis representing reflectance and the horizontal axis representing the number of photodetection elements 10 (branching sections 4). For example, the smaller the number on the horizontal axis shown in setting RS3, the closer the number is to the photodetection elements 10 (branching sections 4) that are closer to the introduction section 2. For example, the branching section 4 for which the value on the horizontal axis shown in setting RS3 corresponds to 0 corresponds to the reflectance of the branching section 4 (branching section 4a in the photon number detector 1E in Figure 9) that corresponds to the photodetection element 10 closest to the introduction section 2.

Line L31 in setting RS3 shows the reflectance of the branching section 4 corresponding to each light detection element 10 when the reflectance R is exponentially decayed. The reflectance R of the branching section 4 corresponding to each light detection element 10 shown on line L31 is calculated by the following formula (3). For example, line L31 shows a case where the branching section 4 is configured with a beam splitter or the like having a different reflectance for each detector. Below, the result when the reflectance is exponentially decayed as shown on line L31 is referred to as the "first result."

For example, the formula (3) shows a case where _R0 is 99%, α is a predetermined coefficient, and _x0 is 144. Note that the formula (3) is merely an example, and when the reflectance of the branching portion 4 is to be attenuated, the reflectance R of the branching portion 4 may be calculated by any function.

Furthermore, line L32 in setting RS3 shows the reflectance of the branching section 4 corresponding to each light detection element 10 when it is set to a constant reflectance. The reflectance R of the branching section 4 corresponding to each light detection element 10 shown on line L32 is set to 97%. For example, line L32 shows the case where the branching section 4 is configured with a beam splitter or the like having a constant reflectance (97%). Below, the result when the reflectance is constant as shown on line L32 is referred to as the "second result".

Next, the results of Figures 19 and 20 will be explained. Figure 19 shows the probability (correct rate) that the number of input and output photons matches depending on the difference in the optical branching unit (branching unit 4). Result RS4 in Figure 19 is a graph with the probability on the vertical axis and the number of input and output photons on the horizontal axis. Line L41 in result RS4 shows the result (first result) when the reflectance of branching unit 4 is different. Line L42 in result RS4 shows the result (second result) when the reflectance of branching unit 4 is constant (97%).

Figure 20 shows the maximum rate of measurement events where the number of input and output photons match due to differences in the optical branching section (branching section 4). Result RS5 in Figure 20 is a graph with the measurement rate on the vertical axis and the number of input and output photons on the horizontal axis. Line L51 in result RS5 shows the result (first result) when the reflectance of branching section 4 is different. Line L52 in result RS5 shows the result (second result) when the reflectance of branching section 4 is constant (97%). As shown in Figures 19 and 20, it was confirmed that in both indicators, performance is further improved when the reflectance R is exponentially decayed.

<1-15. Others>
For example, an information processing device such as a control device that controls the above-mentioned photon number detectors 1 to 1F, etc., and collects (acquires) information detected (measured) by the photon number detectors 1 to 1F, etc., may be realized by a dedicated computer system or a general-purpose computer system.

For example, a program for executing the above-mentioned operations is stored on a computer-readable recording medium such as an optical disk, semiconductor memory, magnetic tape, or flexible disk and distributed. Then, for example, the program is installed on a computer and the above-mentioned process is executed to configure a control device. In this case, the control device may be an external device (for example, a personal computer) such as the photon number detector 1. The control device may also be an internal device (for example, a processor) such as the photon number detector 1.

The above program may also be stored on a disk device provided in a server on a network such as the Internet, so that it can be downloaded to a computer. The above functions may also be realized by cooperation between an OS (Operating System) and application software. In this case, parts other than the OS may be stored on a medium and distributed, or parts other than the OS may be stored on a server so that they can be downloaded to a computer.

Furthermore, among the processes described in the above embodiments, all or part of the processes described as being performed automatically can be performed manually, or all or part of the processes described as being performed manually can be performed automatically using known methods. In addition, the information including the processing procedures, specific names, various data and parameters shown in the above documents and drawings can be changed as desired unless otherwise specified. For example, the various information shown in each drawing is not limited to the information shown in the drawings.

Furthermore, each component of each device shown in the figure is a functional concept, and does not necessarily have to be physically configured as shown in the figure. In other words, the specific form of distribution and integration of each device is not limited to that shown in the figure, and all or part of them can be functionally or physically distributed and integrated in any unit depending on various loads, usage conditions, etc. This distribution and integration configuration may also be performed dynamically.

The above-described embodiments can be combined as appropriate in areas where the processing content is not contradictory. The order of the steps shown in the flowcharts of the above-described embodiments can be changed as appropriate.

Furthermore, for example, this embodiment can be implemented as any configuration that constitutes an apparatus or system, such as a processor as a system LSI (Large Scale Integration), a module using multiple processors, a unit using multiple modules, a set in which a unit has been further enhanced with other functions, etc. (i.e., a configuration that constitutes part of an apparatus).

In this embodiment, a system refers to a collection of multiple components (devices, modules (parts), etc.), regardless of whether all the components are in the same housing. Therefore, multiple devices housed in separate housings and connected via a network, and a single device in which multiple modules are housed in a single housing, are both systems.

Furthermore, for example, this embodiment can be configured as a cloud computing system in which a single function is shared and processed collaboratively by multiple devices via a network.

For example, an information processing device such as the control device described above is realized by a computer having the configuration shown below. The computer has a CPU, RAM, ROM (Read Only Memory), HDD (Hard Disk Drive), a communication interface, and an input/output interface. Each part of the computer is connected by a bus.

The CPU operates based on the programs stored in the ROM or HDD and controls each part. For example, the CPU loads the programs stored in the ROM or HDD into the RAM and executes the processes corresponding to the various programs.

ROM stores boot programs such as the Basic Input Output System (BIOS), which is executed by the CPU when the computer starts up, and programs that depend on the computer's hardware.

The HDD is a computer-readable recording medium that non-temporarily records programs executed by the CPU and data used by such programs. Specifically, the HDD is a recording medium that records information processing programs such as control programs for controlling the photon number detectors 1, 1A-1F, etc., according to the present disclosure, which are examples of program data.

A communications interface is an interface that allows a computer to connect to an external network (such as the Internet). For example, a CPU can receive data from other devices and send data generated by the CPU to other devices via a communications interface.

An input/output interface is an interface for connecting an input/output device to a computer. For example, a CPU receives data from an input device such as a keyboard or mouse via the input/output interface. The CPU also transmits data to an output device such as a display, speaker, or printer via the input/output interface. The input/output interface may also function as a media interface that reads programs and the like recorded on a specific recording medium. Media include, for example, optical recording media such as DVDs (Digital Versatile Discs) and PDs (Phase change rewritable Disks), magneto-optical recording media such as MOs (Magneto-Optical Disks), tape media, magnetic recording media, or semiconductor memories.

For example, when a computer functions as a control device in photon number detectors 1, 1A-1F, etc., the computer's CPU executes an information processing program loaded onto the RAM to realize the functions of the control device's control unit, etc. Also, the HDD stores the information processing program according to the present disclosure and data in the storage unit of the control device. The CPU reads and executes program data from the HDD, but as another example, these programs may be obtained from other devices via an external network.

Although each embodiment of the present disclosure has been described above, the technical scope of the present disclosure is not limited to the above-mentioned embodiments as they are, and various modifications are possible without departing from the gist of the present disclosure. Furthermore, components from different embodiments and modified examples may be combined as appropriate.

Furthermore, the effects of each embodiment described in this specification are merely examples and are not limiting, and other effects may also be present.

2. Effects of the Present Disclosure
As described above, the photon number detector according to the present disclosure (photon number detector 1 in the embodiments, etc.; the same applies below) comprises a detection unit (detection unit 5 in the embodiments, etc.; the same applies below) having at least one photodetection element (photodetection element 10 in the embodiments, etc.; the same applies below) capable of single photon detection, and a branching unit that probabilistically transmits input light and branches it into a first path (first path FP in the embodiments, etc.; the same applies below) toward the photodetection element, and a second path (second path SP in the embodiments, etc.; the same applies below) different from the first path.

In this way, the photon number detector of the present disclosure probabilistically transmits input light and branches it into a first path toward the photodetection element and a second path different from the first path, making it possible to adjust the probability that each photon will be detected by the photodetection element. For example, by lowering the probability of branching to the first path, the photon number detector can estimate that when one photodetection element detects something, the detection is due to one photon. In this way, the photon number detector can adjust the detection by one photodetection element to be one photon. Therefore, the photon number detector can make it possible to realize a flexible configuration for photon number detection.

In addition, in the photon number detector disclosed herein, the branching portion is a reflective film, and the photodetection elements are pixels.

As a result, the photon number detector of the present disclosure probabilistically transmits input light using the reflective film, which is a branching section, and branches the light into a first path toward the pixel, which is the light detection element, and a second path different from the first path, making it possible to adjust the probability that each photon will be detected by the pixel. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

In addition, in the photon number detector disclosed herein, the branching portion is disposed along the surface where the light of the light detection element is input, and the light detection element detects the light transmitted through the branching portion.

As a result, the photon number detector of the present disclosure can stochastically transmit the input light through the branching section arranged along the surface where the light of the photodetector is input, and detect the light transmitted through the branching section, thereby arranging combinations of branching sections and photodetector elements side by side. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

The photon number detector according to the present disclosure also includes a waveguide section (waveguide section 3 in the embodiment, etc.; the same applies below) that guides light toward the photodetector element, and an introduction section (introduction sections 2, 2A, etc. in the embodiment, etc.; the same applies below) that introduces light in a photon number state to be measured into the waveguide section, and the branching section stochastically transmits the light input from the waveguide section and branches it into a first path and a second path that reflects the light to the waveguide section.

As a result, the photon number detector disclosed herein can guide the introduced light toward the photodetector element, probabilistically transmit the guided light, and branch into a first path and a second path that reflects the light toward the waveguide section, thereby appropriately guiding the light.

In addition, in the photon number detector according to the present disclosure, the detection section has a plurality of photodetection elements, a plurality of branch sections are provided corresponding to the plurality of photodetection elements, and the waveguide section guides the light introduced from the introduction section toward each of the plurality of branch sections.

As a result, the photon number detector of the present disclosure can appropriately detect multiple photons by probabilistically transmitting input light and using multiple photon detection elements. For example, by lowering the probability of each photon detection element being branched to the first path, the photon number detector can estimate that when one photon detection element performs detection, the detection is due to one photon. This allows the photon number detector to estimate the number of photon detection elements as the number of photons. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

In addition, in the photon number detector disclosed herein, the detection section has a plurality of photodetection elements arranged along one direction corresponding to the guiding of light by the waveguide section.

As a result, the photon number detector according to the present disclosure can appropriately arrange multiple photodetection elements in accordance with the waveguiding by arranging multiple photodetection elements along one direction corresponding to the waveguiding of light. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

In addition, in the photon number detector disclosed herein, the multiple branch sections are disposed between the waveguide section and the detection section.

As a result, the photon number detector according to the present disclosure can appropriately arrange the multiple branch sections in accordance with the waveguiding by arranging the multiple branch sections between the waveguide section and the detection section. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

The photon number detector according to the present disclosure also includes a first detection unit that is a detection unit arranged on one side of a direction intersecting with one direction, and a second detection unit that is a detection unit arranged on the other side of the direction intersecting with the one direction.

As a result, the photon number detector according to the present disclosure can efficiently arrange the detection units by arranging the detection units on both sides that intersect in one direction corresponding to the light waveguiding (opposite arrangement). Therefore, the photon number detector can realize a flexible configuration for photon number detection.

In addition, in the photon number detector disclosed herein, the detection unit is disposed on one side of a direction that intersects with one direction.

As a result, the photon number detector according to the present disclosure can appropriately position the detection unit in accordance with the guided wave by positioning the detection unit on one side in a direction intersecting one direction. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

The photon number detector according to the present disclosure also includes a reflector (in the embodiment, reflector 9, 90, etc.; the same applies below) that is disposed on the other side of the direction intersecting the one direction and reflects light toward the detector.

As a result, the photon number detector according to the present disclosure can position the reflector on the other side of a direction that intersects with one direction, and reflect light toward the detection unit, thereby allowing the reflector to be positioned appropriately in accordance with the guided wave. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

In addition, in the photon number detector disclosed herein, multiple photodetection elements are arranged two-dimensionally along a surface.

As a result, the photon number detector according to the present disclosure can efficiently arrange multiple photodetection elements. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

The photon number detector according to the present disclosure also includes light guides (light guides 41, 42, etc. in the embodiments; the same applies below) at both ends of the multiple light detection elements in one direction along the surface, which shift the light in the other direction along the surface and reflect it in the one direction.

As a result, the photon number detector of the present disclosure can appropriately guide light to each of a plurality of photodetection elements arranged in two dimensions by shifting the light in another direction along the surface and reflecting it in one direction. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

Furthermore, in the photon number detector disclosed herein, the multiple branches have the same reflectance.

As a result, the photon number detector disclosed herein can match the detection probability of each photodetector by using multiple branching sections with the same reflectance. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

Furthermore, in the photon number detector disclosed herein, at least some of the multiple branches have different reflectivities.

As a result, the photon number detector disclosed herein can individually adjust the detection probability of each photodetector element by using multiple branching sections, at least some of which have different reflectivities. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

Furthermore, in the photon number detector disclosed herein, each of the multiple branches has a reflectance equal to or less than the reflectance of the branch that reflects the input light.

As a result, the photon number detector according to the present disclosure can reduce the reflectance closer to the end by making the reflectance of each branch equal to or lower than the reflectance of the branch to which light is input. This allows the photon number detector to increase the possibility of detecting photons, for example, in a light detection element closer to the end, and reduce missed detection of photons.

In addition, in the photon number detector disclosed herein, each of the multiple branches has a reflectance that attenuates with increasing distance from the introduction section.

As a result, the photon number detector according to the present disclosure can reduce the reflectance closer to the end by attenuating the reflectance of each branch section as it moves away from the introduction section. This allows the photon number detector to increase the possibility of detecting photons, for example, in a photodetector element closer to the end, and reduce missed detection of photons.

The photon number detector according to the present disclosure also includes a photon branching section (in the embodiment, a beam splitter 6, a wavelength-dependent beam splitter 7, an optical switch 8, etc.; the same applies below) that branches the light before it is introduced into the waveguide section, and multiple waveguide sections are provided corresponding to the paths branched by the photon branching section, and multiple detection sections are provided corresponding to the multiple waveguide sections.

As a result, the photon number detector according to the present disclosure uses a photon branching section that branches the light before it is introduced into the waveguide section, making it possible to arrange multiple detection sections in response to the branching at the photon branching section, and thus enabling efficient arrangement of the detection sections. Therefore, the photon number detector can realize a flexible configuration for photon number detection.

The present technology can also be configured as follows.
(1)
a detection unit having at least one photodetection element capable of single photon detection;
a branching section that stochastically transmits input light and branches it into a first path toward the light detection element and a second path different from the first path;
A photon number detector comprising:
(2)
the branching portion is a reflective film,
The photon number detector according to (1), wherein the photodetection elements are pixels.
(3)
the branching portion is disposed along a surface of the photodetector element into which light is input,
The photon number detector according to (1) or (2), wherein the light detection element detects light transmitted through the branching portion.
(4)
a waveguide portion that guides light toward the photodetector;
an introduction section that introduces light of a photon number state to be measured into the waveguide section;
Equipped with
The photon number detector according to any one of (1) to (3), wherein the branching section stochastically transmits the light input from the waveguide section and branches the light into the first path and the second path that reflects the light to the waveguide section.
(5)
The detection unit has a plurality of light detection elements,
a plurality of the branching portions are provided corresponding to the plurality of light detecting elements;
The photon number detector according to (4), wherein the waveguide section guides the light introduced from the introduction section toward each of a plurality of branch sections.
(6)
The photon number detector according to (5), wherein the detection section has the plurality of light detection elements arranged along one direction corresponding to the guiding of light by the waveguide section.
(7)
The photon number detector according to (6), wherein the plurality of branching sections are disposed between the waveguide section and the detection section.
(8)
A first detection unit that is the detection unit arranged on one side in a direction intersecting the one direction;
A second detection unit that is the detection unit disposed on the other side in a direction intersecting the one direction;
The photon number detector according to (6) or (7), comprising:
(9)
The photon number detector according to (6) or (7), wherein the detection unit is disposed on one side in a direction intersecting the one direction.
(10)
a reflecting portion that is disposed on the other side of a direction intersecting the one direction and that reflects light toward the detecting portion;
The photon number detector according to (9) comprising:
(11)
The photon number detector according to (9) or (10), wherein the plurality of photodetection elements are arranged two-dimensionally along a surface.
(12)
a light guide portion at both ends of the plurality of light detection elements in one direction along the surface, the light guide portion shifting the light in another direction along the surface and reflecting the light in the one direction;
The photon number detector according to (11) above,
(13)
The photon number detector according to any one of (5) to (12), wherein the plurality of branching sections have the same reflectance.
(14)
The photon number detector according to any one of (5) to (12), wherein at least some of the plurality of branching sections have different reflectivities.
(15)
The photon number detector according to (14), wherein each of the plurality of splitting sections has a reflectance equal to or lower than a reflectance of a splitting section that reflects input light.
(16)
The photon number detector according to (14) or (15), wherein each of the plurality of branch sections has a reflectance that attenuates with increasing distance from the introduction section.
(17)
a photon splitter that splits light before it is introduced into the waveguide;
Equipped with
a plurality of the waveguide sections are provided corresponding to paths branched by the photon branching section;
The photon number detector according to any one of (4) to (16), wherein a plurality of the detection sections are provided corresponding to a plurality of the waveguide sections.

1, 1A to 1F Photon number detector (detection system)
2, 2A Introduction section 3, 30 Waveguide section 4 Branch section 5, 50 Detection section 6 Beam splitter 7 Wavelength-dependent beam splitter 8 Optical switch 9, 90 Reflection section 10 Photodetection element 20, 21 Detection unit 41, 42 Light guide section

Claims (17)

a detection unit having at least one photodetection element capable of single photon detection;
a branching section that stochastically transmits input light and branches it into a first path toward the light detection element and a second path different from the first path;
A photon number detector comprising: the branching portion is a reflective film,
The photon number detector of claim 1 , wherein the photodetection elements are pixels. the branching portion is disposed along a surface of the photodetector element into which light is input,
The photon number detector according to claim 1 , wherein the light detection element detects light transmitted through the branching portion. a waveguide portion that guides light toward the photodetector;
an introduction section that introduces light of a photon number state to be measured into the waveguide section;
Equipped with
The photon number detector according to claim 1 , wherein the branching section stochastically transmits the light input from the waveguide section and branches the light into the first path and the second path that reflects the light to the waveguide section. The detection unit has a plurality of light detection elements,
a plurality of the branching portions are provided corresponding to the plurality of light detecting elements;
The photon number detector according to claim 4 , wherein the waveguide section guides the light introduced from the introduction section toward each of a plurality of branch sections. The photon number detector according to claim 5 , wherein the detection section has the plurality of photodetection elements arranged along one direction corresponding to the guiding of light by the waveguide section. The photon number detector according to claim 6 , wherein the plurality of branching sections are disposed between the waveguide section and the detection section. A first detection unit that is the detection unit arranged on one side in a direction intersecting the one direction;
A second detection unit that is the detection unit disposed on the other side in a direction intersecting the one direction;
7. The photon number detector of claim 6, comprising: The photon number detector according to claim 6 , wherein the detection unit is disposed on one side in a direction intersecting the one direction. a reflecting portion that is disposed on the other side of a direction intersecting the one direction and that reflects light toward the detecting portion;
10. The photon number detector of claim 9 comprising: The photon number detector according to claim 9 , wherein the plurality of photodetection elements are arranged two-dimensionally along a surface. a light guide portion at both ends of the plurality of light detection elements in one direction along the surface, the light guide portion shifting the light in another direction along the surface and reflecting the light in the one direction;
12. The photon number detector of claim 11 comprising: The photon number detector according to claim 5 , wherein the plurality of branching sections have the same reflectance. The photon number detector of claim 5 , wherein at least some of the multiple branches have different reflectivities. The photon number detector according to claim 14 , wherein each of the plurality of splitting sections has a reflectance equal to or lower than a reflectance of a splitting section that reflects input light. The photon number detector according to claim 14 , wherein each of the plurality of branch sections has a reflectance that attenuates with increasing distance from the introduction section. a photon splitter that splits light before it is introduced into the waveguide;
Equipped with
a plurality of the waveguide sections are provided corresponding to paths branched by the photon branching section;
The photon number detector according to claim 4 , wherein a plurality of the detection sections are provided corresponding to a plurality of the waveguide sections.

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