JP2010258208A - Superconductive single photon detecting element, method of manufacturing the superconductive single photon detecting element, and method of mounting components of superconductive single photon detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a superconductive single photon detecting element excellent in both photocoupling property with photons and optical absorbing property. <P>SOLUTION: The superconductive single photon detecting element 100 includes a magnesium oxide substrate 10, a niobium nitride wiring 13 formed on a front surface of the substrate 10, a cavity layer 12 formed on the niobium nitride wiring 13, a reflection layer 11 formed on the cavity layer 12, and an antireflection layer 14 formed on a rear surface of the substrate 10. The niobium nitride wiring 13 is connected with a bias source via a transmission line 15 to flow a predetermined bias current through it to be used in a superconductive state. Photons P are detected one by one based on a change in resistance of the niobium nitride wiring 13 when the photons P are incident on the niobium nitride wiring 13 from the rear surface side of the substrate 10. The thickness L1 of the substrate 10 is set in a thickness range L1<SB>opt</SB>of the substrate 10 with which a photocoupling efficiency P<SB>c</SB>is saturated. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a superconducting single photon detector, a method of manufacturing a superconducting single photon detector, and a superconducting single photon detector (hereinafter referred to as “SSPD”). Is sometimes abbreviated as “)”. In particular, the present invention relates to an improvement in a superconducting single photon detection element using a niobium nitride wiring in a superconducting state made of niobium nitride (NbN) (hereinafter sometimes abbreviated as “nanowire”).

  Superconducting single-photon detection elements that can detect photons one by one are expected to be used in the field of quantum communication such as quantum cryptography that makes wiretapping impossible.

  As an example of such a superconducting single photon detection element, a single layer structure detection element (hereinafter referred to as “single layer structure element”) in which a single niobium nitride layer is formed on a magnesium oxide substrate (MgO substrate). ) Has already been developed.

  In this single layer structure output element, meander-like (meandering) nanowires made of a niobium nitride layer are arranged on the surface of the MgO substrate, so that light can be incident on the surface of the MgO substrate. Therefore, the single-layer structure element can easily collect the light emitted from the optical fiber onto the nanowire, and has excellent optical coupling properties with photons.

  However, the single layer structure element has a drawback that the light absorption efficiency is limited by the light reflection and light transmission characteristics of the nanowire. That is, light is reflected and transmitted in the nanowire of a thin film (thickness of about 6 nm), and the light absorption by the nanowire is not good (light absorption efficiency is predicted to be about 30% at the maximum). This causes a decrease in the photodetection efficiency of SSPDs using the.

  In this specification, “light detection efficiency” refers to the detection efficiency of the SSPD system, and refers to the probability that the photon can be detected when a single photon is inserted into the photon detection system. Specifically, the light detection efficiency R is represented by the following formula (1).

          R = Pc × Pa × Pd (1)

  In formula (1), “Pc” is the optical coupling efficiency corresponding to the rate at which photons can be coupled to the nanowires. “Pa” is the light absorption efficiency corresponding to the photon absorption ratio in the nanowire. Pd is an element detection efficiency corresponding to the rate at which a signal is generated when a photon enters the light receiving element.

  By the way, in order to improve the light detection efficiency R, a superconducting single photon detection element (hereinafter referred to as “conventional multilayer structure element”) having a multilayer structure (optical cavity structure) is proposed instead of a single layer structure element. (For example, refer nonpatent literature 1).

  In this conventional multilayer structure element, a niobium nitride layer (microfabrication into meander-like nanowires in a later process) and a dielectric cavity layer (for example, a cavity layer made of hydrogen silsesquioxane (HQS) are formed on the surface side of the sapphire substrate. ) And a reflective layer (for example, an Au (gold) layer).

  With the above configuration, since photons can be confined in the cavity layer, the absorption efficiency Pa is improved as compared with the single-layer structure element.

23 January 2006 / Vol.14, No 2 / OPTICS EXPRESS 527-534

  However, in the conventional laminated structure element, it is necessary to irradiate light from the back side opposite to the surface of the sapphire substrate on which the nanowires constituting the light receiving element are formed. Therefore, it is difficult to collect incident light on a nanowire element having a small light receiving area, and there is a difficulty in optical coupling with photons. For this reason, in the conventional multilayer structure element, the optical coupling efficiency Pc is deteriorated as compared with the single layer structure element, and on the contrary, the light detection efficiency R may not be improved.

  As described above, both the single layer structure element and the conventional multilayer structure element have advantages and disadvantages in improving the light detection efficiency R.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a superconducting single-photon detection element that is excellent in both optical coupling with a photon and light absorption.

  Another object of the present invention is to provide a method for manufacturing such a superconducting single photon detection element.

  Another object of the present invention is to provide a method for mounting an SSPD component on which such a superconducting single photon detection element is mounted.

In order to solve the above problems, the present invention provides a substrate made of magnesium oxide, a niobium nitride wiring formed on the surface of the substrate, a cavity layer formed on the niobium nitride wiring, and formed on the cavity layer And a reflection layer formed on the back surface of the substrate,
The niobium nitride wiring is used in a superconducting state, connected to a bias source via a transmission line so that a predetermined bias current flows.
Based on the resistance change of the niobium nitride wiring when the photon is incident on the niobium nitride wiring from the back side of the substrate, the photons are detected one by one,
Provided is a superconducting single-photon detection element in which the thickness of the substrate is set within the thickness range of the substrate where the optical coupling efficiency is saturated.

  With the above configuration, in the superconducting single photon detection element having the optical cavity structure of the present invention, the thickness of the substrate is appropriately set, which can contribute to the improvement of the optical coupling efficiency. As a result, the light detection efficiency of SSPD is improved as compared with the conventional stacked structure element.

  In the superconducting single photon detection element of the present invention, the thickness of the substrate may be set near the maximum value in the thickness range.

  With the above configuration, the mechanical strength of the substrate of the superconducting single photon detection element can be maintained.

  The present invention also includes a step of epitaxially growing a niobium nitride layer on the surface of a substrate made of magnesium oxide, a step of forming a meandered niobium nitride wiring by patterning the niobium nitride layer, and covering the niobium nitride wiring. After the step of forming a cavity layer on the substrate, the step of forming a reflective layer on the cavity layer, the step of adjusting the thickness of the substrate by scraping the back surface of the substrate, and the step of adjusting the thickness of the substrate And a step of forming an anti-reflection layer on the back surface of the substrate.

  With the above manufacturing method, the substrate can be shaved while monitoring the thickness of the substrate in real time, so that the substrate can be easily controlled to a desired thickness.

  The present invention is also a method of mounting a superconducting single photon detector component in which the superconducting single photon detection element and the optical transmission means described above are mounted on a package block, which is formed on the package block. A step of placing a clearance adjustment member in close contact with the dummy substrate in the through-hole by covering the through-hole with a dummy substrate, and inserting the optical transmission means into the through-hole. A step of bringing the tip of the transmission means into contact with the clearance adjustment member, and removing the dummy substrate so that the antireflection layer of the substrate of the superconducting single photon detection element faces the tip of the light transmission means. And a step of covering the through hole with the superconducting single photon detection element.

  With the above-described method of mounting the superconducting single photon detection element component, the clearance corresponding to the distance between the tip of the optical transmission means and the substrate (antireflection layer) of the superconducting single photon detection element can be adjusted to an appropriate amount. .

  Further, the SSPD can be configured in a compact manner by the packaging method mounting method described above. Therefore, this SSPD is excellent in cost responsiveness in multi-channeling when considering use in the quantum communication field such as quantum cryptography communication.

  Further, in the superconducting single photon detector component mounting method of the present invention, the optical transmission means is made of resin material for the package block in a state where the tip of the optical transmission means is in contact with the clearance adjusting member. And a step of fixing.

  Thereby, the optical transmission means is appropriately fixed, and the clearance is maintained at an appropriate amount.

  In the method of mounting a superconducting single photon detector component according to the present invention, the superconducting single photon detection element is placed on the package block after the superconducting single photon detection element is put on the through-hole. You may further include the process of fixing using a wax.

  With the above electron wax, the superconducting single photon detection element is appropriately fixed to the package block, and the clearance is maintained at an appropriate amount. Further, the thermal conductivity between the superconducting single photon detecting element and the package block is appropriately improved.

  According to the present invention, it is possible to obtain a superconducting single-photon detection element that is excellent in both optical coupling with a photon and light absorption.

  Moreover, according to this invention, the manufacturing method of such a superconducting single photon detection element is also obtained.

  Furthermore, according to the present invention, a method of mounting an SSPD component on which such a superconducting single photon detection element is mounted is also obtained.

It is the figure which showed typically the example of 1 structure of the superconducting single photon detection element of embodiment of this invention. It is a schematic diagram used for description of the detection method of the photon by the superconducting single photon detection element of embodiment of this invention. It is sectional drawing which showed each process of the manufacturing method of the superconducting single photon detection element of embodiment of this invention. It is a figure used for description of the structure of SSPD of embodiment of this invention, and description of the mounting method of the component of SSPD. It is the block diagram which showed an example of the optical interference measurement system typically. It is the figure which showed each photodetection efficiency of SSPD (400 micrometers) and SSPD (45 micrometers). It is a figure for demonstrating that optical coupling efficiency improves when the thickness of a MgO board | substrate is thin. It is the figure which showed the dependence of the thickness (optical path length) of a MgO board | substrate about the optical coupling efficiency (light detection efficiency) of SSPD. It is the figure which showed the grinding | polishing system used for grinding | polishing of the MgO board | substrate with which the light receiving element of the superconducting single photon detection element of this embodiment was mounted. It is the figure which published the photograph by which the optical fiber was mounted in the block for optical fiber holding. It is the figure which published the photograph with which the MgO board | substrate with which the light receiving element was mounted was mounted in the element holding block.

  Hereinafter, a superconducting single photon detection element 100 according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram schematically showing a configuration example of a superconducting single photon detection element according to an embodiment of the present invention. FIG. 1A is a plan view of the periphery of a light receiving element of a superconducting single photon detecting element from above, and FIG. 1B is a diagram of a portion along the line BB in FIG. It is sectional drawing which showed the cross section.

  As shown in FIG. 1, in the superconducting single photon detection element 100 of this embodiment, photons P are incident from the back side of a magnesium oxide (MgO) single crystal substrate 10 (hereinafter abbreviated as “MgO substrate 10”). It has a back-illuminated optical cavity structure.

  First, the planar structure of the light receiving element S of the superconducting single photon detecting element 100 will be described.

  The light receiving element S of the superconducting single photon detecting element 100 includes meandering (meandering) nanowires 13 in a plan view, as shown in FIG. The nanowire 13 is formed, for example, at a predetermined pitch (here, 160 nm pitch) with a line width of several tens to several hundreds of nanometers (here, 100 nm), and an appropriate cooling means (eg, GM refrigerator) is used. Used in the superconducting state. For example, the size of the light receiving element S may be about 15 × 15 μm square.

  The nanowire 13 can be manufactured by patterning a niobium nitride layer 13A having a thickness of several nanometers (here, 6 nm) using an electron beam or the like.

Further, the nanowire 13 is connected to a substantially U-order transmission path 15 (thickness: 150 nm) and a rectangular transmission path 15 (thickness: 150 nm) as shown in FIGS. 1 (a) and 1 (b). Has been. The nanowire 13 is connected to an output terminal of a bias source (not shown) through the transmission path 15 so that a desired bias current slightly lower than the critical current flows. As described above, the transmission path 15 functions as a path for supplying a bias current to the nanowire 13.

  The transmission path 15 may be made of the same material as the niobium nitride layer 13A for the purpose of making it difficult for the superconducting single photon detection element 100 to be destroyed at the contact surface with the niobium nitride layer 13A.

  Next, the laminated structure of the light receiving element S of the superconducting single photon detecting element 100 will be described.

  As shown in FIG. 1B, the superconducting single-photon detection element 100 has a MgO substrate 10, a laminate 101 provided on the surface of the MgO substrate 10, and a back surface of the MgO substrate 10 in a cross-sectional view. And an antireflection layer 14 formed. And the light receiving element S is comprised by the laminated body 101 explained in full detail below.

  The superconducting single photon detection element 100 of the present embodiment is characterized in that the MgO substrate 10 is formed into a thin plate having a thickness L1 of about 45 μm by polishing a thick plate of about 400 μm, for example. The reason for forming the MgO substrate 10 in a thin plate will be described later. The size of the MgO substrate 10 is preferably about 3 mm square, for example.

  As shown in FIG. 1B, the stacked body 101 covers a nanowire 13 (niobium nitride wiring) formed by patterning a solid niobium nitride layer 13 </ b> A deposited on the MgO substrate 10 and covers the nanowire 13. And a reflective layer 11 formed on the cavity layer 12.

  Here, the reflective layer 11 of the stacked body 101 is a thin film reflective mirror made of metal (for example, gold (Au)) having a thickness of about 120 nm. However, other members (for example, a silver (Ag) thin film reflecting mirror) may be used as the reflecting layer 11 as long as the member has an excellent reflectance.

  Here, the cavity layer 12 of the multilayer body 101 is a layer made of silicon monoxide (SiO) and functioning as an optical resonator having a thickness of about 250 nm. However, another dielectric material (for example, silicon dioxide) may be used as the cavity layer 12.

  By the reflective layer 11 and the cavity layer 12 described above, the photons P incident on the light receiving element S from the back surface of the MgO substrate 10 are appropriately confined in the cavity layer 12.

  The antireflection layer 14 is made of an amorphous fluororesin (product name “Cytop”, manufactured by Asahi Glass Co., Ltd.), has a thickness of about 200 to 300 nm, and is used when the photons P enter the light receiving element S. It is a non-reflective coating layer that prevents reflection.

  Next, a method for detecting the photon P in the light receiving element S of the superconducting single photon detecting element 100 will be outlined.

  FIG. 2 is a schematic diagram used for explaining a photon detection method by the superconducting single photon detection element of the present embodiment.

  As shown in FIG. 2, when a photon P (single photon) is incident on the nanowire 13, it exceeds the cap energy at the location where the photon P is incident, and as a result, a normal conduction region A (high resistance region) called a hot spot. Will occur. In this case, as shown in the enlarged view of FIG. 2, the current C flows intensively through the nanowires 13 on both sides of the region A so as to bypass the high resistance region A. Then, the current C flowing around the region A exceeds the critical current, the portions on both sides of the region A are also in the normal conduction state, and the region A in the normal conduction state temporarily extends over the entire width direction of the nanowire 13. spread. In this way, based on the change in resistance across the entire width of the nanowire 13 in the process of generating the normal conduction region A and restoring the normal conduction region A to the superconducting state, And the voltage signal is taken out from the transmission path 15 to the outside.

  Note that a detection method of a superconducting single photon detection element that can detect the photons P one by one by biasing the nanowire 13 in the vicinity of the critical current is known (for example, “IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL.11, NO.1, MARCH 2001 P574-577 ”). Therefore, detailed description of this detection method is omitted.

  Further, in the superconducting single photon detection element 100 of the present embodiment, as described above, the nanowire 13 is formed so that the detectable region of the light receiving element S is expanded as much as possible to facilitate optical coupling with the photon P. Although it is formed in a meander shape, meander-type nanowires are also described in the above-mentioned known documents and the above-mentioned Non-Patent Document 1. Therefore, detailed description of the meandered nanowire is also omitted.

  Next, a manufacturing method of the superconducting single photon detection element 100 of this embodiment will be described.

  FIG. 3 is a cross-sectional view showing each step of the method of manufacturing the superconducting single photon detecting element according to the embodiment of the present invention.

  However, in FIG. 3, some of the constituent elements of the superconducting single photon detection element 100 (for example, the transmission path 15 and the antireflection layer 14) are not shown.

  First, solid nitride is formed on the (100) surface (front surface) of a mother substrate 10A (hereinafter referred to as “MgO substrate 10A”) made of single crystal magnesium oxide (MgO) and having a thickness of about 400 μm. The niobium layer 13A is epitaxially deposited by direct current reactive sputtering using an Nb (niobium) target. In this case, argon gas may be used as the discharge gas, and nitrogen gas may be used as the reaction gas.

  Next, when the niobium nitride layer 13A is patterned using a fine processing technique utilizing electron beam lithography or the like, the nanowire 13 can be formed as shown in FIG.

  Next, as shown in FIG. 3B, a window 103 for depositing the cavity layer 12 is formed in the photoresist 102 by using a photolithography technique.

  Then, as shown in FIG. 3C, the MgO substrate is formed by vacuum deposition so that the cavity layer 12 made of silicon monoxide (SiO) having a thickness of about 250 nm covers the nanowire 13 in the window 103 of the photoresist 102. A film is formed on 10A.

  Further, as shown in FIG. 3C, a reflective layer 11 made of gold (Au) having a thickness of about 120 nm is formed on the cavity layer 12 of the MgO substrate 10A by vacuum deposition in the window 103 of the photoresist 102. Be filmed.

  Next, as shown in FIG. 3D, when the photoresist 102 is removed, the light receiving element S (laminated body 101) is obtained.

  Next, although not shown, the thickness of the MgO substrate 10A is adjusted by polishing the back surface of the MgO substrate 10A. Note that an example of a method for polishing the MgO substrate 10A will be described in Examples described later.

  Thereby, the MgO substrate 10 (see FIG. 1) having a thickness L1 of about 45 μm is obtained.

  Thereafter, an antireflection layer 14 (see FIG. 1) is formed on the back surface of the MgO substrate 10.

  In this way, the superconducting single photon detection element 100 (see FIG. 1) can be manufactured.

  Next, the SSPD 200 on which the above superconducting single photon detection element 100 is mounted will be described with reference to the drawings.

  FIG. 4 is a diagram used for explaining the configuration of the SSPD and the mounting method of the components of the SSPD according to the embodiment of the present invention.

  Note that the configuration of the SSPD 200 is shown in FIG. 4D, but here, illustration of some of the components (for example, the antireflection layer 14) of the superconducting single photon detection element 100 is omitted. ing.

  As shown in FIG. 4D, the main parts (components) of the SSPD 200 include a copper package block 22, an optical fiber 23 (optical transmission means), and a superconducting single photon detecting element 100. .

  The optical fiber 23 includes an optical fiber core wire 23A having a fiber core diameter (diameter) of about 9 μm and a ferrule 23B (optical connector) that covers the optical fiber core wire 23A.

  The package block 22 includes an element holding block 20 used for holding the superconducting single photon detecting element 100 and an optical fiber holding block 21 used for holding the optical fiber 23. The both end faces 20A, 20B of the element holding block 20 and the both end faces 21A, 21B of the optical fiber holding block 21 are surface-finished and smooth.

  Furthermore, a through hole 21C is formed at the center of the optical fiber holding block 21 to such an extent that the ferrule 23B of the optical fiber 23 can be inserted. Further, the central portion of the element holding block 20 has a penetration that is larger than the outer dimension of the film-like clearance adjustment member 25 having a thickness of about 20 μm and smaller than the outer dimension of the dummy flat substrate 26 (dummy substrate). A hole 20C is formed. The outer dimension of the through hole 20C is larger than the outer dimension of the through hole 21C. Further, the outer dimension of the flat substrate 26 is the same as the outer dimension of the MgO substrate 10.

  Next, a mounting method of components of the SSPD 200 for mounting the superconducting single photon detection element 100 and the optical fiber 23 on the package block 22 will be described with reference to the drawings.

  As shown in FIGS. 4A and 4B, the fixing bolts (not shown) are provided so that the end surface 20B of the element holding block 20 and the end surface 21A of the optical fiber holding block 21 are in close contact with each other. Thus, the package block 22 is used. In this case, the element holding block 20 and the optical fiber holding block are arranged so that the central axis of the through hole 21C of the optical fiber holding block 21 and the central axis of the through hole 20C of the element holding block 20 substantially coincide with each other. The blocks 21 are fixed to each other. Therefore, the pair of through holes 20 </ b> C and 21 </ b> C function as a through hole formed in the package block 22.

  The element holding block 20 can also be moved slightly in the in-plane direction of the end face 20A by play of a bolt fastening portion (not shown). For this reason, in the SSPD 200 of FIG. 4D, by emitting light from the optical fiber 23, the position between the light receiving element S and the optical fiber 23 in the in-plane direction can be adjusted by visual observation of the emitted light. .

  In the above-described package block 22, as shown in FIG. 4B, the clearance adjustment member 25 closely attached on the surface of the flat substrate 26 is put in the through-hole 20 </ b> C by covering the through-hole 20 </ b> C with the flat substrate 36. Arrange.

  Next, as shown in FIG. 4B, the tip (fiber end) of the ferrule 23B is brought into contact with the clearance adjusting member 25 by inserting the ferrule 23B of the optical fiber 23 into the through holes 21C and 20C.

  Next, as shown in FIG. 4 (c), the ferrule 23B of the optical fiber 23 is in contact with the clearance adjustment member 25, and the rear portion of the ferrule 23B is used and the epoxy resin 24 is used for the optical fiber holding block 21. And fix. Thereby, the optical fiber 23 is appropriately fixed to the optical fiber holding block 21, and the clearance L2 is maintained at an appropriate amount.

  Finally, as shown in FIG. 4 (d), the flat substrate 26 is removed, and the antireflection layer 14 (not shown in FIG. 4) formed on the MgO substrate 10 of the superconducting single photon detection element 100 is light. The superconducting single photon detection element 100 is placed over the through hole 20C so as to face the tip of the ferrule 23B of the fiber 23.

  By the above-described mounting method of the components of SSPD 200, the clearance L2 corresponding to the distance between the MgO substrate 10 (antireflection layer 14) and the tip of the ferrule 23B (fiber end) is adjusted to an appropriate amount (here, about 20 μm). The obtained SSPD 200 is obtained.

  Such a packaging type SSPD 200 does not require an expensive stage as compared with a nano-stage type mounting technology (detailed description is omitted), and can be configured compactly. Therefore, this SSPD 200 is excellent in cost responsiveness in multi-channeling when considering use in the quantum communication field such as quantum cryptography communication.

  The light detection efficiency R (optical coupling efficiency Pc) of the superconducting single photon detection element 100 can be improved by adjusting the thickness L1 of the MgO substrate 10 and the clearance L2, and this reason will be described later.

  Further, after superconducting single photon detection element 100 is put on through hole 20C, MgO substrate 10 of superconducting single photon detection element 100 is fixed to element holding block 20 using electron wax (details will be described later). Then, since the heat conductivity between both can be improved, it is convenient. Therefore, when the package block 22 made of copper is cooled to an extremely low temperature (about 4K) using a GM refrigerator (not shown), the superconducting single photon detection element 100 can be efficiently put into a superconducting state.

  Next, the dependence of the optical coupling efficiency Pc (light detection efficiency R) of the superconducting single photon detection element 100 of this embodiment on the thickness L1 of the MgO substrate 10 will be described.

  First, in SSPD using the MgO substrate 10A (hereinafter, abbreviated as “SSPD (400 μm)”), the light detection efficiency R is measured, and then the back surface of the MgO substrate 10A is polished to thereby remove the MgO substrate 10. In the used SSPD (hereinafter abbreviated as “SSPD (45 μm)”), the light detection efficiency R was measured again.

  In the above SSPD (400 μm) and SSPD (45 μm), the antireflection layer 14 is not provided. Thereby, both light detection efficiency R can be compared easily.

  Prior to the actual measurement of the light detection efficiency R, it was confirmed that the thickness L1 and the clearance L2 of the SSPD (400 μm) and the SSPD (45 μm) had the desired values by a known optical interference measurement technique. .

  For example, FIG. 5 shows an example of an optical interference measurement system 300.

  In the optical interference measurement system 300 of FIG. 5, incident light having different wavelengths by the wavelength tunable laser 41 enters the SSPD through the optical circulator 42. Then, the incident light is reflected by the end face of the optical fiber 23, the surfaces of the MgO substrates 10 and 10A, and the back surfaces of the MgO substrates 10 and 10A. The reflected light passes through the optical circulator 42 and enters the optical power meter 40, where the intensity of the reflected light is measured.

  In this way, optical interference of the reflected light reflected on the end face of the optical fiber 23, the surfaces of the MgO substrates 10 and 10A, and the back surface of the MgO substrates 10 and 10A occurs, and the intensity of the reflected light varies due to the optical interference. The thickness L1 and clearance L2 in SSPD (400 μm) and SSPD (45 μm) can be confirmed.

  FIG. 6 is a diagram showing the light detection efficiencies of SSPD (400 μm) and SSPD (45 μm).

  A dark count (c / s; count / second) corresponding to the noise of the system is taken on the horizontal axis in FIG. 6, and a light detection efficiency R (%) is taken on the vertical axis in FIG.

As shown in FIG. 6, at a desired dark count (10 ² c / s), the light detection efficiency R of SSPD (400 μm) was 0.5%, whereas the light detection efficiency of SSPD (45 μm). R was 2.1%. Further, as can be easily understood from FIG. 6, the light detection efficiency R of SSPD (45 μm) is higher than the light detection efficiency R of SSPD (400 μm) over all dark counts.

  From the examination experiment described above, it was found that the light detection efficiency R is improved when the thickness L1 of the MgO substrate is thin. This can be understood based on the spread of the Gaussian beam, as shown in FIG. 7, assuming that the light emitted from the fiber end (core diameter: about 9 μm) of the optical fiber 23 can be approximated by the Gaussian beam shape.

  That is, in SSPD (400 μm), as shown in FIG. 7A, since the distance between the fiber end of the optical fiber 23 and the light receiving element S is long (L1 + L2 = 420 μm), a part of the Gaussian beam (near the tail) Cannot enter the light receiving element S. For this reason, it is considered that the light detection efficiency R of SSPD (400 μm) is lowered.

  In contrast, in SSPD (45 μm), the distance between the fiber end of the optical fiber 23 and the light receiving element S is short (L1 + L2 = 65 μm) as shown in FIG. The light can enter the light receiving element S. For this reason, it is considered that the light detection efficiency R of SSPD (45 μm) is increased.

  Thus, adjustment of the thickness L1 of the MgO substrate 10 and adjustment of the clearance L2 are important for improving the light detection efficiency R of the SSPD 200. The thickness L1 of the MgO substrate 10 can be adjusted appropriately by a polishing method (described later) of the MgO substrate 10A. The clearance L2 can be adjusted appropriately by the mounting method shown in FIG. The thickness L1 and the clearance L2 can be appropriately managed by the optical interference measurement system 300 shown in FIG.

  Next, the following examination experiment and theoretical calculation were performed.

  FIG. 8 is a diagram showing the dependency of the thickness (optical path length) of the MgO substrate on the optical coupling efficiency (light detection efficiency) of SSPD.

  In this examination experiment, four SSPDs using MgO substrates having thicknesses L1 of 400 μm, 200 μm, 100 μm, and 45 μm were manufactured by gradually polishing the MgO substrate 10A.

  Note that the clearance L2 is fixed to 20 μm, the antireflection layer 14 is not disposed, and the optical interference measurement system 300 is used to confirm the thickness L1 and the clearance L2. This is the same as the examination experiment described in.

  Then, the optical coupling efficiency Pc (light detection efficiency R) of each SSPD was measured and plotted as MgO (400 μm), MgO (200 μm), MgO (100 μm), and MgO (45 μm) (● in FIG. 8). In FIG. 8, each light coupling efficiency Pc (light detection efficiency R: where actual measurement data is normalized) is shown.

  The horizontal axis in FIG. 8 represents the optical path length (propagation distance) from the fiber end of the optical fiber 23 to the light receiving element S. This is the MgO substrate (refractive index of MgO: 1.8). The following equation (2) corresponds to the thickness L1.

      Optical path length = L2 (20 μm) + L1 / 1.8 (2)

  When the light emitted from the fiber end of the optical fiber 23 is approximated to a Gaussian beam shape, the theoretical calculation of the optical coupling efficiency Pc (light detection efficiency R) from the Gaussian beam approximation is performed as follows. The resulting profile is shown in FIG. 8 (see dotted line in FIG. 8).

When the mode field diameter (MFD) at the fiber end of the optical fiber 23 is ω ₀ , ω ₀ is 10.4 μm in the case of a single-mode optical fiber (SMF) for a communication path having a communication wavelength of 1550 nm most suitable for optical fiber transmission. It will be about.

  At this time, the radius of the Gaussian beam when the Gaussian beam propagates by the optical path length (propagation distance) X is expressed by the following equation (3).

.... (3)

  Further, the power of the Gaussian beam in a circle having a radius r in the vertical plane at the optical path length (propagation distance) X is approximately expressed by the following equation (4).

.... (4)

  When the light receiving area of the light receiving element S is 15 × 15 μm square, the optical coupling efficiency Pc with respect to the optical path length (propagation distance) X is calculated in the equation (4) within the circle of r = 10.7 μm. The result is shown by the dotted line in FIG.

  From the above examination, it can be seen that the change in the plot (marked with ● in FIG. 8) and the theoretical calculation profile (dotted line in FIG. 8) almost coincide. As a result, as shown in FIG. 8, it was possible to obtain knowledge that there is a suitable substrate thickness range “L1opt” of the MgO substrate in which the optical coupling efficiency Pc (light detection efficiency R) is saturated. For example, even when the thickness L1 of the MgO substrate is 45 μm (in the vicinity of the maximum value of the thickness range “L1opt”), the optical coupling efficiency Pc (light detection efficiency R) is predicted to reach 99% or more.

  As described above, in the superconducting single photon detection element 100 having the optical cavity structure of the present embodiment, the preferred substrate thickness range “L1opt” in which the optical coupling efficiency Pc (light detection efficiency R) is saturated with the thickness of the MgO substrate 10. By setting it in, it can contribute to the improvement of the optical coupling efficiency Pc. As a result, the light detection efficiency R of the SSPD 200 is improved as compared with the conventional stacked structure element.

  In particular, considering the mechanical strength of the MgO substrate 10 of the superconducting single-photon detection element 100, the thickness of the MgO substrate 10 is set near the maximum value of this substrate thickness range “L1opt” (here, about 45 μm). It is preferable to do.

  In the present specification, the quantum communication field has been cited as an application of the superconducting single photon detection element 100 of the present embodiment, but this is merely an example. This superconducting single photon detection element 100 can be applied to various energetic particle detection fields such as terahertz (THz) electronics technology, X-ray observation technology, mass spectrometry technology, etc., in addition to use in the field of quantum communication.

  Hereinafter, an example of a polishing method for the MgO substrate 10A and a mounting example of the superconducting single photon detection element 100 of the present embodiment will be described.

(Polishing method of MgO substrate 10A)
FIG. 9 is a diagram showing a polishing system used for polishing an MgO substrate on which the light receiving element of the superconducting single photon detection element of this embodiment is mounted.

  As shown in FIG. 9, the MgO substrate 10 </ b> A on which the attendance element S (not shown in FIG. 9) is mounted is set on the pedestal 61. The MgO substrate 10A on the pedestal 61 is mechanically polished by the polishing sheet 60, and the polishing amount of the MgO substrate 10A is monitored by the micrometer 62 in real time.

  The details of the polishing system here are as follows.

Polishing machine: ALLIED MULTI PREP ^TM (Trademark) system Polishing sheet 60: ALLIED DIAMOND LAPPING FILM (8-inch DISK),
Diamond particle size in polishing sheet 60: 3 μm or 5 μm
Stage rotation speed: 120 rpm
Polishing rate: 3 μm / min (when using 5 μm polishing sheet 60)
: 1 μm / min (when using 3 μm polishing sheet 60)

  According to the above polishing method of the MgO substrate 10A, the MgO substrate 10A can be polished while monitoring the thickness L1 of the MgO substrate 10A in real time, so that the MgO substrate 10A can be easily controlled to a desired thickness.

(Installation example of SSPD200)
In FIG. 10, a photograph in which the optical fiber 23 is mounted on the optical fiber holding block 21 is shown.

  FIG. 11 shows a photograph in which the MgO substrate 10 on which the light receiving element S is mounted is mounted on the element holding block 20.

  The SSPD 200 shown in FIG. 10 is configured in a compact manner using a packaging method using a parsing method. Therefore, this SSPD 200 is excellent in cost responsiveness in multi-channeling when considering use in the quantum communication field such as quantum cryptography communication.

  As shown in FIG. 11, a recess having the same dimension as the outer dimension of the MgO substrate 10 is formed on the surface of the element holding block 20, and the MgO substrate 10 is fitted into the recess. Electron wax 50 is disposed so as to straddle the four corners of the MgO substrate 10 and the surface of the element holding block 20. With this electron wax 50, the MgO substrate 10 is appropriately fixed to the element holding block 20, and the clearance L2 is maintained at an appropriate amount. Further, the thermal conductivity between the MgO substrate 10 and the element holding block 20 is appropriately improved.

  According to the present invention, it is possible to obtain a superconducting single-photon detection element that is excellent in both optical coupling with a photon and light absorption.

  Therefore, the present invention can be used as a detection element in the field of quantum communication such as quantum cryptography communication, for example, by improving optical coupling efficiency at a communication wavelength most suitable for optical fiber transmission.

10, 10A MgO substrate 11 Reflective layer 12 Cavity layer 13 Nanowire 13A Niobium nitride layer 14 Antireflection layer 15 Transmission path 20 Element holding block 21 Optical fiber holding block 22 Package block 23 Optical fiber 23A Optical fiber core wire 23B Ferrule 24 Epoxy Resin 25 Clearance adjustment member 26 Flat substrate 40 Optical power meter 41 Wavelength variable laser 42 Optical circulator 50 Electron wax 60 Polishing sheet
61 Pedestal 62 Micrometer 101 Laminate 102 Photoresist 103 Photoresist Window 100 Superconducting Single Photon Detector 200 SSPD
300 Optical Interference Measurement System L1 MgO Substrate Thickness L2 Clearance L1opt Substrate Thickness Range S Photodetector P Photon R Photodetection Efficiency Pc Optical Coupling Efficiency

Claims (6)

A substrate made of magnesium oxide;
A niobium nitride wiring formed on the surface of the substrate;
A cavity layer formed on the niobium nitride wiring;
A reflective layer formed on the cavity layer;
An antireflection layer formed on the back surface of the substrate;
With
The niobium nitride wiring is used in a superconducting state, connected to a bias source via a transmission line so that a predetermined bias current flows.
Based on the resistance change of the niobium nitride wiring when the photon is incident on the niobium nitride wiring from the back side of the substrate, the photons are detected one by one,
A superconducting single photon detection element in which the thickness of the substrate is set within the thickness range of the substrate where the optical coupling efficiency is saturated.   The superconducting single photon detection element according to claim 1, wherein the thickness of the substrate is set near a maximum value in the thickness range. Epitaxially growing a niobium nitride layer on the surface of a substrate made of magnesium oxide;
Forming a meandering niobium nitride wiring by patterning the niobium nitride layer;
Forming a cavity layer so as to cover the niobium nitride wiring;
Forming a reflective layer on the cavity layer;
Adjusting the thickness of the substrate by scraping the back surface of the substrate;
After the step of adjusting the thickness of the substrate, a step of forming an antireflection layer on the back surface of the substrate;
A method of manufacturing a superconducting single photon detecting element including: A method of mounting a superconducting single photon detector component, wherein the superconducting single photon detection element and the optical transmission means according to claim 1 or 2 are mounted on a package block,
Disposing a clearance adjustment member in close contact with the dummy substrate in the through hole by covering the through hole formed in the package block with a dummy substrate;
Inserting the optical transmission means into the through hole to bring the tip of the optical transmission means into contact with the clearance adjustment member;
Removing the dummy substrate, and covering the through-hole with the superconducting single photon detection element so that the antireflection layer of the substrate of the superconducting single photon detection element faces the tip of the optical transmission means; ,
A method of mounting a superconducting single photon detector component comprising:   5. The superconducting single photon detection according to claim 4, further comprising a step of fixing the optical transmission means to the package block using a resin material in a state where the tip of the optical transmission means is in contact with the clearance adjusting member. How to mount the parts of the container.   5. The superconducting single unit according to claim 4, further comprising a step of fixing the superconducting single photon detecting element to the package block using electron wax after the superconducting single photon detecting element is put on the through hole. Mounting method for one-photon detector components.