Quantum converter and quantum conversion method

Abstract

A quantum converter according to the present invention includes: a ferromagnetic topological insulator film having a first surface; a magnetic field application unit that applies a magnetic field, which includes a component perpendicular to the first surface, to the ferromagnetic topological insulator film; and a microwave transmitter / receiver unit that transmits and receives microwaves to and from the ferromagnetic topological insulator film, wherein the first surface of the ferromagnetic topological insulator film is irradiated with laser light. Quantum converters can be used, for example, in quantum computers.

Description

Quantum converter and quantum conversion method

[0001] The present disclosure relates to quantum transducers and methods of quantum transformation.

[0002] Quantum computers can perform quantum conversion between microwave photons and optical photons, and quantum converters have been proposed for quantum conversion.

[0003] JP 2019-512161 A JP 2019-512104 A U.S. Patent Application Publication No. 2022 / 0207405 U.S. Patent Application Publication No. 2021 / 0271999

[0004] R. Hisatomi et al., Phys. Rev. B 93, 174427 (2016)Tse & MacDonald, Phys. Rev. Lett. 105 057401 (2010)J. Maciejko et al., Phys. Rev. Lett. 105 166803 (2010)

[0005] Conventional quantum converters have low conversion efficiency in quantum conversion.

[0006] An object of the present disclosure is to provide a quantum converter and a quantum conversion method that can improve conversion efficiency.

[0007] According to one aspect of the present disclosure, there is provided a quantum converter comprising: a ferromagnetic topological insulator film having a first surface; a magnetic field application unit that applies a magnetic field including a component perpendicular to the first surface to the ferromagnetic topological insulator film; and a microwave transceiver unit that transmits and receives microwaves to and from the ferromagnetic topological insulator film, wherein laser light is irradiated onto the first surface of the ferromagnetic topological insulator film.

[0008] According to the present disclosure, conversion efficiency can be improved.

[0009] FIG. 1 is a schematic diagram showing a quantum converter according to a first embodiment. FIG. 2 is a schematic diagram showing a quantum converter according to a reference example. FIG. 3 is a diagram showing the relationship between the thickness of a ferromagnetic topological insulator film and the ratio of conversion efficiencies. FIG. 4 is a diagram showing the relationship between the thickness of a ferromagnetic topological insulator film, the atomic percentage of ferromagnetic material, and the ratio of conversion efficiencies. FIG. 5 is a schematic diagram showing a quantum converter according to a second embodiment. FIG. 6 is a schematic diagram showing a quantum converter according to a third embodiment.

[0010] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant description may be omitted.

[0011] (First Embodiment) A first embodiment will be described. The first embodiment relates to a quantum converter. Fig. 1 is a schematic diagram showing a quantum converter according to the first embodiment.

[0012] As shown in FIG. 1, the quantum converter 100 according to the first embodiment includes a microwave resonator 30 , a laminate 10 , an S pole 51 , an N pole 52 , and an antenna 40 .

[0013] The stack 10 includes a ferromagnetic topological insulator film 11 and a first film 12. The ferromagnetic topological insulator film 11 has a first surface 11A and a second surface 11B opposite to the first surface 11A. The first film 12 is in contact with the first surface 11A. The first film 12 is made of, for example, a non-magnetic insulator. The first film 12 is made of, for example, SrTiO ３ , InP or Al ２ O ３ or a substrate containing any combination thereof. The ferromagnetic topological insulator film 11 contains a topological insulator and a ferromagnetic material doped in the topological insulator. For example, the topological insulator contains Bi, Sb, and Te, and the ferromagnetic material contains Cr or V, or both. The composition of the ferromagnetic topological insulator film 11 is, for example, Cr ｘ (Bi １－ｙ Sb ｙ ) ２－ｘ Te ３ , V ｘ (Bi１－ｙ Sb ｙ ) ２－ｘ Te ３ For example, the value of x is 0.1 or more and 0.6 or less, and the value of y is 0.7 or more and 0.9 or less. The thickness of the ferromagnetic topological insulator film 11 is, for example, 5 nm or more and 100 μm or less. The ferromagnetic topological insulator film 11 has a square planar shape with a side length of approximately 1 mm when viewed from a direction perpendicular to the first surface 11A. The planar shape of the ferromagnetic topological insulator film 11 is not limited to a square shape, and may be a circular shape, etc.

[0014] The south pole 51 and the north pole 52 are provided on the outer wall surface of the microwave resonator 30. The south pole 51 faces the first surface 11A, and the north pole 52 faces the second surface 11B. A magnetic field H directed from the south pole 51 to the north pole 52 is generated between the south pole 51 and the north pole 52. The south pole 51 and the north pole 52 act as magnetic field application units and apply the magnetic field H, which includes a component perpendicular to the first surface 11A, to the ferromagnetic topological insulator film 11.

[0015] The antenna 40 is provided on the outer wall surface of the microwave resonator 30. The antenna 40 serves as a microwave transmitting / receiving unit, transmitting and receiving microwaves to and from the ferromagnetic topological insulator film 11.

[0016] The microwave resonator 30 is provided with an inlet 31 and an outlet 32. The inlet 31 faces the first surface 11A, and the outlet 32 ​​faces the second surface 11B. An optical fiber is connected to the inlet 31, and laser light L1 is irradiated from the outside toward the ferromagnetic topological insulator film 11 through the inlet 31. The laser light L1 is irradiated onto the first surface 11A of the ferromagnetic topological insulator film 11. An optical fiber is connected to the outlet 32, and laser light L2 that has transmitted through the ferromagnetic topological insulator film 11 is extracted to the outside through the outlet 32. The laser light L1 is linearly polarized laser light.

[0017] The polarization and the like of the laser light L2 is detected when it is extracted to the outside through the extraction port 32. In this way, the microwave photons of the microwaves irradiated onto the ferromagnetic topological insulator film 11 through the antenna 40 are quantum converted into optical photons.

[0018] Here, the characteristics of the ferromagnetic topological insulator film 11 will be described.

[0019] In general, the photon conversion efficiency η of a ferromagnetic material using the Faraday effect is approximately Ｆ ２ / N Ｓ ２ " where "φ Ｆ " is the Faraday rotation angle, and "N Ｓ " is the total number of spins contained in the ferromagnetic material.

[0020] In general, the Faraday rotation angle φ in a ferromagnetic topological insulator film Ｆ is "tan －１ α (rad)" where α is the fine structure constant and "tan －１ α (rad)” value, i.e., the Faraday rotation angle φ Ｆ is approximately 1 / 137 rad.

[0021] The total number of spins contained in the ferromagnetic topological insulator film 11, N Ｓ is the spin density per unit volume of the ferromagnetic topological insulator film 11, Ｓ (mm －３ ), and the volume of the ferromagnetic topological insulator film 11 is V (mm ３ ) then "n Ｓ The spin density per unit volume of the ferromagnetic material contained in the ferromagnetic topological insulator film 11 is n Ｓ０ (mm －３ ), the atomic ratio of the ferromagnetic material in the ferromagnetic topological insulator film 11 is γ Ｓ Then, the spin density per unit volume of the ferromagnetic topological insulator film 11 is n Ｓ is "γ Ｓ ×n Ｓ０ (mm －３ )"

[0022] As an example, the composition is Cr ０．１５ (Bi ０．１ Sb ０．９ ) １．８５ Te ３ When (x=0.15, y=0.9), the spin density n Ｓcan be calculated as follows: (Bi ０．１ Sb ０．９ ) ２ Te ３ In the case of Cr, 40 atomic % is Bi or Sb. ０．１５ (Bi ０．１ Sb ０．９ ) １．８５ Te ３ So, (Bi ０．１ Sb ０．９ ) ２ Te ３ 15 atomic % of Bi or Sb contained in is substituted with Cr. ０．１５ (Bi ０．１ Sb ０．９ ) １．８５ Te ３ The atomic percentage of Cr in the Cr layer is 6 atomic %. Therefore, the spin density per unit volume of Cr is n Ｓ０，Ｃｒ Then, the composition is Cr ０．１５ (Bi ０．１ Sb ０．９ ) １．８５ Te ３ The spin density n of the ferromagnetic topological insulator film 11 when Ｓ is "0.06 x n Ｓ０，Ｃｒ (mm －３ From the above, it can be seen that the composition is Cr ０．１５ (Bi ０．１ Sb ０．９ ) １．８５ Te ３ The conversion efficiency η of the ferromagnetic topological insulator film 11 when ＴＩ is "(1 / 137) ２ / (0.06 x n Ｓ０，Ｃｒ ×V) ２ " The spin density n Ｓ０，Ｃｒ is 2.1 x 10 １９ mm －３ Therefore, the thickness d of the ferromagnetic topological insulator film 11 is 30 nm, and the area S of the first surface 11A is 1 mm ２ When , the conversion efficiency η of the ferromagnetic topological insulator film 11 ＴＩ is 1.14 x 10 －２ That's about it.

[0023] Next, a reference example will be described for comparison with the first embodiment. The reference example differs from the first embodiment mainly in the configuration of the ferromagnetic material and the arrangement of the magnetic poles. Fig. 2 is a schematic diagram showing a quantum converter according to the reference example.

[0024] As shown in Figure 2, the quantum converter 100X according to the reference example has a ferromagnetic sphere 10X instead of the stack 10. The sphere 10X is made of yttrium iron garnet (YIG) and has a diameter of 0.75 mm. The south pole 51 and the north pole 52 are arranged so that a magnetic field H is formed in a direction perpendicular to the propagation direction of the laser light L1. The other configurations are the same as those of the first embodiment.

[0025] Faraday rotation angle φ of sphere 10X Ｆ is expressed as "γ × d (rad)" where γ is the Verdet constant (rad / mm) and d is the diameter (mm). The Verdet constant of YIG is 0.38 (rad / mm).

[0026] Total number of spins contained in sphere 10X: N Ｓ is the spin density per unit volume of the sphere 10X, Ｓ (mm －３ ), the volume of the sphere 10X is V (mm ３ ) then "n Ｓ ×V". The entire sphere 10X is made of YIG, which is a ferromagnetic material, and has a spin density of n Ｓ is the spin density per unit volume of YIG, n Ｓ０ (mm －３ From the above, the spin density per unit volume of YIG is equal to n Ｓ０ to n Ｓ０，ＹＩＧ Then, the conversion efficiency η of the sphere 10X made of YIG is (0.38) ２ / (n Ｓ０，ＹＩＧ ×V) ２ " The spin density n Ｓ０，ＹＩＧ is 2.1 x 10 １９ mm －３ Therefore, the volume V is 0.2209 mm ３ (diameter is 0.75 mm), the conversion efficiency η of the sphere 10X ＹＩＧ is 10 －１０ That's about it.

[0027] Therefore, the conversion efficiency η of the ferromagnetic topological insulator film 11 in the first embodiment is 1 / 2 times the conversion efficiency η of the sphere 10X in the reference example. ＹＩＧ 10 of ８ About twice as big.

[0028] Next, the thickness d of the ferromagnetic topological insulator film 11 and the atomic ratio γ of the ferromagnetic material Ｓ and the ratio of conversion efficiencies (η ＴＩ / η ＹＩＧ 3 shows the relationship between the thickness d of the ferromagnetic topological insulator film 11 and the conversion efficiency ratio (η ＴＩ / η ＹＩＧ 3 shows the relationship between the atomic ratio γ of the ferromagnetic material and the atomic ratio γ of the ferromagnetic material. Ｓ 4 shows the relationship between the thickness d of the ferromagnetic topological insulator film 11 and the atomic fraction γ of the ferromagnetic material. Ｓ and the ratio of conversion efficiencies (η ＴＩ / η ＹＩＧ 3 and 4, the atomic ratio γ of the ferromagnetic material is Ｓ If is constant, the smaller the thickness d of the ferromagnetic topological insulator film 11, the greater the conversion efficiency ratio (η ＴＩ / η ＹＩＧ ) becomes larger. If the thickness d of the ferromagnetic topological insulator film 11 is constant, the atomic ratio γ Ｓ The smaller the conversion efficiency ratio (η ＴＩ / η ＹＩＧ ) becomes larger. For example, the atomic ratio γ Ｓ is 6% or more and 25% or less, and the thickness d of the ferromagnetic topological insulator film 11 is 5 nm or more and 30 nm or less, the ratio of conversion efficiencies (η ＴＩ / η ＹＩＧ ) is 1 x 10 ８ 1x10 or more ９ It is about the following.

[0029] If the thickness d of the ferromagnetic topological insulator film 11 is less than 5 nm, the ferromagnetic topological insulator film 11 behaves as a two-dimensional material, which may make it difficult to obtain the Faraday effect. On the other hand, if the thickness d is 5 nm or more, the ferromagnetic topological insulator film 11 can be easily and stably formed.

[0030] Second Embodiment A second embodiment will be described. The second embodiment differs from the first embodiment mainly in the configuration of the stack. Fig. 5 is a schematic diagram showing a quantum converter according to the second embodiment.

[0031] As shown in FIG. 5, a quantum converter 200 according to the second embodiment has a stack 20 instead of the stack 10 .

[0032] The stack 20 includes a plurality of ferromagnetic topological insulator films 11 and a plurality of first films 12. A plurality of ferromagnetic topological insulator films 11 and a plurality of first films 12 are stacked alternately. Between two ferromagnetic topological insulator films 11 adjacent to each other in the stacking direction, the first surface 11A of one film faces the second surface 11B of the other film.

[0033] Other configurations of the second embodiment are similar to those of the first embodiment.

[0034] The second embodiment also makes it possible to obtain high conversion efficiency, similar to the first embodiment. In the first embodiment, the spin density per unit volume of the ferromagnetic topological insulator film 11, n Ｓ When the total number of spins N contained in the ferromagnetic topological insulator film 11 is low, Ｓ In contrast, in the second embodiment, since a plurality of ferromagnetic topological insulator films 11 are stacked, the spin density n Ｓ Even if the conversion efficiency η in the second embodiment is low, the total number of spins contained in the stack 10 can be sufficiently obtained, and a ferromagnetic resonance state can be realized. ＴＩ is expressed as "(n×φ)" where n is the number of ferromagnetic topological insulator films 11. Ｆ ) ２ / (n × N Ｓ ) ２ ", so "φ Ｆ ２ / N Ｓ ２ "

[0035] The thickness of the first film 12 sandwiched between the two ferromagnetic topological insulator films 11 is, for example, equal to or less than the thickness d of the ferromagnetic topological insulator film 11 .

[0036] Third Embodiment A third embodiment will be described. The third embodiment differs from the second embodiment mainly in the polarization of the laser light irradiated onto the ferromagnetic topological insulator film 11. Fig. 6 is a schematic diagram showing a quantum converter according to the third embodiment.

[0037] 6 , in the quantum converter 300 according to the third embodiment, laser light L3 is irradiated from the outside toward the ferromagnetic topological insulator film 11 through an inlet 31. The laser light L3 includes two types of laser light whose deflection angles are orthogonal to each other. The laser light L3 is irradiated onto the first surface 11A of the ferromagnetic topological insulator film 11. The outlet 32 ​​does not necessarily have to be provided.

[0038] In the third embodiment, microwaves corresponding to the polarization of the laser light L3 are emitted from the stack 20 and output to the outside through the antenna 40. In this way, the optical photons of the laser light L3 irradiated onto the ferromagnetic topological insulator film 11 are quantum converted into microwave photons of the microwaves.

[0039] Similarly to the second embodiment, the third embodiment can also provide high conversion efficiency.

[0040] In the third embodiment, the laminate 10 may be used in place of the laminate 20 .

[0041] The quantum converter according to the present disclosure can be used, for example, for communication between superconducting qubits housed in a plurality of refrigerators. However, the use of the quantum converter according to the present disclosure is not limited to communication between superconducting qubits. The quantum converter can also be used for quantum computing.

[0042] Although the preferred embodiments have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the claims.

[0043] 10: Laminate 11: Ferromagnetic topological insulator film 11A: First surface 11B: Second surface 12: First film 20: Laminate 30: Microwave resonator 31: Inlet 32: Outlet 40: Antenna 51: South pole 52: North pole 100, 200, 300: Quantum converter

Claims

1. a ferromagnetic topological insulator film having a first surface; a magnetic field applying unit that applies a magnetic field including a component perpendicular to the first plane to the ferromagnetic topological insulator film; a microwave transceiver that transmits and receives microwaves to and from the ferromagnetic topological insulator film; and A quantum converter, wherein the first surface of the ferromagnetic topological insulator film is irradiated with laser light.

2. a microwave resonator; The quantum converter of claim 1 , wherein the ferromagnetic topological insulator film is disposed within the microwave resonator.

3. 3. A quantum converter according to claim 2, wherein said microwave resonator is provided with an inlet through which said laser light is introduced from the outside.

4. 4. The quantum converter according to claim 3, wherein the microwave resonator has an outlet through which the laser light transmitted through the ferromagnetic topological insulator film is guided to the outside.

5. The ferromagnetic topological insulator film is Topological insulators and a ferromagnet doped in the topological insulator; and Including, 5. The quantum converter according to claim 1, wherein the spin density of the ferromagnetic topological insulator film is 6% to 25% of the spin density of the ferromagnetic material.

6. 5. The quantum converter according to claim 1, wherein the thickness of the ferromagnetic topological insulator film is 5 nm to 100 μm.

7. 5. The quantum converter according to claim 1, wherein the thickness of the ferromagnetic topological insulator film is 5 nm or more and 30 nm or less.

8. 5. The quantum converter according to claim 1, further comprising a first film in contact with the ferromagnetic topological insulator film.

9. 9. The quantum converter according to claim 8, wherein a plurality of the ferromagnetic topological insulator films and the first films are alternately stacked.

10. a ferromagnetic topological insulator film having a first surface; a magnetic field applying unit that applies a magnetic field including a component perpendicular to the first plane to the ferromagnetic topological insulator film; a microwave transceiver that transmits and receives microwaves to and from the ferromagnetic topological insulator film; A quantum transformation method using a quantum converter having A quantum conversion method comprising the step of irradiating the first surface of the ferromagnetic topological insulator film with laser light.