Nanomagnets for spin-based quantum dot qubits

Abstract

The invention relates to a nanomagnet for spin-based quantum dot qubits. A quantum computing device (20) is provided comprising a plurality of spin-based quantum dot qubits (22), each qubit comprising one or more quantum dots (25, 26). The plurality of spin-based quantum dot qubits also each include a nanomagnet (28) that includes an amorphous ferromagnetic alloy.

Description

Technical Field

[0001] The present disclosure relates generally to the field of quantum computing, and more particularly to quantum computing devices including spin-based quantum dot qubits. Background Art

[0002] In a quantum computing device, computations are performed on information encoded in quantum states, called qubits. In some quantum computing devices, called spin-based quantum computing devices, qubits are physically embodied as the spins of charge carriers. The charge carriers can be electrons or electron holes in semiconductor devices. The electrons or electron holes used as qubits in spin-based quantum computing devices are confined in quantum dots. When performing quantum computations, the quantum state of the qubit is modified by affecting the spin of the electron or electron hole. Summary of the invention

[0003] According to one aspect of the present disclosure, a quantum computing device is provided, which includes a plurality of spin-based quantum dot qubits, each of which includes one or more quantum dots. The plurality of spin-based quantum dot qubits also each include a nanomagnet, the nanomagnet including an amorphous ferromagnetic alloy,

[0004] According to another aspect of the present disclosure, a nanomagnet is provided. The nanomagnet is configured to apply an external magnetic field to one or more quantum dots included in a spin-based quantum computing device. The nanomagnet includes an amorphous ferromagnetic alloy.

[0005] According to another aspect of the present disclosure, a spin-based quantum dot qubit is provided. The spin-based quantum dot qubit includes a first silicon layer where a first quantum dot and a second quantum dot are located. The spin-based quantum dot qubit also includes a silicon-germanium SiGe alloy layer located above the first silicon layer in the thickness direction. The spin-based quantum dot qubit also includes a second silicon layer located above the SiGe alloy layer in the thickness direction. The spin-based quantum dot qubit also includes a plurality of embolism gates and a plurality of barrier gates located above the second silicon layer in the thickness direction. The spin-based quantum dot qubit also includes a nanomagnet located above the plurality of embolism gates and the plurality of barrier gates in the thickness direction. The nanomagnet includes an amorphous ferromagnetic alloy. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 An energy level diagram showing the energy levels of quantum dots as a function of magnetic field strength according to one example.

[0007] Figure 2A An example top view of a quantum computing device including a qubit grid and a measurement device is shown according to one example.

[0008] Figure 2BShown in more detail Figure 2A A grid of quantum bits.

[0009] Figure 3 Show according to Figure 2A A vertical cross-sectional view of an example of a spin-based quantum dot qubit.

[0010] Figure 4A Show according to Figure 2A An example cross-sectional view of a portion of a conventional nanomagnet formed from a crystalline ferromagnetic material.

[0011] Figure 4B Show according to Figure 2A An example cross-sectional view of a portion of an example nanomagnet comprising an amorphous ferromagnetic alloy. DETAILED DESCRIPTION

[0012] Spin-based quantum computing devices exploit the Zeeman effect when measuring the spin of charge carriers. The Zeeman effect is the influence on the energy levels of charged particles that occurs when they are subjected to a magnetic field. Figure 1 Energy level diagram 10 shows the energy levels of quantum dots as a function of magnetic field strength. Figure 1 In the example of , electrons are confined within a quantum dot. When a magnetic field is applied, the energy E of the electrons confined within the quantum dot is split into two different energy levels. These energy levels correspond to electrons with 1 / 2 spin and -1 / 2 spin, respectively. The spin of the electrons confined within the quantum dot is controlled to perform quantum computations at the quantum computing device.

[0013] Figure 2A An example top view of a portion of a quantum computing device 20 according to one example is shown. Figure 2A As shown, the quantum computing device 20 includes a quantum bit grid 21 in which a plurality of quantum bits 22 are embodied. The quantum bit grid 21 is connected to a measuring device 23, which includes a plurality of measuring circuits 24 connected to the corresponding quantum bits 22. Therefore, the measuring circuit 24 is configured to obtain a classical bit as an output of a quantum computation performed at the quantum bit grid 21 by measuring the quantum state of the quantum bit 22. By performing the measurement at the quantum bit grid 21, the quantum state of the quantum bit 22 is also modified during the execution of the quantum algorithm. In some examples, the measuring circuit 24 is configured to measure the tunneling between the quantum dot and the corresponding charge reservoir. In other examples, the measuring circuit 24 is configured to obtain a charge state corresponding to the spin state of the electron confined at the quantum dot.

[0014] Despite Figure 2AIn the example of FIG. 1 , multiple qubits 22 are arranged in a rectangular grid, but in other quantum computing device configurations, multiple qubits 22 may be arranged in some other pattern. Additionally or alternatively, other configurations of multiple measurement circuits 24 may be used in other example quantum computing devices.

[0015] Figure 2B In more detail, according to an example Figure 2A The quantum bit grid of 21. Figure 2B In the quantum bit grid 21 depicted in FIG. 1 , a plurality of quantum bits 22 are spin-based quantum dot quantum bits. Each of the spin-based quantum dot quantum bits 22 includes one or more quantum dots. Figure 1 In the example of FIG. 1 , each of the plurality of spin-based quantum dot qubits 22 is a dual quantum dot qubit including a first quantum dot 25 and a second quantum dot 26. In other examples, the spin-based quantum dot qubit 22 may be a single quantum dot qubit.

[0016] exist Figure 2B In the example quantum bit grid 21 of FIG. 1 , a plurality of electrical leads 27 are electrically connected to the first quantum dot 25 and the second quantum dot 26. The electrical leads 27 are configured to apply a voltage to the first quantum dot 25 and the second quantum dot 26. In addition, as discussed in more detail below, a plurality of gate electrodes 29 are disposed at the electrical leads 27. The gate electrodes 29 are configured to be opened and closed to adjust the energy levels of the first quantum dot 25 and the second quantum dot 26.

[0017] exist Figure 2B In the example qubit grid 21 shown, the spin-based quantum dot qubits 22 also include corresponding nanomagnets 28. Figure 2B In the example of FIG. 2 , the nanomagnet 28 is disposed above the surface of the quantum bit grid 21 . Figure 2B In the example of FIG. 1 , each quantum bit 22 has a corresponding nanomagnet 28 located above it. In other examples, multiple quantum bits 22 have a shared nanomagnet. Via the nanomagnet 28, the first quantum dot 25 and the second quantum dot 26 are subjected to a magnetic field, and the magnetic field generates the above reference Figure 1 The energy level splitting in question.

[0018] In some examples, nanomagnet 28 is formed via sputtering deposition during fabrication of quantum computing device 20. Nanomagnet 28 may be subjected to an external magnetic field when sputtering deposition is performed to align magnetic moments of atoms included in nanomagnet 28, as discussed in more detail below.

[0019] Figure 3 A vertical cross-sectional view of a spin-based quantum dot qubit 22 included in a quantum computing device 20 is shown according to one example. Figure 3In the example of FIG. 2 , the spin-based quantum dot qubit 22 includes a stack of multiple layers. Figure 3 The thickness direction 30 of the stack is shown.

[0020] The plurality of layers includes one or more silicon layers. Figure 3 In an example of the invention, the spin-based quantum dot qubit 22 includes a substrate layer 31. For example, the substrate layer can be formed of silicon and thus includes silicon.

[0021] The spin-based quantum dot qubit 22 further includes a buffer layer 32 located above the substrate layer 31 in the thickness direction 30. The buffer layer may be a silicon germanium (SiGe) alloy layer, which may include, for example, Si 0.7 Ge 0.3 In other examples, buffer layer 32 may have some other composition. Buffer layer 32 provides electron confinement between layers of spin-based quantum dot qubit 22.

[0022] The spin-based quantum dot qubit 22 further includes a quantum well layer 33 located above the buffer layer 32 in the thickness direction 30. The first quantum dot 25 and the second quantum dot 26 are located in the quantum well layer 33. For example, the quantum well layer 33 may be a silicon layer.

[0023] In the example where the buffer layer 32 is a SiGe alloy layer, the ratio of silicon to germanium is selected so that the electron confinement in the quantum well layer 33 is balanced with the material strain. The electron confinement increases with increasing germanium content. The material strain decreases with increasing silicon content, thereby reducing the dislocation density of the buffer layer 32. The example composition Si discussed above 0.7 Ge 0.3 Provides high levels of electron confinement and low dislocation density.

[0024] Figure 3 The illustrated spin-based quantum dot qubit 22 also includes a top barrier layer 34 located above the quantum well layer in the thickness direction 30. Similar to the buffer layer 32, the top barrier layer 34 provides electron confinement between the layers of the spin-based quantum dot qubit 22. The top barrier layer 34 may be a SiGe layer and may have a composition of SiGe, for example. 0.7 Ge 0.3 .

[0025] The spin-based quantum dot qubit 22 also includes a gate dielectric layer 35 located above the top barrier layer 34 in the thickness direction 30. The gate dielectric layer 35 is an insulator that insulates the electrical lead 27 and the gate electrode 29 from the top barrier layer 34. For example, the gate dielectric layer 35 can be a silicon dioxide (SiO2) layer.

[0026] The spin-based quantum dot qubit 22 further includes a plurality of gate electrodes located above the gate dielectric layer 35 in the thickness direction 30. The plurality of gate electrodes 29 includes a plurality of plunger gates 29A and a plurality of barrier gates 29B. Additionally, the source lead 27A and the drain lead 27B are located above the second silicon layer 33 in the thickness direction 30. The source lead 27A and the drain lead 27B are electrical leads through which electrons are transferred between the quantum dot and the electron source region. The source lead 27A and the drain lead 27B are respectively configured to supply electrons to the quantum dot and receive electrons from the quantum dot. In some examples, the electron source region is the injection region of the qubit grid 21.

[0027] As discussed above with reference to Figure 2B the spin-based quantum dot qubit 22 further includes a nanomagnet 28 located above the plurality of plunger gates 29A and the plurality of barrier gates 29B in the thickness direction 30. Thus, the nanomagnet 28 is located in a layer of the quantum computing device 20 disposed above the quantum dots 25 and 26 in the thickness direction 30 and is on the opposite side of the substrate layer 31 relative to the quantum dots 25 and 26.

[0028] The nanomagnet 28 includes an amorphous ferromagnetic alloy. As Figure 3 shown in the example of, the amorphous ferromagnetic alloy is a cobalt iron boron (CoFeB) alloy. In this example, the CoFeB alloy has a composition of Co 1-x Fe x B y where 0 < x < 1 and 0.2 < y < 1.

[0029] Using an alloy rather than an elemental material as the material of the nanomagnet 28 allows for a stronger magnetic field to be achieved. Figure 4A An example cross-sectional view showing a portion of a conventional nanomagnet 40 formed of a crystalline ferromagnetic material is shown. For example, the conventional nanomagnet 40 can be a crystalline cobalt nanomagnet. The conventional nanomagnet 40 is shown during manufacturing while being subjected to an external magnetic field B. The external magnetic field B is applied to align the magnetic moments 42 included in the alloy within the conventional nanomagnet 40.

[0030] Figure 4A A plurality of grains 41 are shown in. The grains 41 of the conventional nanomagnet 40 exhibit magnetocrystalline anisotropy. Since the crystal structure of cobalt is hexagonal, the magnetocrystalline anisotropy exhibited by the grains 41 of the conventional nanomagnet 40 is uniaxial. Within each grain 41, the crystalline cobalt has a corresponding easy axis of magnetization 43. When the external magnetic field B is applied, the magnetic moments 42 of the atoms included in each grain 41 are at least partially aligned with the external magnetic field B.

[0031] The easy axis of magnetization 43 of the grains 41 of the crystalline cobalt nanomagnet typically has H c<100Oe coercive field strength. Each grain 41 also has a hard magnetization axis 44 perpendicular to the easy magnetization axis 43 and generally has H c >4000 Oe coercive field strength. In addition, due to the variability of the angle of the magnetic moment 42, the conventional nanomagnet 40 generates a non-uniform magnetic field.

[0032] Figure 4B Show Figure 2B An example cross-sectional view of a portion of a nanomagnet 28. Figure 4A , showing a nanomagnet 28 when an external magnetic field B is applied during manufacture. Figure 4B As shown in the example of FIG. 2 , since the nanomagnet 28 is formed of an amorphous ferromagnetic alloy, the magnetic moments 45 of the atoms included in the nanomagnet 28 align themselves with the direction of the external magnetic field B without rotating in the direction of the easy axis of magnetization. Therefore, the degree to which the magnetic moments 45 of the atoms included in the nanomagnet 28 are aligned with the external magnetic field B is greater than Figure 4A of conventional nanomagnets 40. Figure 4B Nanomagnet 28 thus exerts a stronger magnetic field on quantum dots 25 and 26 than would be exerted by conventional nanomagnet 40. During qubit operations performed at quantum computing device 20, the magnetic field applied to the quantum dots typically has an intensity between 0.3 T and 1 T. In some experiments performed at quantum computing device 20, the maximum applied magnetic field reached an intensity between 5 T and 10 T.

[0033] Since the formation of the nanomagnet 28 included in the spin-based quantum computing device 20 from an amorphous ferromagnetic alloy such as CoFeB results in an increase in magnetic field strength, the splitting between the energy levels of the quantum dots 25 and 26 due to the Zeeman effect is also increased. In some examples, in addition to the smaller size of the nanomagnet 28, the nanomagnet 28 generates a magnetic field of equivalent strength to the magnetic field that a conventional nanomagnet 40 would generate. This size reduction allows the overall size of the spin-based quantum dot qubit 22 to be reduced. In addition, since the magnetic field generated by the nanomagnet 28 is decoupled from the crystallographic axis of the grains 41, the nanomagnet 28 generates a magnetic field that is equal in strength to that of the conventional nanomagnet 40. Figure 4A The nanomagnet 28 discussed above allows the spins of the quantum dots 25 and 26 to be controlled with greater reliability by providing a stronger and more uniform magnetic field than the conventional nanomagnet 40.

[0034] Furthermore, the present disclosure includes configurations according to the following clauses.

[0035] Clause 1. A quantum computing device comprising: a plurality of spin-based quantum dot qubits, each of which comprises: one or more quantum dots; and a nanomagnet comprising an amorphous ferromagnetic alloy.

[0036] Clause 2. The quantum computing device according to Clause 1, wherein the amorphous ferromagnetic alloy is a cobalt iron boron (CoFeB) alloy.

[0037] Clause 3. The quantum computing device according to Clause 2, wherein the CoFeB alloy has a composition of Co 1-x Fe x B y where 0 < x < 1 and 0.2 < y < 1.

[0038] Clause 4. The quantum computing device according to Clause 1, wherein each of the plurality of spin-based quantum dot qubits includes a stack of multiple layers having a thickness direction; and the nano-magnet is located in a layer among the multiple layers that is disposed above one or more quantum dots in the thickness direction.

[0039] Clause 5. The quantum computing device according to Clause 1, wherein each of the plurality of spin-based quantum dot qubits is a double quantum dot qubit including a first quantum dot and a second quantum dot.

[0040] Clause 6. The quantum computing device according to Clause 1, wherein each spin-based quantum dot qubit further includes a plurality of barrier gates.

[0041] Clause 7. The quantum computing device according to Clause 1, wherein each spin-based quantum dot qubit further includes a plurality of plug gates.

[0042] Clause 8. The quantum computing device according to Clause 1, wherein each spin-based quantum dot qubit further includes one or more silicon layers.

[0043] Clause 9. The quantum computing device according to Clause 1, wherein each spin-based quantum dot qubit further includes one or more silicon germanium (SiGe) alloy layers.

[0044] Clause 10. The quantum computing device according to Clause 1, wherein the one or more quantum dots are formed of silicon.

[0045] Clause 11. The quantum computing device according to Clause 1, wherein the nano-magnet is formed by sputtering.

[0046] Clause 12. A nano-magnet configured to apply an external magnetic field to one or more quantum dots included in a spin-based quantum computing device, wherein the nano-magnet includes an amorphous ferromagnetic alloy.

[0047] Clause 13. The nano-magnet according to Clause 12, wherein the amorphous ferromagnetic alloy is a cobalt iron boron (CoFeB) alloy.

[0048] Clause 14. The nanomagnet according to Clause 13, wherein the CoFeB alloy has Co 1-x Fe x B y composition, where 0 < x < 1 and 0.2 < y < 1.

[0049] Clause 15. The nanomagnet according to Clause 12, wherein the nanomagnet is located in a layer disposed above one or more quantum dots in the thickness direction of a spin-based quantum computing device.

[0050] Clause 16. The nanomagnet according to Clause 15, wherein the nanomagnet is located on the opposite side of the substrate layer with respect to one or more quantum dots.

[0051] Clause 17. The nanomagnet according to Clause 12, wherein the nanomagnet is formed by sputtering.

[0052] Clause 18. A spin-based quantum dot qubit, comprising: a substrate layer; a buffer layer located above the substrate layer in the thickness direction; a quantum well layer where a first quantum dot and a second quantum dot are located, wherein the quantum well layer is located above the buffer layer in the thickness direction; a top barrier layer located above the quantum well layer in the thickness direction; a gate dielectric layer located above the top barrier layer in the thickness direction; a plurality of plug gates and a plurality of barrier gates located above the gate dielectric layer in the thickness direction; and a nanomagnet located above the plurality of plug gates and the plurality of barrier gates in the thickness direction, wherein the nanomagnet comprises an amorphous ferromagnetic alloy.

[0053] Clause 19. The spin-based quantum dot qubit according to Clause 18, wherein the amorphous ferromagnetic alloy is a cobalt iron boron CoFeB alloy.

[0054] Clause 20. The spin-based quantum dot qubit according to Clause 19, wherein the CoFeB alloy has Co 1-x Fe x B y composition, where 0

Claims

1. A quantum computing device (20), comprising: A plurality of spin-based quantum dot qubits (22), each of the spin-based quantum dot qubits comprising: one or more quantum dots (25, 26); and A nanomagnet (28) comprising an amorphous ferromagnetic alloy.

2. The quantum computing device according to claim 1, wherein: The amorphous ferromagnetic alloy is a cobalt-iron-boron CoFeB alloy.

3. The quantum computing device according to claim 2, wherein: The CoFeB alloy has the composition of Co 1-x Fe x B y where 0 < x < 1 and 0.2 < y < 1.

4. The quantum computing device according to claim 1, wherein: Each of the plurality of spin-based quantum dot qubits comprises a stack of a plurality of layers, the stack having a thickness direction (30); and The nanomagnet is located in a layer among the plurality of layers that is disposed above the one or more quantum dots in the thickness direction.

5. The quantum computing device according to claim 1, wherein: Each of the plurality of spin-based quantum dot qubits is a dual quantum dot qubit comprising a first quantum dot (25) and a second quantum dot (26).

6. The quantum computing device according to claim 1, wherein: Each of the spin-based quantum dot qubits also includes a plurality of barrier gates (29B).

7. The quantum computing device according to claim 1, wherein: Each of the spin-based quantum dot qubits also includes a plurality of plug gates (29A).

8. The quantum computing device according to claim 1, wherein: Each of the spin-based quantum dot qubits also includes one or more silicon layers (31, 33).

9. The quantum computing device according to claim 1, wherein: Each of the spin-based quantum dot qubits also includes one or more silicon-germanium (SiGe) alloy layers (32, 34).

10. The quantum computing device according to claim 1, wherein: The one or more quantum dots are formed from silicon.

11. The quantum computing device according to claim 1, wherein: The nanomagnets are formed via sputtering.

12. A nanomagnet (28) configured to apply an external magnetic field B to one or more quantum dots (25, 26) included in a spin-based quantum computing device (20), wherein: The nanomagnet includes an amorphous ferromagnetic alloy.

13. The nanomagnet according to claim 12, wherein: The amorphous ferromagnetic alloy is a cobalt-iron-boron CoFeB alloy.

14. The nanomagnet according to claim 13, wherein: The CoFeB alloy has the composition of Co 1-x Fe x B y , where 0 < x < 1 and 0.2 < y < 1.

15. The nanomagnet according to claim 12, wherein: The nanomagnet is located in a layer of the spin-based quantum computing device disposed above the one or more quantum dots in a thickness direction (30).

16. The nanomagnet according to claim 15, wherein: The nanomagnet is located on the opposite side of the substrate layer (31) relative to the one or more quantum dots.

17. The nanomagnet according to claim 12, wherein: The nanomagnets are formed via sputtering.

18. A spin-based quantum dot qubit (22), the spin-based quantum dot qubit comprising: A substrate layer (31); a buffer layer (32) located above the substrate layer in the thickness direction (30); A quantum well layer (33) where the first quantum dot (25) and the second quantum dot (26) are located, wherein the quantum well layer is located above the buffer layer in the thickness direction; A top barrier layer (34) located above the quantum well layer in the thickness direction; A gate dielectric layer (35) located above the top barrier layer in the thickness direction; A plurality of plug gates (29A) and a plurality of barrier gates (29B) located above the gate dielectric layer in the thickness direction; and A nanomagnet (28) is located above the plurality of plug gates and the plurality of barrier gates in the thickness direction, wherein the nanomagnet comprises an amorphous ferromagnetic alloy.

19. The spin-based quantum dot qubit of claim 18, wherein: The amorphous ferromagnetic alloy is a cobalt-iron-boron CoFeB alloy.

20. The spin-based quantum dot qubit of claim 19, wherein: The CoFeB alloy has a composition of Co 1-x Fe x B y where 0 < x < 1 and 0.2 < y < 1.

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