Quantum state generation device and method for generating quantum state

Abstract

Present invention includes: a magnetic tunnel junction element having an insulating layer between a magnetization movable magnetic layer and a magnetization fixed magnetic layer; and a voltage application unit for applying a voltage to the magnetic tunnel junction element. The voltage application unit applies an alternating voltage having a frequency twice the resonance frequency of a magnetic body forming the magnetization movable magnetic layer.

Description

Quantum state generation device and quantum state generation method CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Application No. 2024-144265 filed on August 26, 2024, the contents of which are incorporated herein by reference.

[0002] The present invention relates to a quantum state generation device and a quantum state generation method.

[0003] Quantum computers are expected to be used in a wide range of fields, including pharmaceuticals and materials development. In order for quantum computers to achieve superior computing performance, it is essential that they have high quantum error tolerance, and to achieve this, it is said that an integration of around 100 million quantum bits is necessary.

[0004] For example, Patent Document 1 discloses an example of a quantum computer that uses superconducting materials as quantum bits.

[0005] Re-table 2011 / 162172 publication

[0006] However, in the superconducting quantum computer disclosed in Patent Document 1, it is difficult to miniaturize elements to nano-size, and it has been practically difficult to create a highly integrated quantum computer.

[0007] An object of the present invention is to realize a highly integrated quantum computer.

[0008] The quantum state generating device of the present invention comprises a magnetic tunnel junction element having an insulating layer between a magnetization movable magnetic layer and a magnetization fixed magnetic layer, and a voltage application unit that applies a voltage to the magnetic tunnel junction element, wherein the voltage application unit applies an AC voltage having a frequency twice the resonant frequency of the magnetic material that forms the magnetization movable magnetic layer.

[0009] The quantum state generating method according to the present invention includes applying a voltage to a magnetic tunnel junction element having an insulating layer between a magnetization movable magnetic layer and a magnetization fixed magnetic layer, and the voltage applied to the magnetic tunnel junction element is an AC voltage having a frequency twice the resonant frequency of the magnetic material forming the magnetization movable magnetic layer.

[0010] According to the present invention, a highly integrated quantum computer can be realized.

[0011] FIG. 1 is a diagram showing an outline of the configuration of a quantum state generating device 1 according to an embodiment of the invention. FIG. 2 is a diagram schematically showing the structure of a magnetic tunnel junction element 11 according to an embodiment of the invention. FIG. 3 is a diagram explaining the tunnel magnetoresistance effect of the magnetic tunnel junction element 11. FIG. 4 is a diagram explaining the change in magnetic anisotropy due to voltage of the magnetic tunnel junction element 11. FIG. 5 is a diagram explaining a method of applying an external magnetic field to cause parametric excitation in the magnetic tunnel junction element 11 according to an embodiment of the invention. FIG. 6 is a schematic diagram showing a circuit for measuring parametric excitation of the magnetic tunnel junction element 11 according to an embodiment of the invention. FIG. 7 is a diagram showing the results of detecting a parametric excitation signal. FIG. 8 is a diagram explaining the relationship between the power of an excitation microwave and the state of a magnon.

[0012] 1 is a diagram showing an outline of the configuration of a quantum state generating device 1 according to an embodiment of the present invention. As shown in FIG. 1, the quantum state generating device 1 includes a magnetic tunnel junction element 11, electrodes 12, and a power supply unit (voltage application unit) 13.

[0013] FIG. 2 is a diagram schematically illustrating the structure of the magnetic tunnel junction element 11 according to this embodiment. As shown in FIG. 2, the magnetic tunnel junction element 11 has a magnetization movable magnetic layer 111, an insulating layer 112, and a magnetization fixed magnetic layer 113. In this embodiment, the magnetization movable magnetic layer 111 and the magnetization fixed magnetic layer 113 are formed of an iron-cobalt alloy (cobalt-iron-boron: Co—Fe(—B) in the example of FIG. 2), and the insulating layer 112 may be formed of magnesium oxide (MgO). The thickness of each layer may be, for example, 1 nm for the magnetization movable magnetic layer 111, 2 nm for the insulating layer 112, and 0.8 nm for the magnetization fixed magnetic layer 113. In particular, it is desirable that the insulating layer 112 be 2 nm or more. If the insulating layer 112 is 2 nm or more thick, the influence of the voltage that causes the tunnel effect can be increased. Furthermore, the area resistance (RA) of the magnetic tunnel junction element 11 is 10 Ωμm ２ It is desirable that this is the case.

[0014] Magnetic tunnel junction (MTJ) elements are known as elements used in magnetoresistive random access memories (MRAMs). When a voltage is applied to a magnetic tunnel junction element, a current flows through the insulating layer due to the tunnel effect. It is generally known that resistance is low when the magnetization directions of two magnetic layers are the same and high when they are opposite, and MRAMs are memories that utilize this change in electrical resistance. Magnetic tunnel junction elements are minute elements on the nanoscale (approximately several tens of nanometers), and in MRAMs, the integration of 10 to the power of 8 magnetic tunnel junction elements has been realized.

[0015] As shown in Fig. 3, when an external magnetic field is applied to the magnetic tunnel junction element 11, the magnetization direction M of the magnetization movable magnetic layer 111 changes. When the magnetization direction M of the magnetization movable magnetic layer 111 changes, the relative angle with the magnetization direction of the magnetization fixed magnetic layer 113 changes, and the resistance of the magnetic tunnel junction element 11 changes. This phenomenon in which the resistance value of the element changes due to an external magnetic field is the tunnel magnetoresistance effect. Fig. 4 is a diagram showing the relationship between the value of the external magnetic field (horizontal axis) and the resistance value of the element (vertical axis). In Fig. 4, the voltage V applied to the magnetic tunnel junction element 11 ｄｃ The graph shows the results of measurements in which the value of Ω was changed in 14 steps from -0.70 V to 0.70 V. As shown in FIG. 4, regardless of the applied voltage, the resistance changes from a high state to a low state as the external magnetic field changes. Meanwhile, the magnitude of the external magnetic field required to change the resistance (change the direction of magnetization) changes depending on the magnitude of the applied voltage. In other words, FIG. 4 shows that the coercivity of the magnetization movable magnetic layer 111 changes depending on the applied voltage, and that the magnetic anisotropy can be controlled by voltage.

[0016] In this embodiment, a quantum state is created in the magnetic tunnel junction element 11 by controlling the voltage applied to the magnetic tunnel junction element 11, allowing the magnetic tunnel junction element 11 to be used as a quantum bit. The advantage of using magnons (quasiparticles corresponding to spin waves) of a magnetic material as quantum bits is that, as described above, the size of one bit is very small, making it suitable for integration. When magnetization precesses, the distribution of thermal fluctuations and quantum fluctuations, which are expressed as probability density functions in the phase space of the magnons, can be used as quantum information.

[0017] Specifically, parametric excitation is induced in the magnetic tunnel junction element 11 to generate a squeezed state. A squeezed state is a state in which fluctuations have an asymmetric shape in phase space (fluctuations in one direction are large), and by superposing two squeezed states, it is possible to create entanglement, a resource necessary for quantum information processing. If a squeezed state can be generated in the magnetic tunnel junction element 11, it can be used as a quantum bit.

[0018] Normally, the magnetization direction of the magnetization movable magnetic layer 111 of the magnetic tunnel junction element 11 is the same direction or opposite to the magnetization direction of the magnetization fixed magnetic layer 113. Even if magnons, i.e., precession of magnetization, are excited in this state, the displacement component due to the precession does not affect the change in the resistance value of the magnetic tunnel junction element 11. On the other hand, as shown in Figure 5, when a weak external magnetic field is applied in the transverse direction to the magnetic tunnel junction element 11, the magnetization direction of the magnetization movable magnetic layer 111 becomes perpendicular to the magnetization direction of the magnetization fixed magnetic layer 113 (Figure 5(B)). When magnons are excited in this state, the displacement component due to the precession oscillates between the same direction and the opposite direction to the magnetization direction of the magnetization fixed magnetic layer 113 (Figures 5(A) to 5(C)), causing a large change in the resistance value of the magnetic tunnel junction element 11.

[0019] 6 is a schematic diagram showing a circuit for detecting a parametric excitation signal of the magnetic tunnel junction element 11 under the conditions shown in FIG. 6 . As shown in FIG. 6 , an excitation microwave (AC voltage) of frequency 2f (f is the resonant frequency of the magnetic tunnel junction element 11) is applied to the magnetic tunnel junction element 11 from an excitation signal wave source 21. This causes precession of frequency f in the magnetization movable magnetic layer 111. The reflected excitation microwave of frequency 2f and a voltage due to the tunnel magnetoresistance effect of frequency f generated from the precession of frequency f are returned to the combiner 22 from the magnetic tunnel junction element 11. Of these, the voltage of frequency 2f is removed by a bandpass filter 23, and only the voltage of frequency f is supplied to the mixer 24. Furthermore, a voltage of frequency (1f-Ω) is supplied to the mixer 24 from a signal wave source 25 as a reference wave. Only the voltage of the differential frequency Ω is output from the mixer 24. Therefore, when a voltage of frequency Ω is further supplied from the signal wave source 27 as a reference wave and detection is performed by the lock-in amplifier 28, only the voltage of frequency Ω is extracted and a parametric excitation signal is detected.

[0020] 7 is a diagram showing the power spectral density of a signal detected when an excitation microwave of frequency 2f is applied to the magnetic tunnel junction element 11 in the circuit shown in FIG. 7 . In the example of FIG. 7 , the frequency f is 4.1675 GHz. As shown in FIG. 7 , when the power of the excitation microwave was set to 18.4 dBm, a wave peak of frequency f (=4.1675 GHz) was detected, i.e., a parametric excitation signal was observed. On the other hand, when the power of the excitation microwave was set to 12.0 dBm, no peak was detected.

[0021] 8 is a diagram showing the relationship between the power of the excitation microwave, the power of the magnon state excitation microwave (horizontal axis), and the peak voltage value (vertical axis). As shown in Fig. 8(D), there is a nonlinear relationship between the power of the excitation microwave and the peak voltage value.

[0022] Figure 8 illustrates the relationship between the power of the excitation microwave and the magnon state. The graph on the right shows the relationship between the excitation microwave power (horizontal axis) and the peak voltage value (vertical axis). Also, (A) to (C) in the figure show the magnon probability density function (Wigner function) in phase space when the excitation microwave power is 12 dBm, 17.2 dBm, and 18.4 dBm, respectively. (A) shows a state in which there is equal fluctuation in all directions centered on the origin. (B) shows an increase in fluctuation in a specific direction, indicating the occurrence of a squeezed state. (C) shows the probability density function with two peaks, indicating a mixed state. Thus, there is a correlation between the magnon state and the excitation microwave power. In the example shown in Figure 8, it can be seen that the optimal power for generating a squeezed state is 17.2 dBm.

[0023] In the above embodiment, an AC voltage having a frequency twice the resonant frequency is applied to the magnetic tunnel junction element 11 to induce parametric excitation in the magnetic tunnel junction element 11, thereby generating a magnon state such as a squeezed state. However, a constant DC voltage may be applied superimposed on the AC voltage. By applying the DC voltage, the direction of the squeezed state generated by the AC voltage can be rotated.

[0024] Alternatively, the magnetic tunnel junction element 11 may be disposed in an electromagnetic wave resonator. By disposing the magnetic tunnel junction element 11 in the electromagnetic wave resonator and applying an AC voltage at twice the resonant frequency, parametric excitation can be generated more efficiently. By disposing the magnetic tunnel junction element 11 in the electromagnetic wave resonator, it becomes possible to connect multiple magnetic tunnel junction elements 11 to perform arithmetic processing. Furthermore, while the introduction of microwaves increases the temperature, the microwave power that can be input to maintain an environment of approximately 100 mK, the operating temperature of a quantum computer, is typically approximately -60 dBm. Therefore, to achieve parametric excitation with weak microwave power, it is necessary to confine and increase the electric field. For this purpose, disposing the magnetic tunnel junction element 11 in an electromagnetic wave resonator is effective. The electromagnetic wave resonator is preferably a cavity resonator made of oxygen-free copper or a superconducting resonator made of a superconductor (typically Al, NbN, or Nb).

[0025] As described above, according to this embodiment, by applying an AC voltage of frequency 2f, which is twice the resonance frequency f of the magnetization movable magnetic layer 111, to the magnetic tunnel junction element 11, precession of frequency f can be caused in the magnetization movable magnetic layer 111. This causes parametric excitation in the magnetic tunnel junction element 11, and the magnetic tunnel junction element 11 can be put into a squeezed state. The magnetic tunnel junction element 11 in the squeezed state can be used as a quantum bit. By using a nano-sized element such as the magnetic tunnel junction element 11 as a quantum bit, a highly integrated quantum computer can be created.

[0026] Furthermore, the orientation of the squeezed state generated by the AC voltage can be rotated by applying a DC voltage superimposed on an AC voltage of frequency 2f to the magnetic tunnel junction device 11. By superimposing two squeezed states whose orientations have been changed in this way, it is expected that entanglement can also be generated.

[0027] 1...Quantum state generating device, 11...Magnetic tunnel junction element, 12...Electrode, 13...Power supply device, 21...Excitation signal wave source, 22...Combiner, 23...Band-pass filter, 24...Mixer, 25...Signal wave source, 27...Signal wave source, 28...Lock-in amplifier, 111...Moveable magnetization magnetic layer, 112...Insulating layer, 113...Fixed magnetization magnetic layer

Claims

1. A quantum state generating device comprising: a magnetic tunnel junction element having an insulating layer between a magnetization movable magnetic layer and a magnetization fixed magnetic layer; and a voltage application unit that applies a voltage to the magnetic tunnel junction element, wherein the voltage application unit applies an AC voltage having a frequency twice the resonant frequency of the magnetic material that forms the magnetization movable magnetic layer.

2. A quantum state generating device according to claim 1, wherein the voltage application unit applies a constant DC voltage superimposed on the AC voltage.

3. A quantum state generating device according to claim 1 or 2, wherein the magnetization movable magnetic layer and the magnetization fixed magnetic layer are made of cobalt iron boron, and the insulating layer is made of magnesium oxide.

4. The quantum state generating device according to claim 1 or 2, wherein the magnetic tunnel junction element is disposed in an electromagnetic wave resonator.

5. The area resistance (RA) of the magnetic tunnel junction element is 10 Ωμm ２ The quantum state generating device according to claim 1 or 2.

6. A quantum state generating method comprising applying a voltage to a magnetic tunnel junction element having an insulating layer between a magnetization movable magnetic layer and a magnetization fixed magnetic layer, wherein the voltage applied to the magnetic tunnel junction element is an AC voltage having a frequency twice the resonant frequency of the magnetic material forming the magnetization movable magnetic layer.

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