Superconducting quantum circuit elements and superconducting quantum computers

Abstract

[Problem] To provide a magnetic flux-type superconducting quantum circuit element that has a practically tolerable anharmonicity and enables densification by avoiding addition of a shunt capacitor, thereby achieving an increased coherence time, and a superconducting quantum computer in which the element is used. [Solution] The problem is solved by a large-area junction magnetic flux-type superconducting quantum circuit element characterized in that the superconducting quantum circuit element is based on a magnetic flux-type superconducting quantum circuit element which comprises three Josephson junctions for one loop and into which an α junction ratio is introduced such that the area of one Josephson junction among the three Josephson junctions is α (0.3≦α≦0.7) times the area of the other Josephson junctions having an equal area, wherein the junction cross-sectional area of the three Josephson junctions is set to be large at β (2≦β≦200) times the junction cross-sectional area of the basic magnetic flux-type superconducting quantum circuit element while the α junction ratio is maintained, whereby an increased coherence time is achieved.

Description

[Technical Field]

[0001] This invention relates to a superconducting quantum circuit element (so-called superconducting qubit) and a superconducting quantum computer (so-called superconducting quantum computer) using said element. [Background technology]

[0002] Superconducting qubits, which have Josephson junctions and are used in superconducting quantum computers, can create a quantum mechanical two-level system. This allows for the realization of a state that simultaneously takes on both "0" and "1" (quantum superposition state) within a single physical system, which is said to exhibit quantum parallelism. By configuring the system to accommodate a large number of quantum superposition states, it becomes possible to store a significantly increased number of different states. Therefore, it is considered a promising technology for future computer applications, potentially replacing digital signal processing using voltage differences.

[0003] Figure 7 illustrates this phenomenon by showing the energy levels of a superconducting qubit with a Josephson junction. The horizontal axis represents the phase φ, and the vertical axis represents the energy level E. The height labeled "n=0" represents the ground state, the height labeled "n=1" represents the energy level of the first excited state, and the height labeled "n=2" represents the energy level of the second excited state. As shown in Figure 7, in a superconducting qubit with a Josephson junction, the energy difference ε between the second excited state and the first excited state is significant. 12 and the energy difference ε between the first excited state and the ground state 01 They are adjusted to be of different magnitudes. The reason for this is that when alternating electromagnetic waves are applied to control the energy state, the energy difference ε 12 and energy difference ε 01 If they are different, then only the ground state and the first excited state are possible, whereas the energy difference ε 12 and energy difference ε 01This is because if they are equal, there is a risk of taking on a second excited state. In other words, a Josephson junction can be described as a circuit element for realizing non-equally spaced energy levels. However, there are several types of superconducting qubits that have a Josephson junction, and the degree of non-equal spacing of the energy levels differs depending on the type. The greater the degree of non-equal spacing, the more stable the qubit becomes. This is called anharmonicity. |ε 12 -ε 01 | This is represented by a coefficient, and the larger this value, the more stable the qubit will be in operation.

[0004] The main types of superconducting qubits are known to be charge-type superconducting qubits and flux-type superconducting qubits. Figure 8(a) is an explanatory diagram showing the structure of a charge-type superconducting qubit, and Figure 8(b) is an explanatory diagram showing the structure of a flux-type superconducting qubit. In both figures, a thin insulator EI is joined between two superconductors SC, creating a weak coupling between the two superconductors. This coupling structure is a Josephson junction. In the superconducting state, electron pairs pass through the insulator EI by tunneling effect, and as a result, a zero-resistance current called a Josephson current flows in the Josephson junction. For superconducting qubits, the coherence time, which is the duration of the quantum superposition state, and the anharmonicity required for the qubit to operate stably are important. Currently, charge-type superconducting qubits are advantageous in terms of coherence time, while flux-type superconducting qubits are advantageous in terms of anharmonicity. Specifically, as shown in Figure 8(a), a charge-type superconducting qubit has two Josephson junctions (regions defined by two insulators (EI,EI)), but there may be only one Josephson junction, and the difference between one and two does not significantly affect the coherence time or anharmonicity. On the other hand, the flux-type superconducting qubit shown in Figure 8(b) has three Josephson junctions (regions defined by three insulators (EI1,EI2,EI3)), and the size of the junction area is EI2=EI3, and EI1=αEI2. In this case, α is 0.3≦α≦0.7, and typically 0.4≦α≦0.5. The principle is omitted here, but the introduction of these three Josephson junctions and the junction area ratio α (called the α junction ratio) improves anharmonicity.

[0005] In the practical application of superconducting qubits, at first, the coherence time was an issue. The superposition state of the first excited state and the ground state is lost over time. If the time it takes to be lost is shorter than the time required for state control, it cannot be used as a computer. In order to address the issue of improving this coherence time, it has been reported that by introducing a shunt capacitor, the electrostatic energy in the Josephson junction is effectively reduced, improving the coherence time (Non-Patent Document 1). Also, it has been reported that by using an epitaxially grown nitride Josephson junction, noise sources are eliminated, improving the coherence time (Non-Patent Document 2).

Prior Art Documents

Non-Patent Documents

[0006]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] In the case of charge-type superconducting qubits that have been developed previously, it has become an impossible situation in principle to improve the anharmonicity while ensuring an effective coherence time. As described above, it is known that for anharmonicity, a flux-type superconducting qubit having three Josephson junctions is more advantageous than a charge-type superconducting qubit. However, according to Non-Patent Document 1 and Non-Patent Document 2, it has been confirmed that if a shunt capacitor is added to improve the coherence time, even in the case of a flux-type superconducting qubit, a situation where the anharmonicity is reduced occurs. Also, as shown in Non-Patent Document 1 and Non-Patent Document 2, when a shunt capacitor is added, there is a problem that an increase in the footprint, which is the occupied area of one qubit, is inevitable and it is difficult to achieve high density.

[0008] Therefore, an object of the present invention is to provide a flux-type superconducting quantum circuit element that enables high density without adding a shunt capacitor and realizes an improvement in coherence time while making the anharmonicity practically acceptable, and a superconducting quantum computer using the element.

Means for Solving the Problems

[0009] The device for a large-area junction flux-type superconducting quantum circuit of the present invention comprises at least the following configurations. A device for a superconducting quantum circuit, wherein the device for a superconducting quantum circuit has three Josephson junctions in one loop, and an α-junction ratio is introduced, where the area of one of the three Josephson junctions is α times the area of the other two Josephson junctions of equal area. Based on the flux-type superconducting quantum circuit device, while maintaining the α-junction ratio, the junction cross-sectional area of the three Josephson junctions is set to be β times the junction cross-sectional area of the flux-type superconducting quantum circuit device used as a basis, so as to improve the coherence time. Here, for α, 0.3 ≤ α ≤ 0.7, preferably 0.4 ≤ α ≤ 0.5. As the lower limit, α ≥ 0.3, preferably α ≥ 0.4. As the upper limit, α ≤ 0.7, preferably α ≤ 0.5. For β, 2 ≤ β ≤ 200. Regarding the upper limit, specifically, in order for the Josephson junction to function, the line β ≤ 200 is realistic.

[0010] Also, the device for a large-area junction flux-type superconducting quantum circuit of the present invention is based on a flux-type superconducting quantum circuit device having three Josephson junctions in one loop, and an α-junction ratio (0.3 ≤ α ≤ 0.7) is introduced, where the area of one of the three Josephson junctions is α times the area of the other two Josephson junctions of equal area. While maintaining the α-junction ratio, the junction cross-sectional area of the three Josephson junctions is set to be β times (2 ≤ β ≤ 200) the junction cross-sectional area of the flux-type superconducting quantum circuit device used as a basis, and the areas of the three Josephson junctions are all 0.5 μm 2 or more, 50.0 μm 2 and are set to be as follows.

[0011] Furthermore, the large-area junction flux-type superconducting quantum circuit element of the present invention is based on a flux-type superconducting quantum circuit element in which there are three Josephson junctions in one loop, and an α-junction ratio is introduced in which the area of ​​one of the three Josephson junctions is α times (0.3 ≤ α ≤ 0.7) the area of ​​the other two equal-area Josephson junctions. However, the junction cross-sections of the three Josephson junctions are set to be β times (2 ≤ β ≤ 200) the junction cross-section of the base flux-type superconducting quantum circuit element while maintaining the α-junction ratio. In the case of a shunted flux-type superconducting quantum circuit element with a shunt capacitor added to improve the coherence time of the flux-type superconducting quantum circuit element, the junction cross-sections of the three Josephson junctions are set to be large enough to obtain a combined capacitance value equivalent to that of the shunted flux-type superconducting quantum circuit element, and the shunt capacitor is not actually added.

[0012] Furthermore, the large-area junction flux-type superconducting quantum circuit element of the present invention is based on a flux-type superconducting quantum circuit element in which there are three Josephson junctions in one loop, and the area of ​​one of the three Josephson junctions is α times (0.3 ≤ α ≤ 0.7) the area of ​​the other two equal-area Josephson junctions. However, the junction cross-sectional areas of the three Josephson junctions are set to be β times (2 ≤ β ≤ 200) the junction cross-sectional area of ​​the base flux-type superconducting quantum circuit element, while maintaining the α junction ratio. The α junction ratio, the area S of the two equal-area Josephson junctions, and the Josephson critical current density J are set to prioritize the improvement of anharmonicity. C It is characterized by being adjusted.

[0013] Furthermore, the large-area junction flux-type superconducting quantum circuit element of the present invention is based on a flux-type superconducting quantum circuit element in which there are three Josephson junctions in one loop, and the area of ​​one of the three Josephson junctions is α times (0.3 ≤ α ≤ 0.7) the area of ​​the other two equal-area Josephson junctions. However, the junction cross-sectional areas of the three Josephson junctions are set to be β times (2 ≤ β ≤ 200) the junction cross-sectional area of ​​the base flux-type superconducting quantum circuit element while maintaining the α junction ratio, and the α junction ratio, the area S of the two equal-area Josephson junctions, and the Josephson critical current density J are set to prioritize the improvement of coherence time. C It is characterized by being adjusted.

[0014] The large-area junction flux superconducting quantum computer of the present invention comprises at least the following configurations. A superconducting quantum computer is characterized by having a flux-type superconducting quantum circuit element that is based on a flux-type superconducting quantum circuit element in which there are three Josephson junctions in one loop and an α-junction ratio is introduced in which the area of ​​one of the three Josephson junctions is α times (0.3 ≤ α ≤ 0.7) the area of ​​the other two equal-area Josephson junctions, while maintaining the α-junction ratio, and is set to be β times (2 ≤ β ≤ 200) the junction cross-sectional area of ​​the base flux-type superconducting quantum circuit element, thereby improving the coherence time. [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic diagram illustrating the structure of a flux-type superconducting qubit, which is a prerequisite for this technology. [Figure 2] This is a schematic diagram illustrating the structure of a shunt-equipped flux-type superconducting qubit, which is a prerequisite for this concept. [Figure 3] This is a schematic diagram illustrating the structure of a large-area junction flux-type superconducting qubit according to one embodiment of the present invention. [Figure 4]This is a schematic diagram illustrating the structure of a large-area junction flux type superconducting qubit according to another embodiment of the present invention. [Figure 5] This table compares the characteristics of embodiments of the present invention with those of various conventional superconducting qubits. [Figure 6] This is an explanatory diagram showing a superconducting quantum computer according to an embodiment of the present invention. [Figure 7] This figure shows the energy levels of a superconducting qubit. [Figure 8] This is an explanatory diagram showing the structures of charge-type superconducting qubits and flux-type superconducting qubits. [Modes for carrying out the invention]

[0016] The present invention is based on a flux-type superconducting quantum circuit element, and has a small footprint and can be made high-density. In fact, it can obtain the same advantages as when a shunt capacitor is present, even though it does not have a shunt capacitor. From this perspective, it can be called a self-shunted flux-type superconducting quantum circuit element (SSFQ).

[0017] To understand the technical concept of this invention, it is essential to first understand the advantages and disadvantages of incorporating a shunt capacitor into a flux-type superconducting quantum circuit element. Therefore, this point will be explained first.

[0018] The following explanation uses diagrams, but these diagrams are created for explanatory purposes and may intentionally omit components that are not necessary for the explanation in order to make them easier to understand. Also, components may be intentionally enlarged or reduced in size for explanatory purposes and are not diagrams that show an accurate scale.

[0019] (Regarding the underlying flux-type superconducting qubit) Figure 1 is a schematic diagram illustrating the structure of the flux-type superconducting qubit 3 shown in Figure 8(b). The flux-type superconducting qubit 3 has a configuration in which a magnetic flux F passes through a loop formed by three Josephson junctions: the first Josephson junction 31, the second Josephson junction 32, and the third Josephson junction 33. Of these, the junction cross-sectional area of ​​the second Josephson junction 32 is equal to the junction cross-sectional area of ​​the third Josephson junction 33. Furthermore, the cross-sectional area of ​​the first Josephson junction 31 is α times (where 0.3 ≤ α ≤ 0.7) the cross-sectional areas of the second Josephson junction 32 and the third Josephson junction 33, with the former having a smaller cross-sectional area than the latter. This ratio of Josephson junction areas is called the α-junction ratio.

[0020] Incidentally, a Josephson junction can be thought of as having two types of energy. One is the junction energy E shown in [Equation 1]. J That is the case.

number

[0021] On the other hand, a Josephson junction can be considered a capacitor because it sandwiches an insulator between metals. In other words, the other of the two types of energy that a Josephson junction possesses is the electrostatic energy E shown in [Equation 2]. C That's how it works.

number

[0022] Now, in a charged superconducting qubit, the ratio of these two energies is E J / E C It is known that the ratio of E to E determines the charge noise immunity. The larger this ratio, the higher the charge noise immunity. Also, the higher the charge noise immunity, the more the coherence time tends to increase. That is, the ratio of E to E is the ratio of E to E. J / E C This serves as an indicator of the magnitude of the coherence time.

[0023] On the other hand, E C It is known that reducing E also reduces anharmonicity. However, such E C The relationship between coherence time and anharmonicity is shown as a strong tendency in charge-type superconducting qubits, but not as strongly in flux-type superconducting qubits. For this reason, one possible method to improve the coherence time of flux-type superconducting qubits is to add a shunt capacitor, which is also used in charge-type superconducting qubits.

[0024] (Regarding the prerequisite shunt-type flux-type superconducting qubit) Figure 2 is a schematic diagram illustrating the structure of the shunted flux-type superconducting qubit 2. The shunted flux-type superconducting qubit 2 has a configuration in which a magnetic flux F passes through a loop formed by three Josephson junctions: the first Josephson junction 21, the second Josephson junction 22, and the third Josephson junction 23. In this respect, it is the same as the structure of the flux-type superconducting qubit described above. Also, the junction cross-section of the second Josephson junction 22 and the junction cross-section of the third Josephson junction 23 are equal, and the cross-section of the first Josephson junction 21 is α times the cross-section of the second Josephson junction 22 and the third Josephson junction 23 (where 0.3 ≤ α ≤ 0.7), which is also the same as the structure of the flux-type superconducting qubit. The difference from the flux-type superconducting qubit is that a shunt capacitor CS is connected in parallel to the loop. By connecting a shunt capacitor CS with a large capacitance, E shown in [Equation 2] CSince this is a small value, E is the ratio of the two energies of the Josephson junction. J / E C It can take on a large value.

[0025] E in a shunt-equipped flux-type superconducting qubit J / E C It can take a value greater than 10, and the coherence time is improved. In addition to this, as mentioned above, in flux-type superconducting qubits, E C Even if the value is reduced, the anharmonicity is not impaired to the same extent as when a shunt capacitor is added to a charge-type superconducting qubit. However, it cannot be denied that the anharmonicity is lower than that of a flux-type superconducting qubit. Furthermore, the biggest weakness of a flux-type superconducting qubit with a shunt is that, in addition to the loop formed by the Josephson junction, a connection pattern is required to provide a separate capacitor, which, as mentioned above, leads to an increase in footprint. For these reasons, the inventors have conducted diligent studies and have found that, without using a shunt capacitor, E J / E C This led to the development of a large-area junction flux type superconducting qubit, an approach that significantly increases the size of the qubit.

[0026] (One embodiment of the present invention) Figure 3 is a schematic diagram illustrating the structure of a large-area junction flux-type superconducting qubit 1A according to one embodiment of the present invention. The large-area junction flux-type superconducting qubit 1A has a configuration in which a magnetic flux F passes through a loop formed by three Josephson junctions: a first Josephson junction 11A, a second Josephson junction 12, and a third Josephson junction 13. In this respect, it is the same as the structure of the flux-type superconducting qubit 3 and the shunt-type flux-type superconducting qubit 2 described above. Furthermore, the junction cross-sectional area of ​​the second Josephson junction 12 and the junction cross-sectional area of ​​the third Josephson junction 13 are equal, and the cross-sectional area of ​​the first Josephson junction 11A is α times the cross-sectional area of ​​the second Josephson junction 12 and the third Josephson junction 13 (how α is set will be described later). This is also the same as the structure of the flux-type superconducting qubit 3 and the shunt-type flux-type superconducting qubit 2. As shown in the diagram, it does not have a shunt capacitor. On the other hand, the difference from a flux-type superconducting qubit is that the area of ​​the Josephson junction is increased by β times (β≧2). This feature is the reason why it is called a large-area junction flux-type superconducting qubit. Furthermore, due to this feature, the ratio of the two energies of the Josephson junction, E J / E C For example, it can take very large values, such as 300 or more. Furthermore, needless to say, there is no problem with an increased footprint. While it is possible to change the capacitance of a capacitor by altering the thickness and material of the insulating layer within the Josephson junction, this would result in a change in the Josephson critical current density. Therefore, increasing the area of ​​the Josephson junction is a method that can solve this problem. Furthermore, the thickness of the insulating layer within the Josephson junction is considered when controlling the Josephson critical current density, as will be discussed later.

[0027] In the large-area junction flux-type superconducting qubit 1A, the cross-sectional areas of the first Josephson junction 11A, the second Josephson junction 12, and the third Josephson junction 13 are set to be β times the cross-sectional areas of the first Josephson junction 31, the second Josephson junction 32, and the third Josephson junction 33 of the flux-type superconducting qubit 3, respectively, so that the combined capacitance value obtained is equivalent to that of the combined capacitance of the shunt-equipped flux-type superconducting qubit 2. The value of β is precisely determined by numerically calculating the entire Hamiltonian describing this qubit, but it can be roughly determined as follows. In a shunted flux-type superconducting qubit, the combined capacitance is approximately the sum of the shunt capacitor CS and the Josephson junction capacitor CJ (CS+CJ). In this invention, as mentioned above, this is set to an equivalent value using the Josephson junction capacitor. Typically, the shunt capacitor CS is often approximately equal to or greater than CJ (CS+CJ≧2CJ), so the cross-sectional area of ​​the Josephson junction in this invention is set under the guideline of being more than twice that of a conventional flux-type qubit (β≧2). At that time, the Josephson critical current I C Since it also becomes β times, the Josephson critical current density J C By adjusting E J / E C By adjusting this, it is possible to design element characteristics such as anharmonics. Note that the Josephson critical current density J C This can be adjusted by the thickness of the insulating layer included in the Josephson junction, etc. In this invention, as mentioned above, if α is 0.3 ≤ α ≤ 0.7, the same as in the structures of flux-type superconducting qubit 3 and flux-type superconducting qubit 2 with a shunt, it can operate as a flux-type superconducting qubit. However, because the area of ​​the Josephson junction is β times (β ≥ 2), the range of α that satisfies this condition and is particularly suitable for improving anharmonicity may be limited. In this invention, α ≤ α ≤ 0.5 is a range that yields particularly good performance, and a suitable design can be created. Therefore, a preferred lower limit is α ≥ 0.4, and a preferred upper limit is α ≤ 0.5. However, as can be understood from the existence of research examples that attempt to change the composition of the Josephson junction from typical amorphous aluminum-based materials to crystalline materials, it is conceivable that performance can be maintained by other factors such as materials, so even considering feasibility, it is not necessary to limit it to 0.4 ≤ α ≤ 0.5. Furthermore, the junction area of ​​a typical flux-type superconducting qubit is generally 0.25 μm². 2Although smaller than the usual value, in this invention it is β times (β≧2) larger than usual. According to the setting of the capacitor value under the aforementioned guidelines, the area of ​​the Josephson junction is 0.5 μm². 2 The above is preferable. On the other hand, the upper limit of β is sufficient as long as it is a movable size for a superconducting qubit, but the area of ​​the Josephson junction is 50.0 μm². 2 The system is sufficiently functional if it is below the specified range, so a value of approximately β times (200 ≥ β) or less is acceptable. Furthermore, as mentioned above, by converting to a crystalline composition, problems such as noise sources due to defects can be overcome, so it is expected that a larger β value, closer to 200, can be adopted.

[0028] Large-area junction flux-type superconducting qubits have an α-junction ratio, a junction area S that is set larger than that of conventional flux-type superconducting qubits, and a Josephson critical current density J. C By appropriately adjusting these three values, it becomes possible to maintain high anharmonicity and achieve long coherence times without the need for a shunt capacitor.

[0029] This section describes various numerical values ​​such as the size and characteristics of a large-area junction flux-type superconducting qubit 1A according to one embodiment of the present invention. In one embodiment, α = 0.484, and the area of ​​the two equal-area Josephson junctions is 1.62 μm². 2 It is said that the junction area of ​​a typical flux-type superconducting qubit is 0.25 μm². 2 Considering that it is smaller than β, the junction area is more than 6 times larger (β≧6: β is the ratio of the flux-type superconducting qubit to the Josephson junction cross-section).

[0030] A large-area junction flux type superconducting qubit 1A according to one embodiment of the present invention, given such element size, α-junction ratio, and junction area S, has a Josephson critical current density J C By adjusting this separately, the minimum bonding area can be reduced to 0.78 μm². 2 The critical current density is 11.0 A / cm². 2This design achieves characteristics such as an anharmonicness of 1.0 GHz and a qubit energy change in response to flux fluctuations (smaller values ​​indicate higher coherence), which is an indicator of immunity to flux noise affecting coherence time, of 24.4 MHz. While the anharmonicness is not as high as that of flux-type superconducting qubits, this value is still practically acceptable. Furthermore, since it does not require a large shunt capacitor with sides of several tens to several hundreds of micrometers, as is the case with shunt-type qubits, the footprint can be significantly reduced. Furthermore, while the minimum size (diameter or side length) of a Josephson junction is typically 0.5 μm or less, in one embodiment, the diameter of a circular junction is 1.0 μm (Josephson junction area 0.78 μm). 2 ) and (even in the present invention, the area is 0.5 μm 2 (In the case of circular junctions, the diameter is 0.8 μm or more), and conventionally, these could only be formed using electron beam lithography equipment, but one embodiment (or the present invention) can be formed using less expensive exposure equipment, thus making it possible to manufacture them at a lower cost. Furthermore, the shape of the Josephson joint can be any shape, such as a square, rectangle, circle, or ellipse.

[0031] (Another embodiment of the present invention) Figure 4 is a schematic diagram illustrating the structure of a large-area junction flux-type superconducting qubit 1B according to another embodiment of the present invention. The large-area junction flux-type superconducting qubit 1B has a configuration in which a magnetic flux F passes through a loop formed from three Josephson junctions: a first Josephson junction 11B, a second Josephson junction 12, and a third Josephson junction 13. In this respect, it is the same as the structure of the large-area junction flux-type superconducting qubit 1A according to one embodiment. Also, the fact that the junction cross-sectional area of ​​the second Josephson junction 12 and the junction cross-sectional area of ​​the third Josephson junction 13 are equal is also the same as the structure of the large-area junction flux-type superconducting qubit 1A according to one embodiment. However, the α-junction ratio between the cross-sectional area of ​​the first Josephson junction 11A and the cross-sectional areas of the second Josephson junction 12 and the third Josephson junction 13, and the value of β, which is the ratio of the flux-type superconducting qubit to the Josephson junction cross-sectional area, differ from the large-area junction flux-type superconducting qubit 1A according to one embodiment.

[0032] This section describes various numerical values ​​such as the size and characteristics of a large-area junction flux type superconducting qubit 1B according to another embodiment of the present invention. The α-junction ratio, junction area S, and Josephson critical current density J are also described. C By appropriately adjusting these three values, it is possible to maintain high anharmonicity and achieve a long coherence time. In another embodiment, with an emphasis on improving magnetic flux noise immunity, α is set to 0.407, and the area of ​​the two equal-area Josephson junctions is 1.93 μm². 2 The junction area of ​​a typical flux-type superconducting qubit is 0.25 μm². 2 Considering that it is smaller than β, the junction area is more than 7 times larger (β≧7: β is the ratio of the flux-type superconducting qubit to the Josephson junction cross-section).

[0033] In another embodiment of the present invention, a large-area junction flux type superconducting qubit 1B, given such element size, α-junction ratio, and junction area S, has a Josephson critical current density J C By adjusting this separately, the minimum bonding area can be reduced to 0.79 μm². 2 The critical current density is 6.77 A / cm². 2This results in characteristics such as an anharmonic of 453 MHz and a qubit energy change in response to flux fluctuations (smaller values ​​indicate higher coherence), which is an indicator of flux noise immunity that affects coherence time, of 4.65 MHz. It can be seen that while the anharmonic is worse than that of the 1A, the flux noise immunity is improved. Furthermore, since it does not require a large shunt capacitor with sides of several tens to several hundreds of micrometers, as is the case with shunt-equipped types, the footprint can be significantly reduced.

[0034] (Further embodiments of the present invention) Although not shown in the figures, various numerical values ​​such as the size and characteristics of a large-area junction flux-type superconducting qubit 1C according to yet another embodiment of the present invention will be described. In yet another embodiment, with emphasis on improving flux noise immunity, α = 0.437 is set, and the area of ​​the two equal-area Josephson junctions is 1.80 μm². 2 This is more than seven times the junction area of ​​a typical flux-type superconducting qubit (β≧7: β is the ratio of the junction cross-section of a flux-type superconducting qubit to the Josephson junction cross-section).

[0035] In another embodiment of the present invention, a large-area junction flux type superconducting qubit 1C, given such element size, α-junction ratio, and junction area S, has a Josephson critical current density J C By adjusting this separately, the minimum bonding area can be reduced to 0.79 μm². 2 The critical current density is 9.3 A / cm². 2 This results in characteristics such as an anharmonic value of 630 MHz and a qubit energy change in response to flux fluctuations (smaller values ​​indicate higher coherence), which is an indicator of flux noise immunity that affects coherence time, of 9.42 MHz. Embodiment 1C can be understood as a balanced type that emphasizes both anharmonicness and flux noise immunity, as it is an intermediate between Embodiment 1A and Embodiment 1B.

[0036] This paper examines the differences in characteristics between embodiments of the present invention (1A, 1B, and 1C) and conventional flux-type superconducting qubits, flux-type superconducting qubits with shunts, and Transmon, which are charge-type superconducting qubits with shunts. The table shown in Figure 5 is a comparative table of the characteristics of these various superconducting qubits.

[0037] First, 10,000 μm 2 The absence of shunt capacitors of this order of magnitude contributes significantly to reducing the footprint and, consequently, to increasing density. While Transmon and shunted flux superconducting qubits aimed to improve energy relaxation time at the expense of density, the embodiments of the present invention, although at the theoretical upper limit, all surpass these energy relaxation times. Furthermore, the anharmonicity values ​​are comparable to those of shunted flux superconducting qubits, which were previously considered more advantageous than Transmon. Thus, the embodiments of the present invention are comprehensively superior to Transmon and shunted flux superconducting qubits.

[0038] Figure 6 shows a superconducting quantum computer equipped with a large-area junction flux superconducting quantum circuit element according to an embodiment of the present invention. The large-area junction flux superconducting quantum circuit element 1 is arranged along the microwave control and readout line ML. Since sufficient coherence time is achieved and anharmonicity is also ensured, it is possible to simplify the adjustment of control parameters such as microwave intensity and frequency, thereby reducing the probability of error.

[0039] As described above, the elements for large-area junction flux type superconducting quantum circuits (large-area junction flux type superconducting qubits) according to embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to these embodiments, and any design changes, etc., that do not depart from the gist of the present invention are also included. In particular, the α-junction ratio, junction area S, and Josephson critical current density J. CBy adjusting this appropriately, it becomes possible to design the system with the freedom to prioritize improvements in coherence time, improvements in anharmonicity, or both. This will contribute to the further expansion of application technologies in the field of computing in the future. In charge-type superconducting qubits, there is an idea to improve coherence time by focusing on the junction cross-section. However, improving coherence time significantly impairs anharmonicity, and the two are in a trade-off relationship. This invention overcomes such a trade-off relationship, achieving both improved coherence time and improved anharmonicity, and provides a completely new element that solves various problems of superconducting qubits at once, with a small footprint, the possibility of high density, and low manufacturing cost. It should be fully understood that this is the significance of this invention. [Explanation of symbols]

[0040] 1A Large-area junction flux superconducting qubit (element for large-area junction flux superconducting quantum circuits) 1B Large-area junction flux superconducting qubit (element for large-area junction flux superconducting quantum circuits) 11A First Josephson junction 11B First Josephson junction 12. Second Josephson junction 13. Third Josephson junction 2. Shunt-equipped flux-type superconducting qubits 21. First Josephson junction 22. Second Josephson junction 23 Third Josephson Junction 3. Magnetic flux superconducting qubits 31. First Josephson Junction 32. Second Josephson Junction 33 Third Josephson Junction F magnetic flux

Claims

1. A superconducting quantum circuit element, The superconducting quantum circuit element is based on a flux-type superconducting quantum circuit element in which a single loop has three Josephson junctions, and an α-junction ratio is introduced in which the area of ​​one of the three Josephson junctions is α times the area of ​​the other two equal-area Josephson junctions, The cross-sectional areas of the three Josephson junctions are set to be β times the cross-sectional area of ​​the base flux-type superconducting quantum circuit element, while maintaining the α-junction ratio, thereby improving the coherence time. A large-area junction flux-type superconducting quantum circuit element characterized by the following: However, 0.3 ≤ α ≤ 0.7 and 2 ≤ β ≤ 200.

2. The area of ​​each of the three Josephson junctions is 0.5 μm². 2 Above, 50.0μm 2 It is set to be as follows: The element for a large-area junction flux type superconducting quantum circuit according to feature 1.

3. Assuming that a shunted flux-type superconducting quantum circuit element with a shunt capacitor added to improve the coherence time of the aforementioned flux-type superconducting quantum circuit element is established, The cross-sectional areas of the three Josephson junctions are set to be large enough to obtain a composite capacitance value equivalent to that of the shunt-equipped flux-type superconducting quantum circuit element, and no shunt capacitor is actually added. The element for a large-area junction flux type superconducting quantum circuit according to feature 1.

4. Prioritizing the improvement of anharmonicity, the α-junction ratio, the area S of the two equal-area Josephson junctions, and the Josephson critical current density J are used. C It is being adjusted. The element for a large-area junction flux type superconducting quantum circuit according to feature 1.

5. Prioritizing the improvement of coherence time, the α-junction ratio, the area S of the two equal-area Josephson junctions, and the Josephson critical current density J are used. C It is being adjusted. The element for a large-area junction flux type superconducting quantum circuit according to feature 1.

6. A superconducting quantum computer equipped with a large-area junction flux-type superconducting quantum circuit element according to any one of claims 1 to 5.

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