Quantum gate device

Abstract

The quantum gate device 10 of the present invention includes a first superconducting circuit 11 resonating at a first resonance frequency, a second superconducting circuit 12 resonating at a second resonance frequency, and a connection part 13 connecting the first superconducting circuit 11 and the second superconducting circuit 12. The first superconducting circuit 11 includes a first Josephson element 111, a second Josephson element group 112, and a first capacitor 114. The second Josephson element group 112 is formed by connecting n Josephson elements in series using wiring made of a superconductor, and the Josephson energy of each of the n Josephson elements is greater than n times the Josephson energy of the first Josephson element 111. A first wiring 115 forms a local superconducting circuit 113 by connecting the first Josephson element 111 and the second Josephson element group 112 in a loop, and then connects the local superconducting circuit 113 in parallel with the first capacitor 114. The quantum gate device 10 further has a magnetic field application part 14 for applying a static magnetic field to the inside of the local superconducting circuit 113, and an electromagnetic wave irradiation part (first electromagnetic wave irradiation part 151) for irradiating an electromagnetic wave having a difference frequency, which is the difference between the first resonance frequency and the second resonance frequency, to the first superconducting circuit 11 and / or the second superconducting circuit 12.

Description

Technical Field

[0001] The present invention relates to a quantum gate device as a constituent element of a quantum computer. Background Art

[0002] In recent years, research and development of quantum computers has been prevalent. In existing computers, data represented by either of two values (generally "0" and "1") is used as the minimum unit for calculation. In contrast, in a quantum computer, by using an overlapping state of two states treated by quantum mechanics as the minimum unit for calculation, higher computing power is expected.

[0003] In existing computers, an operation called a logic gate is performed, that is, input data is subjected to logical operations such as logical AND (AND), logical OR (OR), logical NOT (NOT), etc., and the result is output. Similarly, in a quantum computer, an operation called a quantum gate is performed, that is, input data is subjected to a set logical operation and then the result is output. A device that performs such an operation is called a quantum gate device. In a quantum gate device, as a prerequisite for realizing an overlapping state of two states, it is necessary to selectively take two states (not taking states other than the two) from a plurality of states discretized quantum mechanically.

[0004] In Non-Patent Document 1, a circuit called a superconducting qubit (transmon) is described as a constituent element of a quantum gate device. A superconducting qubit is a circuit formed by connecting one Josephson element and one capacitor in a ring shape using wiring made of a superconductor. Here, the Josephson element is composed of a thin film made of an insulator sandwiching two superconductors.

[0005] If the quantum gate device is cooled to a temperature at which the superconductors of the Josephson element and the wiring become superconducting, then due to the Josephson effect, a current flows through the superconducting qubit in a manner passing through the Josephson element. As a result, the superconducting qubit functions as a resonant circuit, and this resonant circuit takes any one of a plurality of discretized energy states through the action of quantum mechanics. Due to the presence of the Josephson element in this resonant circuit, the plurality of energy states are arranged at non-uniform intervals. Therefore, if an electromagnetic wave having an energy corresponding to the minimum interval is made to incident on the Josephson element, the superconducting qubit can selectively take only two energy states, the ground state and one excited state (in addition, in a circuit in which the Josephson element is removed from the superconducting qubit, the plurality of energy states are arranged at uniform intervals, and if an electromagnetic wave having an energy corresponding to this interval is made to incident, three or more energy states are taken). In this way, a plurality of superconducting qubits each taking only two energy states are combined, or one or more superconducting qubits are combined with other circuits to form a quantum gate device.

[0006] [Prior Art Documents]

[0007] [Non-Patent Literature]

[0008] Non-Patent Literature 1: J. Koch et al., "Charge insensitive qubit design derived from the Cooper pair box", Physical Review A, (USA), published by the American Physical Society, October 12, 2007, Vol. 76, Article No. 042319. Summary of the Invention

[0009] [Problems to be Solved by the Invention]

[0010] In a quantum gate device, since the energy state generated by quantum mechanical action can be maintained only for a limited time (the average value of this time is called "coherence time"), if the operation takes time, the probability of misoperation increases. In a quantum computer, although a function for correcting such errors is provided in the quantum gate device, a lower error generation rate, that is, a shorter operation time results in fewer correction processes and thus higher performance. Of course, a shorter operation time itself also contributes to improving the performance of the quantum computer. For this reason, in a quantum gate device, it is desirable to shorten the time for transitioning from one energy state to another energy state.

[0011] The problem to be solved by the present invention is to provide a quantum gate device that can transition from one energy state to another energy state at high speed between two energy states.

[0012] [Technical Means for Solving the Problems]

[0013] The quantum gate device of the present invention completed to solve the above problems includes:

[0014] a) A first superconducting circuit, which includes:

[0015] a-1) One first Josephson element as a Josephson element;

[0016] a-2) A second Josephson element group, which is formed by connecting n Josephson elements in series using wiring made of a superconductor, and each of the n Josephson elements has a Josephson energy greater than n times the Josephson energy of the above first Josephson element;

[0017] a-3) A first capacitor; and

[0018] a-4) A first wiring, which forms a local superconducting circuit by connecting the first Josephson element and the second Josephson element group in a loop, and connects the local superconducting circuit in parallel with the first capacitor, and is composed of a superconductor; and the first superconducting circuit resonates at a first resonance frequency;

[0019] b) A second superconducting circuit, which includes at least one Josephson element, a second capacitor, and a second wiring composed of a superconductor, and resonates at a second resonance frequency;

[0020] c) A connection part, which has a connection part capacitor and a third wiring composed of a superconductor connected to both poles of the connection part capacitor respectively, and connects the first superconducting circuit and the second superconducting circuit;

[0021] d) A magnetic field applying part, which applies a static magnetic field to the inside of the local superconducting circuit; and

[0022] e) An electromagnetic wave irradiating part, which irradiates the first superconducting circuit with an electromagnetic wave having a difference frequency, which is the difference between the first resonance frequency and the second resonance frequency.

[0023] The quantum gate device of the present invention has a circuit formed by connecting two superconducting circuits, namely a first superconducting circuit and a second superconducting circuit, using a connection part. The two superconducting circuits respectively function as qubits having 1-bit information as described below. As in the present invention, a quantum gate device having two qubits is generally referred to as a "2-qubit gate device".

[0024] The first superconducting circuit has a local superconducting circuit formed by connecting the first Josephson element and the second Josephson element group in a loop through a first wiring, and a first capacitor. By applying a static magnetic field from the magnetic field applying part to the local superconducting circuit (from a state where no magnetic field is applied), a current is generated in the first superconducting circuit to maintain the state of the static magnetic field, and the first superconducting circuit functions as a resonance circuit. At this time, the inductive energy U of the local superconducting circuit is set to be equal for all the second Josephson element groups, and is expressed as

[0025] U(φ) = -E J1 cos(φ) - nE J2 cos((Φ ex -φ) / n)…(1)

[0026] Here, E J1 represents the Josephson energy of the first Josephson element, and E J2 represents the Josephson energy of each second Josephson element. Φ ex is the magnetic flux Φ introduced into the local superconducting circuit by the static magnetic field applied by the magnetic field applying part and a constant Φ0 called the magnetic flux quantum, with Φex is defined by = 2πΦ / Φ0. The flux quantum Φ0 is expressed as Φ0 = h / 2e in terms of the Planck constant h and the elementary charge e. φ represents the phase difference between the two superconductors of the first Josephson element. Equation (1) is approximated by a Taylor expansion around the vicinity where U(φ) takes the minimum value as

[0027] U(φ) ~ a2φ 2 + a3φ 3 + a4φ 4 …(2)

[0028] a2, a3, and a4 are constants respectively. Thus, the inductance energy in the first superconducting circuit, in addition to having terms of φ 2 and terms of φ 4 also has terms of φ 3 . In contrast, since the above superconducting qubit has only one Josephson element, its inductance energy becomes a Taylor expansion of a cosine function, approximated by the sum of terms of φ 2 and terms of φ 4 and does not have terms of φ 3 .

[0029] The quantum gate device of the present invention functions as follows due to the inductance energy U of the first superconducting circuit having terms of φ 3 . Generally speaking, if only two qubits are connected, their resonance frequencies are different, so they do not combine (but only operate individually) and do not function as a two-qubit gate device. In contrast, due to the inductance energy U of the first superconducting circuit of the quantum gate device of the present invention having terms of φ 3 , the resonance frequency can be modulated by applying an oscillating electric field from the outside of the first superconducting circuit. The electromagnetic wave irradiation unit irradiates the first superconducting circuit with an electromagnetic wave having the difference between the resonance frequency of the first superconducting circuit (the first resonance frequency, denoted as ω1) in the state without the oscillating electric field and the resonance frequency of the second superconducting circuit (the second resonance frequency, denoted as ω2), that is, the difference frequency |ω2 - ω1| as such an oscillating electric field. Thereby, the resonance frequency is modulated, and the first superconducting circuit and the second superconducting circuit resonate and interact with each other, so that the quantum gate device of the present invention functions as a two-qubit gate device.

[0030] In order to express the inductance energy of the first superconducting circuit by a Taylor expansion as in Equation (2), when U(φ) in Equation (1) is arbitrary Φ exIn this case, it must have a unique minimum value within -nπ < φ < nπ. For this purpose, each of the n Josephson elements constituting the second Josephson element group (hereinafter, each Josephson element is collectively referred to as "each second Josephson element") must have a Josephson energy greater than n times the Josephson energy of the first Josephson element (that is, the same number as the number of Josephson elements in the second Josephson element group). Here, the Josephson energy refers to the energy of the tunnel junction between the two superconductors of the Josephson element.

[0031] In the first superconducting circuit, the energy is discretized by the action of quantum mechanics, and the energy intervals between adjacent energy states are different. Thus, two energy states with the smallest energy interval can be selected and transitions can occur between the two energy states. Hereinafter, the lower of the two energy states in the first superconducting circuit is expressed as the ground state "g1", and the higher one is expressed as the first excited state "e1". In the first superconducting circuit, 1-bit information is represented by the two energy states.

[0032] The second superconducting circuit can use, for example, the above-mentioned superconducting qubit or the following charge qubit, flux qubit, fluxonium, etc. Among the examples, the superconducting qubit in the above example is preferably in terms of having a longer coherence time compared to other examples. The second superconducting circuit includes at least 1 Josephson element, a second capacitor, and a second wiring made of a superconductor. Due to the non-linearity of the Josephson element, multiple energy states are arranged at non-equidistant intervals, and two adjacent energy states with the smallest energy interval among the multiple energy states can be selectively taken (the lower one is expressed as the ground state "g2", and the higher one is expressed as the first excited state "e2"). In the second superconducting circuit, 1-bit information is represented by the energy states.

[0033] As an example of the operation of the quantum gate in the quantum gate device of the present invention, the operation called the "SWAP gate" is described. When the combination of the energy states of the first superconducting circuit and the second superconducting circuit is either the state "g1e2" or "e1g2", the electromagnetic wave irradiation unit irradiates the first superconducting circuit with an electromagnetic wave having a difference frequency |ω2 - ω1|. Thus, during the irradiation of the electromagnetic wave with this difference frequency, the combination of the energy states of the quantum gate device alternately takes the states "g1e2" and "e1g2" at a fixed period. Therefore, by irradiating the electromagnetic wave for only a half-integer multiple of this period, the state can be changed from one of "g1e2" and "e1g2" to the other. This operation is called the "SWAP gate" because it is equivalent to swapping the ground state (g1 or g2) and the first excited state (e1 or e2) between the first superconducting circuit and the second superconducting circuit.

[0034] In addition, the quantum gate device of the present invention exchanges the ground state (g1 or g2) and the first excited state (e1 or e2) between the first superconducting circuit and the second superconducting circuit by changing the phase of the electromagnetic wave irradiated to the first superconducting circuit and / or the second superconducting circuit by 90°, and inverts the phase in any one of the first superconducting circuit and the second superconducting circuit, that is, changes "g1e2" to "ie1g2" (i is the imaginary unit) and "e1g2" to "ig1e2" respectively. This operation of the quantum gate is generally referred to as an "iSWAP gate".

[0035] Furthermore, when the combination of the energy states of the quantum gate device is "e1e2", if an electromagnetic wave having a difference frequency |ω2 - α2 - ω1| is irradiated to the first superconducting circuit and / or the second superconducting circuit, the two states of "e1e2" and "g1f2" are alternately taken. Here, "f2" refers to the second excited state in which the second superconducting circuit takes an energy magnitude second only to the first excited state, and ω2 - α2 refers to the resonance frequency of the resonance between the "f2" state and the "e2" state (in this case, this resonance frequency is set as the above-mentioned second resonance frequency). If "e1e2" is changed to "g1f2" once by this operation and then the irradiation is continued to return to the original state, the state changes by 180° in phase to "-e1e2". This operation is called a "CZ gate (CZgate)".

[0036] As described above, the quantum gate device of the present invention can function as any one or two or all (three) of the three quantum gates of a swap gate, an iSWAP gate, and a CZ gate.

[0037] In an existing two-qubit gate device (for example, combining two superconducting qubits), it takes more than 100 nanoseconds to transition from one energy state to another. In contrast, as described below, the quantum gate device of the present invention was fabricated and experimented, and as a result, it was possible to transition between two energy states in about 16 nanoseconds, which is shorter than the prior art. Thus, as a reason for shortening the time for transitioning between two energy states, it can be cited that the inductive energy U(φ) of the local superconducting circuit has a non-linear and low-degree term with respect to φ 3 so that the interaction between the first superconducting circuit and the second superconducting circuit becomes stronger.

[0038] In addition, the intensity of the static magnetic field described above is not particularly limited as long as it is less than the magnetic field at which the superconductivity of the superconductors of each Josephson element (the first Josephson element and each second Josephson element) and the superconductors of the first wiring of the partial superconducting circuit is destroyed. In fact, it is preferably an intensity of magnetic flux that is 5 times or less of the magnetic flux quantum generated in the partial superconducting circuit, and more preferably an intensity of magnetic flux that is 1 time or less of the magnetic flux quantum generated in the partial superconducting circuit.

[0039] The superconductors of the first Josephson element, each second Josephson element, the first wiring, the second wiring, and the third wiring may be of the same type or different types. In addition, the insulators for the junction of the first Josephson element and the junctions of the second Josephson elements may be of the same type or different types.

[0040] When the first Josephson element is formed by sandwiching a junction having a first thin film made of an insulator between two superconductors, and each second Josephson element is formed by sandwiching a junction having a second thin film made of an insulator of the same type as the first thin film between two superconductors, the value of the tunnel resistance in the first Josephson element is preferably larger than n times the value of the tunnel resistance in each second Josephson element. Thereby, the Josephson energy of each second Josephson element can be made larger than n times the Josephson energy of the first Josephson element. In addition, the tunnel resistance (R) is obtained by dividing the value of the voltage (V) applied to the junction by the value of the current (I) flowing through it (R = V / I).

[0041] In addition, the above-described electromagnetic wave irradiation unit may irradiate the second superconducting circuit with electromagnetic waves in addition to irradiating the first superconducting circuit with electromagnetic waves.

[0042] Generally, in a quantum gate device that uses two superconducting circuits to perform the operation of a quantum gate, after the operation of the quantum gate is executed, an undesired interaction called residual interaction is generated between the electrons of one superconducting circuit and the electrons of the other superconducting circuit, and it may not be possible to maintain the state formed during the operation of the quantum gate. Therefore, preferably, the quantum gate device of the present invention further includes:

[0043] A residual interaction eliminating electromagnetic wave irradiation unit that irradiates the first superconducting circuit with an electromagnetic wave that inverts the phase of the qubit, that is, a residual interaction eliminating electromagnetic wave; and

[0044] An irradiation adjustment unit that performs irradiation with the electromagnetic wave from the above-described electromagnetic wave irradiation unit, and then performs irradiation with the residual interaction elimination electromagnetic wave from the above-described residual interaction elimination electromagnetic wave irradiation unit twice at a set time interval, thereby adjusting the irradiation timings of the electromagnetic wave irradiation unit and the residual interaction elimination electromagnetic wave irradiation unit in this manner.

[0045] According to the quantum gate device of the present invention including the above-described residual interaction elimination electromagnetic wave irradiation unit and the irradiation adjustment unit, after performing the operation of the quantum gate by irradiating the first superconducting circuit with the electromagnetic wave having a difference frequency using the electromagnetic wave irradiation unit, the residual interaction elimination electromagnetic wave that inverts the phase of the qubit is irradiated only twice to any one of the first superconducting circuit and the above-described second superconducting circuit. By the first irradiation with the residual interaction elimination electromagnetic wave, only the phase of the qubit in the first superconducting circuit and the second superconducting circuit that is irradiated with the residual interaction elimination electromagnetic wave is inverted, and thereby, the residual interaction between the electrons in the first superconducting circuit and the electrons in the second superconducting circuit is cut off. Thereafter, by the second irradiation with the residual interaction elimination electromagnetic wave, the phase of the qubit in the superconducting circuit irradiated with the residual interaction elimination electromagnetic wave is further inverted and restored to the original state formed in the operation of the quantum gate. By the above operations, the influence of the residual interaction can be excluded and the state formed in the operation of the quantum gate can be maintained.

[0046] As the residual interaction elimination electromagnetic wave, for example, a pulsed electromagnetic wave called a "π pulse" or "180° pulse" that has been used in the spin echo method for performing magnetic resonance measurement can be used. In this case, by appropriately setting the pulse width (time) according to the frequency and amplitude intensity of the pulsed electromagnetic wave, the phase of the qubit can be inverted.

[0047] The above-described set time interval is not particularly limited and can be optimized by performing preliminary experiments as described below. In addition, the time interval between the irradiation with the electromagnetic wave by the electromagnetic wave irradiation unit and the first irradiation with the residual interaction elimination electromagnetic wave is not particularly limited.

[0048] The above-described residual interaction elimination electromagnetic wave irradiation unit can be configured by the same device as the above-described electromagnetic wave irradiation unit. Since the electromagnetic wave irradiation unit and the residual interaction elimination electromagnetic wave irradiation unit perform electromagnetic wave irradiation at different timings, when irradiating the first superconducting circuit with the residual interaction elimination electromagnetic wave, the electromagnetic wave irradiation unit and the residual interaction elimination electromagnetic wave irradiation unit can use the same device. Thereby, the configuration of the quantum gate device can be simplified. On the other hand, the electromagnetic wave irradiation unit and the residual interaction elimination electromagnetic wave irradiation unit can also be configured by different devices.

[0049] In addition, in addition to irradiating the first superconducting circuit with the residual interaction eliminating electromagnetic wave, the above-mentioned residual interaction eliminating electromagnetic wave irradiating unit may also irradiate the second superconducting circuit with the residual interaction eliminating electromagnetic wave.

[0050] [Effects of the Invention]

[0051] According to the quantum gate device of the present invention, it is possible to rapidly transition from one energy state to another energy state between two energy states. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 is a schematic configuration diagram showing a first embodiment of the quantum gate device of the present invention.

[0053] Figure 2A is a diagram showing the first Josephson element included in the quantum gate device of the first embodiment.

[0054] Figure 2B is a diagram showing the second Josephson element included in the quantum gate device of the first embodiment.

[0055] Figure 3 is a micrograph showing an example of manufacturing the quantum gate device of the first embodiment, and a partial enlarged view thereof.

[0056] Figure 4A is a diagram for explaining the transition of energy states in the quantum gate device of the first embodiment, and is a diagram showing a swap gate and an i-swap gate.

[0057] Figure 4B is a diagram for explaining the transition of energy states in the quantum gate device of the first embodiment, and is a diagram showing a CZ gate.

[0058] Figure 5 is a graph showing the results of an experiment for measuring the energy states of the first superconducting circuit and the second superconducting circuit when an electromagnetic wave having a set frequency is irradiated for only a set time in the quantum gate device of the first embodiment.

[0059] Figure 6 is a schematic configuration diagram showing a second embodiment of the quantum gate device of the present invention.

[0060] Figure 7 is a sequence diagram showing the operation of the quantum gate device of the second embodiment.

[0061] Figure 8 is a sequence diagram showing the operation of a preliminary experiment in the quantum gate device of the second embodiment.

[0062] Figure 9A ​​​​​​​​​​​is a graph showing data when τ is fixed at a certain value in an example of a preliminary experiment for determining the time interval τ of two residual interaction cancellation electromagnetic waves in the quantum gate device of the second embodiment.

[0063] Figure 9B is a graph showing an example of the final result of a preliminary experiment for determining τ.

[0064] Figure 10 is a schematic configuration diagram showing an example of an integrated quantum circuit formed by integrating a plurality of quantum gate devices of the first embodiment.

[0065] Figure 11A is the second superconducting circuit used in the quantum gate device of the modified embodiment, and is a diagram showing a charge qubit.

[0066] Figure 11B is the second superconducting circuit used in the quantum gate device of the modified embodiment, and is a diagram showing a flux qubit.

[0067] Figure 11C is the second superconducting circuit used in the quantum gate device of the modified embodiment, and is a diagram showing FLUXONIUM. Detailed implementation manners

[0068] Use Figures 1 to 1 1 to describe the embodiments of the quantum gate device of the present invention.

[0069] (1) Configuration of the quantum gate device of the first embodiment of the present invention

[0070] Figure 1 is a schematic diagram showing the configuration of the quantum gate device 10 of the first embodiment of the present invention. The quantum gate device 10 includes a first superconducting circuit 11, a second superconducting circuit 12, a connection part 13, a magnetic field application part 14, a first electromagnetic wave irradiation part 151, and a second electromagnetic wave irradiation part 152. In addition, the first electromagnetic wave irradiation part 151 in the first embodiment corresponds to the "electromagnetic wave irradiation part" in the present invention. The second electromagnetic wave irradiation part 152 is provided to confirm the operation of the quantum gate device 10.

[0071] The first superconducting circuit 11 includes a first Josephson element 111, a second Josephson element group 112, a first capacitor 114, and a first wiring 115.

[0072] The first Josephson element 111 is composed of a junction portion in which two superconductors 111S1 and 111S2 sandwich a first thin film 111J made of an insulator ( Figure 2A ​​​​​)。The second Josephson element group 112 is formed by connecting n (n is an integer of 2 or more) second Josephson elements 1121, 1122,... 112n in series using wiring 112L made of a superconductor. In addition, Figure 1 is shown by taking the case of n = 2 as an example, but n can also be an integer of 3 or more. Each second Josephson element 112k (k is each integer in the range of 1 to n) is composed of two superconductors 112S1, 112S2 sandwiching a second thin film 112J made of an insulator of the same type as the first thin film 111J ( Figure 2B ). Here, the thickness and area of the first thin film 111J and the second thin film 112J are set such that the tunnel resistance value in the first Josephson element 111 is n times larger than the tunnel resistance value in each second Josephson element 112k. Thus, the Josephson energy of each second Josephson element 112k is n times larger than the Josephson energy of the first Josephson element 111.

[0073] The first Josephson element 111 and the second Josephson element group 112 are connected in a ring shape through the first wiring 115, thereby forming a local superconducting circuit 113. Furthermore, the local superconducting circuit 113 and the first capacitor 114 are connected in parallel through the first wiring 115.

[0074] With the above configuration, the first superconducting circuit 11 functions as a resonant circuit. Let the resonant frequency of this first superconducting circuit 11 be the "first resonant frequency ω1". The first resonant frequency ω1 is, for example, around 2π×4 GHz to 2π×8 GHz.

[0075] The second superconducting circuit 12 uses a superconducting qubit in the first embodiment. This second superconducting circuit 12 is formed by connecting one Josephson element 121 and one second capacitor 124 in a ring shape using second wiring 125 made of a superconductor. The second superconducting circuit 12 is a resonant circuit, and its resonant frequency is set as the "second resonant frequency". The second superconducting circuit 12 can generate two resonances, namely, the resonance between the ground state g2 and the first excited state e2, and the resonance between the first excited state e2 and the second excited state f2. Let the resonant frequency in the former resonance be ω2, and the resonant frequency in the latter resonance be ω2 - α2, both of which are referred to as the "second resonant frequency". The second resonant frequency ω2 is, for example, around 2π×4 GHz to 2π×8 GHz.

[0076] The connection part 13 connects the first superconducting circuit 11 and the second superconducting circuit 12, and includes a connection part capacitor 134 and a third wiring 135 made of a superconductor.

[0077] The magnetic field application unit 14 applies a static magnetic field to the local superconducting circuit 113. The magnitude of the applied static magnetic field is not particularly limited as long as it is less than the magnetic field that destroys the superconducting states of the superconductors 111S1, 111S2, 112S1, 112S2 of the first Josephson element 111 and each second Josephson element 112k and the superconductor of the first wiring 115, but a smaller value is preferred. For example, it is preferably a magnetic flux intensity of 5 times or less of the magnetic flux quantum generated in the local superconducting circuit 113, and more preferably a magnetic flux intensity of 1 time or less of the magnetic flux quantum generated in the local superconducting circuit 113. The magnitude of the preferred static magnetic field is, for example, about 10 μT.

[0078] The first electromagnetic wave irradiation unit 151 selects an electromagnetic wave having either of the two frequencies |ω2−ω1| and |ω2−α2−ω1| and irradiates it to the first superconducting circuit 11. In Figure 1 , only a capacitor is depicted as the first electromagnetic wave irradiation unit 151, but this capacitor is connected to a microwave generator (not shown), and the microwave supplied from the microwave generator is irradiated (applied) to the first superconducting circuit 11 as the above-mentioned electromagnetic wave through the capacitor.

[0079] In the first embodiment, in order to evaluate the quantum gate device 10, the first electromagnetic wave irradiation unit 151 also has a function of irradiating the first superconducting circuit 11 with an electromagnetic wave having a first resonance frequency ω1. In addition, in order to evaluate the quantum gate device 10, the second electromagnetic wave irradiation unit 152 has a function of irradiating the second superconducting circuit 12 with an electromagnetic wave having a second resonance frequency ω2. These functions are not essential functions in the quantum gate device of the present invention.

[0080] The quantum gate device 10 of the first embodiment further includes a first readout cavity 171 connected to the first superconducting circuit 11 and a second readout cavity 172 connected to the second superconducting circuit 12. Capacitors 161 and 162 are respectively provided between the first superconducting circuit 11 and the first readout cavity 171 and between the second superconducting circuit 12 and the second readout cavity 172.

[0081] In addition, the quantum gate device 10 has a cooling device (not shown) that cools the superconductors of the first superconducting circuit 11, the second superconducting circuit 12, and the connection portion 13 to below the superconducting transition temperature.

[0082] Figure 3 An example of actually fabricating the quantum gate device 10 of the first embodiment is shown using a micrograph. However, the magnetic field application unit 14 among the constituent elements of the quantum gate device 10 is not shown in Figure 3 and is disposed at a position away from the local superconducting circuit 113 in a direction perpendicular to the plane of the figure. In Figure 3In the figure, an upper diagram showing the overall quantum gate device 10 is shown, and a lower diagram showing an enlarged view of a portion surrounded by a dashed line in the upper diagram is shown. In Figure 3 In, the portion surrounded by a line of a gray darker than the light gray of the background is the position where the surface of the substrate made of silicon appears. The light gray portion is made of niobium, and the white portion is made of aluminum. Niobium and aluminum are superconductors. The thin film of the insulating system that each Josephson element has but does not appear in this micrograph is alumina obtained by oxidizing aluminum. In addition, in Figure 3 In, the symbols of the first capacitor 114 and the second capacitor 124 are not marked, but the first capacitor 114 is formed between the first wiring 115 and the ground, and the second capacitor 124 is formed between the second wiring 125 and the ground.

[0083] In addition, in the present invention, the superconductor and the insulator are not limited to the above examples, and any material can be used.

[0084] (2) Operation of the quantum gate device of the first embodiment

[0085] The operation of the quantum gate device 10 of the first embodiment will be described. First, the quantum gate device 10 is cooled to a temperature below the above-mentioned superconducting transition temperature by a cooling device, and a static magnetic field is applied to the local superconducting circuit 113 from the magnetic field application unit 14.

[0086] In the swap gate, first, when the energy state of the first superconducting circuit 11 is e1 and the energy state of the second superconducting circuit 12 is g2 (this state is denoted as "e1g2"), or when the energy state of the first superconducting circuit 11 is g1 and the energy state of the second superconducting circuit 12 is e2 (g1e2), an electromagnetic wave with a difference frequency |ω2 - ω1| is irradiated to the first superconducting circuit 11 from the first electromagnetic wave irradiation unit 151. As a result, the resonance frequency is modulated, and the first superconducting circuit 11 interacts with the second superconducting circuit 12, and the combination of the energy states of the two superconducting circuits alternately takes the two states of e1g2 and g1e2 at a set period. Therefore, by irradiating the electromagnetic wave with the difference frequency |ω2 - ω1| for only a time that is a half-integer multiple of this period, it is possible to change from one state of e1g2 and g1e2 to the other state. This operation corresponds to a swap gate.

[0087] Figure 5 is shown in Figure 3In the quantum gate device 10 shown, the experiment results of measuring the energy states of the first superconducting circuit 11 and the second superconducting circuit 12 while changing the irradiation time and irradiating the electromagnetic wave with the difference frequency |ω2−ω1| from the first electromagnetic wave irradiation unit 151 for only the set irradiation time are presented. In addition, in this experiment, first, in order to form the state of e1g2 or g1e2, an electromagnetic wave with the first resonance frequency ω1 is irradiated from the first electromagnetic wave irradiation unit 151 to the first superconducting circuit 11, or an electromagnetic wave with the second resonance frequency ω2 is irradiated from the second electromagnetic wave irradiation unit 152 to the second superconducting circuit 12, and then, the electromagnetic wave with the difference frequency |ω2−ω1| is irradiated. In Figure 5 , it is shown that in the range where the value of the vertical axis is negative, it is in the ground state (g1 or g2), and in the range where the value of the vertical axis is positive, it is in the first excited state (e1 or e2). According to the results of this experiment, it is shown that if one of the first superconducting circuit 11 and the second superconducting circuit 12 is in the ground state, the other is in the first excited state. That is, in this quantum gate device 10, during the irradiation of the above electromagnetic wave, the energy state changes in such a way that the two states of the e1g2 state and the g1e2 state are alternately taken ( Figure 4A ). If the change period of this energy state is set as T, the time T / 2 required to change from one state to the other state is about 16 nanoseconds according to the Figure 5 shown experiment results. Generally, in existing quantum gate devices using superconducting qubits or the like, the change between two states requires a time of 100 nanoseconds or more. In contrast, according to the first embodiment, it is possible to transition between two energy states in a shorter time (at high speed). In addition, in Figure 4A and the following Figure 4B , it is described that the energy state of g1e2 is higher than that of e1g2, but depending on the configuration of the first superconducting circuit 11 and the second superconducting circuit 12, the energy state of e1g2 can be higher than that of g1e2.

[0088] In addition, the quantum gate device 10 of the first embodiment can exchange the ground state and the first excited state between the first superconducting circuit 11 and the second superconducting circuit 12 by changing the phase of the electromagnetic wave irradiated to the first superconducting circuit 11 and / or the second superconducting circuit 12 by 90°, and can invert the phase in either the first superconducting circuit 11 or the second superconducting circuit 12, that is, operate as an i-exchange gate that changes g1e2 to ie1g2 or changes e1g2 to ig1e2 respectively.

[0089] On the other hand, the quantum gate device 10 of the first embodiment can also operate as a CZ gate. In the CZ gate, when the combination of the energy states of the two superconducting circuits is in the e1e2 state, an electromagnetic wave with the difference frequency |ω2−α2−ω1| is irradiated from the first electromagnetic wave irradiation unit 151 to the first superconducting circuit 11. As a result, the e1e2 state and the g1f2 state are alternately taken (Figure 4B )。 Here, f2 means that the second superconducting circuit 12 is in the second excited state. When e1e2 is changed to g1f2 in this operation and then directly restored to e1e2, a phase is obtained and the state changes to -e1e2. This operation is equivalent to a CZ gate.

[0090] As described above, according to the quantum gate device 10 of the first embodiment, operations of three quantum gates, namely, a swap gate, an i-swap gate, and a CZ gate, can be performed.

[0091] In Figure 3 the quantum gate device 10 shown, when operating as a swap gate, an i-swap gate, and a CZ gate respectively, an experiment is conducted to confirm the accuracy of the gate operation using a method called a random horizontal point test. As a result, the probability is 99.3% for the swap gate, 99.2% for the i-swap gate, and 99.1% for the CZ gate that the quantum gate operation is performed correctly. That is, the error generation rate in the operation of the quantum gate is suppressed to less than 1%. If the error generation rate can be suppressed this low, when the quantum gate device 10 is installed in a quantum computer, the error correction mechanism can operate without problems and the correction process becomes less, thereby improving the performance of the quantum computer.

[0092] Furthermore, in the description of the quantum gate device 10 of the first embodiment described above, after operating as a swap gate, an i-swap gate, or a CZ gate, the residual interaction between the electrons in the first superconducting circuit 11 and the electrons in the second superconducting circuit 12 is regarded as small enough to be ignored.

[0093] (3) Quantum gate device of the second embodiment of the present invention

[0094] Figure 6 is a schematic diagram showing the configuration of a quantum gate device 20 of the second embodiment of the present invention. This quantum gate device 20 replaces the first electromagnetic wave irradiation unit (electromagnetic wave irradiation unit) 151 in the configuration of the quantum gate device 10 of the first embodiment with the following first electromagnetic wave irradiation unit (electromagnetic wave irradiation unit) and residual interaction elimination electromagnetic wave irradiation unit 251, and is additionally provided with an irradiation adjustment unit 21. Therefore, the configuration other than the first electromagnetic wave irradiation unit and residual interaction elimination electromagnetic wave irradiation unit 251 and the irradiation adjustment unit 21 is the same as the configuration of the quantum gate device 10 of the first embodiment, and thus detailed description is omitted.

[0095] The first electromagnetic wave irradiation unit and residual interaction elimination electromagnetic wave irradiation unit 251 includes a microwave generator 2511 and a capacitor 2512. The microwave generator 2511 generates electromagnetic waves (microwaves) having any one of three frequencies of |ω2 - ω1|, |ω2 - α2 - ω1|, and ω1. The first electromagnetic wave irradiation unit and residual interaction elimination electromagnetic wave irradiation unit 251 irradiates (applies) the electromagnetic waves generated by the microwave generator 2511 to the first superconducting circuit 11 through the capacitor 2512. Among the above three frequencies, |ω2 - ω1| and |ω2 - α2 - ω1| are used when the first electromagnetic wave irradiation unit and residual interaction elimination electromagnetic wave irradiation unit 251 functions as the above electromagnetic wave irradiation unit, and ω1 is used when the first electromagnetic wave irradiation unit and residual interaction elimination electromagnetic wave irradiation unit 251 functions as the above residual interaction elimination electromagnetic wave irradiation unit.

[0096] The irradiation adjustment unit 21 adjusts the irradiation timing of the electromagnetic waves of the first electromagnetic wave irradiation unit and residual interaction elimination electromagnetic wave irradiation unit 251. By the operation of the irradiation adjustment unit 21, when the first electromagnetic wave irradiation unit and residual interaction elimination electromagnetic wave irradiation unit 251 functions as the electromagnetic wave irradiation unit, it outputs electromagnetic waves having a frequency of |ω2 - ω1| or |ω2 - α2 - ω1|, and when it functions as the residual interaction elimination electromagnetic wave irradiation unit, it outputs the residual interaction elimination electromagnetic waves having a frequency of ω1 twice at a set time interval. As the residual interaction elimination electromagnetic wave, for example, pulsed electromagnetic waves (π pulses, 180° pulses) having a pulse width equal to the time of the phase inversion of the qubit in the first superconducting circuit 11 can be used.

[0097] Use Figure 7 , the operation of the quantum gate device 20 of the second embodiment will be described. Figure 7 It represents the operations generated in the first superconducting circuit 11 and the second superconducting circuit 12 respectively at each time as time progresses from left to right. In the quantum gate device 20, as described below, after the operation of the quantum gate, an operation for cutting off the residual interaction is performed.

[0098] The operations of the quantum gates in the quantum gate device 20 are the same as those in the quantum gate device 10 of the first embodiment for any of the swap gate, i-swap gate, and CZ gate. That is, the quantum gate device 20 is cooled below the above-mentioned superconducting transition temperature by a cooling device, and after a static magnetic field is applied to the local superconducting circuit 113 from the magnetic field application unit 14, electromagnetic waves with a set difference frequency are irradiated from the first electromagnetic wave irradiation unit and residual interaction elimination electromagnetic wave irradiation unit 251 to the first superconducting circuit 11. Here, the difference frequency is |ω2 - ω1| for the swap gate and i-swap gate, and |ω2 - α2 - ω1| for the CZ gate. Thus, in the first superconducting circuit 11 and the second superconducting circuit 12, the operations of the quantum gates are generated in the same manner as in the first embodiment ( Figure 7 the symbol 31 in).

[0099] After a first set time t0 has elapsed since the operation of the quantum gate, the first residual interaction elimination electromagnetic wave 32 with a frequency ω1 is irradiated from the first electromagnetic wave irradiation unit and residual interaction elimination electromagnetic wave irradiation unit 251 to the first superconducting circuit 11. As a result, the phase of the qubit in the first superconducting circuit 11 is inverted, and the residual interaction with the second superconducting circuit 12 is cut off. Subsequently, after a second set time τ has elapsed since the irradiation of the first residual interaction elimination electromagnetic wave 32, the second residual interaction elimination electromagnetic wave 33 with a frequency ω1 is irradiated to the first superconducting circuit 11. As a result, the phase of the qubit in the first superconducting circuit 11 is restored to its original state. Through the above operations, the residual interaction can be cut off after the operation of the quantum gate.

[0100] Here, the length of the first set time t0 is not particularly limited. The length of the second set time τ is also not particularly limited, but it is preferably determined by performing the preliminary experiment described below.

[0101] Use Figure 8 and FIG. 9 to illustrate an example of the preliminary experiment for determining the second set time τ. In the preliminary experiment of this example, first, electromagnetic waves 34 with a frequency ω2 that change the phase of the qubit in the second superconducting circuit 12 by 90° are irradiated from the second electromagnetic wave irradiation unit 152 to the second superconducting circuit 12 (see Figure 8)。As such electromagnetic wave 34, generally, a pulse electromagnetic wave known as a "π / 2 pulse" or "90° pulse" that has been used when performing the spin echo method in magnetic resonance measurements can be applied. After irradiating the electromagnetic wave 34, the operation 31 of the quantum gate is performed. In addition, the time length between the irradiation of the electromagnetic wave 34 and the operation 31 of the quantum gate is not limited. Subsequently, after the first set time t0 (here set as "t / 2") has elapsed since the operation 31 of the quantum gate, the irradiation of the first residual interaction canceling electromagnetic wave 32 is performed, and thereafter, the irradiation of the second residual interaction canceling electromagnetic wave 33 is not performed. As an alternative, after a time (τ + t / 2) has elapsed since the irradiation of the first residual interaction canceling electromagnetic wave 32, an electromagnetic wave (e.g., π / 2 pulse) 35 having a frequency of ω2 and causing a 90° phase change in the qubit of the second superconducting circuit 12 is irradiated from the second electromagnetic wave irradiation unit 152 to the second superconducting circuit 12. After irradiating the electromagnetic wave 35, the measurement 36 of the output signal of the capacitor 162 is performed. The above measurement is set as the first preliminary experiment. This first preliminary experiment is performed for various t and τ.

[0102] A second preliminary experiment is performed together with the first preliminary experiment. In the second preliminary experiment, first, an electromagnetic wave (e.g., π pulse) 37 that inverts the phase of the qubit of the first superconducting circuit 11 is irradiated from the first electromagnetic wave irradiation unit and residual interaction canceling electromagnetic wave irradiation unit 251 to the first superconducting circuit 11 (shown by a dashed line in Figure 8 ). Thereafter, by the same method as the first preliminary experiment, the operations from the irradiation of the electromagnetic wave 34 to the measurement 36 of the output signal of the capacitor 162 are performed. In addition, the time length between the irradiation of the electromagnetic wave 37 and the irradiation of the electromagnetic wave 34 is not limited. This second preliminary experiment is performed for various t and τ.

[0103] Figure 9A shows the results obtained by performing the first and second preliminary experiments for various t with τ fixed at a certain value. In the said preliminary experiment, the operation of the gate is performed for 34 nanoseconds. In Figure 9A , the data recorded as "without irradiation of the electromagnetic wave 37" are the data of the first preliminary experiment, and the data recorded as "with irradiation of the electromagnetic wave 37" are the data of the second preliminary experiment. Any of the data changes in such a way that the output value becomes a sine wave as t changes, but a phase difference θR is generated between the two data.

[0104] If the phase difference θR is obtained for various τ respectively, then as shown in Figure 9B , a function of θR with τ as a variable can be obtained. In Figure 9B , the left side of the two solid lines with a negative slope shows the function of θR when performing the operation of the swap gate, and the right side shows the function of θR when performing the operation of the CZ gate. In addition, in Figure 9BThe dashed line with a negative slope in [the figure] is a function of θR in the case where the operation of the quantum gate is not performed. Among the three functions, the values of θR in the case where the operation of the SWAP gate is performed and in the case where the operation of the gate is not performed are shown on the left vertical axis, and the value of θR in the case where the operation of the CZ gate is performed is shown on the right vertical axis. In the function, the value of time τ when θR = 0 or 180° (π) is the optimal value τ0 of τ.

[0105] In the quantum gate device 20 of the second embodiment, a random level point test was conducted. As a result, the probability was (97.1 ± 0.1)% in the SWAP gate, (97.2 ± 0.1)% in the iSWAP gate, and (96.4 ± 0.1)% in the CZ gate, and the operation of the quantum gate was executed without error. These values are lower (higher error generation rate) than the case of the first embodiment, and it is considered that the reason is that since the operation of irradiating the electromagnetic wave for eliminating the residual interaction is performed twice after the operation of the quantum gate, time is required.

[0106] (4) Example of an integrated quantum circuit formed by integrating the quantum gate device of the present invention

[0107] Figure 10 shows an example of an integrated quantum circuit formed by integrating a plurality of the quantum gate devices 10 of the first embodiment. This integrated quantum circuit 50 is formed by connecting a plurality of quantum gate devices 10 two-dimensionally (in a matrix shape), and adjacent quantum gate devices 10 are connected to each other by a capacitor 51. Here, an example of using the quantum gate device 10 of the first embodiment is shown, but an integrated quantum circuit can also be formed by integrating a plurality of the quantum gate devices 20 of the second embodiment.

[0108] (5) Modified embodiments

[0109] The present invention is not limited to the above embodiments. For example, in the quantum gate devices 10 and 20 of the above first and second embodiments, the second superconducting circuit 12 is used as the superconducting qubit, but instead of the second superconducting circuit 12, any one of the second superconducting circuits 12A, 12B, and 12C shown in Figures 11A to 11C can be used. Figure 11A The second superconducting circuit 12A shown in [the figure] is called a charge qubit, and the Josephson element 121A and the capacitor 124A are connected in series through a second wiring 125A made of a superconducting material. Figure 11B The second superconducting circuit 12B shown in [the figure] is called a flux qubit, and a plurality of (three in the example of Figure 11B ) Josephson elements 121B are connected in series, and capacitors 124B are connected in parallel to each Josephson element 121B. Each component of the second superconducting circuit 12B is connected by a second wiring 125B made of a superconducting material. Figure 11CThe second superconducting circuit 12C shown is called FLUXONIUM, and the Josephson element 121C, the capacitor 124C, and the coil 126C are connected in parallel through the second wiring 125C made of a superconducting material.

[0110] In the quantum gate device 20 of the second embodiment, the first electromagnetic wave irradiation unit and the residual interaction elimination electromagnetic wave irradiation unit 251 are used. However, as an alternative, the first electromagnetic wave irradiation unit (for example, the same as the first electromagnetic wave irradiation unit 151 used in the quantum gate device 10 of the first embodiment) and the residual interaction elimination electromagnetic wave irradiation unit may be provided separately.

[0111] The electromagnetic wave irradiation unit (the first electromagnetic wave irradiation unit 151) and / or the residual interaction elimination electromagnetic wave irradiation unit (the first electromagnetic wave irradiation unit and the residual interaction elimination electromagnetic wave irradiation unit 251 or the residual interaction elimination electromagnetic wave irradiation unit that does not have the function of the first electromagnetic wave irradiation unit) may irradiate the electromagnetic wave only to the first superconducting circuit, or may irradiate the electromagnetic wave to the second superconducting circuit in addition to irradiating the electromagnetic wave to the first superconducting circuit.

[0112] Furthermore, an integrated quantum circuit may be formed by integrating a plurality of quantum gate devices formed by combining the configurations of the above-described embodiments and modified embodiments or by combining the configurations of the above-described embodiments and modified embodiments.

[0113] Explanation of reference numerals:

[0114] 10, 20: Quantum gate device

[0115] 11: First superconducting circuit

[0116] 111: First Josephson element

[0117] 111J: First thin film

[0118] 112: Second Josephson element group

[0119] 1121, 1122: Second Josephson element

[0120] 112J: Second thin film

[0121] 112L: Wiring

[0122] 113: Local superconducting circuit

[0123] 114: First capacitor

[0124] 115: First wiring

[0125] 12, 12A, 12B, 12C: Second superconducting circuit

[0126] 121, 121A, 121B, 121C: Josephson element

[0127] 124: Second capacitor

[0128] 124A, 124B, 124C, 161, 162, 21: Capacitor

[0129] 125, 125A, 125B, 125C: Second wiring

[0130] 126C: Coil

[0131] 13: Connection part

[0132] 134: Connection part capacitor

[0133] 135: Third wiring

[0134] 14: Magnetic field application part

[0135] 151: First electromagnetic wave irradiation part (electromagnetic wave irradiation part)

[0136] 152: Second electromagnetic wave irradiation part

[0137] 171: First readout cavity

[0138] 172: Second readout cavity

[0139] 21: Irradiation adjustment part

[0140] 251: First electromagnetic wave irradiation part (electromagnetic wave irradiation part) and residual interaction elimination electromagnetic wave irradiation part

[0141] 2511: Microwave generator

[0142] 2512: Capacitor of first electromagnetic wave irradiation part and residual interaction elimination electromagnetic wave irradiation part

[0143] 31: Operation of quantum gate

[0144] 32: First residual interaction elimination electromagnetic wave

[0145] 33: Second residual interaction elimination electromagnetic wave

[0146] 34, 35, 37: Electromagnetic waves in preliminary experiments

[0147] 36: Measurement of output signal

[0148] 50: Integrated quantum circuit

[0149] 51: Capacitor.

Claims

1. A quantum gate device, characterized in that Comprising: a) A first superconducting circuit, comprising: a-1) A first Josephson element as a Josephson device; a-2) A second group of Josephson elements, which serially connect n Josephson elements using wiring made of a superconductor, and each of the n Josephson elements has a Josephson energy greater than n times the Josephson energy of the first Josephson element; a-3) A first capacitor; and a-4) A first wiring, which forms a local superconducting circuit by connecting the first Josephson element and the second group of Josephson elements in a loop, and connects the local superconducting circuit in parallel with the first capacitor, and is made of a superconductor; and The first superconducting circuit resonates at a first resonance frequency; b) A second superconducting circuit, which includes at least one Josephson element, a second capacitor, and a second wiring made of a superconductor, and resonates at a second resonance frequency; c) A connection part, which has a connection part capacitor and a third wiring made of a superconductor respectively connected to both poles of the connection part capacitor, and connects the first superconducting circuit and the second superconducting circuit; d) A magnetic field application part, which applies a static magnetic field to the inside of the local superconducting circuit; and e) An electromagnetic wave irradiation part, which irradiates the first superconducting circuit with an electromagnetic wave having a difference frequency, which is the difference between the first resonance frequency and the second resonance frequency.

2. The quantum gate device according to claim 1, wherein The second superconducting circuit is a superconducting qubit formed by connecting one Josephson element and one second capacitor in a loop through the second wiring.

3. The quantum gate device according to claim 1 or 2, wherein The first Josephson element is composed of a junction in which two superconductors sandwich a first thin film made of an insulator, Each of the Josephson elements constituting the second group of Josephson elements is composed of a junction in which two superconductors sandwich a second thin film made of the same kind of insulator as the first thin film, The value of the tunnel resistance in the first Josephson element is greater than n times the value of the tunnel resistance in each of the Josephson elements constituting the second group of Josephson elements.

4. The quantum gate device according to claim 1 or 2, further comprising: A residual interaction cancellation electromagnetic wave irradiation part, which irradiates the first superconducting circuit with an electromagnetic wave that inverts the phase of the qubit, namely a residual interaction cancellation electromagnetic wave; And An irradiation adjustment part, which performs irradiation of the electromagnetic wave using the electromagnetic wave irradiation part, and then performs irradiation of the residual interaction cancellation electromagnetic wave using the residual interaction cancellation electromagnetic wave irradiation part twice at a set time interval, thereby adjusting the irradiation time points of the electromagnetic wave irradiation part and the residual interaction cancellation electromagnetic wave irradiation part.

5. The quantum gate device according to claim 4, wherein The residual interaction cancellation electromagnetic wave irradiation part is constituted by the same device as the electromagnetic wave irradiation part.

6. An integrated quantum circuit, which includes a plurality of quantum gate devices according to any one of claims 1 to 5.

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