RU2780666C1 - Dual-electrode fast flow-tuned superconducting qubit based on kinetic inductance - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: dual-electrode fast flow-tuned superconducting qubit based on kinetic inductance relates to systems for quantum information processing based on superconducting qubits. The technical result is achieved by the qubit containing two kinetic inductors; wherein the first electrode of the tunnelling Josephson junction is connected to the first electrode of the first kinetic inductor; the second electrode of the tunnelling Josephson junction is connected to the first electrode of the second kinetic inductor; and the second electrodes of the first and second kinetic inductors are interconnected and grounded.

EFFECT: improvement in the coherent properties of the qubit, simplified system for controlling the qubit, and facilitated scaling.

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Description

A two-electrode flux-tunable superconducting qubit based on kinetic inductance (hereinafter referred to as a two-electrode qubit) refers to quantum information processing systems based on superconducting qubits and can be used as a basic element (qubit) for building and creating superconducting quantum processors and (or) quantum simulators operating at temperatures below 20 mK, with improved coherence and controllability.

Known device (superconducting qubit), described in Hanhee Paik, et. al., "Observation of High Coherence in Josephson Junction Qubits Measured in a Three-Dimensional Circuit QED Architecture)", Phys. Rev. Lett. 107, 240501, which is a non-tunable superconducting transmon qubit made of aluminum on a dielectric substrate and placed in the center of a three-dimensional microwave resonator. The equivalent electrical circuit of the device includes a non-linear Josephson junction with Josephson energy E _J , shunted with a large charge energy capacitance E _C . The parameters of the equivalent electrical circuit of the device are chosen in such a way that its Hamiltonian is equivalent to the Hamiltonian of a weakly anharmonic oscillator. The described device relates to quantum information processing systems based on superconducting qubits and can be used as a basic element (qubit) for building and creating superconducting quantum processors with a fundamental transition frequency above 6.5 GHz and anharmonicity of the order of several hundred megahertz.

The disadvantages of this device are the high frequency of the fundamental transition of the qubit-transmon, which entails a decrease in its coherent properties due to high dielectric losses in the surface layer, and the need to use a heterodyne circuit to generate microwave pulse signals exciting the qubit, as well as the absence of individual frequency control lines. qubit and the use of a 3D resonator to read system states, making it difficult to scale up to create multi-qubit circuits. The low anharmonicity of the transmon qubit also limits the performance of single-qubit and two-qubit gate operations.

Known devices are described in Place, A.P.M., Rodgers, L.V.H., Mundada, P. et al. "New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds)). Nat Commun 12, 1779 (2021), used as non-tunable superconducting transmon qubits, made in the form of an integrated electric circuit on a dielectric substrate, capacitively coupled to an individual coplanar resonator to read the states of the qubits. The equivalent electrical circuit of the system includes a Josephson junction providing non-linearity shunted by two large capacitances made of tantalum on a sapphire dielectric substrate. The parameters of the system are chosen in such a way that its Hamiltonian is equivalent to the Hamiltonian of a weakly anharmonic oscillator. The described devices relate to quantum information processing systems based on superconducting qubits and can be used as a basic element (qubit) for building and creating superconducting quantum processors with a fundamental transition frequency in the range from 3.1 to 5.5 GHz and anharmonicity of the order of several hundred megahertz .

The disadvantages of this system are the high frequency of the fundamental transition of qubits, which entails high dielectric losses in the surface layer and the need to use a heterodyne circuit to generate microwave pulse signals that excite the qubit, as well as the absence of individual qubit frequency control lines, which complicates the scaling process for creating multi-qubit circuits. . The low anharmonicity of the system also limits the performance of one-qubit and two-qubit gate operations.

A known device is described in J. Koch et al, "Charge-insensitive qubit design derived from the Cooper pair box," Phys. Rev. A 76, 042319 (2007), which is a superconducting transmon qubit that allows capacitive coupling with an individual coplanar resonator to read the states of the qubit, as well as the possibility of being implemented as an integrated electric circuit on a dielectric substrate. The equivalent electrical circuit of the device includes two tunnel Josephson junctions providing non-linearity, connected in parallel to form a closed loop, and shunted with a high capacitance. The magnetic flux through the closed loop is used to control the frequency of the qubit and can be generated using an inductively coupled flux line. The parameters of the system are chosen in such a way that its Hamiltonian is equivalent to the Hamiltonian of a weakly anharmonic oscillator. The described device relates to quantum information processing systems based on superconducting qubits and can be used as a basic element (qubit) for building and creating superconducting quantum processors with a tunable fundamental transition frequency above 4 GHz and anharmonicity of the order of several hundred megahertz.

The disadvantages of this system are the high frequency of the fundamental transition (above 4 GHz), which entails high dielectric losses in the surface layer and the need to use a heterodyne circuit to generate microwave pulse signals exciting the qubit. The low anharmonicity of the system limits the speed of single-qubit and two-qubit gate operations.

A known device is described in Yan, F., Gustavsson, S., Kamal, A. et al. The flux qubit revisited to enhance coherence and reproducibility. Nat Commun 7, 12964 (2016), which is a superconducting capacitively shunted flow qubit, made in the form of an integrated electrical circuit on a dielectric substrate, capacitively coupled to an individual coplanar resonator for reading the states of the qubit. The system equivalent circuit diagram includes a non-linear Josephson junction shunted with high capacitance and connected in parallel with two larger Josephson junctions in series to form a closed loop. The magnetic flux penetrating the closed counter is used to control the frequency of the flux qubit. The described device relates to quantum information processing systems based on superconducting qubits and can be used as a basic element (qubit) for building and creating superconducting quantum processors with a tunable fundamental transition frequency above 4 GHz (the minimum frequency is reached at the magnetic flux degeneration point, when the magnetic the flux in the system circuit is equal to half of the magnetic flux quantum), and the anharmonicity is higher than several gigahertz.

The disadvantages of this system are the high frequency of the main transition (above 4 GHz), which entails high dielectric losses in the surface layer and the need to use a heterodyne circuit to generate microwave pulse signals exciting the qubit, as well as the use of a magnetic loop installed above the chip directly near the location qubit to create a magnetic flux in a closed loop, which makes it difficult to scale the system, since the magnetic field generated by such a loop is not local.

A device is known, described in US patent 20200287540 A1, which is an inductively shunted transmon qubit. The equivalent electrical circuit of the device includes a charge qubit consisting of a non-linear Josephson junction with Josephson energy E _J shunted by one or more capacitors with capacitive energy E _C such that the ratio E _J /E _C is greater than 5, and a chain of several larger Josephson junctions with Josephson energy E _J' , such that E _J' is greater than E _J , connected in parallel with the Josephson junction of the charge qubit and one or more capacitances of the charge qubit, such that the chain of Josephson junctions and the Josephson junction of the charge qubit form a closed circuit, while the total inductive energy of the chain of these contacts E _L for different embodiments of the system can lie within the range from 0.8E _J to 1.2 E _J . The described device relates to quantum information processing systems based on superconducting qubits and can be used as a basic element (qubit) for building and creating superconducting quantum processors with a tunable fundamental transition frequency above 4 GHz (the minimum frequency is reached at the magnetic flux degeneration point, when the magnetic the flux in the system circuit is equal to half of the magnetic flux quantum) and an anharmonicity of the order of several hundred megahertz.

The disadvantages of this system are the high frequency of the main transition (above 4 GHz), which entails high dielectric losses in the surface layer, and the need to use a heterodyne circuit to generate microwave pulse signals exciting the qubit. The low anharmonicity of the system also limits the performance of one-qubit and two-qubit gate operations.

A known device is described in Pop, I., Geerlings, K., Catelani, G. et al. Coherent suppression of electromagnetic dissipation due to superconducting quasiparticles. Nature 508, 369-372 (2014), which is a fluxonium qubit made of aluminum on a dielectric substrate, inductively coupled to a microwave antenna through common Josephson junctions, and located in the center of a copper three-dimensional waveguide. The equivalent electrical circuit of the device includes a small non-linear Josephson junction with a Josephson energy E _J connected in parallel with a superinductor made from a linear array of large Josephson junctions with a Josephson energy of each contact greater than E _J , such that a closed loop is formed for creating an external magnetic flux by a coil located outside the copper waveguide, and the total inductive energy of the superinductor E _L is much less than E _J . The described device relates to quantum information processing systems based on superconducting qubits and can be used as a basic element (qubit) for building and creating superconducting quantum processors with a tunable fundamental transition frequency of less than 1 GHz (the minimum frequency is reached at the magnetic flux degeneration point in the system circuit ) and anharmonicity above 1 GHz.

The disadvantage of this system is the complexity of scaling due to the use of a three-dimensional waveguide to read the states of the system, as well as the absence of individual lines for monitoring the state of the qubit and individual lines for quick tuning of the qubit frequency, instead of which an external magnetic coil is used.

A device is known, described in US patent 20180054201 A1, which is a flux-tunable qubit with several Josephson junctions and two circuits for creating an external magnetic flux. The equivalent electrical circuit of the device includes an inductor connected between the first circuit node and the second circuit node; a capacitor connected in parallel with the inductor between the first and second circuit nodes, a first Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node; and a second Josephson junction connected in parallel with the inductor between the first circuit node and the second circuit node, so that two closed loops are formed to create an external magnetic flux in the system. The system parameters can be chosen in such a way that, depending on the frequency of the main transition (between the ground and the first excited state) on the external magnetic flux, several points will be observed at which the frequency of the main transition will be in the first order insensitive to changes in the external magnetic flux. The described device relates to quantum information processing systems based on superconducting qubits and can be used as a basic element (qubit) for building and creating superconducting quantum processors, while the main transition frequency in the magnetic flux degeneracy region can be obtained less than 1 GHz, and anharmonicity in the time can also be above 1 GHz.

The disadvantage of this system is the excessive complexity of control, since in order to fully control the frequency of the system and realize all the advantages of the design, either simultaneous control of the magnetic flux in two circuits is required, for which it is necessary to have two lines of tuning of the external magnetic flux inductively coupled to the system, or compliance with the Josephson ratios is required. transition energies and/or contour areas equal to integer values with an accuracy of 10%, which is not a simple technological task. The use of additional points that are not sensitive to changes in the external magnetic flux implies operation at a frequency above 1 GHz, which necessitates the use of a heterodyne circuit for generating microwave pulse signals that excite the qubit, in contrast to the magnetic flux degeneracy point, at which the system frequency can be obtained below 1 GHz. Inductive connection with the tuning lines of the external magnetic flux implies a small amount of mutual induction of the system circuit and the control line, which leads to the need to supply large control currents.

A known device (superconducting qubit) is described in Long B. Nguyen, et al., High-Coherence Fluxonium Qubit, Phys. Rev. X 9, 041041, Nov 2019, which is a high-capacity shunted fluxonium qubit fabricated on a chip and placed in a 3D copper resonator to read its states. The equivalent circuit of the device includes a tunnel Josephson junction providing non-linearity, shunted with a large capacitance, and connected in parallel with a kinetic inductor formed from a linear array of larger Josephson junctions, so that a closed loop is formed to generate an external magnetic flux by a coil located outside the three-dimensional resonator. With a special choice of the parameters of the electric circuit, the frequency of the main qubit transition at the magnetic flux degeneracy point can lie below 1 GHz, while the anharmonicity can at the same time be above 1 GHz. At a low frequency of the fundamental transition of the qubit, dielectric losses in the surface layer decrease, as a result of which the coherent properties of the system improve (the coherence time and relaxation time are more than 200 μs). The qubit frequency is controlled by changing the external magnetic flux penetrating the closed circuit of the qubit. In this device, to create and change the external magnetic flux, an inert magnetic coil is used, which is installed outside the three-dimensional resonator and creates a magnetic field in its entire volume. The described device relates to quantum information processing systems based on superconducting qubits and can be used as a basic element (qubit) for building and creating superconducting quantum processors.

The disadvantage of the device is the complexity of scaling due to the use of a three-dimensional resonator to read the states of the system, as well as the absence of individual lines for monitoring the state of the qubit (excitation lines), and individual lines for fast individual tuning of its frequency (flux bias lines), instead of which an inert external magnetic field is used. coil.

The closest technical solution to the proposed (prototype) is the device described in Helin Zhang, et. al. "Universal Fast-Flux Control of a Coherent, Low-Frequency Qubit", Phys. Rev. X 11, 011010, 15 January 2021, which is a high capacitance shunted fluxonium qubit fabricated in planar architecture and capacitively coupled to a coplanar resonator to read qubit states. The equivalent circuit of the device includes a tunnel Josephson junction providing non-linearity, shunted with a large capacitance, and connected in parallel with a kinetic inductor formed from a linear array of larger Josephson junctions, so that a closed loop is formed to create an external magnetic flux by an inductively coupled flux control line. The described device relates to quantum information processing systems based on superconducting qubits and can be used as a basic element (qubit) for building and creating superconducting quantum processors, while the frequency of the main transition in the region of magnetic flux degeneracy is several tens of megahertz, and the anharmonicity of the system is higher 1 GHz. The low frequency of the fundamental transition of the qubit leads to a decrease in dielectric losses in the surface layer, as a result of which the coherent properties of the system improve (the coherence time and relaxation time are more than 200 μs).

The disadvantage of the prototype is the too low fundamental transition frequency of 14 MHz, which leads to a large thermal population of the first excited state even when operating in a dissolution cryostat at temperatures of the order of several tens of millikelvins and requires the implementation of protocols for preliminary initialization of the qubit in the ground state, the prototype used high energy levels of the system , for which two microwave signals were simultaneously applied at a frequency above several gigahertz, which implies the use of a heterodyne circuit for the formation of microwave pulses, which means the use of an additional source of a microwave signal and additional channels of an arbitrary waveform generator. The small value of the inductive coupling of the qubit circuit and the flow line leads to the need to supply large control currents to create a constant magnetic flux in the qubit circuit, in contrast to the case of using galvanically coupled flow lines, this also led to the need to use an additional microwave channel in the dissolution cryostat for fast restructuring the magnetic flux in the qubit circuit and performing single-qubit operations. The low fundamental transition frequency of the qubit limits the speed of single-qubit gate operations, as mentioned in the article, the time of single-qubit operations varies from 17.87 ns to 78.11 ns.

The purpose of the invention is to improve the coherent properties of the qubit, simplify the qubit control system and simplify the scaling process.

The technical result is the development of a two-electrode qubit with a fundamental transition frequency below 1 GHz and an anharmonicity above 1 GHz, which has improved coherent properties, which allows scaling to create multi-qubit systems, as well as having the ability to implement and use individual lines for monitoring the state of the qubit and/or individual galvanic built-in lines for quick tuning of the qubit frequency, which simplifies the qubit control system.

The technical result is achieved by the fact that the qubit contains two kinetic inductors, while the first electrode of the Josephson tunnel junction is connected to the first electrode of the first kinetic inductor, the second electrode of the Josephson tunnel junction is connected to the first electrode of the second kinetic inductor, and the second electrodes of the first and second kinetic inductors are connected between and grounded, so that the two kinetic inductors and the tunnel Josephson junction form a closed loop, and in the closed loop between the two kinetic inductors a magnetic flux control line is galvanically built in, in addition, the distributed mutual capacitances to the ground of each of the electrodes of the tunnel Josephson junction are used for direct capacitive coupling with an individual readout resonator and direct capacitive coupling with neighboring qubits.

The essence of the invention is illustrated by drawings:

in fig. 1 - equivalent circuit of a two-electrode qubit;

in fig. 2 - spectrum (energy levels) of a two-electrode qubit depending on the external magnetic flux Ф ^x for characteristic parameters;

in fig. 3 - an example of connecting an individual galvanically built-in line for fast tuning of the qubit frequency;

где:

- квант магнитного потока,

- постоянная планка, с - скорость света, е - заряд электрона), С_1,2=70,1 фФ, C_J=1,3 фФ и величины взаимной индуктивности М=20 пГн;in fig. 4 - calculation of the dependence of the relaxation time of the two-electrode qubit on the external magnetic flux Ф ^x in the qubit circuit for the characteristic parameters of the system: E _J =2.24 GHz,

where:

- quantum of magnetic flux,

- bar constant, c - speed of light, e - electron charge), C _1.2 \u003d 70.1 fF, C _J \u003d 1.3 fF and mutual inductance M \u003d 20 pH;

in fig. 5 - heterodyne circuit for generating control microwave signals for qubits with a frequency above 1 GHz;

in fig. 6 is a simplified diagram of the formation of microwave pulse signals for qubits with a frequency below 1 GHz;

in fig. 7a - two-electrode qubit capacitively coupled to a quarter-wave resonator (similar coupling is possible with other qubits);

in fig. 7b is an example topology (not to scale) of a two-electrode qubit capacitively coupled to a quarter-wavelength resonator.

The equivalent electrical circuit of a two-electrode qubit is shown in Fig. one,

where 1 and 2 - the first and second electrodes of the tunnel Josephson contact J _f providing nonlinearity, with energy E _J and equivalent capacitance C _J ; C ₁ and C ₂ distributed mutual capacitances to the ground electrodes 1 and 2 tunnel Josephson junction, respectively;

L ₁ and L ₂ are kinetic inductors (made of materials with high kinetic inductance or line arrays of large area Josephson tunnel junctions, for example 1000×500 nm, with more than 20 contacts) connected to the first and second electrodes of the Josephson tunnel junction, thus that a closed circuit is formed, and the electrode between L ₁ and L ₂ on the opposite side of the Josephson tunnel contact is grounded;

Ф ^x - external magnetic flux in the circuit of the qubit, which allows you to tune the frequency of the main transition.

и записывается ее лагранжиан как: L=Т-U, в результате получим:To determine the parameters of the system, such as the fundamental transition frequency and anharmonicity, it is necessary to write down its Hamiltonian. This chain has two degrees of freedom. First, generalized flow variables are set for each node (nodes 1 and 2 in Fig. 1) of the system ϕ ₁ , ϕ ₂ corresponding to stresses

and its Lagrangian is written as: L=T-U, as a result we get:

связанным с нормальными модами линеаризованной схемы в отсутствие беспорядка (под отсутствием беспорядка подразумевается равенство кинетических индукторов L₁=L₂=L, и распределенных взаимных емкостей С₁=С₂=С, обозначения соответствуют введенным на фиг. 1). Переход осуществляется с помощью соотношений:After that, the transition to physically more substantiated variables is performed

associated with the normal modes of the linearized circuit in the absence of disorder (the absence of disorder means the equality of the kinetic inductors L ₁ =L ₂ =L, and the distributed mutual capacitances C ₁ =C ₂ =C, the designations correspond to those introduced in Fig. 1). The transition is carried out using the relations:

связана с разностью фаз на джозефсоновском контакте (обозначен J_f - на фиг. 1) и, таким образом, является нелинейной, мода

не смещает переход и, следовательно, является полностью гармонической модой. Таким образом, в отсутствие беспорядка гамильтониан системы может быть записан как:Fashion

is related to the phase difference across the Josephson junction (denoted by J _f - in Fig. 1) and, thus, is non-linear, the mode

does not shift the transition and is therefore a fully harmonic mode. Thus, in the absence of disorder, the Hamiltonian of the system can be written as:

- оператор Гамильтона квантового осциллятора;where

is the Hamilton operator of the quantum oscillator;

- оператор Гамильтона традиционного кубита-флуксониума;

is the Hamiltonian operator of the traditional fluxonium qubit;

- операторы числа куперовских пар, канонически сопряженные

соответственно.

are operators of the number of Cooper pairs, canonically conjugate

respectively.

(обозначение Ф^х соответствует введенному на фиг. 1), индуктивная энергия E_L=(Ф₀/2π)²/L (L - индуктивность), эквивалентная зарядовая энергия моды флуксониума

где C_f=(C+C_J)/2, эквивалентная зарядовая энергия гармонической моды

Мода флуксониума играет роль кубита.Here we also introduced the dimensionless variables of the external magnetic flux

(the designation Ф ^x corresponds to that introduced in Fig. 1), inductive energy E _L =(Ф ₀ /2π) ² /L (L is the inductance), the equivalent charge energy of the fluxonium mode

where C _f =(C+C _J )/2, the equivalent charge energy of the harmonic mode

The fluxonium mode plays the role of a qubit.

The spectrum of the system shown in Fig. 2, depending on the external flow Ф ^x obtained for the numerical model with parameters: E _J =2.24 GHz,

где n_h - число заполнения гармонической моды, а

- число заполнения моды флуксониума. Из графика видно, что частота гармонической моды (переход |0,0> - >|1,0>) равна 2,0 ГГц, частота основного перехода моды флуксониума (переход |0,0> - >|0,1>) изменяется от 625 МГц в точке вырождения внешнего магнитного потока

до 3,31 ГГц при нулевом значении внешнего магнитного потока, ангармонизм моды флуксониума (разность частот переходов |0,1> - >|0,2> и |0,0> - >|0,1>) в точке вырождения внешнего магнитного потока

составляет величину порядка 1,9 ГГц.Eigenvalues on the graph are marked

where n _h is the occupation number of the harmonic mode, and

is the filling number of the fluxonium mode. It can be seen from the graph that the frequency of the harmonic mode (transition |0.0> - >|1.0>) is 2.0 GHz, the frequency of the main transition of the fluxonium mode (transition |0.0> - >|0.1>) changes from 625 MHz at the degeneracy point of the external magnetic flux

up to 3.31 GHz at zero value of the external magnetic flux, the anharmonicity of the fluxonium mode (the difference between the frequencies of the transitions |0.1> - >|0.2> and |0.0> - >|0.1>) at the point of degeneracy of the external magnetic flow

is about 1.9 GHz.

As can be seen from the graph (see Fig. 2), the frequency of the main transition of the system fluxonium mode at the magnetic flux degeneracy point is 625 MHz, and the anharmonicity is about 1.9 GHz. Thus, the system makes it possible to implement a superconducting qubit with a fundamental transition frequency below 1 GHz, which will reduce dielectric losses in the surface layer, and anharmonicity above 1 GHz, which reduces the limitation on the execution time of single-qubit operations to several nanoseconds. In FIG. 3 shows a variant of galvanic connection of a flux line for creating and controlling a magnetic flux in the system circuit with a mutual inductance M such that M << L _1.2 , the designation corresponds to that entered in FIG. 2. L _f - inductance of the filter installed in the magnetic flux control line (may be absent in the real version).

The limitation of the qubit relaxation time due to galvanic coupling with the flow line is estimated according to the formula:

- матричный элемент потока моды флуксониума.where

is the matrix element of the fluxonium mode flux.

The result of the numerical calculation, for the system parameters used to obtain the spectrum in FIG. 2 and for the value M=20 pH (estimation of the inductance of a superconducting wire with a length of 30 μm, made in planar geometry) is presented in FIG. 4. The relaxation time at the magnetic flux degeneracy point is above 1.9 ms.

; I,Q - квадратурный смеситель для формирования импульсного сигнала на частоте кубита ω_d=ω_q=ω_LO+ω_AWG.In FIG. Figure 5 shows an image of a heterodyne circuit for generating control microwave signals for qubits with a frequency above 1 GHz, where: LO is a sinusoidal signal source operating in continuous mode at a frequency ω _LO , AWG is an arbitrary waveform generator generating pulses at a frequency ω _AWG with a given shape and given pulse duration

; I,Q - quadrature mixer for generating a pulsed signal at the qubit frequency ω _d =ω _q =ω _LO +ω _AWG .

In FIG. Figure 6 shows a diagram of the formation and supply of microwave pulses to control the state of qubits with a frequency below 1 GHz, where: AWG is an arbitrary waveform generator that generates pulses at a qubit frequency ω _AWG =ω _q with a given shape and a given pulse duration

From a comparison of Fig. 5 and FIG. 6, the possibility of simplifying the qubit control system is obvious by eliminating the source of a sinusoidal signal, quadrature mixers, and using only one channel of the arbitrary waveform generator to generate microwave pulses controlling the qubit in the case of using qubits with a fundamental transition frequency below 1 GHz. Moreover, in the case of a galvanic connection to the flow line, the same channel of the arbitrary waveform generator can be used to provide a flow control signal. As a result, only one channel of the arbitrary waveform generator can be used both to control the frequency of the qubit and to control its states.

To demonstrate the scaling capabilities, an example of the implementation of capacitive coupling of a qubit with a readout resonator is shown in Fig. 7.

Thus, the claimed technical result is achieved, namely, the possibility of implementing a superconducting qubit with a fundamental transition frequency below 1 GHz and anharmonicity above 1 GHz, which has improved coherent properties, which allows scaling to create multi-qubit systems, and also allows the implementation and use of individual lines for monitoring the state of the qubit and/or individual galvanically built-in lines for fast tuning of the qubit frequency, which simplifies the qubit control system.

This technical solution improves the coherent properties of the qubit and simplifies the qubit control system.

Claims (3)

1. A two-electrode flux-tunable superconducting qubit based on kinetic inductance, implemented as an equivalent electrical circuit that includes two kinetic inductors with two electrodes each, a Josephson tunnel junction that provides nonlinearity, both electrodes of which have distributed capacitances to ground, while the first electrode of the tunnel Josephson junction is connected to the first electrode of the kinetic inductor, the second electrode of the tunnel Josephson junction is connected to the first electrode of the second kinetic inductor, and the second electrodes of the first and second kinetic inductors are interconnected and grounded, so that the two kinetic inductors and the tunnel Josephson junction form a closed loop. 2. A two-electrode qubit according to claim 1, characterized in that a magnetic flux control line is galvanically built into a closed circuit between two kinetic inductors. 3. A two-electrode qubit according to claim 1, characterized in that the distributed mutual capacitances to ground of each of the electrodes of the Josephson tunnel junction are used for direct capacitive coupling with an individual readout resonator and direct capacitive coupling with neighboring qubits.

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