Frequency-adjustable superconducting quantum bit capable of naturally resisting magnetic flux noise

Abstract

The invention discloses a frequency-adjustable superconducting quantum bit capable of naturally resisting magnetic flux noise. The frequency-adjustable superconducting quantum bit comprises a direct-current equal-arm SQUIDs and a bias control line for adjusting the critical current of the SQUIDs, the SQUIDs of the direct current equal arms are of a standard in-plane loop structure, and the two arms of the SQUIDs intersect with each other to form 2N identical superconducting loops, namely sub-loops. The bias control line changes the magnetic flux in the loop by applying a bias magnetic field, so that the critical current of the SQUIDs is changed, and the frequency of the superconducting quantum bits can be regulated and controlled; on the basis of the SQUIDs structure of a direct-current equal arm, the space geometric structure of the SQUIDs is improved, so that magnetic flux noise entering a loop of the SQUIDs is mutually counteracted, and net magnetic flux accumulated in the loop due to the magnetic field noise is zero. When N is equal to 1, magnetic flux disturbance in the two sub-loops is phi 0 and-phi 0 respectively, and the phi 0 and the-phi 0 counteract each other, so that the superconducting critical current of the SQUIDs cannot be interfered, and the capability of eliminating the magnetic flux noise by bits is realized.

Description

Technical Field

[0001] The present invention relates to a design and preparation method of a frequency-adjustable superconducting quantum bit structure that is naturally resistant to magnetic flux noise, and belongs to the technical field of quantum computing. Background Art

[0002] The key to manufacturing a quantum computer is to develop a multi-qubit processor. A core issue in building a multi-qubit processor is the design and processing of high-quality qubits. Among the various quantum computing solutions, the qubit (processor) solution based on superconducting circuits has outstanding advantages, with flexible parameter design, strong controllability, convenient readout, and good scalability. It is considered to be one of the preferred solutions for realizing universal quantum computing and has received widespread attention. At present, the application of superconducting quantum circuit quantum computing solutions has developed to quantum simulation research, quantum optimization algorithm exploration, and research on basic problems of quantum mechanics, and the progress is particularly rapid. Superconducting quantum circuits are artificially constructed controllable quantum systems. Therefore, by operating superconducting quantum circuits, they can be made to undergo the same regular state evolution as the quantum system to be studied, and finally obtain its relevant evolution information.

[0003] Currently, common superconducting quantum bits usually use SQUIDs structures to provide anharmonic quantized energy levels for superconducting quantum circuits, so that the overall structure can satisfy the Divincenzo criterion (a condition that quantum bits must meet to form a quantum computer) to form a quantum bit, and the frequency of the quantum bit can be controlled by adjusting the critical current of the SQUIDs structure. Superconducting quantum bits are a type of quantum bit designed using the unique properties of superconducting materials. They are generally composed of a bit readout resonant cavity, a coupling resonant cavity, a control line, a coplanar capacitor, and SQUIDs. Among them, SQUIDs is a planar ring superconducting circuit containing a pair of Josephson junctions, and the change of its superconducting critical current can be controlled by adjusting the magnetic flux in the ring. However, SQUIDs are very sensitive to magnetic flux signals. Slight magnetic field noise disturbances will cause the critical current to shift, which in turn causes the quantum bit frequency to change, making it difficult to measure the bit and even reducing the performance of the bit. At the same time, when multiple quantum bits work together, the generation of magnetic field noise is inevitable, because the control signal of each bit will cause crosstalk to the adjacent bits, leading to problems such as bit decoherence and virtual excitation, greatly reducing the fidelity of the bit gate operation, and bringing huge challenges to the measurement and control of multi-bit chips.

[0004] In order to overcome the relevant difficulties, a variety of solutions have been proposed, including using semiconductor materials to prepare Josephson junctions from a material perspective to reduce the system's sensitivity to magnetic field noise; from a measurement and control perspective, using an algorithm to automatically calibrate the bit frequency to track changes in the bit frequency; and at the algorithm application level, using quantum error mitigation or error correction algorithms to maintain the bit gate fidelity at a high level. However, on the one hand, the preparation process of multi-bit chips based on semiconductor Josephson junctions is not yet mature, and its own coherence time is still relatively short. On the other hand, improvements based on measurement and control solutions will increase the overhead of measurement and control hardware and software, and will increase the difficulty of measurement and control of multi-bit chips.

[0005] In summary, it is very necessary to propose a superconducting quantum bit that is still based on SQUIDs to provide anharmonic quantized energy levels and has the ability to resist magnetic perturbations under general conditions. Summary of the invention

[0006] Purpose of the present invention: In order to resist the interference of magnetic noise on superconducting quantum bits, the present invention proposes a frequency-adjustable superconducting quantum bit that can naturally resist magnetic flux noise. In the DC equal-arm SQUIDs structure, by improving the spatial geometric structure of SQUIDs, the magnetic flux noise entering the SQUIDs loop is offset, achieving the immunity of the bit to magnetic flux noise under general conditions, thereby improving the performance of superconducting quantum bits.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is a frequency-adjustable superconducting quantum bit that is naturally resistant to magnetic flux noise. The composition of the bit includes a DC equal-arm SQUIDs and a bias control line for adjusting the critical current of the SQUIDs. Among them, the DC equal-arm SQUIDs, by changing its spatial geometric structure, makes the magnetic flux noise entering the SQUIDs loop cancel each other out, so that the net magnetic flux accumulated in the loop due to magnetic field noise is zero, forming the immunity of the bit to magnetic flux noise under general conditions. At the same time, a shunt bias control line (Z line) is set to achieve that in the case of immunity to magnetic flux noise, the bit frequency can still be adjusted by actively applying magnetic flux bias.

[0008] The design of the frequency-adjustable superconducting quantum bit geometric structure that is naturally resistant to magnetic flux noise in the present invention allows the magnetic flux noise entering the SQUIDs loop to cancel each other out, so that the net magnetic flux accumulated in the loop due to magnetic field noise is zero. On superconducting quantum chips, the most common magnetic flux noise is the uniformly changing magnetic field noise in the environment, and the magnetic flux noise that changes with the first-order gradient in space due to crosstalk caused by the control circuit. This kind of noise can be eliminated on the frequency-adjustable superconducting quantum bit that is naturally resistant to magnetic flux noise through the corresponding bit design.

[0009] In the DC equal-arm SQUIDs structure, by improving the spatial topological geometry of SQUIDs and utilizing spatial overlap to make the two arms of the loop cross each other, the overall structure presents 2N (N is a positive integer) "sub-loops". The improved SQUIDs can actively adjust the bias flux entering the loop to change the frequency of the bit by adopting a shunt bias control line (Z line), thereby achieving frequency adjustability. In the process of actively applying the bias, the induced currents of the N sub-loops in the SQUIDs flow clockwise, and the induced currents of the other N sub-loops flow counterclockwise, forming a loop as a whole. The two groups of N sub-loops are geometrically congruent to each other, so that the magnetic flux disturbances caused by the magnetic field noise in the two groups of N sub-loops cancel each other out, and will not interfere with the superconducting critical current of the SQUIDs, thus achieving the bit's immunity to magnetic flux noise under general conditions.

[0010] When N=1, the areas inside the two "sub-loops" are equal, and the improved SQUIDs is still a superconducting loop containing a pair of Josephson junctions and connected end to end. The magnetic flux noise entering the SQUIDs loop causes a disturbance of Φ0 in one loop and -Φ0 in the other loop, and the two cancel each other out. The SQUIDs part of the bit consists of a "loop" in which the current flows clockwise and a "loop" in which the current flows counterclockwise, which can resist the interference of zero-order gradient magnetic field (uniform magnetic field) noise on the bit, and can also resist the interference of first-order gradient magnetic field noise in a specific direction. The overall shape of the two "sub-loops" can form an approximate "8" shape, and its sub-loops can also be polygons, triangles, rounded polygonal ellipses, etc.

[0011] When N=2, the SQUIDs part of the bit consists of two "loops" with clockwise current flow and two "loops" with counterclockwise current flow, and the internal areas of the four "loops" are equal. The magnetic field noise in any first-order gradient direction can be decomposed into the superposition of vectors in the X direction and the Y direction, so that the magnetic flux noise entering the SQUIDs loop generates magnetic flux disturbances in two of the loops, which are opposite to the magnetic flux disturbances generated by the other two loops, and the two cancel each other out, eliminating the interference of the first-order gradient magnetic field noise on the bit. It can resist the interference of the first-order gradient magnetic field noise on the bit, and can also resist the interference of the second-order gradient magnetic field noise in a specific direction; the same can be said for the case of N>3. Since most of the on-chip magnetic field noise is approximately uniform magnetic field noise and first-order gradient magnetic field noise, this bit design can resist most of the interference caused by magnetic flux noise, becoming a frequency-adjustable superconducting quantum bit that is naturally resistant to magnetic flux noise.

[0012] The frequency-adjustable superconducting qubit of the present invention improves the SQUIDs structure and utilizes spatial overlap to cross the two arms of the SQUIDs, so that the overall structure presents 2N (N is a positive integer) sub-loops under a macroscopic magnetic field and has the ability to naturally cancel out magnetic field noise. This frequency-adjustable superconducting qubit that naturally cancels out magnetic flux noise only requires the same generalized process preparation conditions and test control conditions as common superconducting qubits, and surpasses the performance of the prior art because after being exempted from magnetic field noise, the bit frequency is more stable, and the relaxation time T1 and phase decoherence time T2 are higher. The frequency-adjustable superconducting qubit of the present invention that is naturally resistant to magnetic flux noise provides an effective way to realize bits that are exempt from magnetic field noise and to analyze a more general superconducting qubit noise model.

[0013] Beneficial effects: The present invention can construct a frequency-adjustable superconducting quantum bit with the characteristic of naturally resisting magnetic flux noise. Based on the SQUIDs part of the superconducting quantum bit component, its geometric structure is innovatively designed, and mature micro-nano processing technology is fully utilized to prepare SQUIDs containing 2N (N is a positive integer) sub-loops. The improved SQUIDs can actively adjust the bias flux entering the loop to change the frequency of the bit by adopting a shunt bias control line (Z line) to achieve frequency adjustability. In the process of actively applying the bias, the induced current of the N sub-loops in the SQUIDs flows clockwise, and the induced current of the other N sub-loops flows counterclockwise, forming a loop as a whole. The two groups of N sub-loops are geometrically congruent to each other, so that the magnetic flux disturbances caused by the magnetic field noise in the two groups of N sub-loops cancel each other. It will not interfere with the superconducting critical current of the SQUIDs, and the bit can cancel and eliminate the magnetic flux noise under general circumstances. The present invention can fully adopt the current superconducting quantum bit preparation method and measurement and control method, and is suitable for running various mature quantum algorithms and quantum simulation processes.

[0014] The present invention provides a frequency-adjustable superconducting quantum bit that is naturally resistant to magnetic flux noise. By improving the spatial topological structure of SQUIDs, the superconducting quantum bit itself is exempted from the time-varying magnetic field noise of spatial gradient changes, and has universal applicability. Based on this frequency-adjustable superconducting quantum bit that is naturally resistant to magnetic flux noise, the preparation and measurement and control capabilities of large-scale superconducting quantum chips will be further optimized and improved, which is of great significance to accelerate the realization of universal quantum computing.

[0015] The present invention utilizes spatial crossing and superposition to make the two arms of SQUIDs cross each other, so that the overall structure presents 2N (N is a positive integer) sub-loops under a macroscopic magnetic field and has the ability to naturally resist magnetic field noise. It constitutes a frequency-adjustable superconducting quantum bit that naturally resists magnetic flux noise; it only requires generalized process preparation conditions and test control conditions, and can superconduct the general performance level of current superconducting quantum bits, because after eliminating magnetic field noise, the bit frequency is more stable, and the relaxation time T1 and phase decoherence time T2 are higher. The improved SQUIDs can use a current bias control line (Z line) that can cause a gradient change in the spatial distribution of the spatial magnetic field. For SQUIDs containing 2N sub-loops, an N-order gradient change magnetic field is required to actively adjust the bias flux entering the loop to change the frequency of the bit, while avoiding magnetic field fluctuation noise in the environment with a gradient less than the Nth order, so that the bit is exempted from flux noise and its frequency is adjustable.

[0016] The present invention also provides an effective way to analyze a more general superconducting quantum bit noise model. The present invention provides an effective way to realize superconducting quantum bits that are free of magnetic field noise, relieve the pressure of large-scale superconducting quantum chip measurement and control, and realize the construction of larger-scale superconducting quantum chips. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of an embodiment of the present invention: N=1 SQUIDs;

[0018] Figure 2 N=1SQUIDs principle diagram for eliminating magnetic field noise;

[0019] Figure 3 Schematic diagram of the shunt bias control line for N=1SQUIDs;

[0020] Figure 4 N=1SQUIDs real electron microscope image;

[0021] Figure 5 Schematic diagram of the air bridge preparation process; Figure 5 a, b, c, and d are specific preparation processes;

[0022] Figure 6 Schematic diagram of the dielectric layer preparation process; Figure 6 a, b, c, and d are specific preparation processes;

[0023] Figure 7 Schematic diagram of the packaging buried wire solution process; Figure 7 a, b, c, and d are specific preparation processes;

[0024] Figure 8 Schematic diagram of the structure for eliminating in-plane magnetic field noise of N=1SQUIDs;

[0025] Fig. 9 Schematic diagram of magnetic field noise elimination inside the torus;

[0026] Fig.10 Schematic diagram of N=2SQUIDs;

[0027] Fig.11 N=2 loop space topology diagram. DETAILED DESCRIPTION

[0028] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0029] The terms used in the following embodiments are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0030] The present invention proposes a frequency-adjustable superconducting quantum bit that is naturally resistant to flux noise; in the DC equal-arm SQUIDs structure, by improving the spatial geometry of the SQUIDs, the flux noise entering the SQUIDs loop cancels each other out, achieving the bit's ability to be immune to flux noise under general conditions, thereby improving the performance of the superconducting quantum bit. In the specific design, by improving the SQUIDs space, the two arms of the loop are crossed or superimposed, so that the overall structure presents 2N (N is a positive integer) sub-loops under a macroscopic field. The improved SQUIDs can actively adjust the bias flux entering the loop to change the frequency of the bit by adopting a shunt-type bias control line (Z line), thereby achieving frequency adjustability. In the process of actively applying the bias, the induced currents of the N sub-loops in the SQUIDs flow clockwise, and the induced currents of the other N sub-loops flow counterclockwise, forming a loop as a whole. The two groups of N sub-loops are geometrically identical to each other, so that magnetic flux disturbances caused by magnetic field noise in the two groups of N sub-loops cancel each other out.

[0031] In the structure of superconducting quantum bits, since the component of SQUIDs is a simple planar superconducting loop, it contains two identical Josephson junctions. In this case, the magnetic flux noise in the environment will cause a net magnetic flux change in the SQUIDs, affecting the critical current of the SQUIDs and causing disturbances to the frequency of the bit. The frequency-tunable superconducting quantum bits that are naturally resistant to magnetic flux noise mainly cancel each other out by designing the bit geometry, so that the net magnetic flux accumulated in the loop due to magnetic field noise is zero. On superconducting quantum chips, the most common magnetic flux noise is the uniformly changing magnetic field noise in the environment, and the magnetic flux noise that changes with the first-order gradient in space due to crosstalk caused by the control circuit. This type of noise can be eliminated on frequency-tunable superconducting quantum bits that are naturally resistant to magnetic flux noise through corresponding bit design.

[0032] The preparation and processing steps are basically the same as those of common superconducting quantum bits. The first step is the deposition of superconducting thin films. In order to reduce the interface loss between the substrate and the superconducting film, magnetron sputtering, electron beam evaporation, molecular beam epitaxy and other methods are used in combination with different substrate processing processes to grow high-quality superconducting thin films. The second step is the preparation of basic circuits. Basic circuits refer to a series of planar signal transmission lines in superconducting quantum bits, except for the Josephson junction, mainly including readout resonant cavities, coupling resonant cavities, control lines and coplanar capacitors of bits. The third step is the preparation of Josephson junctions, which are the core devices of superconducting quantum bits. They provide anharmonic energy levels for bits and achieve stable encoding of |0> and |1> quantum states.

[0033] Preparation of Josephson junction: Josephson junction is usually composed of two superconductors sandwiched by a very thin barrier layer (such as an insulator or semiconductor). The superconductor can be Nb, Al, etc., and the barrier layer can be obtained by oxidation. Thin film deposition: Superconducting materials are deposited on the substrate by methods such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). The thickness, uniformity and crystal quality of the film need to be controlled during the deposition process. Photolithography and etching: Use photolithography technology to transfer the designed pattern to the film, and then remove the excess film through an etching process to form the desired superconducting Josephson junction.

[0034] The improved SQUIDs can use a current bias control line (Z line) that can cause a gradient change in the spatial distribution of the spatial magnetic field. For SQUIDs containing 2N sub-loops, an N-order gradient-changing magnetic field is required to actively adjust the bias flux entering the loop to change the frequency of the bit, while avoiding the magnetic field fluctuation noise with a gradient less than N order in the environment, thereby achieving the exemption of the bit from flux noise and adjustable frequency.

[0035] Since the two groups of N sub-loops are geometrically identical, the induced currents caused by magnetic field fluctuations (0th order magnetic field noise) cancel each other out and do not affect the bit frequency, thus achieving the bit immunity to magnetic flux noise under general conditions. This setting can also reduce or even eliminate Z-line crosstalk between adjacent bits.

[0036] The crossing and superposition of SQUIDs loops are achieved through any method that can achieve a non-conductive crossing structure, such as air bridges, blocking dielectric layers, or encapsulated buried wires. The air bridge process is to use air bridge processing technology to make one arm of the SQUIDs suspended across the other arm after the basic circuit is prepared, and the suspended part presents the shape of an "arch bridge". This is a very common process in the micro-nano processing process. The dielectric layer method is to grow an insulator on one arm of the SQUIDs as a dielectric layer after the basic circuit is prepared, and then grow the other arm of the SQUIDs to achieve spatial crossing, wherein the two arms of the SQUIDs are insulated from each other due to the presence of the dielectric layer. The buried wire method is to form one arm of the SQUIDs on the lower layer of the substrate through a combination of micro-nano processing methods such as etching and coating after the basic circuit is prepared, and fill it with an insulating dielectric layer, and then grow the other arm of the SQUIDs to achieve spatial crossing, wherein the two arms of the SQUIDs are insulated from each other due to the presence of the insulating filling layer, and the overall structure is relatively flat.

[0037] It still has an effective shielding effect for the magnetic field noise in the in-plane direction. Since the improved SQUIDs overlap in space, there is also an effective magnetic flux area in the in-plane direction of the superconducting loop plane. Special design and related micro-nano processing steps are required to achieve the offset of the effective magnetic flux area and achieve the shielding effect of the in-plane magnetic field noise.

[0038] The following case uses zero-order gradient magnetic field noise as an example to further illustrate a frequency-tunable superconducting qubit that is naturally resistant to flux noise:

[0039] In the DC equal-arm SQUIDs structure, by improving the spatial topological geometry of SQUIDs and utilizing spatial overlap, the two arms of the loop are crossed with each other, so that the overall structure presents 2N (N is a positive integer) sub-loops. The improved SQUIDs can actively adjust the bias flux entering the loop to change the frequency of the bit by adopting a shunt bias control line (Z line), thereby achieving frequency adjustability. In the process of actively applying the bias, the induced currents of the N sub-loops in the SQUIDs flow clockwise, and the induced currents of the other N sub-loops flow counterclockwise, forming a loop as a whole. The two groups of N sub-loops are geometrically congruent to each other, so that the flux disturbances caused by the magnetic field noise cancel each other out, and will not interfere with the superconducting critical current of the SQUIDs, thereby achieving the bit's immunity to flux noise under general conditions. When N=1, the overall structure is as follows Figure 1 As shown in the figure, the improved SQUIDs is still a superconducting loop containing a pair of Josephson junctions connected end to end. The magnetic flux noise entering the SQUIDs loop will be distributed in two "sub-loops" of equal area. Due to the global nature of the magnetic field noise, the magnetic flux disturbances caused are Φ0 and -Φ0 respectively, and the two cancel each other out, as shown in the figure. Figure 2 shown.

[0040] Frequency-tunable superconducting quantum bits that are naturally resistant to flux noise, due to the special structure of the improved SQUIDs, need to use a shunt bias control line to achieve equal bias for the two groups of N sub-loops, so that the bias fluxes entering the two groups of N sub-loops can be superimposed on each other. When N = 1, the overall structure of the SQUIDs shunt bias control line is as follows: Figure 3 shown.

[0041] The frequency-tunable superconducting quantum bits that are naturally resistant to flux noise have basically the same preparation and processing steps as common superconducting quantum bits. The first step is the deposition of superconducting thin films. In order to reduce the interface loss between the substrate and the superconducting film, magnetron sputtering, electron beam evaporation, molecular beam epitaxy and other methods can be used in conjunction with different substrate processing processes to grow high-quality superconducting thin films. The second step is the preparation of basic circuits. Basic circuits refer to a series of planar signal transmission lines in superconducting quantum bits other than Josephson junctions, mainly including readout resonant cavities, coupling resonant cavities, control lines and coplanar capacitors of bits. The third step is the preparation of Josephson junctions. Josephson junctions are the core devices of superconducting quantum bits. They provide anharmonic energy levels for the bits and achieve stable encoding of |0> and |1> quantum states. Electron microscope images of physical samples prepared through micro-nano processing are shown below. Figure 4 shown.

[0042] Frequency-tunable superconducting quantum bits that are naturally resistant to magnetic flux noise can achieve the overlap of SQUIDs loops through air bridges, dielectric layers, packaged buried wires and other structures during the preparation process. The air bridge process is to use air bridge processing technology after the basic circuit preparation is completed, so that one arm of the SQUIDs is suspended in the air and crosses the other arm. The suspended part presents the shape of an "arch bridge", so it is called the air bridge process. This is a very common process in the micro-nano processing process. The specific preparation process is as follows Figure 5 shown.

[0043] The dielectric layer method is to grow an insulator as a dielectric layer on one arm of the SQUIDs after the basic circuit is prepared, and then grow the other arm of the SQUIDs to achieve spatial crossing. The two arms of the SQUIDs are insulated from each other due to the presence of the dielectric layer. The specific preparation process is as follows: Figure 6 shown.

[0044] The encapsulation buried wire method is to form one arm of SQUIDs on the lower layer of the substrate through a combination of micro-nano processing methods such as etching and coating after the basic circuit is prepared, and then fill it with an insulating dielectric layer, and then grow the other arm of SQUIDs to achieve spatial crossing. The two arms of SQUIDs are insulated from each other due to the existence of the insulating filling layer, and the overall structure is relatively flat. This process has high requirements on the structure of the substrate material. Its specific preparation process is as follows Figure 7 shown.

[0045] The frequency-tunable superconducting quantum bit, which is naturally resistant to magnetic flux noise, still has an effective shielding effect on the magnetic field noise in the in-plane direction. The improved SQUIDs have spatial overlap, and they also have an effective magnetic flux area in the in-plane direction. Through special design and related micro-nano processing methods, the magnetic flux of SQUIDs along the in-plane direction can be offset, so that Φ1 = -(Φ2 + Φ3), achieving the shielding effect on the in-plane magnetic field noise. The schematic diagram of its structure is shown in Figure 8 As shown, the exemption principle can be seen in the cross-sectional diagram of the structural diagram, such as Fig. 9 shown.

[0046] In a large-scale superconducting quantum bit chip, when multiple bits work simultaneously, crosstalk will occur between them, causing the bit frequency to be disturbed or offset. These crosstalk noises are mainly generated by the magnetic field induced by the control circuit, showing a first-order gradient change in space.

[0047] The following case uses the first-order gradient magnetic field noise as an example to further illustrate a frequency-tunable superconducting quantum bit that is naturally resistant to magnetic flux noise:

[0048] The frequency-tunable superconducting quantum bit that is naturally resistant to magnetic flux noise, when N = 2, the SQUIDs part of the bit consists of 4 sub-loops connected end to end. Under normal conditions, the currents of 2 "loops" flow clockwise, and the currents of 2 "loops" flow counterclockwise. The internal areas of the 4 "loops" are equal, such as Fig.10 As shown in Figure 2, the spatial overlap of each sub-loop is as follows: Fig.11 As shown. The magnetic field noise in any first-order gradient direction can be decomposed into the vector superposition of the X-direction and the Y-direction. The magnetic flux disturbance caused by the magnetic field noise in the X-direction will cancel each other out in the two groups of sub-loops in the orthogonal direction (Y direction); the magnetic flux disturbance caused by the magnetic field noise in the Y-direction will cancel each other out in the two groups of sub-loops in the orthogonal direction (X direction). In this way, the magnetic flux noise entering the SQUIDs loop can cancel each other out because the magnetic flux disturbance it brings is equal in magnitude and opposite in direction in each sub-loop, thus resisting the interference of the first-order gradient magnetic field noise on the bit.

[0049] The frequency-adjustable superconducting qubit of the present invention that is naturally resistant to magnetic flux noise improves the SQUIDs structure and utilizes spatial overlap to cross the two arms of the SQUIDs, so that the overall structure presents 2N (N is a positive integer) sub-loops under a macroscopic magnetic field and has the ability to naturally resist magnetic field noise. A frequency-adjustable superconducting qubit that is naturally resistant to magnetic flux noise only requires the same generalized process preparation conditions and test control conditions as common superconducting qubits, and exceeds the performance of the current version, because after eliminating magnetic field noise, the bit frequency is more stable, and the relaxation time T1 and phase decoherence time T2 are higher. Based on a frequency-adjustable superconducting qubit that is naturally resistant to magnetic flux noise, the present invention provides an effective way to realize bits that are exempt from magnetic field noise and analyze a more general superconducting qubit noise model.

[0050] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and changes can be made without departing from the principles of the present invention. These improvements and changes should also be regarded as the scope of protection of the present invention.

Claims

1. A frequency-tunable superconducting quantum bit that is naturally resistant to flux noise, characterized in that: The composition of superconducting quantum bits includes a DC equal-arm SQUIDs and a bias control line for adjusting the critical current of SQUIDs; wherein, the DC equal-arm SQUIDs is a standard in-plane loop structure, and the two arms of the SQUIDs cross each other to form 2N identical superconducting loops, i.e., sub-loops, where N is a positive integer; the bias control line changes the magnetic flux in the loop by applying a bias magnetic field, causing the critical current of the SQUIDs to change, thereby being able to regulate the frequency of the superconducting quantum bits; in the DC equal-arm SQUIDs structure, by improving the spatial geometric structure of the SQUIDs, the magnetic flux noise entering the SQUIDs loop cancels each other out, making the net magnetic flux accumulated in the loop due to the magnetic field noise zero.

2. The frequency-tunable superconducting quantum bit according to claim 1, which is naturally resistant to magnetic flux noise, is characterized in that In the planar superconducting loop where the SQUIDs of each superconducting quantum bit are located, the two arms of the SQUIDs cross each other, so that the overall structure of the superconducting quantum bit presents 2N planar superconducting loops, i.e., sub-loops, under the macroscopic field; the two groups of N sub-loops are geometrically identical to each other, so that the induced currents caused by magnetic field fluctuations, i.e., 0th-order magnetic flux noise, cancel each other out and do not affect the bit frequency, while reducing or even eliminating the direct Z-line crosstalk between adjacent bits.

3. The frequency-tunable superconducting quantum bit that is naturally resistant to flux noise according to claim 1, characterized in that: When N=1, under the active bias of the bias control line (Z line), the SQUIDs part in the superconducting quantum bit consists of one "loop" in which the current flows clockwise and one "loop" in which the current flows counterclockwise, and the internal areas of these two "loops" are equal; the same applies to the case of N≥2.

4. The frequency-tunable superconducting quantum bit according to claim 1, which is naturally resistant to flux noise, is characterized in that S The crossing and superposition of QUIDs loops are achieved by using air bridges, blocking dielectric layers or encapsulated buried wires to achieve a non-conductive crossing structure.

5. The method for preparing a frequency-tunable superconducting quantum bit that is naturally resistant to magnetic flux noise according to any one of claims 1 to 4, characterized in that: The first step is the deposition of superconducting thin films, using magnetron sputtering, electron beam evaporation, molecular beam epitaxy and other methods in combination with different substrates to grow high-quality superconducting thin films; the second step is the preparation of basic circuits, which refers to a series of planar signal transmission lines in superconducting quantum bits, except for the Josephson junction, including readout resonant cavities, coupling resonant cavities, control lines and coplanar capacitors of the bits; the third step is the preparation of the Josephson junction, which is the core device of the superconducting quantum bit and provides the bit with anharmonic energy levels, realizing the stable encoding of |0> and |1> quantum states.

6. The preparation method according to claim 5, characterized in that S The crossing and superposition of QUIDs loops are achieved through any method that can achieve a non-conductive crossing structure, such as air bridges, blocking dielectric layers, or packaged buried wires. The air bridge process is to use air bridge processing technology after the basic circuit is prepared so that one arm of the SQUIDs is suspended and crosses the other arm, and the suspended part presents a shape of an "arch bridge". The dielectric layer method is to grow an insulator on one arm of the SQUIDs to serve as a dielectric layer after the basic circuit is prepared, and then grow the other arm of the SQUIDs to achieve spatial crossing, wherein the two arms of the SQUIDs are insulated from each other due to the presence of the dielectric layer; the buried wire method is to form one arm of the SQUIDs on the lower layer of the substrate through a combination of micro-nano processing methods such as etching and coating after the basic circuit is prepared, and fill it with an insulating dielectric layer, and then grow the other arm of the SQUIDs to achieve spatial crossing, wherein the two arms of the SQUIDs are insulated from each other due to the presence of the insulating filling layer, and the overall structure is relatively flat.

7. The method according to claim 5, characterized in that: The improved SQUIDs have spatial overlap, and there is also an effective magnetic flux area in the in-plane direction of the superconducting loop plane. Through the combined use of air bridges, blocking dielectric layers or packaging buried wire micro-nano processing solutions, the effective magnetic flux area in the in-plane direction of the ring can also be offset, thereby achieving shielding of the in-plane magnetic field noise.

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