Method for reducing impact of quasi-particle defects on bits and use thereof

Abstract

The present invention relates to the field of quantum devices, in particular to a method for reducing impact of quasi-particle defects on bits and the use thereof. In the present invention, for an Xmon bit, on one hand, adaptation optimization is performed on the structure thereof, such that a bit capacitor is designed as a radiation-insensitive structure, which effectively inhibits the generation of quasi-particles at the Josephson junction of the Xmon bit; and on the other hand, infrared shielding technology is used to place a wave-absorbing material in the terahertz frequency band above a quantum chip in a package, which can effectively improve the electromagnetic environment of the quantum chip and achieve good compatibility with various quantum chips. The method can significantly reduce the negative impact of quasi-particle defects on quantum bits.

Description

A method for reducing the influence of quasi-particle defects on qubits and its application Technical Field

[0001] This invention relates to the field of quantum devices (IPC classification: H01L), and more particularly to a method for reducing the influence of quasiparticle defects on qubits and its application. Background Technology

[0002] Quantum error correction technology assumes that as the scale of superconducting quantum circuits increases, the errors of physical bits can be kept at a small level and sufficiently uncorrelated, thereby exponentially suppressing the error rate of logical bits. However, factors such as ionizing radiation from cosmic rays, absorption of stray photons (greater than twice the superconducting bandgap, on the terahertz scale), and thermal excitation can lead to the generation of quasiparticles. The diffusion of quasiparticles in the chip severely limits the coherence of the bits (the ability of a quantum state to maintain its superposition properties and phase relationship during time evolution). Such large-scale coherent errors may lead to the failure of quantum error correction, thus affecting the application of quantum chips. Therefore, protecting superconducting quantum chips from quasiparticle "poisoning" is crucial for maintaining the stability of superconducting quantum bit energy levels and ensuring the performance of quantum chips. Quasiparticles in superconducting quantum devices have been extensively studied in the past decade. At low temperatures, non-thermal equilibrium quasiparticles can induce phenomena such as relaxation and excitation of qubits, as well as charge parity switching in superconducting islands. Methods to suppress quasiparticle poisoning include improving filtering and shielding techniques, designing the bandgap distribution near the Josephson junction to reduce the probability of quasiparticle tunneling, and using phonon traps to capture quasiparticles in normal metal islands to prevent them from crossing the Josephson junction.

[0003] Current technology, based on flip-chip bonding, uses floating aluminum caps 10μm apart from the capacitor plate to cover the qubits. On the one hand, this approach has high requirements for micro / nano fabrication and is dependent on the structure of the quantum chip. As the number of qubits increases, the number of metal caps also increases, limiting its practical scalability and mass production. On the other hand, shielding materials are typically added to the microwave lines and the outside of the package, without considering the resonant absorption of terahertz electromagnetic waves within the package by the qubits acting as antennas. Furthermore, current solutions only attempt to optimize the design of floating qubits, while other commonly used qubit structures are not fully considered.

[0004] Pan Xianchuang, Zhou Yuxuan, and others, in their paper "Engineering superconducting qubits to reduce quasiparticles and charge noise," investigated the relationship between superconducting qubit structure design and the rate of non-equilibrium quasiparticle generation, and proved the locality of quasiparticle origin. This research demonstrated how to effectively suppress quasiparticle poisoning through qubit structure design. By employing miniaturized qubit design, vertical packaging methods, and quasiparticle potential well design, the stability of the qubits was improved, providing feasible technical support for the research of superconducting quantum chips. However, the design of this technology is complex and its fabrication is difficult, limiting its practical feasibility and still failing to meet the application requirements of superconducting quantum chips.

[0005] Xmon qubits are a type of qubit implementation based on superconducting circuits. They possess advantages such as high coherence, strong coupling, scalability, and high-precision control, and are widely used to build quantum computers, execute complex quantum algorithms, and solve problems that are difficult for conventional computers to handle. However, current technology lacks effective means to suppress the influence of quasiparticle defects on Xmon qubits. Summary of the Invention

[0006] This invention targets the Xmon qubit. On the one hand, it optimizes the structure by designing the qubit capacitor as a radiation-insensitive structure, effectively suppressing the generation of quasi-particles at the Josephson junction of the Xmon qubit. On the other hand, it employs infrared shielding technology to place terahertz-band absorbing material above the quantum chip within the package, which can effectively improve the electromagnetic environment of the quantum chip and has good compatibility with different quantum chips. This method can significantly reduce the negative impact of quasi-particle defects on the quantum bit.

[0007] This invention provides a method for reducing the impact of quasi-particle defects on bits, wherein the bit is an Xmon bit with a cross-shaped capacitor structure.

[0008] The method includes at least: obtaining a shielded bit capacitor by designing the cross-shaped capacitor physical structure of the Xmon bit.

[0009] In some alternative implementations, specifically, the target structure of the cross capacitor is determined by finding the minimum coupling efficiency between the cross capacitor and the Josephson junction.

[0010] A qubit is the fundamental unit in quantum computing, possessing properties such as quantum superposition and entanglement. These properties do not preclude a qubit from having a physical structure; the physical implementations of qubits (such as superconducting circuits, trapped ions, quantum dots, neutral atoms, photons, and topological qubits) embody their physical structures. These physical structures enable the manipulation and reading of the qubit's state within actual physical systems, thereby realizing quantum computing.

[0011] Xmon qubits, a variant of transmon qubits, typically refer to the physical layout and component configuration of superconducting circuits in terms of geometry. The arrangement of these circuit components (such as Josephson junctions and capacitors) in two- or three-dimensional space constitutes the geometry of the qubit. Xmon qubits employ a cross-capacitor structure to enhance their performance. This structure specifically consists of two mutually perpendicular capacitor arms, forming a cross shape, offering advantages such as good scalability and simple fabrication.

[0012] This invention is based on the Xmon cross-capacitor structure. By simulating the radiation impedance of the Xmon bit and calculating its coupling efficiency with the junction impedance, the sensitivity of the Xmon bit to radiation can be determined.

[0013] Optionally, the coupling efficiency of the cross capacitor and the Josephson junction is determined by the reflection coefficient of the cross capacitor, and the two are negatively correlated. By setting the coupling efficiency as a negative function of the reflection coefficient, the sensitivity of the Xmon bit to radiation can be determined through the coupling efficiency, thereby obtaining the parameter information of the cross capacitor corresponding to the lowest coupling efficiency and determining the optimal shielding size of the capacitor's physical structure.

[0014] Optionally, the coupling efficiency of the cross capacitor and the Josephson junction is determined by calculation using the following formula 1. c =1-|Γ| 2 (Formula 1)

[0015] Among them, e c Γ represents the coupling efficiency between the cross capacitor and the Josephson junction, and Γ represents the reflection coefficient of the cross capacitor.

[0016] Optionally, the reflection coefficient of the cross capacitor is determined by the radiation impedance of the cross capacitor and the radiation impedance of the Josephson junction. The reflection coefficient can quantify the matching degree between the radiation impedance of the cross capacitor and the radiation impedance of the Josephson junction, thereby achieving the best signal transmission or energy conversion efficiency.

[0017] Optionally, the reflection coefficient of the cross capacitor is determined by the following formula 2.

[0018] Where Γ is the reflection coefficient of the cross capacitor, Z rad Z is the radiation impedance of the cross capacitor. j Z represents the radiation impedance of the Josephson junction. j * is Z j The conjugate of complex numbers.

[0019] Optionally, the radiation impedance of the cross capacitor is determined through modeling and simulation. Specifically, a model is created based on the initial size information of the cross capacitor, and the model is simulated at the Josephson junction to obtain the radiation impedance of the antenna (i.e., the cross capacitor). By simulating the radiation impedance of the bit capacitor and calculating its coupling efficiency with the Josephson junction impedance, the sensitivity of the bit capacitor to radiation is determined, thereby determining the capacitor size corresponding to the insensitive state.

[0020] Optionally, the initial size information of the cross capacitor can be listed as the length of a single arm of the cross capacitor, the width s of the center strip, and the distance of the center strip from the ground, etc.

[0021] Optionally, the parameters required for modeling also include: substrate height (e.g., 0.5 mm), cross capacitance and film thickness (e.g., 0.1 μm), and substrate material (e.g., sapphire).

[0022] Optionally, a Python script can be created for HFSS (High Frequency Structure Simulator) to import the initial size information of the cross capacitor for modeling and perform simulation at the Josephson junction.

[0023] Optionally, the libraries used by Python are listed below:

[0024] Modeling: pywin32, numpy; Simulation calculation: numpy, scipy, matplotlib, pandas.

[0025] Optionally, the radiation impedance of the Josephson junction is determined by the functional relationship between the electromagnetic wave frequency, the quantum circuit time constant, and the tunneling resistance of the Josephson junction.

[0026] Optionally, the radiation impedance of the Josephson junction is determined by the following formula 3.1.

[0027] Among them, Z j Let R be the radiation impedance of the Josephson junction, ω be the frequency of the electromagnetic wave, τ be the time constant of the quantum circuit, and R be the radiation impedance of the Josephson junction. n This represents the tunneling resistance of the Josephson junction.

[0028] Optionally, the time constant of the quantum circuit is determined by the electrical parameters of the Josephson junction, characterizing the speed of the quantum circuit response. These electrical parameters may include, for example, the tunneling resistance and capacitance of the Josephson junction.

[0029] Optionally, the time constant of the quantum circuit is determined by the following formula 3.2: τ≡R n C j (Formula 3.2)

[0030] Among them, C j The capacitance of the Josephson junction.

[0031] Optionally, the value of ω ranges from 0 to 200 GHz.

[0032] Optionally, the R n The value range is 6000 to 10000Ω, and more preferably 8000Ω.

[0033] Optionally, the C j The range is 1 to 10 fF, more preferably 2 fF.

[0034] This invention automates the design of Xmon bit capacitors to create a radiation-insensitive structure. Specifically, it first determines the radiation impedance of the cross-shaped capacitor through simulation, and then determines its radiation impedance based on the electrical parameters of the Josephson junction. Next, it determines the reflection coefficient of the cross-shaped capacitor using the functional relationship between the two, and further calculates the coupling efficiency between the radiation impedance of the cross-shaped capacitor and the Josephson junction to determine the sensitivity of the bit capacitor to electromagnetic radiation. This yields the capacitor structure parameters corresponding to the lowest coupling efficiency (equivalent to minimizing cost), thus completing the structural design of the shielded bit capacitor.

[0035] In this method, ω and R n C j It can be set according to actual conditions; the design method has strong compatibility.

[0036] Optionally, the method further includes: fabricating the Xmon qubit with shielding bit capacitance into a quantum chip, and then placing the quantum chip together with absorbing material in a quantum package for use to shield against radiation interference.

[0037] Note: The Xmon bit with a specific capacitor structure, combined with absorbing materials, further enhances the radiation shielding effect.

[0038] Optionally, the absorbing material can absorb electromagnetic waves with frequencies above 80 GHz. This invention has found that selecting an absorbing material capable of absorbing electromagnetic waves exceeding twice the frequency of the superconducting band (i.e., greater than 80 GHz) can further enhance the radiation shielding effect.

[0039] Further optionally, the absorbing material is capable of absorbing electromagnetic waves with frequencies of 100 to 10000 GHz (i.e., 0.1 to 10 THz in the terahertz band).

[0040] Optionally, the absorbing material includes one or more combinations of polymer absorbing materials, ferrite absorbing materials, electromagnetic metamaterials, iron-cobalt-ruthenium alloy superconducting materials, and polycrystalline iron fiber absorbing materials.

[0041] Optionally, the absorbing material includes polymer absorbing materials or electromagnetic metamaterials.

[0042] Optionally, the polymeric microwave absorbing material includes a mesh-like open-cell polyurethane foam.

[0043] The reticulated open-cell polyurethane foam can be commercially available, for example... A series of microwave absorbing materials.

[0044] As a conductive, lightweight, broadband absorbing material, it is made of a mesh-like open-cell polyurethane foam with a continuous conductivity gradient. It exhibits excellent absorption performance for electromagnetic waves ranging from a few GHz to over 100 GHz, meeting the frequency requirements of absorbing materials for stray photons. The material's thickness can be listed as 10 mm, 15 mm, or 20 mm, ensuring dimensional compatibility with quantum packaging materials (such as superconducting quantum chip packaging boxes) in practical applications.

[0045] Electromagnetic metamaterials are artificial structural materials with unique electromagnetic properties, composed of subwavelength periodic structural units. They exhibit unconventional material properties, such as negative permittivity and negative permeability. The absorption characteristics of electromagnetic metamaterials are not limited by the quarter-wavelength limitation of traditional absorbing structures, allowing for a significant reduction in structural thickness. Furthermore, the electromagnetic parameters of electromagnetic metamaterials are flexibly adjustable, enabling highly efficient absorption of submillimeter waves.

[0046] The mesh-like open-cell polyurethane foam or electromagnetic metamaterial of this invention can be adapted to most superconducting transporter qubits, without the need for frequent updates to adapt to chip iterations, and is highly feasible.

[0047] A second aspect of the present invention provides an application of the method described above for reducing the influence of quasiparticle defects on qubits in a quantum package.

[0048] The quantum package includes a concave metal base; the metal base contains, from bottom to top, a quantum chip and a microwave absorbing material that are adapted to the size of the concave shape; and a package cover is provided at the upper end of the metal base.

[0049] The Xmon qubit, possessing shielding properties, is obtained by employing the aforementioned method to reduce the impact of quasiparticle defects on qubits. This qubit is then fabricated into a quantum chip and placed at the bottom of a package. A specific absorbing material is placed on top, and the package is then capped, effectively improving the electromagnetic environment of the quantum chip. Furthermore, this method does not require additional modifications to the package structure; only the dimensions of the absorbing material need to be adjusted according to the package structure's adaptability. It exhibits good compatibility with different quantum chips. This method reduces packaging costs while maintaining shielding effectiveness, offering significant advantages in operability.

[0050] Note: This invention does not impose particular limitations on the structure of the quantum chip, the shape of the quantum package, the material of the package cover, or the packaging method of the quantum package, as long as the design objective is achieved. For example, the quantum package can be a package box; the material of the package cover can be aluminum, copper, superconducting materials, etc. Beneficial effects:

[0051] This invention provides a method for reducing the influence of quasi-particle defects on qubits and its application, which has the following advantages:

[0052] (1) This invention is designed for Xmon bits. The cross capacitor structure of the bit is designed by calculating the coupling efficiency of the cross capacitor and Josephson junction and limiting it to the lowest possible level, so as to determine the optimal shielding size of the cross capacitor, suppress quasi-particle interference to the greatest extent, and improve the performance of Xmon bits.

[0053] (2) When determining the cross capacitor shielding structure of the Xmon bit, the present invention uses Python to model the structure of the cross capacitor and performs simulation at the Josephson junction to obtain the relationship between the radiation impedance of the cross capacitor and the electromagnetic wave frequency, thereby quantifying the matching degree between the radiation impedance of the cross capacitor and the Josephson junction and ensuring the best shielding performance.

[0054] (3) This invention further combines infrared shielding technology with capacitor structure design, selecting polymer absorbing materials (e.g. Or electromagnetic metamaterials, when packaged, are placed above the quantum chip to effectively improve the electromagnetic environment of the quantum chip and suppress the negative interference of electromagnetic radiation on the bits and the chip.

[0055] (4) The improved method of the present invention only requires designing the cross capacitor structure of Xmon and using it together with absorbing materials to effectively reduce quasi-particle defects. It has good adaptability to various types of quantum chips, effectively reducing the difficulty of device design and fabrication, and making it easy to implement.

[0056] (5) The present invention does not require any additional modification to the specifications and dimensions of the quantum chip and the absorbing material, nor does it require any change to the structure of the package. It only requires adapting the dimensions of the quantum chip, the absorbing material and the package to obtain a packaged component with significant shielding effect. The method has strong compatibility. Attached Figure Description

[0057] Figure 1. Sample design flowchart for reducing quasiparticle defects;

[0058] Figure 2. Schematic diagram of a cross capacitor;

[0059] Figure 3. Relationship between radiation impedance of a cross capacitor and frequency (where the horizontal axis is the electromagnetic wave frequency ω, and the vertical axis is the radiation impedance of the cross capacitor; the red line is the real part, and the green line is the imaginary part).

[0060] Figure 4. Schematic diagram of quantum chip structure;

[0061] Figure 5. Schematic diagram of the quantum package (1: metal base; 2: quantum chip; 3: microwave absorbing material; 4: metal cover). Detailed Implementation

[0062] The formula parameters, their definitions, and data sources involved in this method are shown in the table below.

[0063] Example

[0064] This embodiment provides a method for reducing the influence of quasi-particle defects on qubits, as shown in Figure 1. The method includes:

[0065] S1. By designing the cross-shaped capacitor structure of the Xmon bit, a shielded bit capacitor is obtained;

[0066] S2. The Xmon qubit with shielded bit capacitance is made into a quantum chip, and then the quantum chip is placed together with the absorbing material in the packaging assembly to shield against radiation interference.

[0067] In step S1, the optimal shielding size of the capacitor physical structure is determined by determining the coupling efficiency of the cross capacitance and Josephson junction of the Xmon bit.

[0068] The coupling efficiency of the cross capacitor and the Josephson junction is determined by the following formula 1. c =1-|Γ| 2 (Formula 1)

[0069] Among them, e c Γ represents the coupling efficiency between the cross capacitor and the Josephson junction, and Γ represents the reflection coefficient of the cross capacitor.

[0070] The reflection coefficient of the cross capacitor is determined by the following formula 2.

[0071] Where Γ is the reflection coefficient of the cross capacitor, Z rad Z is the radiation impedance of the cross capacitor. j Let be the radiation impedance of the Josephson junction.

[0072] The radiation impedance Z of the cross capacitor rad The model is determined through modeling and simulation. Specifically, based on the initial size information of the cross capacitor, a model is created and simulated at the Josephson junction to obtain the radiation impedance of the antenna (i.e., the cross capacitor). By simulating the radiation impedance of the bit capacitor and calculating its coupling efficiency with the Josephson junction impedance, the sensitivity of the bit capacitor to radiation is determined, thereby determining the capacitor size corresponding to the insensitive state.

[0073] The structural schematic diagram of the cross-shaped capacitor is shown in Figure 2. Its initial dimensions include the length l of one arm, the width s of the central strip, and the distance w of the central strip from the ground. An HFSS script for the cross-shaped capacitor is created using Python, importing the initial dimension data for modeling. Simulation is then performed at the Josephson junction to determine Z. rad .

[0074] The radiation impedance Z of the Josephson junction j It is determined by calculation using the following formula 3.1.

[0075] Among them, Z j Let R be the radiation impedance of the Josephson junction, ω be the frequency of the electromagnetic wave, τ be the time constant of the quantum circuit, and R be the radiation impedance of the Josephson junction. n This represents the tunneling resistance of the Josephson junction.

[0076] The time constant of the quantum circuit is determined by the following formula 3.2: τ≡R n C j (Formula 3.2)

[0077] Among them, C j The capacitance of the Josephson junction.

[0078] Figure 4 shows a schematic diagram of the quantum chip prepared in step S2.

[0079] Selection of absorbing material in step S2 Its material is a mesh-like open-cell polyurethane foam, which can absorb electromagnetic waves with a frequency of 80GHz or higher.

[0080] The second aspect of this embodiment provides an application of the method described above for reducing the influence of quasi-particle defects on qubits in a quantum package.

[0081] The quantum package includes a concave metal base; the metal base contains, from bottom to top, a quantum chip and a wave-absorbing material that are adapted to the size of the concave shape; a package cover is provided at the upper end of the metal base; a schematic diagram of the quantum package is shown in Figure 5.

[0082] The material of the encapsulation cover is Al.

[0083] The metal substrate is made of copper.

[0084] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of reducing the effect of quasi-particle defects on bits, comprising: The bit is an Xmon bit with a cross-shaped capacitor structure; The method includes at least: The physical structure of the cross-shaped capacitor for the Xmon bit is designed to obtain a shielded bit capacitor. The design process of the structure includes: The target structure of the cross capacitor is determined by finding the minimum coupling efficiency between the cross capacitor and the Josephson junction.

2. The method of claim 1, wherein, The coupling efficiency of the cross capacitor and the Josephson junction is determined by the reflection coefficient of the cross capacitor, and the two are negatively correlated.

3. The method of claim 2, wherein the method further comprises: The reflection coefficient of the cross capacitor is determined by the radiation impedance of the cross capacitor and the radiation impedance of the Josephson junction.

4. The method of claim 3, wherein the method further comprises: The method for determining the radiation impedance of the cross capacitor includes: A model of the cross-shaped capacitor of the Xmon bit is constructed, and the model is simulated at the Josephson junction to obtain the radiation impedance of the cross-shaped capacitor. The variables of the model include the single arm length of the cross-shaped capacitor, the width of the center strip of the cross-shaped capacitor, and the distance of the center strip of the cross-shaped capacitor to ground.

5. The method of claim 3, wherein the method further comprises: The radiation impedance of the Josephson junction is determined by the relationship between the electromagnetic wave frequency, the quantum circuit time constant, and the tunneling resistance of the Josephson junction.

6. The method of claim 1, wherein, The method further includes: Xmon bits with shielded bit capacitance are fabricated into a quantum chip, and then the quantum chip is used together with absorbing material in a packaging structure to shield against radiation interference.

7. The method of claim 6, wherein the method further comprises: The absorbing material can absorb electromagnetic waves with frequencies above 80 GHz.

8. The method of claim 7, wherein the method further comprises: The absorbing material includes one or more of the following: polymer absorbing materials, ferrite absorbing materials, electromagnetic metamaterials, iron-cobalt-ruthenium alloy superconducting materials, and polycrystalline iron fiber absorbing materials.

9. The method of claim 8, wherein the method further comprises: The polymer absorbing material includes a mesh-like open-cell polyurethane foam.

10. Use of a method of reducing the contrast of quasi-particle defects on bits according to any one of claims 6 to 9 in a quantum package, characterized in that, The quantum package includes a concave metal base; the metal base contains, from bottom to top, a quantum chip and a microwave absorbing material whose dimensions are adapted to the concave shape. The metal base is provided with an encapsulation cover at its upper end.

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