Method for reducing influence of quasi-particle defects on bits and application thereof

Abstract

The invention relates to the field of quantum devices, in particular to a method for reducing the influence of quasi-particle defects on bits and application of the method. Aiming at the Xmon bit, on one hand, adaptive optimization is performed on the structure of the Xmon bit, a bit capacitor is designed into a structure insensitive to radiation, and generation of quasi-particles at a Josephson junction of the Xmon bit is effectively inhibited; on the other hand, an infrared shielding technology is adopted to place a terahertz-frequency-band wave-absorbing material above the quantum chip in the packaging body, so that the electromagnetic environment of the quantum chip can be effectively improved, and good compatibility is achieved for different quantum chips; according to the method, the negative influence of the quasi-particle defect on the quantum bit can be remarkably reduced.

Description

Technical Field

[0001] The present invention relates to the field of quantum devices (IPC classification number: H01L), and in particular to a method for reducing the influence of quasiparticle defects on bits and an application thereof. Background Art

[0002] Quantum error correction technology assumes that as the scale of superconducting quantum circuits expands, the errors of physical bits can be kept at a small level and sufficiently uncorrelated, so that the error rate of logical bits is exponentially suppressed. However, factors such as ionizing radiation from cosmic rays, absorption of stray photons (greater than twice the superconducting energy gap, terahertz level), and thermal excitation can lead to the generation of quasiparticles, and the diffusion of quasiparticles in the chip severely limits the coherence of the bits (the ability of quantum states to maintain their superposition characteristics and phase relationships during time evolution). Such large-scale coherence errors may cause quantum error correction failures, thereby affecting the application of quantum chips. Therefore, protecting superconducting quantum chips from quasiparticle "poisoning" is crucial to maintaining the stability of superconducting quantum bit energy levels and ensuring the performance of quantum chips. Quasiparticles in superconducting quantum devices have been intensively studied in the past decade. At low temperatures, non-thermal equilibrium quasiparticles induce phenomena such as relaxation and excitation of quantum bits and charge parity switching of superconducting islands. Methods to suppress quasiparticle poisoning include improving filtering and shielding technology, designing the band gap 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] The existing technology is based on flip-chip technology, which covers the quantum bit with a floating aluminum cap 10μm away from the capacitor plate. On the one hand, this solution has relatively high requirements for micro-nano processing and depends on the structure of the quantum chip. As the number of bits increases, the number of metal covers also increases, resulting in limited actual expansion and scale. On the other hand, shielding materials are usually added to the outside of the microwave line and the packaging box, without considering the resonant absorption of terahertz frequency band electromagnetic waves in the internal space of the packaging box by the bit as an antenna. At the same time, the existing scheme only attempts to optimize the design of floating bits, while other commonly used bit structures have not been fully considered.

[0004] In the paper "Engineering superconducting qubits to reduce quasiparticles and charge noise", Pan Xianchuang, Zhou Yuxuan and others studied the relationship between the design of superconducting quantum bit structure and the rate of non-equilibrium quasiparticle generation, and proved the locality of the source of quasiparticles. This study demonstrated that the quasiparticle poisoning phenomenon can be effectively suppressed by designing the quantum bit structure, and the stability of the quantum bit was improved by means of miniaturized bit design, vertical packaging and capping methods, and quasiparticle potential well design, providing feasible technical support for the research of superconducting quantum chips. However, the design of this technology is complex and the processing difficulty is high. Its feasibility is limited in practical application and still cannot meet the application requirements of superconducting quantum chips.

[0005] Xmon bit is a quantum bit (qubit) implementation method based on superconducting circuits. It has the advantages of high coherence, strong coupling, scalability and high-precision control. It is widely used to build quantum computers, execute complex quantum algorithms, and solve problems that are difficult for traditional computers to handle. However, there is currently no technical means to effectively suppress the impact of quasiparticle defects on Xmon bits. Summary of the invention

[0006] The present invention optimizes the structure of the Xmon bit by adapting it, designs the bit capacitor to be a structure insensitive to radiation, and effectively suppresses the generation of quasiparticles at the Josephson junction of the Xmon bit; on the other hand, infrared shielding technology is used to place terahertz frequency band absorbing materials above the quantum chip in 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 quasiparticle defects on quantum bits.

[0007] The present invention provides a method for reducing the influence of quasiparticle defects on bits, wherein the bits are Xmon bits having a cross capacitor structure.

[0008] The method at least comprises: obtaining a shielding bit capacitor by designing a cross capacitor physical structure of an Xmon bit.

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

[0010] The qubit is the basic unit in quantum computing, with properties such as quantum superposition and entanglement, and these properties do not prevent the qubit from having a physical entity structure; the physical implementation of the qubit (such as superconducting circuits, trapped ions, quantum dots, neutral atoms, photons, and topological qubits) is the embodiment of its physical structure. These physical structures help to control and read the state of the qubit in the actual physical system, thereby realizing quantum computing.

[0011] As a variant of the Transmon qubit, the geometry of the Xmon bit usually refers to the physical layout and component configuration of the superconducting circuit; the arrangement of these circuit components (such as Josephson junctions, capacitors, etc.) in two-dimensional or three-dimensional space constitutes the geometry of the qubit. The Xmon bit uses a cross capacitor structure to enhance the performance of the qubit. This structure consists of two mutually perpendicular capacitor arms, forming a shape similar to a "cross", with advantages such as good scalability and simple process.

[0012] The present invention is based on the cross capacitor structure of Xmon, simulates the radiation impedance of the Xmon bit, and calculates the coupling efficiency between the radiation impedance and the junction impedance to judge the sensitivity of the Xmon bit to radiation.

[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. The coupling efficiency is set as a negative function of the reflection coefficient, and the sensitivity of the Xmon bit to radiation can be judged by 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 physical structure.

[0014] Optionally, the coupling efficiency between the cross capacitor and the Josephson junction is determined by calculation using the following formula 1.

[0015] e c =1-|Γ| 2 (Formula 1)

[0016] Among them, e c is the coupling efficiency between the cross capacitor and the Josephson junction, and Γ is the reflection coefficient of the cross capacitor.

[0017] 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 of the radiation impedance of the cross capacitor and the radiation impedance of the Josephson junction, thereby achieving optimal signal transmission or energy conversion efficiency.

[0018] Optionally, the reflection coefficient of the cross capacitor is calculated and determined by the following formula 2.

[0019]

[0020] Where Γ is the reflection coefficient of the cross capacitor, Z rad is the radiation impedance of the cross capacitor, Z j is the radiation impedance of the Josephson junction. j * Z j The complex conjugate of .

[0021] Optionally, the radiation impedance of the cross capacitor is determined by modeling and simulation; specifically, the model is modeled 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, and then the capacitor size corresponding to the insensitive state is determined.

[0022] Optionally, the initial size information of the cross capacitor may include the length of a single arm of the cross capacitor, the width s of the center strip, and the distance between the center strip and the ground.

[0023] Optionally, the parameters required for modeling also include: substrate height (eg, 0.5 mm), cross capacitance and film thickness (eg, 0.1 μm), and substrate material (eg, sapphire).

[0024] Optionally, create an HFSS (High Frequency Structure Simulator) script using Python, import the initial size information data of the cross capacitor for modeling, and perform simulation operations at the Josephson junction.

[0025] Optional, the Python libraries used are listed below:

[0026] Modeling: pywin32, numpy; simulation calculation: numpy, scipy, matplotlib, pandas.

[0027] Optionally, the radiation impedance of the Josephson junction is determined by a functional relationship between the electromagnetic wave frequency, the quantum circuit time constant and the tunneling resistance of the Josephson junction.

[0028] Optionally, the radiation impedance of the Josephson junction is determined by calculation using the following formula 3.1.

[0029]

[0030] Among them, Z j is the radiation impedance of the Josephson junction, ω is the frequency of the electromagnetic wave, τ is the quantum circuit time constant, R n is the tunneling resistance of the Josephson junction.

[0031] Optionally, the quantum circuit time constant is determined by an electrical parameter of a Josephson junction, which characterizes the speed of the quantum circuit response. The electrical parameter may be a tunneling resistance of a Josephson junction or a capacitance of a Josephson junction.

[0032] Optionally, the quantum circuit time constant is calculated and determined by the following formula 3.2.

[0033] τ≡R n C j (Formula 3.2)

[0034] Among them, C j is the capacitance of the Josephson junction.

[0035] Optionally, the value range of ω is 0 to 200 GHz.

[0036] Optionally, the R n The value range of is 6000-10000Ω, and more preferably is 8000Ω.

[0037] Optionally, the C j The range is 1 to 10 fF, and more preferably 2 fF.

[0038] The present invention automatically designs the Xmon bit capacitor as a structure insensitive to radiation. Specifically, the radiation impedance of the cross capacitor is first determined by simulation, and its radiation impedance is determined based on the electrical parameters of the Josephson junction, and then the reflection coefficient of the cross capacitor is determined by the functional relationship between the two, and the coupling efficiency of the cross capacitor and the Josephson junction radiation impedance is further calculated to determine the sensitivity of the bit capacitor to electromagnetic wave radiation, and obtain the corresponding capacitor structure parameter information when the coupling efficiency is the lowest (equivalent to the minimum cost level), thereby completing the structural design of the shielded bit capacitor.

[0039] In this method, ω, R n , C j It can be set according to actual conditions; the design method has strong compatibility.

[0040] Optionally, the method further includes: manufacturing the Xmon bit with shielding bit capacitance into a quantum chip, and then placing the quantum chip and the absorbing material together in a quantum package for use to shield radiation interference.

[0041] Note: The introduction of absorbing materials and Xmon bits with specific capacitor structures can further enhance the radiation shielding effect.

[0042] Optionally, the absorbing material can absorb electromagnetic waves with a frequency of more than 80 GHz. The present invention has found that selecting an absorbing material that can absorb electromagnetic waves with a frequency exceeding twice the superconducting energy gap (ie, greater than 80 GHz) can further improve the radiation shielding effect.

[0043] Further optionally, the absorbing material can absorb electromagnetic waves with a frequency of 100 to 10,000 GHz (ie, 0.1 to 10 THz in the terahertz band).

[0044] Optionally, the absorbing material includes one or more combinations of high molecular polymer absorbing materials, ferrite absorbing materials, electromagnetic metamaterials, iron-cobalt-ruthenium alloy superconducting materials, and polycrystalline iron fiber absorbing materials.

[0045] Optionally, the absorbing material includes a high molecular polymer absorbing material or an electromagnetic metamaterial.

[0046] Optionally, the high molecular polymer absorbing material includes reticular open-cell polyurethane foam.

[0047] The reticulated open-cell polyurethane foam may be commercially available, for example HR series microwave absorbing materials.

[0048] HR is a conductive, lightweight, broadband absorbing material made of open-cell polyurethane foam. It has a continuous conductivity gradient and has good absorption performance for electromagnetic waves from a few GHz to over 100 GHz, which can meet the frequency requirements of absorbing materials for stray photons. The thickness of the absorbing material can be listed as 10mm, 15mm, and 20mm, which can meet the size adaptability of quantum packages (such as superconducting quantum chip packaging boxes) in practical applications.

[0049] Electromagnetic metamaterials refer to artificial structural materials with special electromagnetic properties that are composed of sub-wavelength periodic structural units. Electromagnetic metamaterials have anti-conventional material properties, such as negative dielectric constant and negative magnetic permeability. The absorption characteristics of electromagnetic metamaterials are not limited by the quarter wavelength of traditional wave-absorbing structures, and the thickness of the structure can be greatly reduced; and the electromagnetic parameters of electromagnetic metamaterials are flexible and adjustable, which can achieve efficient absorption of submillimeter waves.

[0050] The reticular open-cell polyurethane foam or electromagnetic metamaterial of the present invention can adapt to most superconducting transmon quantum bits, does not need to be frequently updated to adapt to chip iterations, and has strong feasibility.

[0051] A second aspect of the present invention provides an application of the above-mentioned method for reducing the influence of quasiparticle defects on bits in a quantum package.

[0052] The quantum package comprises a metal base with a concave shape; the metal base contains quantum chips and absorbing materials that match the size of the concave shape in sequence from bottom to top; and a packaging cover is arranged on the upper end of the metal base.

[0053] The above method of reducing the influence of quasiparticle defects on bits is used to obtain Xmon bits with shielding properties. The bits are made into quantum chips and placed at the bottom of the package. A specific absorbing material is placed on top of the bits, and then the package cover is covered to effectively improve the electromagnetic environment of the quantum chip. At the same time, this method does not require additional changes to the structure of the package. It only needs to adjust the size of the absorbing material according to the adaptability of the package structure, and has good compatibility with different quantum chips. This method can reduce the packaging cost while ensuring shielding, and has the advantage of operability.

[0054] Note: The present invention does not specifically limit the structure of the quantum chip, the shape of the quantum package, the material of the package cover, the packaging method of the quantum package, etc., as long as the design purpose can be achieved. For example, the quantum package can be set as a packaging box; the material of the package cover can be selected from aluminum, copper, superconducting materials, etc.

[0055] Beneficial effects:

[0056] The present invention provides a method for reducing the influence of quasiparticle defects on bits and its application, which has the following advantages:

[0057] (1) The present invention designs the cross capacitor structure of the Xmon bit. By calculating the coupling efficiency between the cross capacitor and the Josephson junction and limiting it to the lowest possible level, the optimal shielding size of the cross capacitor is determined, quasiparticle interference is suppressed to the greatest extent, and the performance of the Xmon bit is improved.

[0058] (2) When determining the cross capacitor shielding structure of the Xmon bit, the present invention performs Python modeling on 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 frequency of the electromagnetic wave, and further quantifies the matching degree between the cross capacitor and the Josephson junction radiation impedance to ensure optimal shielding.

[0059] (3) The present invention further combines infrared shielding technology with capacitor structure design, and selects polymer absorbing materials (such as HR) or electromagnetic metamaterials are placed above the quantum chip during packaging, which effectively improves the electromagnetic environment of the quantum chip and suppresses the negative interference of electromagnetic radiation on bits and chips.

[0060] (4) The improvement method of the present invention only requires the design of the cross capacitor structure of Xmon, which can be used together with absorbing materials to effectively reduce quasiparticle defects. It has good adaptability to various types of quantum chips, effectively reduces the difficulty of device design and processing, and is easy to implement.

[0061] (5) The present invention does not require additional changes to the specifications and dimensions of the quantum chip and the absorbing material, nor does it require changes to the structure of the package. It only requires matching the dimensions of the quantum chip, the absorbing material and the package to obtain a package component with significant shielding effect. The method has strong compatibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 .Flowchart of sample design to reduce quasiparticle defects;

[0063] Figure 2 .Schematic diagram of the structure of the cross capacitor;

[0064] Figure 3 .The relationship between the radiation impedance of the cross capacitor and the 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);

[0065] Figure 4 .Schematic diagram of quantum chip structure;

[0066] Figure 5 .Schematic diagram of the structure of the quantum package (1: metal base; 2: quantum chip; 3: absorbing material; 4: metal cover). DETAILED DESCRIPTION

[0067] The formula parameters, definitions and data sources involved in this method are shown in the table below.

[0068]

[0069] Example

[0070] This embodiment provides a method for reducing the impact of quasiparticle defects on bits. Figure 1 , the method comprising:

[0071] S1. By designing the cross capacitor structure of the Xmon bit, a shielded bit capacitor is obtained;

[0072] S2. Make the Xmon bit with shielding bit capacitor into a quantum chip, and then place the quantum chip and the absorbing material together in a packaging assembly to shield radiation interference.

[0073] In the step S1, the optimal shielding size of the capacitor physical structure is determined by determining the coupling efficiency between the cross capacitor and the Josephson junction of the Xmon bit.

[0074] The coupling efficiency between the cross capacitor and the Josephson junction is calculated and determined by the following formula 1.

[0075] e c =1-|Γ| 2 (Formula 1)

[0076] Among them, e c is the coupling efficiency between the cross capacitor and the Josephson junction, and Γ is the reflection coefficient of the cross capacitor.

[0077] The reflection coefficient of the cross capacitor is calculated and determined by the following formula 2.

[0078]

[0079] Where Γ is the reflection coefficient of the cross capacitor, Z rad is the radiation impedance of the cross capacitor, Z j is the radiation impedance of the Josephson junction.

[0080] The radiation impedance Z of the cross capacitor rad Determined by modeling and simulation; specifically, based on the initial size information of the cross capacitor, 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, and then the capacitor size corresponding to the insensitive state is determined.

[0081] The schematic diagram of the cross capacitor structure is shown in Figure 2 The initial size information includes the length of the single arm of the cross capacitor l, the width of the center strip s and the distance w from the center strip to the ground. Use Python to create the HFSS script of the cross capacitor, import the initial size information data of the cross capacitor for modeling, and perform simulation operations at the Josephson junction to determine Z rad .

[0082] The radiation impedance Z of the Josephson junction j It is calculated and determined by the following formula 3.1.

[0083]

[0084] Among them, Z j is the radiation impedance of the Josephson junction, ω is the frequency of the electromagnetic wave, τ is the quantum circuit time constant, R n is the tunneling resistance of the Josephson junction.

[0085] The quantum circuit time constant is calculated and determined by the following formula 3.2.

[0086] τ≡R n C j (Formula 3.2)

[0087] Among them, C j is the capacitance of the Josephson junction.

[0088] The schematic diagram of the structure of the quantum chip prepared in step S2 is shown in Figure 4 .

[0089] The absorbing material selection in step S2 HR, made of reticular open-cell polyurethane foam, can absorb electromagnetic waves with frequencies above 80GHz.

[0090] A second aspect of this embodiment provides an application of the above-mentioned method for reducing the influence of quasiparticle defects on bits in a quantum package.

[0091] The quantum package comprises a concave metal base; the metal base contains quantum chips and absorbing materials that match the size of the concave shape in order from bottom to top; a packaging cover is provided at the upper end of the metal base; the schematic diagram of the structure of the quantum package is shown in Figure 5 .

[0092] The packaging cover is made of Al.

[0093] The material of the metal substrate is copper.

[0094] Finally, it should be noted that the above 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 aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for reducing the impact of quasiparticle defects on bits, characterized in that: The bit is an Xmon bit provided with a cross capacitor structure; The method at least comprises: Design the physical structure of the cross capacitor of the Xmon bit to obtain the shielded bit capacitor; The structure design process includes: The target structure of the cross capacitor is determined by determining the minimum value of the coupling efficiency between the cross capacitor and the Josephson junction.

2. The method for reducing the influence of quasiparticle defects on bits according to claim 1, characterized in that: The coupling efficiency between the cross capacitor and the Josephson junction is determined by the reflection coefficient of the cross capacitor, and the two are in a negative correlation.

3. The method for reducing the influence of quasiparticle defects on bits according to claim 2, characterized in that: 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 for reducing the influence of quasiparticle defects on bits according to claim 3, characterized in that: The method for determining the radiation impedance of the cross capacitor includes: The cross capacitor of the Xmon bit is modeled, and the model is simulated at the Josephson junction to obtain the radiation impedance of the cross capacitor, wherein the variables of the model include the single arm length of the cross capacitor, the width of the center strip of the cross capacitor, and the distance from the center strip of the cross capacitor to the ground.

5. The method for reducing the influence of quasiparticle defects on bits according to claim 3, characterized in that: 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 for reducing the influence of quasiparticle defects on bits according to claim 1, characterized in that: The method further comprises: The Xmon bit with shielding bit capacitor is made into a quantum chip, and then the quantum chip and the absorbing material are placed together in a packaging structure to shield radiation interference.

7. The method for reducing the influence of quasiparticle defects on bits according to claim 6, characterized in that: The absorbing material can absorb electromagnetic waves with a frequency of more than 80 GHz.

8. The method for reducing the influence of quasiparticle defects on bits according to claim 7, characterized in that: The absorbing material includes one or more combinations of high molecular polymer absorbing materials, ferrite absorbing materials, electromagnetic metamaterials, iron-cobalt-ruthenium alloy superconducting materials, and polycrystalline iron fiber absorbing materials.

9. The method for reducing the influence of quasiparticle defects on bits according to claim 8, characterized in that: The high molecular polymer absorbing material comprises a reticular open-cell polyurethane foam.

10. An application of the method for reducing the influence of quasiparticle 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 quantum chips and wave-absorbing materials that match the size of the concave shape in sequence from bottom to top; A packaging cover is arranged on the upper end of the metal base.

Images