Method and device for measuring spatial distribution of two-level defects
阅读说明：本技术 一种二能级缺陷空间分布的测量方法及测量装置 (Method and device for measuring spatial distribution of two-level defects ) 是由 冯加贵 武彪 熊康林 孙骏逸 黄永丹 丁孙安 陆晓鸣 芮芳 于 2019-09-29 设计创作，主要内容包括：本发明公开了一种二能级缺陷空间分布的测量方法及测量装置,该二能级缺陷空间分布的测量方法包括：对超导量子比特施加偏置磁场,并获取所述超导量子比特的频率-偏置磁通的关系曲线；通过二能级缺陷空间分布的测量装置的扫描探针输出的局域扫描电场对超导量子比特表面逐点扫描；若所述超导量子比特的频率随着时间的变换存在震荡变化,则所述扫描探针所处扫描点存在二能级缺陷。本实施例提供一种二能级缺陷空间分布的测量方法及测量装置,以表征超导量子芯片中的二能级缺陷的空间分布情况。(The invention discloses a method and a device for measuring the spatial distribution of two-level defects, wherein the method for measuring the spatial distribution of the two-level defects comprises the following steps: applying a bias magnetic field to the superconducting qubit and obtaining a frequency-bias magnetic flux relation curve of the superconducting qubit; scanning the surface of the superconducting qubit point by a local scanning electric field output by a scanning probe of a measuring device for spatial distribution of the two-level defects; and if the frequency of the superconducting qubit has oscillation change along with the change of time, a scanning point where the scanning probe is located has a two-level defect. The present embodiment provides a method and an apparatus for measuring spatial distribution of two-level defects, so as to characterize spatial distribution of two-level defects in a superconducting quantum chip.)
1. A method for measuring the spatial distribution of two-level defects is characterized by comprising the following steps:
applying a bias magnetic field to the superconducting qubit and obtaining a frequency-bias flux relationship curve of the superconducting qubit;
scanning the surface of the superconducting qubit point by a local scanning electric field output by a scanning probe of a measuring device for spatial distribution of the two-level defects;
and if the frequency of the superconducting qubit has oscillation change along with the change of time, a scanning point where the scanning probe is located has a two-level defect.
2. The method of measuring the spatial distribution of two-level defects according to claim 1, wherein:
the voltage range of the scanning probe is 50-200 mV.
3. The method of measuring the spatial distribution of two-level defects according to claim 1,
the range of the vertical distance between the scanning probe and the scanning point is 50-150 nm.
4. The method of claim 1, wherein before applying the local scanning electric field to the plurality of scanning points on the surface of the superconducting qubit one by the scanning probe of the apparatus for measuring the spatial distribution of two-level defects, the method further comprises:
and placing the superconducting qubit and the means for measuring the spatial distribution of the two-level defects in an environment having a temperature less than a first temperature threshold.
5. The method of measuring the spatial distribution of two-level defects according to claim 4, the first temperature threshold being less than or equal to 50 mk.
6. The method of measuring the spatial distribution of two-level defects according to claim 1, further comprising:
and drawing the positions of the scanning points with the two-level defects to obtain a spatial distribution map of the two-level defects of the superconducting qubits.
7. The method of measuring the spatial distribution of two-level defects according to claim 1,
the scanning dots are uniformly distributed to the surface of the superconducting qubit.
8. A device for measuring the spatial distribution of two-level defects, which is applied to the method for measuring the spatial distribution of two-level defects according to any one of claims 1 to 7, the device comprising: the device comprises a bias magnetic field generating device, a two-level defect measuring device and a processor;
the measuring equipment of the two-level defect comprises a scanning probe, wherein a local scanning electric field output by the scanning probe is used for scanning the surface of the superconducting qubit point by point;
the bias magnetic field generating device is used for applying a bias magnetic field to the superconducting qubit, and the processor is used for acquiring a frequency-bias magnetic flux relation curve of the superconducting qubit;
and the processor is also used for judging that a two-level defect exists in a scanning point where the scanning probe is positioned when the fact that the frequency of the superconducting qubit has oscillation change along with the change of time is measured.
9. The apparatus of claim 8, wherein the scanning probe comprises: a central metal layer, an insulating layer and a surface metal layer;
the insulating layer is arranged to cover the central metal layer; the surface metal layer is arranged to cover the insulating layer;
the central metal layer is electrically connected with the positive electrode of the constant voltage source, and the surface metal layer is electrically connected with the negative electrode of the constant voltage source.
10. The apparatus for measuring the spatial distribution of two-level defects according to claim 9,
the diameter range of the central metal layer is 100-500 nm.
Technical Field
The invention relates to the technical field of superconducting quantum chips, in particular to a method and a device for measuring the spatial distribution of two-level defects.
Background
The superconducting quantum computing is based on circuit quantum electrodynamics as a theoretical basis, a non-resonant quantum oscillation circuit is prepared by adopting a superconducting Josephson junction to form a qubit, and coupling among the qubits, control of the qubit and nondestructive reading and writing are realized by utilizing a planar capacitor and a planar superconducting microwave resonator. Different from the physical realization of other qubits, the working parameters of the superconducting qubits, such as resonance frequency, coupling strength between bits, and the like, can be controllably adjusted by changing the geometrical parameters of the Josephson junction and the planar superconducting microwave device. In addition, the preparation of the superconducting quantum chip is compatible with the traditional material growth process, the semiconductor device processing process, the micro-processing process and the microwave device packaging process, so that the preparation research and the comprehensive performance of the superconducting quantum chip are in the leading position in the field of quantum computing, and the superconducting quantum chip is a quantum computing system which is most hopeful to realize commercial application.
At present, superconducting quantum computation is focused internationally, so that the research scale of superconducting quantum chips is greatly developed in recent years, but the coherent time for limiting the performance of the superconducting quantum chips is not improved in a breakthrough way, and long-term experimental research shows that the material of the superconducting quantum chips and dangling bonds, polar molecules, magnetic molecules, crystal defects and the like on relevant surface interfaces form equivalent quantum 'two-level systems'; the two-level systems are coupled with microwave photons for controlling the qubits to cause energy dissipation and phase decoherence of the qubits, detect the two-level defects of the superconducting quantum chip and avoid the incorporation of the two-level defects in the preparation process of the superconducting quantum chip through various parameter regulation and control, which is the development bottleneck of the conventional superconducting quantum chip.
Disclosure of Invention
The embodiment of the invention provides a method and a device for measuring the spatial distribution of two-level defects, which are used for representing the spatial distribution condition of the two-level defects in a superconducting quantum chip.
In a first aspect, an embodiment of the present invention provides a method for measuring a spatial distribution of a two-level defect, including:
applying a bias magnetic field to the superconducting qubit and obtaining a frequency-bias magnetic flux relation curve of the superconducting qubit;
scanning the surface of the superconducting qubit point by a local scanning electric field output by a scanning probe of a measuring device for spatial distribution of the two-level defects;
and if the frequency of the superconducting qubit has oscillation change along with the change of time, a scanning point where the scanning probe is located has a two-level defect.
In a second aspect, an embodiment of the present invention further provides a device for measuring a spatial distribution of two-level defects, which is suitable for the method for measuring a spatial distribution of two-level defects provided in any embodiment of the present invention, where the device for measuring a spatial distribution of two-level defects includes: the device comprises a bias magnetic field generating device, a two-level defect measuring device and a processor;
the measuring equipment of the two-level defect comprises a scanning probe, wherein a local scanning electric field output by the scanning probe is used for scanning the surface of the superconducting qubit point by point;
the bias magnetic field generating device is used for applying a bias magnetic field to the superconducting qubit, and the processor is used for acquiring a frequency-bias magnetic flux relation curve of the superconducting qubit;
and the processor is also used for judging that a two-level defect exists in a scanning point where the scanning probe is positioned when the fact that the frequency of the superconducting qubit has oscillation change along with the change of time is measured.
In this embodiment, a scanning probe of a measurement apparatus for spatial distribution of two-level defects applies a local scanning electric field to a plurality of scanning points on the surface of a superconducting qubit one by one, and at the same time, applies a bias magnetic field to the superconducting qubit, and obtains a relationship between the frequency of the superconducting qubit and the bias magnetic flux, if there is oscillation transformation in the energy of the superconducting qubit, it is indicated that resonance is generated between the superconducting qubit system and the two-level defect system, thereby indicating that there is a two-level defect at the scanning point where the scanning probe is located, in this embodiment, the measurement apparatus for spatial distribution of two-level defects scans the surface of the superconducting qubit point by point, and can characterize the spatial distribution of the two-level defects on the surface of the superconducting qubit, thereby facilitating exploration of introduction of the two-level defects in the preparation process of the superconducting quantum chip material, therefore, material preparation and device process parameters are continuously optimized, and incorporation of two-level defects in the superconducting qubit preparation process is reduced.
Drawings
FIG. 1 is a schematic flow chart of a method for measuring spatial distribution of two-level defects according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a quantum tunneling two-level defect model according to an embodiment of the present invention;
FIG. 3 is a graph illustrating performance characteristics of an adjustable qubit under different bias magnetic fields according to an embodiment of the present invention;
FIG. 4 is a schematic flow chart of another method for measuring the spatial distribution of two-level defects according to an embodiment of the present invention;
FIG. 5 is a schematic structural diagram of an apparatus for measuring spatial distribution of two-level defects according to an embodiment of the present invention;
fig. 6 is a schematic structural diagram of a scanning probe according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be further noted that, for the convenience of description, only some of the structures related to the present invention are shown in the drawings, not all of the structures.
Although the traditional technology proves that the two-level defects exist in the superconducting quantum chip and are regulated and controlled by an electric field and stress, the two-level defects are mainly distributed on a dielectric material of the superconducting quantum chip and a surface interface related to the dielectric material through the design and statistics of the chip, and the spatial distribution and the specific arrangement position of the two-level defects in the superconducting quantum chip cannot be definitely obtained. For example, the parameters of the two-level defects such as the position points of the two-level defects distributed on the surface of the superconducting quantum chip, the areas with high distribution density, the areas with low distribution density, and the like cannot be known.
In order to solve the above problem, an embodiment of the present invention provides a method for measuring spatial distribution of a two-level defect, as shown in fig. 1, where fig. 1 is a schematic flow chart of the method for measuring spatial distribution of a two-level defect provided in the embodiment of the present invention, and the method for characterizing a two-level defect includes:
s110, applying a bias magnetic field to the superconducting qubit and acquiring a frequency-bias magnetic flux relation curve of the superconducting qubit.
And S120, scanning the surface of the superconducting qubit point by point through a local scanning electric field output by a scanning probe of the measuring device for the spatial distribution of the two-level defects.
A two-level defect refers to a quantum system of atoms, radicals, electrons, spins, small molecules, etc. that hop between two minimum energy positions to form two characteristic energy levels. The defects exist in amorphous materials generally, and the defective crystal materials and the surface interfaces of the crystal materials are easily incorporated into the superconducting quantum chips along with preparation materials and process parameters in the preparation process of the superconducting quantum chips, and are formed on the surfaces of the superconducting quantum chips, particularly the surface interfaces of dielectric materials, the dielectric materials refer to materials which can generate polarization, conductance, loss, breakdown and other phenomena under the action of an external electric field, and the two-level defects are easily generated in the dielectric materials.
To facilitate an understanding of the two-level defects, two potential wells, which are close in energy and can be tunneled, may be used to describe the two-level defects. Fig. 2 is a schematic diagram of a two-level defect model for quantum tunneling according to an embodiment of the present invention, in which fig. 2 shows a graph of energy-position relationship, two potential wells, i.e., a potential well a and a potential well B, exist in fig. 2, and an energy level spacing of a two-level defect depends on an asymmetric energy epsilon and a tunneling energy delta 0 between the two potential wells a and B. Asymmetric energy epsilon fingerWhat is meant by the energy difference between the two potential wells is the tunneling energy Δ 0 required for a microscopic particle to tunnel from potential well a to potential well B, or from potential well B to potential well a. When the asymmetric energy epsilon is far larger than the tunneling energy delta 0, the left potential well and the right potential well can well limit respective eigenstates, namely the two potential wells are relatively stable; when the asymmetric energy e is close to zero, the eigenstate wave functions between the two potential wells overlap, forming two new energy states with a certain energy level spacing, such as new energy states ψ + and ψ -as shown in fig. 2. The energy difference between the two new energy states is
The development of superconducting qubits provides more opportunities for the study of two-level defects in materials, since when two-level defects and superconducting qubits resonate, the two-level defects can be detected by detecting the spectral change of the superconducting qubits, and even the quantum state dynamics of the two-level defects can be controlled and observed. In order to confirm the distribution of the two-level defects on the superconducting quantum chip, the surface of the superconducting quantum bit can be scanned by applying a local scanning electric field through a scanning probe of a measuring device of the spatial distribution of the two-level defects. Optionally, the scanning dots are uniformly distributed on the surface of the superconducting qubit, so that the surface of the superconducting qubit can be uniformly scanned, and the accuracy of the obtained two-level defect distribution can be improved. The dielectric loss of various materials can be estimated by the embodiment, the more the two-level defects exist, the larger the dielectric loss is, generally, the dielectric loss of the amorphous material is far greater than that of the single crystal material, and the distribution quantity of the two-level defects of the amorphous material is greater than that of the single crystal material.
As shown in fig. 3, fig. 3 is a performance characterization diagram of an adjustable qubit provided in an embodiment of the present invention under different bias magnetic fields. Fig. 3(a) is a frequency-bias flux relationship diagram of a superconducting qubit, i.e., a frequency domain representation of an adjustable qubit, and it can be known from fig. 3(a) that the characteristic spectrum of the qubit is continuously changed under continuous control of the bias flux, and after the superconducting qubit is excited by a long-duration microwave pulse, the excited state probability of the superconducting qubit can be measured, so as to obtain the characteristic spectral line of the adjustable superconducting qubit, as shown in fig. 3. When the characteristic spectrum of the superconducting qubit is close to the nearby dual-energy level defect, the two dual-energy level quantum systems resonate to generate a split resonance peak, namely, the horizontal crossing is avoided in the superconducting qubit measurement process; FIG. 3(b) is a graph of frequency-time relationship of a superconducting qubit, i.e., a time-domain representation of an adjustable qubit, in which the superconducting qubit can be excited to an excited state by a pi microwave pulse, modulated for a time Δ t, modulated in frequency to a varying detectable frequency band, and then monitored for changes in its frequency spectrum over time. Referring to fig. 3(b), isolated superconducting qubits exhibit only a simple energy relaxation, showing an exponential decay, as shown by curve L1, whereas if the superconducting qubits interact with and resonate at the two-level defect, energy is redistributed between the superconducting qubit system and the two-level defect system, showing the formation of oscillations, as shown by curve L2.
When the local scanning electric field scans the scanning points one by one, the scanning points stay at microsecond magnitude, spectral lines of a frequency domain and a time domain of the superconducting qubit are detected in the period of time, if the change of the spectral lines can be detected, the fact that a two-level defect capable of resonating with the superconducting qubit exists is proved, therefore, the space distribution representation of the two-level defect is realized, and finally, the accuracy of the representation result can be further confirmed through electric field simulation.
And S130, if the frequency of the superconducting qubit has oscillation change along with the change of time, the scanning point where the scanning probe is located has a two-level defect.
Meanwhile, since the two-level defect is similar to an electric dipole moment, the energy level spacing of the two-level defect can be modulated by locally scanning the electric field, thereby adjusting the energy of the two-level defect. In this embodiment, the local scanning electric field may be fixed, and only the bias magnetic field of the superconducting qubit may be controlled, so that the superconducting qubit system and the two-level defect system resonate, or the bias magnetic field of the superconducting qubit may be fixed, and the local scanning electric field may be adjusted to change, so that the superconducting qubit system and the two-level defect system resonate, or the local scanning electric field and the bias magnetic field may be adjusted at the same time. In any mode, the position and distribution rule of the two-level defects can be obtained through the local scanning electric field output by the scanning probe and the bias magnetic field of the superconducting qubit, so that the generation of the two-level defects is reduced in the subsequent preparation process of the superconducting quantum chip, and the energy dissipation and phase decoherence of the excess qubit caused by the two-level defects are prevented.
In the embodiment of the invention, a scanning probe of a measuring device for spatial distribution of two-level defects applies a local scanning electric field to a plurality of scanning points on the surface of a superconducting qubit one by one, and simultaneously applies a bias magnetic field to the superconducting qubit, and obtains the relationship between the frequency and the bias magnetic flux of the superconducting qubit, if the energy of the superconducting qubit has oscillation transformation, the resonance is generated between a superconducting qubit system and a two-level defect system, thereby indicating that the scanning point where the scanning probe is located has the two-level defects, the embodiment scans the surface of the superconducting qubit point by point through the measuring device for spatial distribution of the two-level defects, the spatial distribution of the two-level defects on the surface of the superconducting qubit can be represented, and the introduction condition of the two-level defects in the preparation process of a superconducting quantum chip material device can be conveniently explored, therefore, material preparation and device process parameters are continuously optimized, and incorporation of two-level defects in the superconducting qubit preparation process is reduced.
The above is the core idea of the present invention, and the technical solution in the embodiment of the present invention will be clearly and completely described below with reference to the drawings in the embodiment of the present invention. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without any creative work belong to the protection scope of the present invention.
In order to ensure the limitation of a local scanning electric field and extremely small tunneling current caused under bias voltage, the distance between the scanning probe and the surface of the superconducting qubit can be controlled in a nanometer level, and optionally, the range of the vertical distance between the scanning probe and a scanning point is 50-150 nm. Illustratively, the vertical distance between the scanning probe and the scanning spot can be controlled in the range of 100 nm.
Similarly, in order to ensure the limitation of the local scanning electric field and the minimum tunneling current caused by the bias voltage, optionally, the voltage range of the scanning probe may be 50 to 200 mV. Illustratively, the vertical distance between the scanning probe and the scanning spot may be controlled in the range of 100 mV.
Referring to fig. 4, fig. 4 is a schematic flow chart of another method for measuring a spatial distribution of a two-level defect according to an embodiment of the present invention, in this example, on the basis of the foregoing embodiment, a position of a scanning point where the two-level defect exists is plotted to obtain a spatial distribution map of the two-level defect of the superconducting qubit, and specifically, as shown in fig. 4, the method for measuring the spatial distribution of the two-level defect includes:
s210, placing the superconducting qubit and the measurement device for the two-level defect space distribution in an environment with the temperature less than a first temperature threshold.
Before the local scanning electric field is applied to the plurality of scanning points on the surface of the superconducting qubit one by one through the scanning probe of the measuring device for the spatial distribution of the two-level defects, the embodiment may further place the superconducting qubit and the measuring device for the spatial distribution of the two-level defects in an environment with a temperature less than the first temperature threshold, that is, the temperature of the measuring environment for the two-level defects is set to be less than the first temperature threshold. It is noted that the energy level spacing corresponds to a thermodynamic temperature above which thermal fluctuations cause energy level broadening, resulting in certain two-level defects not being detected. The temperature of the measurement environment is controlled to be set to be less than the first temperature threshold to enhance the accuracy of the two-level defect measurement. Alternatively, the first temperature threshold may be less than or equal to 50 mk. When the temperature of the measuring environment is less than or equal to 50mk, the distribution position of the two-level defects can be effectively obtained, and the accuracy of the two-level defect measurement is improved.
S220, applying a bias magnetic field to the superconducting qubit and acquiring a frequency-bias magnetic flux relation curve of the superconducting qubit.
And S230, scanning the surface of the superconducting qubit point by point through a local scanning electric field output by a scanning probe of the measuring device for the spatial distribution of the two-level defects.
And S240, if the frequency of the superconducting qubit has oscillation change along with the change of time, the scanning point where the scanning probe is located has a two-level defect.
And S250, drawing the position of the scanning point with the two-level defect to obtain a spatial distribution map of the two-level defect of the superconducting qubit.
When the position point of each two-level defect is obtained, the position of each position point, namely the position of the scanning point with the two-level defect, can be drawn and displayed, so that the spatial distribution of the two-level defects of the superconducting quantum bit is displayed, a user can conveniently research the spatial distribution of the two-level defects, and the spatial distribution rule of the two-level defects is obtained, so that the introduction of the two-level defects is reduced in the production process of the superconducting quantum chip, the performance of the superconducting quantum chip and the decorrelation time of the superconducting quantum bit are improved, and the development of an extensible superconducting quantum computer is promoted.
In the embodiment, by controlling the measurement environment of the two-level defects and drawing the spatial distribution map of the two-level defects, accurate distribution of the two-level defects can be obtained, continuous optimization of relevant parameters of material preparation and device processes is facilitated, incorporation of the two-level defects in the processes is reduced, and the performance of the superconducting quantum chip and the coherence time of the superconducting quantum bit are improved.
Based on the same conception, the embodiment of the invention also provides a measuring device for the spatial distribution of the two-level defects. Fig. 5 is a schematic structural diagram of an apparatus for measuring spatial distribution of two-level defects according to an embodiment of the present invention, and as shown in fig. 5, the apparatus for measuring spatial distribution of two-level defects according to this embodiment includes: a bias magnetic
the bias magnetic
the measuring
the
In the embodiment of the invention, a scanning probe of a measuring device for spatial distribution of two-level defects applies a local scanning electric field to a plurality of scanning points on the surface of a superconducting qubit one by one, and simultaneously applies a bias magnetic field to the superconducting qubit, and obtains the relationship between the frequency and the bias magnetic flux of the superconducting qubit, if the energy of the superconducting qubit has oscillation transformation, the resonance is generated between a superconducting qubit system and a two-level defect system, thereby indicating that the scanning point where the scanning probe is located has the two-level defects, the embodiment scans the surface of the superconducting qubit point by point through the measuring device for spatial distribution of the two-level defects, the spatial distribution of the two-level defects on the surface of the superconducting qubit can be represented, and the introduction condition of the two-level defects in the preparation process of a superconducting quantum chip material device can be conveniently explored, therefore, material preparation and device process parameters are continuously optimized, and incorporation of two-level defects in the superconducting qubit preparation process is reduced.
On the basis of the above embodiment, referring to fig. 6, fig. 6 is a schematic structural diagram of a scanning probe according to an embodiment of the present invention, and the
Fig. 6 also shows a schematic structural diagram of the
Optionally, the diameter of the
It is to be noted that the foregoing is only illustrative of the preferred embodiments of the present invention and the technical principles employed. It will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described herein, but is capable of various obvious changes, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, although the present invention has been described in greater detail by the above embodiments, the present invention is not limited to the above embodiments, and may include other equivalent embodiments without departing from the spirit of the present invention, and the scope of the present invention is determined by the scope of the appended claims.