A method for measuring coupler energy spectrum using sideband transition

Abstract

The invention belongs to the field of quantum technology, and specifically relates to a method for measuring the energy spectrum of a coupler by using sideband transitions. The method comprises coupling a detection bit with a corresponding reading cavity and coupling the reading cavity with a transmission line, using a vector network analyzer to perform continuous wave measurement or a data acquisition card to perform pulse measurement, obtaining information of the reading cavity, obtaining information of the detection bit by dispersion measurement, and applying non-resonant strong driving to induce high-order sideband transitions to obtain energy spectrum information of the coupler. The conditions for dispersion measurement are as follows: the coupling strength between the detection bit and the coupler must be much smaller than the detuning between the detection bit and the target bit; the frequency of the non-resonant strong driving is a microwave frequency and a frequency-converted microwave frequency to the detection bit; and the parameters of the target bit that cannot be read are measured by using the detection bit that can read the quantum state. In the direction of superconducting quantum computing, the application of this method can further reduce the number of reading cavities in a superconducting quantum chip.

Description

Technical Field

[0001] The invention belongs to the field of quantum technology, and in particular relates to a method for measuring a coupler energy spectrum by utilizing sideband transition. Background Art

[0002] Quantum entanglement is an important concept in quantum computing. It can bring about a huge acceleration effect and is one of the foundations for achieving exponential growth in quantum computing capabilities. However, achieving large-scale multi-qubit manipulation is a challenging goal. It requires not only highly connected quantum systems and high-fidelity multi-bit gates, but also efficient and precise quantum state measurements. The current sample architecture and measurement methods face many challenges such as complex structure and low efficiency when facing large-scale qubit measurements.

[0003] To address these challenges, we propose a new technique that uses a probe bit to read the target bit's information to reduce the number of large-scale bit-reading cavities, thereby simplifying the structure of the superconducting quantum chip. This technique is particularly suitable for situations where only energy spectrum information is required for the target bit, such as couplers. The measurement of the coupler energy spectrum is crucial for flux crosstalk calibration, waveform distortion calibration, and gate manipulation calibration in superconducting quantum chips. By accurately measuring the coupler energy spectrum and adjusting and optimizing it accordingly, the reliability, accuracy, and performance of quantum computing can be improved.

[0004] At present, the existing measurement schemes are mainly divided into two categories: continuous wave measurement and pulse measurement. Continuous wave measurement uses a vector network analyzer and a continuous microwave signal for measurement. The energy spectrum information of the target bit is obtained by applying a continuous wave to the detection bit and analyzing the read cavity signal. However, there are some problems with continuous wave measurement, such as chip heating, AC Stark shift, and incompatibility with high-precision quantum control of subsequent bits. Another way is pulse measurement, which inputs a timing pulse to the chip and obtains the information of the read cavity and the detection bit according to the demodulation result of the data acquisition card. In order to reduce the number of read cavities on the chip and improve the integration, the coupler's read cavity is generally not designed, but the parameters of the coupler have an important influence on the experiment. Therefore, it is necessary to study and propose new measurement schemes to achieve efficient and accurate coupler energy spectrum measurement. 。 Summary of the invention

[0005] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a method for measuring the energy spectrum of a coupler by utilizing sideband transitions.

[0006] In order to achieve the purpose of the present invention, the present invention will be implemented by adopting the technical solution described below.

[0007] A method for measuring a coupler energy spectrum using sideband transitions includes a continuous wave measurement method or a pulse measurement method, wherein the continuous wave measurement method includes the following steps:

[0008] S11, using a vector network analyzer to measure the scattering parameters of the detection bit to obtain the information of the reading cavity;

[0009] S12, performing dispersion measurement on the detection bit according to the detection bit reading cavity information to obtain the detection bit information;

[0010] S13, inducing high-order sideband transition by applying non-resonant strong driving to the detection bit to obtain the energy spectrum of the target bit; or the pulse measurement method comprises the following steps:

[0011] S21, using a data acquisition card to measure the reading channel of the detection bit to obtain the information of the reading cavity;

[0012] S22, performing dispersion measurement on the detection bit according to the information of the detection bit reading cavity to obtain the information of the detection bit;

[0013] S23, inducing high-order sideband transition by applying non-resonant strong driving to the detection bit to obtain the energy spectrum of the target bit;

[0014] The detection bit and the target bit are adjacent bits; the sideband transition is a blue sideband transition; and the conditions for the dispersion measurement are:

[0015] The coupling strength of the adjacent bits needs to be much smaller than the detuning between the detection bit and the target bit;

[0016] The frequency of the non-resonant strong drive is ω ac =ω q -δ The first beam of microwaves and the variable frequency is ω s =ω a +δ is the second frequency-converted microwave beam to the detection bit, where δ is detuned and controlled at 5MHz~10MHz, ω a is the frequency of the target bit.

[0017] As a preferred embodiment of the present invention, the blue sideband transition refers to a transition between |0g> and |1e>; wherein |0> and |1> are the ground state energy level and the first excited state energy level of the detection bit, respectively; and |g> and |e> are the ground state energy level and the first excited state energy level of the target bit, respectively.

[0018] As a preferred embodiment of the present invention, the effective Hamiltonian of the blue sideband transition is Where Ω ac is the driving strength of the first microwave beam, Ω s is the driving intensity of the second frequency-converted microwave, is the product of the detection bit and target bit generation operator, aσ -is the product of the detection bit and the target bit annihilation operator.

[0019] As a preferred solution of the present invention, the method for constructing the blue sideband transition is to construct the blue sideband transition by adjusting the detuning of the detection bit and the target bit and the driving strength of the first microwave beam and the second frequency-converted microwave beam.

[0020] As a preferred embodiment of the present invention, the construction method specifically includes:

[0021] (1) Designing the architecture of the quantum bit array and naming the detection bits and target bits in the architecture; wherein the detection bits and target bits in the architecture are capacitively coupled and the coupling strength is set; and there is direct capacitive coupling interaction between the detection bits and the detection bits and the coupling strength is set;

[0022] (2) constructing all detection bits and target bits in the architecture using a Josephson-structured superconducting quantum interference device;

[0023] (3) applying a DC bias signal to the Z lines of the detection bit and the target bit to generate a magnetic flux and adjust the frequencies of the detection bit and the target bit;

[0024] (4) Setting the detection bit and cavity frequency;

[0025] (5) Characterize the parameters of the entire quantum bit array architecture.

[0026] As a preferred embodiment of the present invention, the specific operation of the construction method is to obtain the information of the reading cavity by coupling the detection bit with the corresponding reading cavity and coupling the reading cavity with the transmission line, using a vector network analyzer to perform continuous wave measurement or using a data acquisition card to perform pulse measurement, and obtain the information of the detection bit through dispersion measurement.

[0027] As a preferred solution of the present invention, the reading cavity information in S11 or S21 is the measurement points of the reading cavity, and the measurement points include the reading frequency, the reading power and the voltage bias.

[0028] As a preferred solution of the present invention, the sideband transition in S13 or S23 is achieved through virtual energy levels.

[0029] As a preferred solution of the present invention, the detection bit is an adjustable transmission sub-qubit; and the target bit is an adjustable coupler.

[0030] As a preferred solution of the present invention, the energy spectrum measurement of the target bit is achieved by two microwave sources and a vector network analyzer.

[0031] Beneficial Effects

[0032] 1. By using the sideband transition induced by the non-resonant strong driving pulse, the energy spectrum measurement of the adjacent coupler is realized on the bit, without the need to use the reading cavity of the coupler, thus reducing the cost of superconducting quantum circuit design;

[0033] 2. This technology is very suitable for bits that only need energy spectrum information without reading quantum states (such as couplers). By accurately measuring the energy spectrum of the coupler and adjusting and optimizing it accordingly, the reliability, accuracy and performance of quantum computing can be improved;

[0034] 3. This method can be extended to any two bits and is not limited to the measurement of couplers. If the target bit and the detection bit are coupled and the coupling strength and frequency meet the conditions, the target bit information can be obtained by reading the detection bit;

[0035] 4. This method provides new ideas for future multi-bit control and reduces the demand for components on the chip, which helps expand multi-bit systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 A flow chart of a method for measuring a coupler energy spectrum using sideband transitions according to the present invention;

[0037] Figure 2 is a schematic diagram of the sideband transition described in the present invention;

[0038] Figure 3 This is a schematic diagram of the architecture of the 2*3 quantum bit array described in the present invention;

[0039] Figure 4 is a timing diagram of the continuous wave measurement described in the present invention;

[0040] Figure 5 is the general spectrum of the coupler described in the present invention;

[0041] Figure 6 It is a timing diagram of the pulse measurement described in the present invention. DETAILED DESCRIPTION

[0042] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. The following embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] The present invention provides a method for measuring the energy spectrum of a coupler using sideband transitions, without requiring a reading cavity of the coupler. The principle of the method is to apply a non-resonant strong drive to adjacent bits to induce a non-resonant strong drive structure and a sideband transition of the coupler, thereby obtaining the energy spectrum information of the coupler. The implementation process is as follows: Figure 1 shown.

[0044] like Figure 2 As shown, |0> and |1> are the ground state energy level and the first excited state energy level of the detection bit, respectively; |g> and |e> are the ground state energy level and the first excited state energy level of the target bit, respectively. A non-resonant strong drive is applied to the detection bit to induce red and blue sideband transitions. The red sideband transition is the transition between |0e> (the first one represents the energy level of the detection bit, and the second one represents the energy level of the target bit, the same below) to |1g>; the blue sideband transition refers to the transition between |0g> and |1e>. These two transitions are realized through virtual energy levels, which are essentially high-order interactions induced by driving. This method uses blue sideband transitions to measure the coupler energy spectrum. Since the blue sideband transition is a high-order interaction, in order to obtain high-contrast results, it is necessary to suppress parasitic stray coupling and increase the blue sideband transition intensity. The blue sideband transition intensity is related to the coupling strength and detuning between the detection bit and the target bit. First, the dispersion condition needs to be met, that is, the coupling strength g between the detection bit and the target bit needs to be much smaller than the detuning Δ=ω of the two bits. a -ω q The applied frequency is ω ac =ω q -δ The first beam of microwaves and the variable frequency is ω s =ω a +δ is the second beam of microwave to the detection bit, where δ is detuned and controlled at 5MHz~10MHz, ω a is the frequency of the target bit. At this time, the effective Hamiltonian of the two-bit blue sideband transition is Where Ω ac is the driving strength of the first microwave beam, Ω s is the driving intensity of the second frequency-converted microwave, is the product of the detection bit and target bit generation operator, aσ - The method is to detect the product of the bit and the target bit annihilation operator. By adjusting the two-bit detuning and the intensity of the two-beam drive, a high-intensity and high-contrast blue sideband transition can be constructed. The method specifically includes:

[0045] The architecture of a 2*3 quantum bit array is as follows Figure 3As shown, it includes 6 frequency-adjustable transmission sub-qubits Q1, Q2, Q3, Q4, Q5, Q6 and 8 frequency-adjustable couplers C12, C23, C15, C24, C35, C26, C45, C56. C12 is represented as a coupler between Q1 and Q2. There is capacitive coupling between the bit and the coupler, and the coupling strength is about 67MHz, while there is direct capacitive coupling interaction between the bits, and the coupling strength is about 5MHz. The naming, coupling strength and architecture of other couplers are similar to coupler C12. All qubits and couplers are built based on two Josephson structure superconducting quantum interferometers, so the qubit frequency f Qi and adjustable coupler frequency f Ci The external magnetic flux Φ i By applying a DC bias signal to their respective Z lines to generate flux (i.e. Biot-Savart law, current generates magnetic field), thereby adjusting the frequency of the bit or coupler. Since the bit and cavity are controlled based on frequency resonance, the frequencies of the bits designed here are 3.9063GHz, 3.8911GHz, 4.0730GHz, 4.0912GHz, 3.8502GHz and 3.8326GHz, respectively, and the frequencies of the cavity are 6.93955GHz, 7.08130GHz, 6.98355GHz, 7.13165GHz, 7.04185GHz and 7.18975GHz, respectively. According to the actual bit architecture, it is first necessary to characterize the parameters of the entire sample. The following takes Q1 and C12 as examples to explain the specific operation steps, where Q1 is used as the detection bit and C12 is used as the target bit. By coupling Q1 with the corresponding reading cavity R1 and coupling the reading cavity R1 with the transmission line, a vector network analyzer can be used for continuous wave measurement or a data acquisition card can be used for pulse measurement to obtain cavity information, and bit information can be obtained through dispersion measurement. The method uses continuous wave measurement or pulse measurement to achieve energy spectrum measurement of the coupler. The method includes a continuous wave measurement method and a pulse measurement method, wherein:

[0046] (1) Continuous wave measurement method:

[0047] Vector network analyzers can measure network parameters and are used to characterize RF devices. In the field of superconducting quantum circuits, vector network analyzers are used to measure scattering parameters to characterize cavity and even bit information. Ports 1 and 2 of the vector network analyzer are connected to the two ports of the chip transmission line T1 via microwave cables. The S21 function of the vector network analyzer is used to measure the transmission signal (S21 parameter) of the chip transmission line T1. According to the sample design and the low-temperature line attenuation in the refrigerator, the input power of the vector network analyzer is adjusted to measure the frequency information of all cavities on the transmission line T1. By adjusting the voltage of the Z line corresponding to the cavity, the change of the cavity frequency with voltage, that is, the cavity flux modulation information, can be obtained. With the help of the cavity frequency information of the transmission line T1, the measurement point of the reading cavity of the bit Q1 can be obtained, including the reading frequency, reading power and voltage bias.

[0048] By using the measurement point of the known bit Q1, the information of bit Q1 can be further characterized. According to the theory of circuit quantum electrodynamics, the dispersion measurement of the bit is performed according to the influence of the bit on the dispersion cavity. The cavity frequency near the measurement point is scanned by a vector network analyzer, and the frequency of the input microwave is changed. When the input microwave frequency is (nearly) resonant with the bit frequency, the cavity voltage signal will have obvious peaks or valleys, thereby determining the bit frequency. By adjusting the voltage of the Z line, the frequency change of the bit with voltage, that is, the bit flux modulation information, can be obtained.

[0049] The energy spectrum of the target bit (i.e., coupler C12) is achieved by applying non-resonant drive to the detection bit (i.e., Q1) to induce high-order sideband transitions. The energy spectrum of the target bit can be read by using the sideband transition from the detection bit to the target bit. This method does not require a reading cavity for the target bit and can be completed only by manipulating the detection bit. The energy spectrum of the target bit can be measured using two microwave sources and a vector network analyzer. The waveform is as follows: Figure 4 shown.

[0050] According to the known bit Q1 and the flux modulation information of coupler C12, bit Q1 is biased to the frequency ω q , apply continuous microwave to the detection bit, the frequency is ω ac =ω q -δ (δ is the modulation offset, which is 5MHz to 10MHz). Then a continuously variable frequency microwave signal ω is applied s =ω a +δ to the detection bit, where ω a is the frequency of the target bit (i.e., coupler C12), such as Figure 5As shown. Finally, the frequency of the target bit (coupler C12) can be obtained by subtracting the offset δ from the peak frequency. According to the design parameters of coupler C12, the frequency range and flux modulation range of the continuous wave are determined. By adjusting the Z line voltage of coupler C12, the frequency of the coupler can be changed, thereby obtaining information on the change of the coupler frequency with voltage (flux).

[0051] (2) Pulse measurement method:

[0052] Pulse measurement refers to the use of microwave sources and arbitrary waveform generators to generate pulse signals of different timings under the coordination of atomic clocks and triggers. These signals are transmitted to the chip in the refrigerator through room temperature and low temperature cables, then amplified by amplifiers and demodulated by data acquisition cards on computers to obtain chip information.

[0053] For single-bit systems, pulse measurements are usually divided into three channels: read channel, drive channel, and DC bias channel. Similar to continuous wave measurement, it is first necessary to obtain the read cavity information and energy spectrum information for detecting a single bit. On the read channel, a read signal with variable frequency and power is applied to obtain the read cavity measurement point for detecting a single bit. Then, a 30μs variable frequency drive pulse is applied to a fixed measurement point. When the bit frequency is approximately resonant, an obvious peak or valley can be observed to determine the bit frequency.

[0054] By using the known measurement points and energy spectrum information of the detection bit, the energy spectrum information of the target bit (such as the coupler) can be obtained. The timing is as follows: Figure 6 As shown. Take bit Q1 as the detection bit and coupler C12 as the target bit as an example. According to the pre-designed coupler frequency, the bit frequency is determined, and the bit Q1 frequency is biased to ω by adjusting the DC bias signal. q To meet the dispersion condition (i.e., the detuning between the target bit and the detection bit needs to be much greater than the coupling strength between the two). The drive channel needs to apply two equal-length 30μs pulses, one of which has a fixed frequency of ω ac =ω q -δ, where δ is the detuning, which is controlled at 5MHz to 10MHz; another frequency-converting pulse ω s =ω a +δ is applied to the detection bit, where ω a is the frequency of the target bit. According to the designed frequency of the coupler C12 and the flux modulation information, the voltage of the z line on the coupler C12 is changed, and the frequency of the coupler is changed, and the change of the frequency of the coupler with the voltage (flux) can be obtained.

[0055] In the field of superconducting quantum circuits, pulse measurement is usually used instead of continuous wave measurement to characterize bit information. It has the following advantages: First, the timing can be controlled to conduct basic experiments such as Rabi oscillation and Ramsey interference; second, the total power consumption of the chip can be reduced to avoid heating effects. The chip is generally placed in the mixing chamber of the dilution refrigerator, with a temperature of about 10mK and a cooling power of 10-30μW. The power of the pulse signal input into the refrigerator is much smaller than the input power in the continuous wave measurement, which can prevent the chip from being affected by stray heating; third, the influence of AC Stark shift can be avoided, thereby improving the measurement accuracy. During continuous wave measurement, the microwave signal will always affect the bit, and an AC Stark shift will occur at this time.

[0056] The preferred embodiments of the present invention are described above with reference to the accompanying drawings, but the scope of the rights of the present invention is not limited thereto. Any modification, equivalent substitution and improvement made by a person skilled in the art without departing from the scope and essence of the present invention should be within the scope of the rights of the present invention.

Claims

1. A method for measuring a coupler energy spectrum using sideband transitions, characterized in that: The method includes a continuous wave measurement method or a pulse measurement method, wherein the continuous wave measurement method includes the following steps: S11, using a vector network analyzer to measure the scattering parameters of the detection bit to obtain the information of the reading cavity; S12, performing dispersion measurement on the detection bit according to the detection bit reading cavity information to obtain the detection bit information; S13, inducing high-order sideband transition by applying non-resonant strong driving to the detection bit to obtain the energy spectrum of the target bit; or the pulse measurement method comprises the following steps: S21, using a data acquisition card to measure the reading channel of the detection bit to obtain the information of the reading cavity; S22, performing dispersion measurement on the detection bit according to the information of the detection bit reading cavity to obtain the information of the detection bit; S23, inducing high-order sideband transition by applying non-resonant strong driving to the detection bit to obtain the energy spectrum of the target bit; The detection bit and the target bit are adjacent bits; the sideband transition is a blue sideband transition; and the conditions for the dispersion measurement are: The coupling strength of the adjacent bits needs to be much smaller than the detuning between the detection bit and the target bit; The frequency of the non-resonant strong drive is ω ac =ω q -δ The first beam of microwaves and the variable frequency is ω s =ω a +δ is the second frequency-converted microwave beam to the detection bit, where δ is detuned and controlled at 5MHz~10MHz, ω a is the frequency of the target bit.

2. The method for measuring coupler energy spectrum using sideband transition according to claim 1, characterized in that: The blue sideband transition refers to the transition between |0g> and |1e>; wherein |0> and |1> are the ground state energy level and the first excited state energy level of the detection bit, respectively; and |g> and |e> are the ground state energy level and the first excited state energy level of the target bit, respectively.

3. The method for measuring coupler energy spectrum using sideband transition according to claim 2, characterized in that: The effective Hamiltonian for the blue sideband transition is Where Ω ac is the driving strength of the first microwave beam, Ω s is the driving intensity of the second frequency-converted microwave, is the product of the detection bit and target bit generation operator, aσ - is the product of the detection bit and the target bit annihilation operator.

4. The method for measuring coupler energy spectrum using sideband transition according to claim 3, characterized in that: The method for constructing the blue sideband transition is to construct the blue sideband transition by adjusting the detuning of the detection bit and the target bit and the driving strength of the first microwave beam and the second frequency-converted microwave beam.

5. The method for measuring coupler energy spectrum using sideband transition according to claim 4, characterized in that: The construction method specifically includes: (1) Designing the architecture of the quantum bit array and naming the detection bits and target bits in the architecture; wherein the detection bits and target bits in the architecture are capacitively coupled and the coupling strength is set; and there is direct capacitive coupling interaction between the detection bits and the detection bits and the coupling strength is set; (2) constructing all detection bits and target bits in the architecture using a Josephson-structured superconducting quantum interference device; (3) applying a DC bias signal to the Z lines of the detection bit and the target bit to generate a magnetic flux and adjust the frequencies of the detection bit and the target bit; (4) Setting the detection bit and cavity frequency; (5) Characterize the parameters of the entire quantum bit array architecture.

6. The method for measuring coupler energy spectrum using sideband transition according to claim 5, characterized in that: The specific operation of the construction method is to obtain the information of the reading cavity by coupling the detection bit with the corresponding reading cavity and coupling the reading cavity with the transmission line, using a vector network analyzer for continuous wave measurement or using a data acquisition card for pulse measurement, and obtain the information of the detection bit through dispersion measurement.

7. The method for measuring coupler energy spectrum using sideband transition according to claim 1, characterized in that: The reading cavity information in S11 or S21 is the measurement points of the reading cavity, and the measurement points include the reading frequency, the reading power and the voltage bias.

8. The method for measuring coupler energy spectrum using sideband transition according to claim 1, characterized in that: The sideband transition in S13 or S23 is achieved through virtual energy levels.

9. A method for measuring coupler energy spectrum using sideband transition according to claim 1, 2, 3, 4 or 5, characterized in that: The detection bit is an adjustable transmission sub-qubit; the target bit is an adjustable coupler.

10. The method for measuring coupler energy spectrum using sideband transition according to claim 1, characterized in that: The energy spectrum measurement of the target bit is achieved through two microwave sources and a vector network analyzer.

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