A method, system, and quantum computer for optimizing a qubit cz gate

Abstract

The application discloses a method and system for optimizing a quantum bit CZ gate and a quantum computer. The method comprises the following steps: placing two adjacent quantum bits at a working point and closing a coupler between the two adjacent quantum bits; applying a pulse signal to the coupler to obtain the relationship between the occupation probability of the two quantum bits connected with the coupler being in a |1> state and the pulse signal; wherein the pulse signal is used to adjust the working frequency of the coupler; using a Ramsey experiment to obtain the relationship between the conditional phase of two adjacent quantum bits and the pulse signal; obtaining the parameters of the pulse signal when the occupation probability is greater than a first set value and the conditional phase θ control =; wherein the pulse signal is applied to the coupler connecting the two quantum bits, and the pulse signal is a high-frequency pulse and almost a single frequency; in the application, the pulse distortion caused by the line can be ignored, the time consumed by distortion calibration can be saved, and a CZ gate with a high execution degree is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of quantum computing, and in particular to a method and system for optimizing a CZ gate of a quantum bit and a quantum computer. BACKGROUND

[0002] Quantum computing and quantum information is a cross-discipline based on the principles of quantum mechanics to achieve computing and information processing tasks, which is closely related to quantum physics, computer science, information science, etc. In the past two decades, it has developed rapidly. Quantum algorithms based on quantum computers for factorization and unstructured search scenarios have shown much better performance than existing algorithms based on classical computers, and this direction has been expected to exceed the existing computing power. Since quantum computing has the potential to far exceed the performance of classical computers in solving certain problems, in order to realize a quantum computer, a quantum chip containing a sufficient number and quality of quantum bits is needed, and quantum logic gate operations and reading with high fidelity are required. Quantum chip is equivalent to CPU for traditional computer, quantum chip is the core component of quantum computer, and quantum chip is the processor that performs quantum computing. Before each quantum chip is formally put into use, the parameters of the quantum bits in the quantum chip need to be tested and characterized.

[0003] Similar to classical bits, when performing quantum computing using quantum bits, it is inevitable to apply quantum bit logic gates to quantum bits. For quantum bits, quantum bit logic gates actually refer to a series of control signals, and the accuracy of the parameters of these control signals is very important for quantum bits. Quantum bit logic gates are mainly composed of single quantum bit logic gates and two quantum bit logic gates, and two quantum bit logic gates include but are not limited to CNOT gates, swap gates, CZ gates, etc. Taking the CZ gate as an example, the working frequency of two adjacent quantum bits is adjusted to realize the CZ gate. Specifically, the working frequency is adjusted by the pulse signal applied to the Z line of the quantum bit. However, as the number of quantum bits expands, the pulse signal applied to the Z line of the quantum bit to adjust the working frequency will be distorted due to crosstalk, affecting the accuracy of the frequency adjustment of the quantum bit, and thus reducing the fidelity of the CZ gate.

[0004] In the prior art, in order to ensure the fidelity of the CZ gate, waveform distortion tests and calibration operations need to be performed on the Z lines of the two quantum bits first. The waveform distortion test and calibration operation of each quantum bit consumes a lot of time, which greatly affects the execution efficiency of the quantum computer.

[0005] It should be noted that the information disclosed in the background section of this application is only intended to deepen the understanding of the general background of the application and should not be considered as acknowledging or implying in any form that the information constitutes prior art known to those skilled in the art. SUMMARY

[0006] The purpose of the present application is to provide a method for optimizing a quantum bit CZ gate, a system and a quantum computer for obtaining a CZ gate with a higher degree of execution.

[0007] A method for optimizing a quantum bit CZ gate, comprising:

[0008] Placing two adjacent quantum bits at a working point, and closing a coupler between the two adjacent quantum bits;

[0009] Applying a pulse signal to the coupler to obtain the relationship between the occupation probability of the two quantum bits connected to the coupler being in the |1> state and the pulse signal; wherein the pulse signal is used to adjust the operating frequency of the coupler, and the pulse signal is a high-frequency signal and a quasi-single frequency, and the pulse signal as a high-frequency signal and a quasi-single frequency satisfies providing the energy difference of |0, 2> or |2, 0> and |1, 1> to the coupler;

[0010] Two adjacent quantum bits use a Ramsey experiment to obtain the relationship between the conditional phase and the pulse signal;

[0011] When the occupation probability is greater than a first set value and the conditional phase θ control = π, the parameters of the pulse signal are obtained.

[0012] Optionally, closing the coupler between the two adjacent quantum bits comprises:

[0013] Adjusting the energy level of the third quantum bit to be away from the energy levels of the first quantum bit and the second quantum bit, obtaining the coupling strength of a first adjustable coupler connected to the first quantum bit and the second quantum bit, and when the coupling strength is less than a second set value, the magnetic flux size of the first adjustable coupler is a first magnetic flux; the third quantum bit is a quantum bit that is adjacent to the first quantum bit or the second quantum bit in a quantum chip;

[0014] Setting the magnetic flux of the first adjustable coupler to the first magnetic flux, and setting the energy level of the third quantum bit to an initial state, obtaining the coupling strength of a second adjustable coupler connected to the third quantum bit and its adjacent first quantum bit or second quantum bit, and when the coupling strength is less than a second set value, the magnetic flux size of the second adjustable coupler is a second magnetic flux;

[0015] setting a magnetic flux size of the first adjustable coupler to a first magnetic flux and a magnetic flux size of the second adjustable coupler to a second magnetic flux to turn off the coupling between the first quantum bit, the second quantum bit and the third quantum bit.

[0016] Optionally, after the magnetic flux size of the first adjustable coupler is set to the first magnetic flux and the magnetic flux size of the second adjustable coupler is set to the second magnetic flux, the method further comprises:

[0017] obtaining a coupling strength between the first quantum bit and the second quantum bit, and if the coupling strength between the first quantum bit and the second quantum bit is greater than a second set value, updating the value of the first magnetic flux and returning to execute the step of setting the magnetic flux size of the first adjustable coupler to the first magnetic flux and the magnetic flux size of the second adjustable coupler to the second magnetic flux.

[0018] Optionally, the method of applying a pulse signal to the coupler and obtaining a relationship between the occupation probability of the two quantum bits connected to the coupler being in the |1> state and the pulse signal comprises:

[0019] causing the two adjacent quantum bits to be in the |1> state, applying a pulse signal to the coupler, and obtaining a relationship between the occupation probability and the pulse signal excitation time;

[0020] exciting the pulse signal for a set time and drawing a relationship image between the occupation probability and the pulse signal parameter; the pulse signal parameter includes an amplitude parameter and a frequency parameter.

[0021] Optionally, the relationship between the occupation probability and the pulse signal excitation time is obtained by directly measuring the occupation probability corresponding to the pulse signal excitation time.

[0022] Optionally, the two adjacent quantum bits use a Ramsey experiment to obtain a conditional phase θ control and the pulse signal, comprising:

[0023] causing the two adjacent quantum bits to be in the |0> state, performing an X / 2 inversion operation on one of the quantum bits, applying a pulse signal to cause the two quantum bits to be coupled, performing a quantum process tomography operation on the quantum bit subjected to the inversion operation after the pulse signal is applied for a set time, obtaining a phase difference θ corresponding to each time point of the |0> state and the |1> state of the quantum bit subjected to the inversion operation, and drawing a relationship image between the phase difference θ and the pulse signal parameter within the pulse signal excitation set time; the pulse signal parameter includes an amplitude parameter and a frequency parameter.

[0024] The two quantum bits are subjected to an X / 2 inversion operation respectively, and the phase differences corresponding to each time point are θ1 and θ2 respectively.

[0025] obtaining the conditional phase θ control = θ1- θ2, drawing the conditional phase θ control and the pulse signal parameter.

[0026] Optionally, the parameters of the pulse signal satisfying the condition that the occupation probability is greater than the first set value and the conditional phase θ control = π, the parameters including the frequency parameter and the amplitude parameter, include:

[0027] setting the frequency parameter in the pulse signal to a frequency fixed value, the amplitude parameter being a variable value; obtaining the occupation probability greater than the first set value and the corresponding amplitude determination value when the conditional phase θ control = π according to the relationship between the occupation probability and the pulse signal and the relationship between the conditional phase θ control = π and the pulse signal;

[0028] setting the amplitude parameter in the pulse signal to the amplitude determination value, the frequency parameter being a variable value; obtaining the occupation probability greater than the first set value and the corresponding frequency determination value when the conditional phase θ control = π according to the relationship between the occupation probability and the pulse signal and the relationship between the conditional phase θ control = π and the pulse signal.

[0029] Optionally, the parameters of the pulse signal satisfying the condition that the occupation probability is greater than the first set value and the conditional phase θ control = π, the parameters including the frequency parameter and the amplitude parameter, include:

[0030] setting the amplitude parameter in the pulse signal to an amplitude fixed value, the frequency parameter being a variable value; obtaining the occupation probability greater than the first set value and the corresponding frequency determination value when the conditional phase θ control = π according to the relationship between the occupation probability and the pulse signal and the relationship between the conditional phase θ control = π and the pulse signal.

[0031] setting the frequency parameter in the pulse signal to the frequency determination value, the amplitude parameter being a variable value; obtaining the occupation probability greater than the first set value and the corresponding amplitude determination value when the conditional phase θ control = π according to the relationship between the occupation probability and the pulse signal and the relationship between the conditional phase θ control = π and the pulse signal.

[0032] Optionally, the pulse signal Φ p (t) = A(t)cos(ω input t+ φ), when the amplitude parameter A(t) is a variable value, the amplitude function A(t) satisfies:

[0033]

[0034] parameters λ1, λ2, λ3,.... λ 2i , λ 2i+1 values are optimized by Nelder Mead algorithm, T a is the gate width.

[0035] Optionally, the pulse signal Φ p (t) = A(t)cos(ω input t + φ) is used, and the frequency parameter ω input is a variable value, the frequency parameter ω input satisfies that the pulse signal provides the energy difference between |0, 2> or |2, 0> and |1, 1> to the coupler.

[0036] A system for optimizing a quantum bit CZ gate, comprising:

[0037] A coupling shutdown module is configured to shut down the coupler between adjacent quantum bits after the working point is located between the adjacent quantum bits.

[0038] An acquisition probability and pulse signal relationship module is configured to obtain the relationship between the occupation probability of two quantum bits connected to the coupler being in the |1> state and the pulse signal after the pulse signal is applied to the coupler, wherein the pulse signal is used to adjust the working frequency of the coupler.

[0039] A Ramsey module is configured to obtain the relationship between the conditional phase of two adjacent quantum bits and the pulse signal.

[0040] A parameter determination module is configured to obtain the parameters of the pulse signal that satisfy the occupation probability being greater than a first set value and the conditional phase θ control = π.

[0041] A quantum control system uses the above-mentioned method for optimizing a quantum bit CZ gate to optimize the quantum bit CZ gate.

[0042] A quantum computer comprises the above-mentioned quantum control system.

[0043] Compared with the prior art, the present application has the following beneficial effects:

[0044] Based on the method of the application, and wherein the pulse signal is a high-frequency signal and is quasi-single frequency, the pulse signal as a high-frequency signal and quasi-single frequency satisfies the energy difference of |0, 2> or |2, 0> and |1, 1> provided to the coupler; the pulse signal is a high-frequency pulse and almost single frequency, compared with the flat-top Gaussian wave input to the quantum bit in the prior art, the pulse distortion caused by the line in the application can be ignored, the time consumed by the distortion calibration can be saved, and therefore the CZ gate with higher execution degree is realized.

[0045] The quantum bit parameter gate execution system, the quantum control system, the quantum computer and the readable storage medium provided by the application belong to the same inventive concept as the method for optimizing the CZ gate of the quantum bit, and therefore have the same beneficial effects, which will not be repeated here. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 The flowchart of the method for optimizing the CZ gate of the quantum bit provided by the application.

[0047] Figure 2 The flowchart of the method for controlling the coupling relationship between quantum bits in the application.

[0048] Figure 3 The structural schematic diagram of a quantum chip provided by the application.

[0049] Figure 4 The structural schematic diagram of the system for optimizing the CZ gate of the quantum bit provided by the application.

[0050] Figure 5 The structural schematic diagram of the coupling-off module in the system for optimizing the CZ gate of the quantum bit provided by the application.

[0051] Figure 6 The structural schematic diagram of the module for acquiring the relationship between the occupation probability and the pulse signal in the system for optimizing the CZ gate of the quantum bit provided by the application.

[0052] Figure 7 The structural schematic diagram of the Ramsey module in the system for optimizing the CZ gate of the quantum bit provided by the application.

[0053] Figure 8 The structural schematic diagram of one embodiment of the determination module in the system for optimizing the CZ gate of the quantum bit provided by the application.

[0054] Figure 9 The structural schematic diagram of another embodiment of the determination module in the system for optimizing the CZ gate of the quantum bit provided by the application. DETAILED DESCRIPTION

[0055] The specific embodiments of the present application will be described below in greater detail with reference to the accompanying drawings. The advantages and features of the present application will become more apparent from the following description and claims. It should be noted that the drawings are in extremely simplified form and are not drawn to precise scale, and are used merely to facilitate, clarify and aid understanding of the present application, and are not intended to limit the scope of the application.

[0056] In the description of the present application, it should be noted that the terms "center", "upper", "lower", "left", "right" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0057] In addition, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features referred to. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise specifically limited.

[0058] In order to better understand the technical solutions of the present application, the following will first briefly describe the QST, Ramsey experiment and Nelder-Mead algorithm involved in the present application:

[0059] The Ramsey experiment refers to applying two π / 2 quantum logic gate operations to a quantum bit, the time interval of the two operations is τ, and at the same time, a read pulse is applied to the quantum bit after the second π / 2 quantum logic gate operation to obtain the excited state distribution P1(τ) of the quantum bit, and the time interval τ is changed to obtain the process of P1(τ). The result of a typical Ramsey experiment is that P1(τ) satisfies the mathematical model of exponential oscillation decay with respect to the time interval τ as follows:

[0060]

[0061] In formula (1), A and B are fitting coefficients, T0 is the decoherence time of the quantum bit, f d is the carrier frequency of the microwave pulse signal corresponding to the π / 2 quantum logic gate operation, f0 is the oscillation frequency of the quantum bit, and f0 and the real frequency f q of the quantum bit satisfy:

[0062] f0(f d )=|f q -f d | (2)

[0063] In summary and in combination with formula (2), it can be obtained that the result of the Ramsey experiment, that is, the oscillation frequency of the curve, is equal to the difference between the carrier frequency of the quantum logic gate operation and the real frequency of the quantum bit, so that the Ramsey experiment can be used to not only obtain the decoherence time of the quantum bit, but also accurately obtain the real frequency of the quantum bit.

[0064] The Nelder-Mead algorithm adheres to the basic idea of ensuring that each iteration is better than the previous one, first finds a basic feasible solution, and then identifies whether it is the optimal solution, if not, iterates to another improved basic feasible solution according to certain rules, and then identifies again, if it is still not, iterates again, and repeats in this way. Because the number of basic feasible solutions is limited, the optimal solution of the problem can be obtained through a limited number of iterations.

[0065] As can be understood by those skilled in the art, in order to ensure that the CZ gate between the quantum bits is executed more quickly and accurately, it is necessary to first make the coupling strength between the two quantum bits as small as possible, and then obtain the CZ gate on this basis.

[0066] Reference Figure 1 The application provides a method for optimizing a CZ gate of a quantum bit, which comprises the following steps:

[0067] S100, two adjacent quantum bits are brought to a working point, and a coupler between the two adjacent quantum bits is closed; the working point is an initial position of the frequency of the quantum bits when the CZ gate is executed.

[0068] S200, a pulse signal is applied to the coupler to obtain a relationship between the occupation probability of the two quantum bits connected with the coupler being in a |1> state and the pulse signal; wherein the pulse signal is used to adjust the working frequency of the coupler, and the pulse signal Φ p (t)=A(t)cos(ω input t+φ) is a high-frequency signal and is quasi-single-frequency, the pulse signal that is a high-frequency signal and is quasi-single-frequency satisfies the energy difference between |0, 2> or |2, 0> and |1, 1> provided to the coupler; in the scheme, the pulse signal is a high-frequency pulse and is almost single-frequency, compared with the multiple frequency components of the flat-top Gaussian wave input to the quantum bit in the prior art scheme after Fourier transform, the pulse distortion caused by the circuit in the application can be ignored, the time consumed for distortion calibration can be saved, and thus a CZ gate with a higher degree of execution can be realized.

[0069] The conditional phase of two adjacent quantum bits obtained by using the Ramsey experiment is related to the pulse signal;

[0070] S300, parameters of the pulse signal satisfying the occupation probability being greater than a first set value and the conditional phase θ control = π are obtained.

[0071] The difference from the prior art is that the CZ obtaining method of the quantum bit provided by the application first turns off the coupler between the two adjacent quantum bits, then applies a pulse signal to the coupler, obtains the occupation probability of the two quantum bits being in the |1> state, and performs a Ramsey experiment to obtain the parameter of the pulse signal when the occupation probability P 11 is greater than a first set value or the leakage rate is less than a second set value and the condition phase θ control = π. The scheme of the application can effectively reduce the coupling strength between adjacent quantum bits, effectively improve the execution accuracy of the two quantum bit logic gate by applying a pulse signal to the coupler, and to a certain extent, improve the execution accuracy of the quantum computer.

[0072] Specifically, refer to Figure 2 , turn off the coupler between the two adjacent quantum bits, including:

[0073] S110: Adjust the energy level of the third quantum bit to be far away from the energy levels of the first quantum bit and the second quantum bit, obtain the coupling strength of the first adjustable coupler connecting the first quantum bit and the second quantum bit, and when the coupling strength is less than a second set value, the magnetic flux of the first adjustable coupler is a first magnetic flux; the third quantum bit is a quantum bit adjacent to the first quantum bit or the second quantum bit in the quantum chip; the quantum bit adjacent to the first quantum bit or the second quantum bit is processed as the third quantum bit;

[0074] S120: Set the magnetic flux of the first adjustable coupler to the first magnetic flux, and set the energy level of the third quantum bit to an initial state, obtain the coupling strength of the second adjustable coupler connecting the third quantum bit and its adjacent first quantum bit or second quantum bit, and when the coupling strength is less than a second set value, the magnetic flux of the second adjustable coupler is a second magnetic flux; in a theoretical case, the coupling strength between quantum bits needs to be equal to 0, and in actual operation, the required coupling strength is less than 10KHz. In this scheme, three quantum bits are taken as an example, refer to Figure 3 , Figure 3 is a structure diagram of a quantum chip provided by the embodiment, wherein Q1, Q2, and Q3 are the first quantum bit, the second quantum bit, and the third quantum bit, respectively, C1 and C2 are the first adjustable coupler and the second adjustable coupler, Q1 and Q2 are two quantum bits in the quantum chip for executing a two quantum bit logic gate, and Q3 is a quantum bit having a coupling relationship with Q2. The coupling strength between the first quantum bit and the second quantum bit is less than 10KHz, and the coupling strength between the second quantum bit and the third quantum bit is also less than 10KHz.

[0075] S130: set the magnetic flux size of the first adjustable coupler to a first magnetic flux and set the magnetic flux size of the second adjustable coupler to a second magnetic flux to turn off the coupling between the first quantum bit, the second quantum bit and the third quantum bit.

[0076] Specifically, in the embodiment, after the magnetic flux size of the first adjustable coupler is set to the first magnetic flux and the magnetic flux size of the second adjustable coupler is set to the second magnetic flux, the method further comprises:

[0077] obtaining the coupling strength between the first quantum bit and the second quantum bit, and if the coupling strength between the first quantum bit and the second quantum bit is greater than a second set value at this time, updating the value of the first magnetic flux and returning to execute the step of setting the magnetic flux size of the first adjustable coupler to the first magnetic flux and setting the magnetic flux size of the second adjustable coupler to the second magnetic flux. Thus, the magnetic flux size of the first adjustable coupler and the second adjustable coupler is corrected for the last time.

[0078] The scheme of the present application considers not only the first quantum bit and the second quantum bit participating in the execution of the two-qubit logic gate, but also the coupling relationship of the adjacent bit, i.e. the third quantum bit. In the quantum chip, similar to the third quantum bit, other quantum bits can also be turned off by the coupling according to the scheme, eliminating the coupling influence of the adjacent bits.

[0079] Specifically, in the embodiment, the energy level of the third quantum bit is adjusted by the bit frequency of the third quantum bit. Those skilled in the art can understand that the bit frequency refers to the signal frequency size required for the quantum bit to be excited from the |0> state to the |1> state.

[0080] Specifically, in the embodiment, when the coupling strength between the first quantum bit and the second quantum bit is less than the second set value, the magnetic flux size of the first adjustable coupler is the first magnetic flux, comprising:

[0081] setting any one of the two quantum bits in the |0> state and the |1> state, respectively, and obtaining the frequency of the other quantum bit of the two quantum bits by using the Ramsey experiment, the two quantum bits being the first quantum bit and the second quantum bit;

[0082] obtaining the coupling strength between the first quantum bit and the second quantum bit as a first value based on the obtained two frequencies;

[0083] determining whether the first value is less than a second set value, and if so, obtaining the magnetic flux size of the first adjustable coupler at this time as the first magnetic flux;

[0084] If not, the magnetic flux size of the first adjustable coupler is adjusted, and the process returns to setting any one of the two qubits in |0> state and |1> state respectively, and acquiring the frequency of the other qubit by using Ramsey experiment.

[0085] Specifically, in the embodiment, the coupling strength between the first qubit and the second qubit is acquired as a first value based on the acquired two frequencies, which includes:

[0086] The difference between the two frequencies is acquired, and the difference is divided by two to obtain the coupling strength between the first qubit and the second qubit.

[0087] Specifically, when the coupling strength between the third qubit and its adjacent first qubit or second qubit is less than a second set value, the magnetic flux size of the second adjustable coupler is a second magnetic flux, which includes:

[0088] Any one of the two qubits is set in |0> state and |1> state respectively, and the frequency of the other qubit is acquired by using Ramsey experiment, the two qubits being the third qubit and its adjacent first qubit or second qubit;

[0089] The coupling strength between the second qubit and the third qubit is acquired as a second value based on the acquired two frequencies;

[0090] It is judged whether the second value is less than a second set value, if yes, the magnetic flux size of the second adjustable coupler at this time is the second magnetic flux;

[0091] If not, the magnetic flux size of the second adjustable coupler is adjusted, and the process returns to setting any one of the two qubits in |0> state and |1> state respectively, and acquiring the frequency of the other qubit by using Ramsey experiment.

[0092] Specifically, the pulse signal is applied to the coupler to obtain the relationship between the occupation probability of the two qubits connected with the coupler being both in |1> state and the pulse signal, which includes:

[0093] The two adjacent qubits are both in |1> state, the pulse signal is applied to the coupler to obtain the relationship between the occupation probability and the excitation time of the pulse signal; the pulse signal compensates the energy difference between |0, 2> or |2, 0> and |1, 1>, so that the exchange between |0, 2> or |2, 0> and |1, 1> occurs;

[0094] The pulse signal excitation setting time, draw the relationship between the occupation probability and the pulse signal parameters; the pulse signal parameters include amplitude parameters and frequency parameters. In another embodiment, the relationship between the leakage rate and the pulse signal parameters can also be drawn, and the leakage rate = 1-occupation probability, which also belongs to the protection scope of the present application. The pulse signal

[0095] Optionally, the relationship between the occupation probability and the pulse signal excitation time is obtained by directly measuring the occupation probability corresponding to the pulse signal excitation time.

[0096] Optionally, the occupation probability of the pulse signal excitation time can also be obtained by QST (Quantum state tomography). As a commonly used quantum state calibration method, QST can construct a corresponding density matrix by measuring the projection of the quantum state in different directions.

[0097] The two adjacent quantum bits use the Ramsey experiment to obtain the conditional phase θ control The relationship with the pulse signal includes:

[0098] The two adjacent quantum bits are both in the |0> state, an X / 2 inversion operation is performed on one of the quantum bits, the pulse signal is applied to make the two quantum bits coupled, after the pulse signal is applied for a set time, the quantum process tomography operation is performed on the quantum bit subjected to the inversion operation, the phase difference θ corresponding to each time point of the |0> state and the |1> state of the quantum bit subjected to the inversion operation is obtained, and the relationship between the phase difference θ and the pulse signal parameters within the pulse signal excitation setting time is drawn; the pulse signal parameters include amplitude parameters and frequency parameters.

[0099] The two quantum bits are subjected to X / 2 inversion operations respectively, and the phase differences θ1 and θ2 of each time point are obtained respectively; in detail, the first quantum bit is subjected to an X / 2 inversion operation, the phase difference θ1 of each time point of the |0> state and the |1> state of the first quantum bit is obtained, that is, the relationship between the phase difference θ1 and the time is obtained; similarly, the second quantum bit is subjected to an X / 2 inversion operation, the phase difference θ2 of each time point of the |0> state and the |1> state of the second quantum bit is obtained, that is, the relationship between the phase difference θ2 and the time is obtained.

[0100] The phase differences of the same time point are subtracted to obtain the conditional phase θ control = θ1- θ2, the conditional phase and the pulse signal function image are obtained.

[0101] In an embodiment, the parameters of the pulse signal satisfying the conditions that the occupation probability is greater than a first set value and the conditional phase θ control = π are obtained, and the parameters include frequency parameters and amplitude parameters, which include:

[0102] The frequency parameter in the pulse signal is set to a frequency fixed value, and the amplitude parameter is a variable value; according to the relationship between the occupation probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the conditional phase θ control = π is obtained, and the corresponding amplitude determination value is determined.

[0103] The amplitude parameter in the pulse signal is set to the amplitude determination value, and the frequency parameter is a variable value; according to the relationship between the occupation probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the conditional phase θ control = π is obtained, and the corresponding frequency determination value is determined.

[0104] In still another embodiment, the parameter of the pulse signal satisfying the occupation probability greater than the first set value and the conditional phase θ control = π is obtained, and the parameter includes the frequency parameter and the amplitude parameter, including:

[0105] The amplitude parameter in the pulse signal is set to a fixed value, and the frequency parameter is a variable value; according to the relationship between the occupation probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the conditional phase θ control = π is obtained, and the corresponding frequency determination value is determined.

[0106] The frequency parameter in the pulse signal is set to the frequency determination value, and the amplitude parameter is a variable value; according to the relationship between the occupation probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the conditional phase θ control = π is obtained, and the corresponding amplitude determination value is determined.

[0107] In the above two embodiments, the pulse signal Φ p (t) = A(t)cos(ω input t + φ), when the amplitude parameter A(t) is a variable value, the amplitude function A(t) satisfies:

[0108]

[0109] The parameters λ1, λ2, λ3,.... λ 2i , λ 2i+1 The values are optimized by Nelder Mead algorithm, and T a is the gate width.

[0110] In the above two embodiments, the pulse signal Φ p(t) = A(t)cos(ω input t+φ) where the frequency parameter ω input is a variable value, the frequency parameter ω input satisfies that the energy difference between |0,2> or |2,0> and |1,1> is provided to the coupler by the pulse signal.

[0111] Based on the same inventive concept, please refer to Figure 4 The embodiment of the present application also provides a system for optimizing a quantum bit CZ gate, comprising:

[0112] A coupling-off module 100 is configured to turn off a coupler between adjacent quantum bits after the adjacent quantum bits are located at a working point;

[0113] An acquisition probability and pulse signal relationship module 200 is configured to obtain a relationship between an occupation probability and a pulse signal after the pulse signal is applied to the coupler, wherein the pulse signal is used to adjust a working frequency of the coupler; and the occupation probability is that both of two quantum bits connected with the coupler are in a |1> state.

[0114] A Ramsey module 300 is configured to obtain a relationship between a conditional phase of two adjacent quantum bits and the pulse signal.

[0115] A parameter determination module 400 is configured to obtain a parameter of the pulse signal when the occupation probability is greater than a first set value and the conditional phase θ control = π.

[0116] As shown in Figure 5 The coupling-off module 100 comprises:

[0117] A first magnetic flux acquisition unit 110 is configured to adjust an energy level of a third quantum bit to be far away from energy levels of a first quantum bit and a second quantum bit, acquire a coupling strength of a first adjustable coupler connected with the first quantum bit and the second quantum bit, and set a magnetic flux of the first adjustable coupler to a first magnetic flux when the coupling strength is less than a second set value; and the third quantum bit is a quantum bit that is adjacent to the first quantum bit or the second quantum bit in a quantum chip.

[0118] A second magnetic flux acquisition unit 120 is configured to set the magnetic flux of the first adjustable coupler to the first magnetic flux, set the energy level of the third quantum bit to an initial state, acquire a coupling strength of a second adjustable coupler connected with the first quantum bit or the second quantum bit adjacent to the third quantum bit, and set a magnetic flux of the second adjustable coupler to a second magnetic flux when the coupling strength is less than the second set value.

[0119] The coupling turn-off unit 130 is configured to set the magnetic flux size of the first adjustable coupler to a first magnetic flux and set the magnetic flux size of the second adjustable coupler to a second magnetic flux, so as to turn off the coupling among the first quantum bit, the second quantum bit and the third quantum bit.

[0120] As shown in Figure 6 The occupation probability and pulse signal relationship acquisition module 200 includes:

[0121] The occupation probability and pulse signal excitation time acquisition unit 210 is configured to measure the relationship between the occupation probability and the pulse signal excitation time after the pulse signal is applied to the coupler when the two adjacent quantum bits are in the |1> state. Preferably, the occupation probability is directly measured.

[0122] The occupation probability and pulse signal parameter relationship acquisition unit 220 is configured to draw a relationship image of the occupation probability and the pulse signal parameter within the pulse signal excitation time.

[0123] As shown in Figure 7 The Ramsey module 300 includes:

[0124] The phase difference acquisition unit 310 is configured to make the two adjacent quantum bits in the |0> state, perform an X / 2 inversion operation on one of the quantum bits, apply a pulse signal to cause the two quantum bits to be coupled, perform a quantum process tomography operation on the quantum bit subjected to the inversion operation after the pulse signal is applied for a set time, and obtain the phase difference θ corresponding to each time point of the |0> state and the |1> state of the quantum bit subjected to the inversion operation. The two quantum bits are subjected to X / 2 inversion operations respectively, and the phase differences θ1 and θ2 of each time point are obtained respectively.

[0125] The conditional phase and pulse signal parameter relationship acquisition unit 320 is configured to obtain a conditional phase θ control = θ1-θ2 by subtracting the two phase differences, and draw a relationship image of the conditional phase θ control and the pulse signal parameter.

[0126] The parameter determination module includes a frequency determination unit and an amplitude determination unit connected in series through a switching unit.

[0127] In the two different embodiments of the parameter determination module 400 in the present application, the following are respectively:

[0128] As shown in Figure 8 The input end of the switching unit 420 is connected with the output end of the frequency determination unit 410, and the output end is connected with the input end of the amplitude determination unit 430.

[0129] Frequency determination unit 410: sets the frequency parameter in the pulse signal to a fixed value, and the amplitude parameter to a variable value; based on the relationship between the occupancy probability and the pulse signal and the conditional phase θ... control The relationship between the pulse signal and the occupancy probability is greater than a first preset value and the conditional phase θ control The amplitude value corresponding to π;

[0130] Adapter unit 420: sets the amplitude parameter in the pulse signal to the amplitude determination value, and the frequency parameter to a variable value; inputs the corresponding pulse signal to the amplitude determination unit;

[0131] Amplitude determination unit 430: Based on the relationship between the occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupancy probability is greater than a first preset value and the conditional phase θ control The frequency value corresponding to π.

[0132] like Figure 9 As shown, the input terminal of the adapter unit 420 is connected to the output terminal of the amplitude determination unit 430, and the output terminal is connected to the input terminal of the frequency determination unit 410.

[0133] Amplitude determination unit 430: sets the amplitude parameter in the pulse signal to a fixed amplitude value, and the frequency parameter to a variable value; based on the relationship between the occupancy probability and the pulse signal and the conditional phase θ... control The relationship between the pulse signal and the occupancy probability is greater than a first preset value and the conditional phase θ control The determined frequency value corresponding to π;

[0134] Adapter unit 420: Sets the frequency parameter in the pulse signal to the frequency determination value, and the amplitude parameter is a variable value, and inputs it to the frequency determination unit 410;

[0135] Frequency determination unit 410: Based on the relationship between the occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupancy probability is greater than a first preset value and the conditional phase θ control The amplitude value corresponding to π.

[0136] In the two embodiments above, the pulse signal Φ p (t)=A(t)cos(ω input When the amplitude parameter A(t) is a variable value, the amplitude function A(t) satisfies:

[0137]

[0138] Parameters λ1, λ2, λ3, ....λ 2i, λ 2i+1 The values are optimized by Nelder Mead algorithm, T a is the gate width. For convenience of calculation, the amplitude function is:

[0139]

[0140] The pulse signal Φ p (t) = A(t)cos(ω input t + φ), the frequency parameter ω input is a variable, the frequency parameter ω input satisfies that the pulse signal provides the energy difference between |0, 2> or |2, 0> and |1, 1> to the coupler.

[0141] It can be understood that each unit can be combined in one device, or any one of the units can be split into multiple sub-units, or at least part of the functions of the one or more units can be combined with part of the functions of other units, and implemented in one function module. According to the embodiments of the present application, at least one of the units can be at least partially implemented as a hardware circuit, such as a field programmable gate array (FPGA), a programmable logic array (PLA), a system on chip, a system on substrate, a system on package, an application specific integrated circuit (ASIC), or any other reasonable way of integrating or packaging circuits, etc. hardware or firmware, or a suitable combination of software, hardware and firmware. Alternatively, at least one of the units can be at least partially implemented as a computer program module which can perform the functions of the corresponding module when the program is run by a computer.

[0142] Based on the same inventive concept, the embodiments of the present application also propose a quantum control system, which utilizes the method for optimizing a quantum bit CZ gate according to any one of the above feature descriptions, or a system comprising the quantum bit CZ gate according to the above feature descriptions.

[0143] Based on the same inventive concept, the embodiments of the present application also propose a quantum computer, which comprises the quantum control system according to the above feature descriptions.

[0144] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", or "a specific example" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any suitable manner in any one or more embodiments. In addition, those skilled in the art can combine and integrate different embodiments or examples described in the present specification.

[0145] The above are only preferred embodiments of the present application, and do not have any limiting effect on the present application. Any person skilled in the art can make any form of equivalent replacement or modification, etc. to the technical solutions and technical contents disclosed by the present application without departing from the scope of the technical solutions of the present application, which still belongs to the protection scope of the present application.

Claims

1. A method of optimizing a qubit CZ gate, characterized by, The method comprises the following steps: making two adjacent quantum bits at a working point, and closing a coupler between the two adjacent quantum bits; applying a pulse signal to the coupler to obtain a relationship between an occupation probability of the two quantum bits connected with the coupler being in a |1> state and the pulse signal; wherein the pulse signal is used to adjust a working frequency of the coupler, the pulse signal is a high-frequency signal and is quasi-single-frequency, and the pulse signal as a high-frequency signal and quasi-single-frequency satisfies that an energy difference of |0, 2> or |2, 0> and |1, 1> is provided to the coupler; two adjacent quantum bits use a Ramsey experiment to obtain a relationship between a conditional phase and the pulse signal; obtaining a condition phase when the occupation probability of both the two qubits being in |1> state is greater than a first set value parameters of the clock pulse signal; the step of applying the pulse signal to the coupler to obtain the relationship between the occupation probability of the two quantum bits connected with the coupler being in the |1> state and the pulse signal comprises the following steps: making the two adjacent quantum bits in the |1> state, and applying a pulse signal to the coupler to obtain a relationship between an occupation probability and a pulse signal excitation time; pulse signal excitation sets a time, and a relationship graph of the occupation probability and a pulse signal parameter is drawn; the pulse signal parameter comprises an amplitude parameter and a frequency parameter; the step of obtaining the relationship between the occupation probability and the pulse signal excitation time is by directly measuring the occupation probability corresponding to the pulse signal excitation time.

2. The method of claim 1, wherein, the step of closing the coupler between the two adjacent quantum bits comprises the following steps: adjusting an energy level of a third quantum bit to be far away from energy levels of a first quantum bit and a second quantum bit, obtaining a coupling strength of a first adjustable coupler connected with the first quantum bit and the second quantum bit, and when the coupling strength is less than a second set value, a magnetic flux of the first adjustable coupler is a first magnetic flux; the third quantum bit is a quantum bit that is adjacent to the first quantum bit or the second quantum bit in a quantum chip; setting the magnetic flux of the first adjustable coupler to the first magnetic flux, and setting the energy level of the third quantum bit to an initial state, obtaining a coupling strength of a second adjustable coupler connected with the third quantum bit and the adjacent first quantum bit or second quantum bit, and when the coupling strength is less than the second set value, a magnetic flux of the second adjustable coupler is a second magnetic flux; setting the magnetic flux of the first adjustable coupler to the first magnetic flux and the magnetic flux of the second adjustable coupler to the second magnetic flux to close the coupling between the first quantum bit, the second quantum bit and the third quantum bit.

3. The method of claim 2, wherein, after the magnetic flux of the first adjustable coupler is set to the first magnetic flux and the magnetic flux of the second adjustable coupler is set to the second magnetic flux, the method further comprises the following steps: obtaining a coupling strength between the first quantum bit and the second quantum bit, and if the coupling strength between the first quantum bit and the second quantum bit is greater than the second set value at this time, updating a value of the first magnetic flux, and returning to execute the step of setting the magnetic flux of the first adjustable coupler to the first magnetic flux and the magnetic flux of the second adjustable coupler to the second magnetic flux.

4. The method of claim 1, wherein, The two adjacent qubits obtain conditional phase using Ramsey experiment In relation to the pulse signal, including: To achieve the desired state, two adjacent qubits are both in the |0> state. An X / 2 inversion operation is performed on one of the qubits. An applied pulse signal causes the two qubits to couple. After the pulse signal is applied for a set time, a quantum process tomography (QT) operation is performed on the inverted qubit to obtain the phase difference between the |0> and |1> states of the inverted qubit at various time points. The phase difference within a set time period of pulse signal excitation is plotted. A graph showing the relationship between pulse signal parameters and pulse signal parameters; the pulse signal parameters include amplitude parameters and frequency parameters. Two of the quantum bits are respectively subjected to X / 2 inversion operation, and the phase difference of each time point is obtained as and ; differencing the two phase differences to obtain a conditional phase , plotting the conditional phase image of the conditional phase in relation to the pulse signal parameters.

5. The method of claim 1, wherein, obtaining a condition phase satisfying that occupation probability of the two qubits being in |1> state is greater than a first set value parameters of the clock pulse signal, the parameters including a frequency parameter and an amplitude parameter, comprising: The frequency parameter in the pulse signal is set as a fixed value, and the amplitude parameter is a variable value; the occupation probability is obtained according to the relationship between the occupation probability and the pulse signal and the conditional phase When the occupation probability is greater than a first set value and the conditional phase corresponding to the pulse signal is greater than a second set value, the amplitude determination value is determined. The amplitude parameter in the pulse signal is set as the amplitude determined value, and the frequency parameter is a variable value; the occupation probability is obtained according to the relationship between the occupation probability and the pulse signal and the conditional phase The frequency determined value corresponding to the condition that the occupation probability is greater than a first set value and the conditional phase of the pulse signal is greater than a second set value.

6. The method of claim 1, wherein, obtaining a condition phase satisfying that occupation probability of the two qubits being in |1> state is greater than a first set value parameters of the clock pulse signal, the parameters including a frequency parameter and an amplitude parameter, comprising: The amplitude parameter in the pulse signal is set as a fixed amplitude value, and the frequency parameter is a variable value; the occupation probability is obtained according to the relationship between the occupation probability and the pulse signal and the conditional phase When the occupation probability is greater than a first set value and the conditional phase corresponding to the pulse signal is greater than a second set value, the frequency determination value is determined. setting the frequency parameter in the pulse signal to the frequency determined value, the amplitude parameter being a variable value; obtaining the occupation probability according to the relationship between the occupation probability and the pulse signal and the conditional phase corresponding to the amplitude determined value when the occupation probability is greater than a first set value and the conditional phase of the pulse signal.

7. The method of claim 5 or 6, wherein, The pulse signal In the amplitude parameter The amplitude function Satisfies: Parameters , , … Values are optimized by the Nelder Mead algorithm, is the gate width.

8. The method of claim 5 or 6, wherein, The pulse signal In the case where the frequency parameter is a variable value, the frequency parameter satisfies the energy difference of the pulse signal to the coupler provides |0,2> or |2,0> and |1,1>.

9. A system for optimizing a qubit CZ gate, the system comprising: The method comprises the following steps: a coupling closing module is configured to close a coupler between adjacent quantum bits after the adjacent quantum bits are at a working point; The obtaining-occupation-probability-and-pulse-signal-relation module is configured to obtain a relation between an occupation probability of two qubits connected with the coupler being in |1> state and a pulse signal after the pulse signal is applied to the coupler, wherein the pulse signal is used to adjust a working frequency of the coupler. The Ramsey module is configured to obtain a relation between a conditional phase of two adjacent qubits and the pulse signal. a parameter determination module, configured to obtain a parameter of the clock pulse signal, wherein the parameter satisfies a condition that occupation probabilities of the two qubits being in |1> states are greater than a first set value and a conditional phase of the two qubits is less than a second set value. The obtaining-occupation-probability-and-pulse-signal-relation module is configured to obtain a relation between an occupation probability of two qubits connected with the coupler being in |1> state and a pulse signal after the pulse signal is applied to the coupler, wherein the pulse signal is used to adjust a working frequency of the coupler. The Ramsey module is configured to obtain a relation between a conditional phase of two adjacent qubits and the pulse signal. The obtaining-occupation-probability-and-pulse-signal-relation module is configured to obtain a relation between an occupation probability of two qubits connected with the coupler being in |1> state and a pulse signal after the pulse signal is applied to the coupler, wherein the pulse signal is used to adjust a working frequency of the coupler. The Ramsey module is configured to obtain a relation between a conditional phase of two adjacent qubits and the pulse signal.

10. A quantum control system, characterized by, The obtaining-occupation-probability-and-pulse-signal-relation module is configured to obtain a relation between an occupation probability of two qubits connected with the coupler being in |1> state and a pulse signal after the pulse signal is applied to the coupler, wherein the pulse signal is used to adjust a working frequency of the coupler.

11. A quantum computer, characterized by The Ramsey module is configured to obtain a relation between a conditional phase of two adjacent qubits and the pulse signal. The obtaining-occupation-probability-and-pulse-signal-relation module is configured to obtain a relation between an occupation probability of two qubits connected with the coupler being in |1> state and a pulse signal after the pulse signal is applied to the coupler, wherein the pulse signal is used to adjust a working frequency of the coupler. The Ramsey module is configured to obtain a relation between a conditional phase of two adjacent qubits and the pulse signal. The obtaining-occupation-probability-and-pulse-signal-relation module is configured to obtain a relation between an occupation probability of two qubits connected with the coupler being in |1> state and a pulse signal after the pulse signal is applied to the coupler, wherein the pulse signal is used to adjust a working frequency of the coupler. The Ramsey module is configured to obtain a relation between a conditional phase of two adjacent qubits and the pulse signal. The obtaining-occupation-probability-and-pulse-signal-relation module is configured to obtain a relation between an occupation probability of two qubits connected with the coupler being in |1> state and a pulse signal after the pulse signal is applied to the coupler, wherein the pulse signal is used to adjust a working frequency of the coupler. The Ramsey module is configured to obtain a relation between a conditional phase of two adjacent qubits and the pulse signal. The obtaining-occupation-probability-and-pulse-signal-relation module is configured to obtain a relation between an occupation probability of two qubits connected with the coupler being in |1> state and a pulse signal after the pulse signal is applied to the coupler, wherein the pulse signal is used to adjust a working frequency of the coupler. The Ramsey module is configured to obtain a relation between a conditional phase of two adjacent q

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