Method and system for optimizing quantum bit CZ gate and quantum computer

Abstract

The invention discloses a method and system for optimizing a quantum bit CZ gate and a quantum computer, and the method comprises the steps: enabling two adjacent quantum bits to be located at a working point, and closing a coupler between the two adjacent quantum bits; applying a pulse signal to the coupler, and obtaining two quantum bits which are both 1gt and connected with the coupler; a relationship between the occupancy probability of the state and the pulse signal; wherein the pulse signal is used for adjusting the working frequency of the coupler; using a Ramsey experiment on two adjacent quantum bits to obtain a relation between a conditional phase and the pulse signal; and obtaining a parameter of the pulse signal when the occupancy probability is greater than a first set value and the condition phase theta < contro > is equal to the condition phase theta < contro >. The pulse signal is applied to the coupler connected with the two quantum bits, the pulse signal is high-frequency pulse and almost single-frequency pulse, pulse distortion caused by the circuit can be ignored, time consumed by distortion calibration can be saved, and a CZ gate with high execution degree is achieved.

Description

Technical Field

[0001] The present invention relates to the field of quantum computing technology, and in particular to a method and system for optimizing a quantum bit CZ gate, and a quantum computer. Background Art

[0002] Quantum computing and quantum information are interdisciplinary subjects that use the principles of quantum mechanics to achieve computing and information processing tasks. They are closely related to quantum physics, computer science, informatics and other disciplines. They have developed rapidly in the past two decades. Quantum algorithms based on quantum computers in scenarios such as factorization and unstructured search have shown performance far beyond that of existing algorithms based on classical computers, and this direction has been expected to exceed existing computing capabilities. Since quantum computing has the potential to far exceed the performance of classical computers in solving specific problems, in order to realize quantum computers, it is necessary to obtain a quantum chip that contains a sufficient number and quality of quantum bits, and can perform quantum logic gate operations and read quantum bits with extremely high fidelity. Quantum chips are to quantum computers what CPUs are to traditional computers. Quantum chips are the core components of quantum computers, and quantum chips are processors that perform quantum computing. Before each quantum chip is officially put into use, it is necessary to test and characterize the various parameters of the quantum bits in the quantum chip.

[0003] Similar to classical bits, when using qubits to perform quantum computing, it is inevitable to apply qubit logic gates to the qubits. For qubits, qubit logic gates actually refer to a series of control signals. The accuracy of the various parameters of these control signals is very important for qubits. Qubit logic gates are mainly composed of two types: single-qubit logic gates and two-qubit logic gates. Two-qubit logic gates include but are not limited to CNOT gates, exchange gates, CZ gates, etc. Taking the CZ gate as an example, the CZ gate is realized by adjusting the operating frequencies of two adjacent qubits. Specifically, the operating frequency is adjusted by the pulse signal applied to the Z line of the qubit. However, as the number of qubits increases, the pulse signal applied to the Z line of the qubit to adjust the operating frequency will be distorted due to crosstalk, affecting the accuracy of the frequency adjustment of the qubit, thereby reducing the fidelity of the CZ gate.

[0004] In the prior art, in order to ensure the fidelity of the CZ gate, it is necessary to first perform waveform distortion testing and calibration operations on the Z lines of the two quantum bits. The waveform distortion testing and calibration operations for each quantum bit take 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 technology section of this application is only intended to deepen the understanding of the general background technology of this application, and should not be regarded as an admission or suggestion in any form that the information constitutes prior art already known to those skilled in the art. Summary of the invention

[0006] The object of the present invention is to provide a method, system and quantum computer for optimizing a quantum bit CZ gate, so as to obtain a CZ gate with a higher execution degree.

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

[0008] Make the two adjacent quantum bits at the working point and close the coupler between the two adjacent quantum bits;

[0009] Applying a pulse signal to the coupler to obtain a 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, the pulse signal is a high-frequency signal and is quasi-single-frequency, and the pulse signal as a high-frequency signal and a quasi-single-frequency signal satisfies the requirement of providing the coupler with an energy difference of |0, 2> or |2, 0> and |1, 1>;

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

[0011] Obtain the condition that the occupation probability is greater than the first set value and the phase θ control =π is the parameter of the pulse signal.

[0012] Optionally, closing a coupler between two adjacent quantum bits includes:

[0013] The energy level of the third quantum bit is adjusted to be away from the energy levels of the first quantum bit and the second quantum bit, and the coupling strength of the first adjustable coupler connecting the first quantum bit and the second quantum bit is obtained. When the coupling strength is less than the second set value, the magnetic flux of the first adjustable coupler is the first magnetic flux; the third quantum bit is a quantum bit in the quantum chip that is adjacent to the first quantum bit or the second quantum bit;

[0014] The magnetic flux of the first adjustable coupler is set to the first magnetic flux, and the energy level of the third quantum bit is set to an initial state, and the coupling strength of the second adjustable coupler connecting the third quantum bit and its neighboring first quantum bit or second quantum bit is obtained, and when the coupling strength is less than a second set value, the magnetic flux of the second adjustable coupler is the second magnetic flux;

[0015] The magnetic flux size of the first adjustable coupler is set to a first magnetic flux, and the magnetic flux size of the second adjustable coupler is set to a second magnetic flux, so as to cut off the coupling among 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 includes:

[0017] Obtain the coupling strength between the first quantum bit and the second quantum bit. If the coupling strength between the first quantum bit and the second quantum bit is greater than a second set value, update the value of the first magnetic flux, and return 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, applying a pulse signal to the coupler to obtain a relationship between an occupation probability of two quantum bits connected to the coupler being in a |1> state and the pulse signal includes:

[0019] Make two adjacent quantum bits both in the |1> state, apply a pulse signal to the coupler, and obtain the relationship between the occupation probability and the pulse signal excitation time;

[0020] The pulse signal excitation setting time is used to draw a relationship graph between the occupation probability and the pulse signal parameters; the pulse signal parameters include amplitude parameters and frequency parameters.

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

[0022] Optionally, the two adjacent quantum bits use the Ramsey experiment to obtain a conditional phase θ control The relationship with the pulse signal includes:

[0023] Make two adjacent quantum bits both in the |0> state|, perform an X / 2 inversion operation on one of the quantum bits, apply a pulse signal to couple the two quantum bits, apply the pulse signal for a set time, perform a quantum process tomography operation on the quantum bit undergoing the inversion operation, obtain the phase difference θ corresponding to each time point between the quantum bit |0> state and |1> state of the inversion operation, and draw a relationship image between the phase difference θ and the pulse signal parameters within the set time of pulse signal excitation; the pulse signal parameters include amplitude parameters and frequency parameters;

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

[0025] Difference of the two phase differences is used to obtain the conditional phase θ control =θ1-θ2, plot the conditional phase θ control Relationship image with the pulse signal parameters.

[0026] Optionally, the acquisition satisfies the condition that the occupancy probability is greater than a first set value and the phase θ control =π, the parameters of the pulse signal include frequency parameters and amplitude parameters, including:

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

[0028] 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 occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding frequency is determined.

[0029] Optionally, the acquisition satisfies the condition that the occupancy probability is greater than a first set value and the phase θ control =π, the parameters of the pulse signal include frequency parameters and amplitude parameters, including:

[0030] The amplitude parameter in the pulse signal is set to a fixed amplitude value, and the frequency parameter is a variable value; according to the relationship between the occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π corresponding frequency determination value;

[0031] 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 occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding amplitude is determined.

[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 , 2i+1 The value is optimized by the Nelder Mead algorithm, T a The door width.

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

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

[0037] A coupling shutoff module, used to shut down the coupler between adjacent quantum bits after the adjacent quantum bits are located at the working point;

[0038] A module for obtaining the relationship between occupation probability and pulse signal, which is used to obtain the relationship between the occupation probability and pulse signal of two quantum bits connected to the coupler in the |1> state after applying the pulse signal to the coupler; wherein the pulse signal is used to adjust the operating frequency of the coupler;

[0039] A Ramsey module, used to obtain the relationship between the conditional phase of two adjacent quantum bits and the pulse signal;

[0040] The parameter determination module is used to obtain the condition that the occupation probability is greater than the first set value and the phase θ control =π is the parameter of the pulse signal.

[0041] A quantum control system, which optimizes the quantum bit CZ gate system using the above-mentioned method for optimizing the quantum bit CZ gate.

[0042] A quantum computer comprises the quantum control system as described above.

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

[0044] Based on the method of the present 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 a quasi-single-frequency satisfies the requirement of providing an energy difference of |0, 2> or |2, 0> and |1, 1> to the coupler; the pulse signal is a high-frequency pulse and is almost single-frequency. Compared with the flat-top Gaussian wave input to the quantum bit in the prior art solution, which has multiple frequency components after Fourier transform, the pulse distortion caused by the line in the present application can be ignored, which can save the time consumed by the distortion calibration, thereby realizing a CZ gate with a higher execution degree.

[0045] The parameter gate execution system, quantum control system, quantum computer and readable storage medium of the quantum bit proposed in the present invention belong to the same inventive concept as the method for optimizing the quantum bit CZ gate, and therefore have the same beneficial effects, which will not be described in detail here. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 A schematic flow chart of the method for optimizing quantum bit CZ gates proposed in the present invention.

[0047] Figure 2 It is a flow chart of the method for controlling the coupling relationship between quantum bits in the present invention.

[0048] Figure 3 This is a schematic diagram of the structure of a quantum chip proposed in the present invention.

[0049] Figure 4 This is a schematic diagram of the structure of the system for optimizing quantum bit CZ gates proposed in the present invention.

[0050] Figure 5 This is a schematic diagram of the structure of the coupling shutdown module in the system for optimizing the quantum bit CZ gate proposed in the present invention.

[0051] Figure 6 This is a schematic diagram of the structure of the module for obtaining the relationship between occupation probability and pulse signal in the system of optimizing quantum bit CZ gate proposed in the present invention.

[0052] Figure 7 This is a schematic diagram of the structure of the Ramsey module in the system of the optimized quantum bit CZ gate proposed in the present invention.

[0053] Figure 8 A schematic diagram of the structure of an embodiment of a system determination module for optimizing a quantum bit CZ gate proposed in the present invention.

[0054] Fig. 9 A schematic diagram of the structure of another embodiment of the system determination module for optimizing the quantum bit CZ gate proposed in the present invention. DETAILED DESCRIPTION

[0055] The specific implementation of the present invention will be described in more detail below in conjunction with the schematic diagram. The advantages and features of the present invention will become clearer from the following description and claims. It should be noted that the drawings are all in a very simplified form and are not in exact proportions, and are only used to facilitate and clearly assist in explaining the purpose of the embodiments of the present invention.

[0056] In the description of the present invention, it is necessary to understand that the terms "center", "up", "down", "left", "right", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as a limitation on the present invention.

[0057] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0058] In order to better understand the technical solution of the present application, the QST, Ramsey experiment and Nelder-Mead algorithm involved in the present application are briefly described below:

[0059] The Ramsey experiment refers to the process of applying two π / 2 quantum logic gate operations to a quantum bit, with the time interval between the two operations being τ, applying a read pulse 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 changing the time interval τ to obtain P1(τ). The result of a typical Ramsey experiment is that P1(τ) satisfies the mathematical model of exponential oscillation decay with 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, and 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 is related to the real frequency f of the quantum bit. q , the carrier frequency of the π / 2 quantum logic gate operation satisfies:

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

[0063] In summary and combined with formula (2), we can obtain: 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. Therefore, in addition to being able to be used to obtain the decoherence time of the quantum bit, the Ramsey experiment can also accurately obtain the real frequency of the quantum bit at the same time.

[0064] The Nelder-Mead algorithm adheres to the basic idea of ​​ensuring that each iteration is better than the previous one. First, find a basic feasible solution to see if it is the optimal solution. If not, it iterates to another improved and better basic feasible solution according to certain rules, and then identifies it. If it is still not the optimal solution, it iterates again and repeats this process. Since the number of basic feasible solutions is limited, the optimal solution to the problem can be obtained after a limited number of iterations.

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

[0066] Please refer to Figure 1 The present invention proposes a method for optimizing a quantum bit CZ gate, comprising:

[0067] S100, making two adjacent quantum bits at a working point, and closing a coupler between the two adjacent quantum bits; the working point is the initial position of the quantum bit frequency when executing the CZ gate.

[0068] S200, applying a pulse signal to the coupler to obtain a relationship between the occupation probability of the two quantum bits connected to the coupler being both 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 Φ p (t) = A(t)cos(ω input t+φ) is a high-frequency signal and is quasi-single-frequency. The pulse signal as a high-frequency signal and quasi-single-frequency satisfies the requirement of providing the coupler with an energy difference of |0, 2> or |2, 0> and |1, 1>; in this scheme, the pulse signal is a high-frequency pulse and is almost single-frequency. Compared with the flat-top Gaussian wave input to the quantum bit in the prior art scheme, which has multiple frequency components after Fourier transform, the pulse distortion caused by the line in this application can be ignored, which can save the time consumed by the distortion calibration, thereby realizing a CZ gate with higher execution degree.

[0069] Two adjacent quantum bits use the Ramsey experiment to obtain the relationship between the conditional phase and the pulse signal;

[0070] S300, obtaining a condition that the occupation probability is greater than the first set value and the phase θ control =π is the parameter of the pulse signal.

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

[0072] For details, please refer to Figure 2 , closing the coupler between two adjacent quantum bits, including:

[0073] S110: 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 the first adjustable coupler connecting the first quantum bit and the second quantum bit, when the coupling strength is less than the second set value, the magnetic flux of the first adjustable coupler is the first magnetic flux; the third quantum bit is a quantum bit in the quantum chip that is adjacent to the first quantum bit or the second quantum bit; the quantum bits that are adjacent to the first quantum bit or the second quantum bit are all 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 the initial state, obtain the coupling strength of the second adjustable coupler connecting the third quantum bit and its neighboring first quantum bit or second quantum bit, when the coupling strength is less than the second set value, the magnetic flux of the second adjustable coupler is the second magnetic flux; in theory, the coupling strength between quantum bits needs to be equal to 0, and in actual operation, the coupling strength is required to be less than 10KHz. In this solution, three quantum bits are taken as an example, please refer to Figure 3 , Figure 3 This is a schematic diagram of the structure of a quantum chip proposed in this 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-qubit logic gate, and Q3 is a quantum bit coupled to 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: Setting the magnetic flux size of the first adjustable coupler to a first magnetic flux and the magnetic flux size of the second adjustable coupler to a second magnetic flux, so as to cut off the coupling among the first quantum bit, the second quantum bit and the third quantum bit.

[0076] Specifically, in this 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 includes:

[0077] The coupling strength between the first qubit and the second qubit is obtained. If the coupling strength between the first qubit and the second qubit is greater than the second set value, the value of the first magnetic flux is updated, and 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. Thus, the magnetic flux of the first adjustable coupler and the second adjustable coupler are calibrated for the last time.

[0078] In addition to considering the first quantum bit and the second quantum bit involved in the execution of the two-qubit logic gate, the solution of the present application also takes into account the coupling relationship of the adjacent bit, that is, the third quantum bit. Other quantum bits in the quantum chip similar to the third quantum bit can use this solution to shut down the coupling, thereby eliminating the coupling influence of the adjacent bits.

[0079] Specifically, in this embodiment, the energy level of the third qubit is adjusted by the bit frequency of the third qubit. It can be understood by those skilled in the art that the bit frequency refers to the signal frequency required for the qubit to be excited from the |0> state to the |1> state.

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

[0081] Setting any one of the two qubits in the |0> state and the |1> state respectively, and obtaining the frequency of the other qubit of the two qubits by using a Ramsey experiment, the two qubits being the first qubit and the second qubit;

[0082] Based on the two acquired frequencies, acquiring a coupling strength between the first quantum bit and the second quantum bit as a first value;

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

[0084] If not, adjust the magnetic flux size of the first adjustable coupler, and return to execute the step of setting any one of the two quantum bits in the |0> state and the |1> state respectively, and use the Ramsey experiment to obtain the frequency of the other quantum bit of the two quantum bits.

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

[0086] The difference between the two frequencies is obtained, and the difference is divided by two, and the result obtained is the coupling strength between the first quantum bit and the second quantum bit.

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

[0088] Setting any one of the two qubits in the |0> state and the |1> state respectively, and obtaining the frequency of the other qubit of the two qubits by using a Ramsey experiment, wherein the two qubits are the third qubit and the first qubit or the second qubit adjacent to the third qubit;

[0089] Based on the two acquired frequencies, acquiring a coupling strength between the second quantum bit and the third quantum bit as a second value;

[0090] Determine whether the second value is less than a second set value, and if so, obtain the magnetic flux size of the second adjustable coupler at this time as the second magnetic flux;

[0091] If not, adjust the magnetic flux size of the second adjustable coupler, and return to execute the step of setting any one of the two quantum bits in the |0> state and the |1> state respectively, and use the Ramsey experiment to obtain the frequency of the other quantum bit of the two quantum bits.

[0092] Specifically, applying a pulse signal to the coupler to obtain a relationship between the occupation probability of two quantum bits connected to the coupler being in the |1> state and the pulse signal includes:

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

[0094] The pulse signal excitation setting time is used to draw a graph of the relationship between the occupancy probability and the pulse signal parameters; the pulse signal parameters include amplitude parameters and frequency parameters. In another embodiment, a graph of the relationship between the leakage rate and the pulse signal parameters can also be drawn, where the leakage rate = 1-occupancy probability, which also falls within the scope of protection of this application. The pulse signal

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

[0096] Optionally, the occupation probability of the pulse signal excitation time can also be obtained through QST. QST (Quantum state tomography) is a commonly used method for quantum state calibration. By projecting and measuring quantum states in different directions, the corresponding density matrix can be constructed.

[0097] The two adjacent qubits obtain the conditional phase θ using the Ramsey experiment 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 applied causes the two quantum bits to couple, and after the pulse signal is applied for a set time, a quantum process tomography operation is performed on the quantum bit undergoing the inversion operation, and a phase difference θ corresponding to each time point between the quantum bit |0> state and the quantum bit |1> state undergoing the inversion operation is obtained, and a relationship image between the phase difference θ and the pulse signal parameters within the set time of the pulse signal excitation is plotted; the pulse signal parameters include amplitude parameters and frequency parameters;

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

[0100] Difference the previous two phase differences at the same time point to obtain the conditional phase θ control =θ1-θ2, and obtain the conditional phase and pulse signal function image.

[0101] In one embodiment, the acquisition satisfies the occupancy probability greater than the first set value and the conditional phase θ control =π, the parameters of the pulse signal include frequency parameters and amplitude parameters, including:

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

[0104] In another embodiment, the acquisition satisfies the condition that the occupancy probability is greater than the first set value and the phase θ control =π, the parameters of the pulse signal include frequency parameters and amplitude parameters, including:

[0105] The amplitude parameter in the pulse signal is set to a fixed amplitude value, and the frequency parameter is a variable value; according to the relationship between the occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding frequency 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 occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding amplitude 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] Parameters λ1, λ2, λ3, ....λ 2i , 2i+1 The value is optimized by the Nelder Mead algorithm, T a The door width.

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

[0111] Based on the same invention concept, please refer to Figure 4 , the embodiment of the present invention further proposes a system for optimizing a quantum bit CZ gate, comprising:

[0112] A coupling shutoff module 100 is used to shut down the coupler between adjacent qubits after the adjacent qubits are at the working point;

[0113] The module 200 for obtaining the relationship between the occupation probability and the pulse signal is used to obtain the relationship between the occupation probability and the pulse signal of the two quantum bits connected to the coupler being in the |1> state after applying the pulse signal to the coupler; wherein the pulse signal is used to adjust the operating frequency of the coupler;

[0114] A Ramsey module 300, for obtaining a relationship between the conditional phases of two adjacent quantum bits and the pulse signal;

[0115] The parameter determination module 400 is used to obtain the condition that the occupation probability is greater than the first set value and the phase θ control =π is the parameter of the pulse signal.

[0116] like Figure 5 As shown, the coupling cut-off module 100 includes:

[0117] The first magnetic flux acquisition unit 110 adjusts 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, and acquires the coupling strength of the first adjustable coupler connecting the first quantum bit and the second quantum bit. When the coupling strength is less than the second set value, the magnetic flux of the first adjustable coupler is the first magnetic flux; the third quantum bit is a quantum bit in the quantum chip that is adjacent to the first quantum bit or the second quantum bit;

[0118] A second magnetic flux acquisition unit 120 is used 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, and obtain the coupling strength of the second adjustable coupler connecting the third quantum bit and its neighboring first quantum bit or second quantum bit. When the coupling strength is less than a second set value, the magnetic flux of the second adjustable coupler is the second magnetic flux;

[0119] The coupling shutoff unit 130 is used to set the magnetic flux size of the first adjustable coupler to a first magnetic flux and the magnetic flux size of the second adjustable coupler to a second magnetic flux, so as to shut off the coupling between the first quantum bit, the second quantum bit and the third quantum bit.

[0120] like Figure 6 As shown, the module 200 for obtaining the relationship between the occupancy probability and the pulse signal includes:

[0121] The occupation probability and pulse signal excitation time acquisition unit 210 measures the relationship between the occupation probability and the pulse signal excitation time after applying a pulse signal to the coupler when two adjacent quantum bits are in the |1> state; the occupation probability is preferably measured directly.

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

[0123] like Figure 7 As shown, the Ramsey module 300 includes:

[0124] The phase difference acquisition unit 310 makes two adjacent quantum bits both in the |0> state|, performs an X / 2 inversion operation on one of the quantum bits, applies a pulse signal to couple the two quantum bits, applies the pulse signal for a set time, performs a quantum process tomography operation on the quantum bit undergoing the inversion operation, and obtains the phase difference θ corresponding to each time point of the quantum bit |0> state and |1> state of the inversion operation; the two quantum bits are respectively subjected to an X / 2 inversion operation, and the corresponding phase differences at each time point are θ1 and θ2 respectively;

[0125] The conditional phase and pulse signal parameter relationship acquisition unit 320 obtains the conditional phase θ by subtracting the two phase differences. control =θ1-θ2, plot the conditional phase θ control Relationship image with the pulse signal parameters.

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

[0127] Two different embodiments of the parameter determination module 400 described in this application are as follows:

[0128] like Figure 8 As shown, the input end of the switching unit 420 is connected to the output end of the frequency determination unit 410 , and the output end is connected to 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 frequency value, and the amplitude parameter to a variable value; according to the relationship between the occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding amplitude is determined;

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

[0131] Amplitude determination unit 430: According to the relationship between the occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding frequency is determined.

[0132] like Fig. 9 As shown, the input end of the switching unit 420 is connected to the output end of the amplitude determination unit 430 , and the output end is connected to the input end 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; according to the relationship between the occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π corresponding frequency determination value;

[0134] The switching unit 420 sets the frequency parameter in the pulse signal as 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: According to the relationship between the occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding amplitude is determined.

[0136] 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:

[0137]

[0138] Parameters λ1, λ2, λ3, ....λ 2i, 2i+1 The value is optimized by the Nelder Mead algorithm, T a is the gate width. For the 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 When it is a variable value, the frequency parameter ω input The pulse signal satisfies the energy difference between |0, 2> or |2, 0> and |1, 1> provided to the coupler.

[0141] It is 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 one or more units can be combined with part of the functions of other units and implemented in one functional module. According to an embodiment of the present invention, 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 a chip, a system on a substrate, a system on a package, an application specific integrated circuit (ASIC), or can be implemented in hardware or firmware such as any other reasonable way to integrate or package the circuit, or in 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, and when the program is run by a computer, the function of the corresponding module can be executed.

[0142] Based on the same inventive concept, an embodiment of the present invention also proposes a quantum control system, using the method of optimizing quantum bit CZ gate described in any one of the above feature descriptions, or a system including the optimized quantum bit CZ gate described in the above feature descriptions.

[0143] Based on the same inventive concept, an embodiment of the present invention further proposes a quantum computer, including the quantum control system described in the above feature description.

[0144] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example" or "specific example" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments in a suitable manner. In addition, those skilled in the art may combine and combine different embodiments or examples described in this specification.

[0145] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any technician in the relevant technical field, without departing from the scope of the technical solution of the present invention, makes any form of equivalent replacement or modification to the technical solution and technical content disclosed in the present invention, which does not depart from the content of the technical solution of the present invention and still falls within the protection scope of the present invention.

Claims

1. A method for optimizing a quantum bit CZ gate, characterized in that: include: Make the two adjacent quantum bits at the working point and close the coupler between the two adjacent quantum bits; Applying a pulse signal to the coupler to obtain a 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, the pulse signal is a high-frequency signal and is quasi-single-frequency, and the pulse signal as a high-frequency signal and a quasi-single-frequency signal satisfies the requirement of providing the coupler with an energy difference of |0, 2> or |2, 0> and |1, 1>; Two adjacent quantum bits use the Ramsey experiment to obtain the relationship between the conditional phase and the pulse signal; Obtain the condition that the occupation probability is greater than the first set value and the phase θ control = parameters of the time pulse signal.

2. The method according to claim 1, characterized in that Closing the coupler between two adjacent qubits includes: The energy level of the third quantum bit is adjusted to be away from the energy levels of the first quantum bit and the second quantum bit, and the coupling strength of the first adjustable coupler connecting the first quantum bit and the second quantum bit is obtained. When the coupling strength is less than the second set value, the magnetic flux of the first adjustable coupler is the first magnetic flux; the third quantum bit is a quantum bit in the quantum chip that is adjacent to the first quantum bit or the second quantum bit; The magnetic flux of the first adjustable coupler is set to the first magnetic flux, and the energy level of the third quantum bit is set to an initial state, and the coupling strength of the second adjustable coupler connecting the third quantum bit and its neighboring first quantum bit or second quantum bit is obtained, and when the coupling strength is less than a second set value, the magnetic flux of the second adjustable coupler is the second magnetic flux; The magnetic flux size of the first adjustable coupler is set to a first magnetic flux, and the magnetic flux size of the second adjustable coupler is set to a second magnetic flux, so as to cut off the coupling among the first quantum bit, the second quantum bit and the third quantum bit.

3. The method according to claim 2, characterized in that 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 includes: Obtain the coupling strength between the first quantum bit and the second quantum bit. If the coupling strength between the first quantum bit and the second quantum bit is greater than a second set value, update the value of the first magnetic flux, and return 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.

4. The method according to claim 1, characterized in that The applying a pulse signal to the coupler to obtain a relationship between the occupation probability of two quantum bits connected to the coupler being in the |1> state and the pulse signal includes: Make two adjacent quantum bits both in the |1> state, apply a pulse signal to the coupler, and obtain the relationship between the occupation probability and the pulse signal excitation time; The pulse signal excitation setting time is used to draw a relationship graph between the occupation probability and the pulse signal parameters; the pulse signal parameters include amplitude parameters and frequency parameters.

5. The method according to claim 4, characterized in that 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.

6. The method according to claim 1, characterized in that The two adjacent qubits obtain the conditional phase θ using the Ramsey experiment control The relationship with the pulse signal includes: Make two adjacent quantum bits both in the |0> state|, perform an X / 2 inversion operation on one of the quantum bits, apply a pulse signal to couple the two quantum bits, apply the pulse signal for a set time, perform a quantum process tomography operation on the quantum bit undergoing the inversion operation, obtain the phase difference θ corresponding to each time point between the quantum bit |0> state and |1> state of the inversion operation, and draw a relationship image between the phase difference θ and the pulse signal parameters within the set time of pulse signal excitation; the pulse signal parameters include amplitude parameters and frequency parameters; The two quantum bits are subjected to an X / 2 inversion operation respectively, and the corresponding phase differences at each time point are θ1 and θ2 respectively; Difference of the two phase differences is used to obtain the conditional phase θ control =1-2, plot the conditional phase θ control Relationship image with the pulse signal parameters.

7. The method according to claim 1, characterized in that The acquisition satisfies the condition that the occupancy probability is greater than the first set value and the phase θ control = parameters of the pulse signal when the pulse signal is φ(t) = φ(t), the parameters including frequency parameters and amplitude parameters, including: The frequency parameter in the pulse signal is set to a fixed frequency value, and the amplitude parameter is a variable value; according to the relationship between the occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding amplitude is determined; 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 occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding frequency is determined.

8. The method according to claim 1, characterized in that The acquisition satisfies the condition that the occupancy probability is greater than the first set value and the phase θ control =π, the parameters of the pulse signal include frequency parameters and amplitude parameters, including: The amplitude parameter in the pulse signal is set to a fixed amplitude value, and the frequency parameter is a variable value; according to the relationship between the occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding frequency is determined; 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 occupancy probability and the pulse signal and the conditional phase θ control The relationship between the pulse signal and the occupation probability is greater than the first set value and the conditional phase θ control =π when the corresponding amplitude is determined.

9. The method according to claim 7 or 8, characterized in that 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: Parameters λ1, λ2, λ3, ....λ 2i , 2i+1 The value is optimized by the Nelder Mead algorithm, T a The door width.

10. The method according to claim 7 or 8, characterized in that The pulse signal Φ p (t) = A(t)cos(ω input t+φ), the frequency parameter ω input When it is a variable value, the frequency parameter ω input The pulse signal satisfies the energy difference between |0, 2> or |2, 0> and |1, 1> provided to the coupler.

11. A system for optimizing a quantum bit CZ gate, characterized in that: include: A coupling shutoff module, used to shut down the coupler between adjacent quantum bits after the adjacent quantum bits are located at the working point; A module for obtaining the relationship between occupation probability and pulse signal, which is used to obtain the relationship between the occupation probability and pulse signal of two quantum bits connected to the coupler in the |1> state after applying the pulse signal to the coupler; wherein the pulse signal is used to adjust the operating frequency of the coupler; A Ramsey module, used to obtain the relationship between the conditional phase of two adjacent quantum bits and the pulse signal; The parameter determination module is used to obtain the condition that the occupation probability is greater than the first set value and the phase θ control = parameters of the time pulse signal.

12. A quantum control system, characterized in that: A method for optimizing a quantum bit CZ gate as described in any one of claims 1 to 10, or a system comprising the optimized quantum bit CZ gate as described in claim 11.

13. A quantum computer, characterized in that: Comprising the quantum control system as claimed in claim 12.

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