Quantum Circuit and Its Control Method, Superconducting Quantum Chip, and Superconducting Quantum Computer

Abstract

The present disclosure provides a quantum circuit, a control method thereof, a superconducting quantum chip, and a superconducting quantum computer, relating to the field of quantum computers, and particularly to the field of superconducting quantum chip technology. The quantum circuit includes: qubits, a readout resonator; and a tunable coupler configured to couple with the qubits and the readout resonator. When initializing the qubits based on the readout resonator, the modulation frequency of the tunable coupler is configured to be equal to the difference between the frequencies of the readout resonator and the qubits.

Description

Technical Field

[0001] The present disclosure relates to the field of quantum computers, particularly to the field of superconducting quantum chip technology, and specifically to a quantum circuit, a control method for the quantum circuit, a superconducting quantum chip, and a superconducting quantum computer. Background Art

[0002] Superconducting quantum computing is currently a route that is generally considered by the industry to be the most likely to achieve a universal quantum computer. In recent years, the number of superconducting quantum bits has been continuously increasing, and the decoherence time of superconducting quantum bits has also been continuously improved, making the performance of superconducting quantum processors also enhanced. Currently, superconducting quantum processors have exceeded classical computers in special computing tasks and achieved quantum computing advantages experimentally. However, the performance of current superconducting quantum processors still far from meets the general needs of the industry, and how to further improve the performance of superconducting quantum processors has become a problem to be solved. Summary of the Invention

[0003] The present disclosure provides a quantum circuit, a control method for the quantum circuit, a superconducting quantum chip, and a superconducting quantum computer.

[0004] According to one aspect of the present disclosure, there is provided a quantum circuit, including: a quantum bit, a readout resonator; and an adjustable coupler configured to couple with the quantum bit and the readout resonator, wherein when initializing the quantum bit based on the readout resonator, the modulation frequency of the adjustable coupler is configured to be equal to the frequency difference between the readout resonator and the quantum bit.

[0005] According to another aspect of the present disclosure, there is provided a method for controlling a quantum circuit, the quantum circuit including a quantum bit, a readout resonator, and an adjustable coupler, wherein the method includes: when initializing the quantum bit based on the readout resonator, adjusting the modulation frequency of the adjustable coupler so that the modulation frequency is equal to the frequency difference between the readout resonator and the quantum bit.

[0006] According to another aspect of the present disclosure, there is provided an electronic device, including: at least one processor; and a memory communicatively connected to the at least one processor; the memory stores instructions executable by the at least one processor, and when executed by the at least one processor, the instructions enable the at least one processor to execute the method described in the present disclosure.

[0007] According to another aspect of the present disclosure, there is provided a non-transitory computer-readable storage medium storing computer instructions for causing a computer to execute the method described in the present disclosure.

[0008] According to another aspect of the present disclosure, there is provided a computer program product including a computer program which, when executed by a processor, implements the method described in the present disclosure.

[0009] According to another aspect of the present disclosure, there is provided a superconducting quantum chip including the quantum circuit described above.

[0010] According to another aspect of the present disclosure, there is provided a superconducting quantum computer including the superconducting quantum chip described above.

[0011] According to one or more embodiments of the present disclosure, effective initialization of superconducting qubits is achieved by reading a resonator, that is, by modulating the coupling strength through a tunable coupler to achieve energy exchange between the superconducting qubit and the reading resonator without changing the qubit frequency, thereby realizing the initialization of the superconducting qubit.

[0012] It should be understood that the content described in this part is not intended to identify the key or important features of the embodiments of the present disclosure, nor is it used to limit the scope of the present disclosure. Other features of the present disclosure will become easily understood through the following description. Description of the Drawings

[0013] The drawings exemplarily illustrate embodiments and form a part of the description, and are used together with the written description of the description to explain the exemplary embodiments of the embodiments. The illustrated embodiments are for illustrative purposes only and do not limit the scope of the claims. In all the drawings, the same reference numerals refer to similar but not necessarily identical elements.

[0014] Figure 1 A schematic diagram of a circuit structure in which a qubit is directly coupled to a reading resonator according to an embodiment of the present disclosure is shown;

[0015] Figure 2 A schematic diagram of a quantum circuit according to an embodiment of the present disclosure is shown;

[0016] Figure 3 A schematic diagram of a superconducting quantum interference device according to an embodiment of the present disclosure is shown;

[0017] Figure 4 A schematic diagram of a numerical simulation result during the qubit initialization process according to an embodiment of the present disclosure is shown;

[0018] Figure 5 A schematic diagram of a circuit structure in which a qubit is coupled to a reading resonator through a tunable coupler according to an embodiment of the present disclosure is shown;

[0019] Figure 6 A schematic diagram of a coupling strength change curve according to an embodiment of the present disclosure is shown;

[0020] Figure 7 shows a flowchart of a quantum circuit control method according to an embodiment of the present disclosure;

[0021] Figure 8 shows a flowchart of a quantum circuit control method of an exemplary embodiment; and

[0022] Figure 9 shows a structural block diagram of an exemplary electronic device capable of implementing the embodiments of the present disclosure. Detailed implementation manners

[0023] The following describes exemplary embodiments of the present disclosure with reference to the accompanying drawings. Various details of the embodiments of the present disclosure are included to assist understanding, and they should be considered merely exemplary. Therefore, those of ordinary skill in the art should recognize that various changes and modifications can be made to the embodiments described herein without departing from the scope of the present disclosure. Similarly, for clarity and conciseness, descriptions of well-known functions and structures are omitted in the following description.

[0024] In the present disclosure, unless otherwise specified, the terms "first", "second", etc. are used to describe various elements and are not intended to limit the positional relationship, timing relationship, or importance relationship of these elements. Such terms are only used to distinguish one element from another. In some examples, the first element and the second element may refer to the same instance of the element, and in certain cases, based on the context description, they may also refer to different instances.

[0025] In the description of various examples in the present disclosure, the terms used are only for the purpose of describing specific examples and are not intended to be limiting. Unless the context clearly indicates otherwise, if the number of elements is not specifically limited, the element may be one or more. In addition, the term "and / or" used in the present disclosure covers any one of the listed items and all possible combinations.

[0026] Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.

[0027] So far, various types of computers in application are based on classical physics as the theoretical basis for information processing, known as traditional computers or classical computers. Classical information systems use the physically easiest-to-implement binary data bits to store data or programs. Each binary data bit is represented by 0 or 1, called a bit or a qubit, as the smallest information unit. Classical computers themselves have inevitable weaknesses: one is the most basic limit of energy consumption in the computing process. The minimum energy required for logic elements or storage units should be several times that of kT to avoid misoperation under thermal fluctuations; the second is information entropy and heat generation energy consumption; the third is that when the wiring density of computer chips is very large, according to the Heisenberg uncertainty relation, when the uncertainty of the electron position is very small, the uncertainty of the momentum will be very large. The electrons are no longer bound, and there will be quantum interference effects, which can even damage the performance of the chip.

[0028] A quantum computer is a physical device that follows the properties and laws of quantum mechanics to perform high-speed mathematical and logical operations, store, and process quantum information. When a device processes and calculates quantum information and runs quantum algorithms, it is a quantum computer. Quantum computers follow unique quantum dynamics laws (especially quantum interference) to achieve a new mode of information processing. For parallel processing of computational problems, quantum computers have an absolute speed advantage over classical computers. The transformation implemented by a quantum computer for each superposition component is equivalent to a classical calculation. All these classical calculations are completed simultaneously and are superimposed according to a certain probability amplitude to give the output result of the quantum computer. This kind of calculation is called quantum parallel computing. Quantum parallel processing greatly improves the efficiency of quantum computers, enabling them to complete tasks that classical computers cannot, such as the factorization of a very large natural number. Quantum coherence is essentially utilized in all quantum superfast algorithms. Therefore, quantum parallel computing using quantum states instead of classical states can achieve an operation speed and information processing function that classical computers cannot match, while saving a large amount of computing resources.

[0029] Extensible qubits, initialization of qubits, long decoherence time, universal quantum gates, and readout of qubits are important criteria for determining whether a physical system is suitable for implementing a universal quantum computer. For example, qubit decoherence not only reduces the fidelity of quantum gates, introducing unnecessary errors in calculations, but also greatly limits the depth of quantum circuits, thereby restricting the computational power of superconducting quantum processors. Currently, in the industry, a readout resonator with a fixed coupling strength to superconducting qubits is commonly used to read the state of superconducting qubits. This fixed coupling method limits the adjustability of the coupling strength between superconducting qubits and the readout resonator, which is not conducive to optimizing the parameter of the coupling strength. In practice, if the coupling strength is too large, it will reduce the decoherence time of superconducting qubits, and if the coupling strength is too small, it will reduce the sensitivity of the readout resonator to superconducting qubits, which is not conducive to reading the state of qubits.

[0030] Exemplarily, in order to read the state of a superconducting qubit, a circuit structure in which the qubit and the readout resonator are directly coupled by a capacitor can be used, such as Figure 1 shown. In Figure 1 , the leftmost is a Transmon-type superconducting qubit with adjustable frequency, and its equivalent circuit consists of a capacitor C q and a Josephson junction J q ; on the right is the readout resonator, which is a resonant circuit used to read the state of the superconducting qubit. The readout resonator does not belong to the control sub-circuit (or control line) in the superconducting quantum circuit. The control sub-circuit is used to implement quantum gates in the superconducting quantum computer, and the readout resonator is used to read the superconducting qubit. The readout resonator and the control sub-circuit are used to perform different functions in the superconducting quantum computer and are both essential parts. In the example in Figure 1 , the superconducting qubit and the readout resonator are directly coupled through a capacitor C qr to generate a fixed coupling strength g.

[0031] In the example where the superconducting qubit is directly coupled to the readout resonator through a capacitor, the superconducting qubit in different quantum states has different effects on the frequency of the readout resonator, thereby realizing the readout of the state of the superconducting qubit. Usually, the qubit readout technology is dispersive readout, that is, the frequency detuning amount (frequency difference) between the qubit and the readout resonator is much larger than the coupling strength g and the linewidth of the readout resonator. The advantage of dispersive readout is that it can achieve quantum non-demolition measurement, so it has been widely used in the readout of qubits.

[0032] However, due to the energy dissipation caused by the coupling between the readout resonator and the external environment, the coupling between the superconducting qubit and it will also result in a fixed dissipation coefficient, which limits the decoherence time of the superconducting qubit. When the qubit does not need to be read or initialized using the readout resonator, the fixed coupling between the superconducting qubit and the readout resonator will still cause unnecessary dissipation, leading to an unnecessary reduction in the coherence time of the qubit and ultimately an unnecessary decrease in the fidelity of quantum computing.

[0033] In addition, when reading the superconducting qubit, the coupling between the qubit and the readout resonator needs to be within a certain optimal range. Under the condition of dispersive readout, the frequency detuning (frequency difference) between the qubit and the readout resonator is much larger than the coupling strength. Therefore, if the coupling strength is too large, it does not meet the conditions required for dispersive readout. On the contrary, if the coupling strength is too small, the influence of the qubit in different states on the readout resonator is relatively small, and the signal-to-noise ratio of the read signal is low, which is not conducive to the resolution and reading of the quantum state. Finally, when the superconducting qubit is not in the initialization operation, its frequency is far detuned from the frequency of the readout resonator (the frequency difference is large enough), and when initializing, the frequency of the superconducting qubit needs to be approximately equal to the frequency of the readout resonator to achieve the energy exchange between the superconducting qubit and the readout resonator. Therefore, when the coupling strength between the readout resonator and the superconducting qubit is fixed, this initialization scheme requires a large range of changes in the frequency of the superconducting qubit, which has a direct impact on the qubit.

[0034] Therefore, according to the embodiments of the present disclosure, as Figure 2 shown, a quantum circuit is also provided, including: a qubit 201, a tunable coupler 202, and a readout resonator 203. The tunable coupler 202 is configured to be coupled to the qubit 201 and the readout resonator 203. When initializing the qubit 201 based on the readout resonator 203, the modulation frequency of the tunable coupler 202 is configured to be equal to the frequency difference between the readout resonator 203 and the qubit 201.

[0035] According to the embodiments of the present disclosure, an effective initialization of the superconducting qubit through the readout resonator is achieved, that is, the coupling strength is modulated through the tunable coupler to achieve the energy exchange between the superconducting qubit and the readout resonator without changing the qubit frequency, thereby realizing the initialization of the superconducting qubit.

[0036] In the present disclosure, the modulation frequency of the coupler refers to the frequency of periodically adjusting the coupling strength (or the coupler frequency, and the coupler frequency and the coupling strength are in one-to-one correspondence), that is, the frequency of the periodic change of the coupling strength (or the coupler frequency) is the modulation frequency of the coupler.

[0037] In some examples, the tunable coupler can be implemented using a frequency-tunable superconducting qubit (but not as a computational qubit), and its key structure is a superconducting quantum interference device (SQUID). According to some embodiments, the superconducting quantum interference device (SQUID) includes at least two Josephson junctions connected in parallel.

[0038] A schematic diagram of the superconducting quantum interference device (SQUID) can be as Figure 3 shown, and it can be equivalent to a tunable nonlinear inductor. Referring to Figure 3 , the SQUID is formed by two Josephson junctions connected in parallel. The nonlinear current response of each Josephson junction is:

[0039] I = I c *sin(φ)

[0040] where φ is the magnetic flux passing through the Josephson junction, and I c is the critical current, and the overall corresponding Josephson energy is E J . For the loop formed by the two Josephson junctions in the SQUID, due to superconducting flux quantization requirements, the sum of all magnetic fluxes passing through the loop must be an integer multiple of the superconducting unit magnetic flux φ 0 . Therefore, considering the magnetic fluxes φ 1 and φ 2 passing through the two branch Josephson junctions of the loop and the externally applied magnetic flux Φ ext , they have the following relationship:

[0041]

[0042] where k is a positive integer. Using this relationship, one degree of freedom φ can be eliminated, and the overall current passing through the SQUID can be obtained as:

[0043]

[0044] Its corresponding equivalent Josephson energy is:

[0045]

[0046] Therefore, the equivalent Josephson energy can be adjusted by adjusting the externally applied magnetic flux Φ ext , and then its frequency can be adjusted. By inserting the tunable coupler structure between the qubit and the readout resonator, the qubit and the readout resonator interact with the coupler through coupling. By adjusting the externally applied magnetic flux and thus changing its frequency, the coupling strength between the qubits can be indirectly adjusted.

[0047] When initializing qubits through an adjustable coupler, the system Hamiltonian of the qubit and the readout resonator can be expressed as:

[0048]

[0049] Denote In the rotating coordinate system, the Hamiltonian can be written as:

[0050]

[0051] To utilize the readout resonator to initialize the qubit, energy exchange between the qubit and the readout resonator is required, that is, to achieve type of interaction. Usually, when initializing the qubit, mainly the qubit frequency is adjusted to make the qubit frequency equal to the readout resonator frequency, that is, |ω q -ω r | = 0.

[0052] In the embodiments of the present disclosure, since the coupling strength g ′ can be adjusted by the frequency of the adjustable coupler, modulation can be applied to the coupling strength (adjusting the external magnetic flux based on the modulation frequency to achieve periodic variation of the coupling strength), the modulation frequency is ω m , when ω m = |ω q -ω r |, That is, when the modulation frequency ω m is equal to the difference between the readout resonator and the superconducting qubit frequencies |ω q -ω r |, energy exchange occurs between the superconducting qubit and the readout resonator. Due to the strong coupling between the readout resonator and the environment, the energy in the readout resonator can be quickly dissipated into the environment, thus realizing the dissipation of the superconducting qubit energy, so the initialization of the superconducting qubit can be achieved.

[0053] Specifically, denote the state of the system composed of the qubit and the resonator as |σ,n>. Since |0,0> itself is in the initialization state, consider the |1,0> and |0,1> states. The system oscillates between the |1,0> and |0,1> states at a rate of the coupling strength g 0 , but at the same time, the energy of the resonator dissipates at a rate of Γ, and the dissipation rate Γ is much greater than the coupling strength g 0 , so the system finally ends up in the |0,0> state, that is, the initialization of the qubit is completed. Figure 4 Shows a schematic diagram of the numerical simulation results during the qubit initialization process. As Figure 4As shown, the mathematical model used is the master equation, and the coupling strength and dissipation rate are selected as g 0 = 2π * 10 MHz, Γ = 500 MHz, where both are angular frequencies. When the system is in the |0,1> state and the |1,0> state, after the coupling between the qubit and the resonator and the dissipation process of the resonator, the qubit will finally be in the |0> state, that is, the initialization of the qubit is completed. Figure 4 The solid line in m = |ω q - ω r | shows the process of qubit initialization. Figure 4 The dashed line part in m is the case of ω m = 0 (when the modulation frequency ω q is much larger than |ω r - ω m |, the effect is the same as ω m = 0), that is, no modulation is applied to the coupling strength, and it can be seen that there is no initialization effect. Figure 4 The dotted line is the intermediate case, that is, the modulation frequency ω q is close to |ω r - ω

[0054] According to an embodiment of the present disclosure, due to the strong coupling between the read resonator and the environment, the energy in the read resonator can be quickly dissipated into the environment, thereby realizing the dissipation of the energy of the superconducting qubit, and thus the initialization of the superconducting qubit can be realized.

[0055] In addition, according to an embodiment of the present disclosure, when reading a qubit based on a read resonator, the coupling strength of the tunable coupler can be configured within a preset range; when the qubit is in an invalid operation, the coupling between the read resonator and the qubit is turned off.

[0056] In some examples, when reading a superconducting qubit, the coupling strength between the qubit and the read resonator needs to be within a certain optimal range. Specifically, when reading a qubit, the coupling strength can be adjusted to maximize the signal-to-noise ratio of the read signal, thereby improving the reading fidelity.

[0057] By introducing an adjustable coupler into a traditional directly-fixed-coupled readout circuit, the coupling strength between the qubit and the readout resonator is adjusted. When not performing readout or using the readout resonator for initialization, the coupling between the qubit and the readout resonator is turned off, thereby reducing the energy dissipation of the qubit and increasing the coherence time of the qubit. When reading the qubit, the coupling strength is adjusted to maximize the signal-to-noise ratio of the readout signal, thereby improving the readout fidelity. When using the readout resonator to initialize the qubit, a modulation is applied to the coupling strength. When the modulation frequency is equal to the difference between the frequencies of the readout resonator and the superconducting qubit, energy exchange occurs between the superconducting qubit and the readout resonator, and thus the initialization of the qubit can be achieved.

[0058] According to an embodiment of the present disclosure, as Figure 5 shown, the adjustable coupler includes: a first capacitor C qr , a second capacitor C qc , a third capacitor C cr , a fourth capacitor C c and a superconducting quantum interference device as Figure 3 shown. The first end of the first capacitor C qr is connected to the qubit, and the second end of the first capacitor C qr is connected to the readout resonator; the first end of the second capacitor C qc is connected to the first end of the first capacitor C qr , and the second end of the second capacitor C qc is connected to the first end of the third capacitor C cr ; the second end of the third capacitor C cr is connected to the second end of the first capacitor C qr ; and the first end of the fourth capacitor C c and the first end of the superconducting quantum interference device are both connected to the second end of the second capacitor C qc , that is, are simultaneously connected to the first end of the third capacitor C cr .

[0059] According to some embodiments, the capacitance value of the first capacitor C qr is less than the capacitance values of the second capacitor C qc and the third capacitor C cr , and the capacitance values of the second capacitor C qc and the third capacitor C cr are less than the capacitance value of the fourth capacitor C c .

[0060] For reading the superconducting qubit, it is necessary to couple the qubit with the readout resonator, and different states of the qubit have different effects on the readout resonator. In the present disclosure, the readout resonator is used to distinguish and read the states of the qubit. The following references Figure 1 and Figure 5To describe the variation range of the coupling strength achievable by the tunable coupler according to the embodiments of the present disclosure.

[0061] As Figure 1 shown, a direct fixed coupling is achieved between the superconducting qubit and the readout resonator through a capacitor. The coupling strength is:

[0062]

[0063] where ω q is the frequency of the qubit, ω r is the frequency of the readout resonator, and the coupling capacitance C qr can be designed to obtain the required coupling strength g. However, once the chip is fabricated, the coupling strength g is fixed. The leftmost frequency-tunable Transmon-type superconducting qubit includes the capacitor C q , the frequency-tunable SQUIDJ q ; the middle tunable coupler structure includes the capacitors C c , C qr , C qc , C cr and the frequency-tunable SQUIDJ c ; the rightmost readout resonator includes the inductor L r and the capacitor C r .

[0064] After inserting the tunable coupler structure, the original direct fixed coupling between the qubit and the readout resonator (as Figure 1 shown) becomes an indirectly tunable coupling that can be adjusted by the tunable coupler (as Figure 5 shown). After introducing the tunable coupler, assuming that the capacitance values satisfy C qr << C cr , C qc << C q , C r , C c , then the coupling strength is:

[0065]

[0066] where Δ 1 = ω q - ω c , Δ 2 = ω r - ω c , Σ 1 = ω q + ω c , Σ 2 = ω r + ω c , η = C qc Ccr / C qr C c ,ω q is the frequency of the qubit, ω r is the frequency of the readout resonator, ω c is the tunable coupler frequency. It can be seen that when the tunable coupler frequency ω c tends to infinity, g ′ ≈g, and it is always possible to find a suitable tunable coupler frequency ω c such that the coupling strength g ′ = 0, that is, the coupling is turned off. In addition, with a suitable combination of capacitances, the adjustment range of the coupling strength is not only from 0 - g, because when the tunable coupler frequency ω c is close to the qubit frequency or the readout resonator frequency, i.e., Δ 1 and Δ 2 are small, the coupling strength will become larger, so that |g ′ | > |g|, but at the same time, it should be noted that Δ 1 and Δ 2 need to be large enough.

[0067] Therefore, according to the tunable coupler of the circuit structure in the embodiments of the present disclosure, a large range of changes in the coupling strength between the qubit and the readout resonator can be achieved, thereby facilitating processes such as qubit readout and initialization, and improving the fidelity of superconducting quantum computing.

[0068] To describe this result more intuitively, numerical simulations can be performed. Exemplarily, the parameters are set as C q = 70 fF, C r = 72 fF, C q = 200 fF, C qc = 4 fF, C rc = 4.2 fF, C qr = 0.1 fF, ω q = 4 GHz, ω r = 4.5 GHz, from which g = 6.3 MHz can be obtained. The curve of the coupling strength g ′ changing with the coupler frequency is as shown in Figure 6 . The top dashed line is the asymptote of the coupling strength g ′ when the coupler frequency tends to infinity, and its value is g = 6.0 MHz. The other dashed line below represents that the coupling strength g ′ is zero.

[0069] It can be seen from the simulation that when adjusting the coupling frequency ω c , the coupling strength g ′It can reach zero value, that is, completely turn off the coupling between the qubit and the readout resonator; when the coupler frequency approaches infinity, it can also ensure that the coupling strength g after inserting the tunable coupler ′ remains the original direct coupling strength (i.e., g ′ ≈g). Moreover, the adjustment range of the coupling strength is not limited to this, and the absolute value of the equivalent coupling strength g ′ can be greater than the original direct coupling strength g. For example, when the coupler frequency ω c = 5 GHz, the equivalent coupling strength g ′ = -9.2 MHz (the negative sign does not affect). In this way, if a tunable coupler is inserted between the qubit and the resonator, resulting in an increase in the distance between the qubit and the resonator, making the capacitance C qr become smaller, thus causing the original coupling strength g to become smaller to g 1 , that is, g 1 <g. When adjusting the tunable coupler frequency to approach infinity, only g ′ ≈g 1 can be obtained, but it is still less than g. At this time, g ′ can be jumped to the negative value region so that |g ′ |≈g>g 1 , and the negative sign does not affect, so it can still reach a level close to the original coupling strength g. For example, if after inserting the tunable coupler, the capacitance C qr becomes half, and the coupling strength g 1 = g / 2≈3 MHz. At this time, if the tunable coupler frequency is adjusted to infinity, then g ′ ≈g 1 ≈3 MHz<g, but if the tunable coupler frequency is adjusted to ω e ≈5.15 GHz, g ′ = -6 MHz, and the absolute value of the coupling strength g ′ can still reach the magnitude of the original direct coupling strength g.

[0070] In some embodiments, the coupling capacitance C qr can be designed to obtain an appropriate coupling strength g. After the chip is fabricated, that is, after the capacitances C qr , C cr , C qc , C q and C r are determined, by fixing the qubit frequency ω q and the readout resonator frequency ω r , the frequency ω c of the tunable coupler can be adjusted by changing the magnetic flux through the SQUID in the tunable coupler to achieve the adjustment of the coupling strength g ′ . The coupling strength g ′It can be adjusted to zero, that is, theoretically, it can achieve a complete turn-off of the coupling between the qubit and the readout resonator, and can also ensure the coupling strength g after inserting the tunable coupler ′ still maintains the original direct coupling strength (i.e., g ′ ≈g), so as to facilitate the readout of the qubit. Of course, it is also noted that the adjustment range of the coupling strength is not limited to this, and the absolute value of the equivalent coupling strength g ′ can be greater than the original coupling strength g, as described above.

[0071] In some embodiments, the coupling capacitance C can be designed qr to obtain an appropriate coupling strength g. After the chip is fabricated, that is, after the capacitances C qr 、C cr 、C qc 、C q and C r are determined, by fixing the qubit frequency ω q and the readout resonator frequency ω r , the magnetic flux through the SQUID in the tunable coupler can be adjusted to change the frequency ω c of the tunable coupler, so as to achieve the adjustment of the coupling strength g ′ . The coupling strength g ′ can be adjusted to zero, that is, theoretically, it can achieve a complete turn-off of the coupling between the qubit and the readout resonator, and can also ensure the coupling strength g after inserting the tunable coupler ′ still maintains the original direct coupling strength (i.e., g ′ ≈g), so as to facilitate the readout of the qubit. Of course, it is also noted that the adjustment range of the coupling strength is not limited to this, and the absolute value of the equivalent coupling strength g ′ can be greater than the original coupling strength g, as described above.

[0072] Therefore, by introducing a tunable coupler structure into the readout circuit of the superconducting qubit, with the tunable coupler located between the qubit and the readout resonator, the change of the coupling strength between the qubit and the readout resonator can be achieved by adjusting the frequency of the coupler. It can either adjust the coupling strength to zero to turn off the coupling between the qubit and the readout resonator, or maintain the coupling strength at the original direct coupling strength and is not limited to this. Therefore, this superconducting qubit readout scheme with adjustable coupling not only retains the basic functions of the traditional superconducting qubit readout scheme, but also has two new functions of turning off the coupling strength and adjusting the coupling strength.

[0073] According to the embodiments of the present disclosure, as Figure 7As shown, a quantum circuit control method 700 is also provided. The quantum circuit includes qubits, a readout resonator, and a tunable coupler. The method 700 includes: when initializing the qubits based on the readout resonator, adjusting the modulation frequency of the tunable coupler so that the modulation frequency is equal to the difference between the frequencies of the readout resonator and the qubits (step 710).

[0074] According to some embodiments, the method 700 further includes: when reading the qubits based on the readout resonator, the coupling strength of the tunable coupler is configured within a preset range; and when the qubits perform invalid operations, the coupling between the readout resonator and the qubits is turned off.

[0075] In an embodiment according to the present disclosure, as Figure 8 shown, it is determined whether the qubits need to be read (step 801), and when reading is required (step 801, "yes"), the frequency of the tunable coupler is adjusted to thereby adjust the coupling strength between the qubits and the readout resonator (step 802). When the signal-to-noise ratio of the read signal reaches the maximum (i.e., adjusted to the optimal value), the read fidelity reaches the highest under certain other conditions, so the current coupling strength is used to read the qubits. It is determined whether the qubits need to be initialized (step 803), and when initialization is required (step 803, "yes"), an oscillating signal is applied to the magnetic flux passing through the coupler to thereby modulate the frequency of the tunable coupler (step 804), that is, the modulation of the coupling strength between the qubits and the readout resonator is achieved, and the modulation frequency is equal to the absolute value of the difference between the qubit frequency and the readout resonator frequency, realizing the initialization of the qubits. In the case where the qubits are neither being read nor initialized (step 805), the frequency of the tunable coupler is adjusted to turn off the coupling between the qubits and the readout resonator, that is, the coupling strength is adjusted to zero. At this time, the coupling between the qubits and the readout resonator is greatly reduced, thereby reducing the energy dissipation rate of the qubits and thus improving the coherence time of the qubits.

[0076] It can be understood that the above implementation steps can be designed according to actual needs, and it is not limited thereto. Moreover, only one or more of the above steps can be implemented, and no limitation is made here.

[0077] According to an embodiment of the present disclosure, a superconducting quantum chip including the above-described quantum circuit, and a superconducting quantum computer including the superconducting quantum chip are also provided.

[0078] According to an embodiment of the present disclosure, an electronic device, a readable storage medium, and a computer program product are also provided.

[0079] Referring to Figure 9, a block diagram of an electronic device 900 that can be a server or a client of the present disclosure will now be described. It is an example of a hardware device that can be applied to various aspects of the present disclosure. The electronic device is intended to represent various forms of digital electronic computer devices, such as, laptop computers, desktop computers, workstations, personal digital assistants, servers, blade servers, mainframe computers, and other suitable computers. The electronic device can also represent various forms of mobile devices, such as, personal digital processors, cellular phones, smart phones, wearable devices, and other similar computing devices. The components shown herein, their connections and relationships, and their functions are merely examples and are not intended to limit the implementation of the present disclosure described and / or claimed herein.

[0080] As Figure 9 shown, the electronic device 900 includes a computing unit 901, which can perform various appropriate actions and processes according to a computer program stored in a read-only memory (ROM) 902 or a computer program loaded from a storage unit 908 into a random access memory (RAM) 903. In the RAM 903, various programs and data required for the operation of the electronic device 900 can also be stored. The computing unit 901, the ROM 902, and the RAM 903 are connected to each other via a bus 904. An input / output (I / O) interface 905 is also connected to the bus 904.

[0081] A plurality of components in the electronic device 900 are connected to the I / O interface 905, including: an input unit 906, an output unit 907, a storage unit 908, and a communication unit 909. The input unit 906 can be any type of device that can input information into the electronic device 900. The input unit 906 can receive input digital or character information, and generate key signal inputs related to the user settings and / or function controls of the electronic device, and can include, but are not limited to, a mouse, a keyboard, a touch screen, a trackpad, a trackball, a joystick, a microphone, and / or a remote control. The output unit 907 can be any type of device that can present information, and can include, but are not limited to, a display, a speaker, a video / audio output terminal, a vibrator, and / or a printer. The storage unit 908 can include, but is not limited to, magnetic disks, optical disks. The communication unit 909 allows the electronic device 900 to exchange information / data with other devices via a computer network such as the Internet and / or various telecommunication networks, and can include, but are not limited to, a modem, a network card, an infrared communication device, a wireless communication transceiver, and / or a chipset, such as a BluetoothTM device, an 802.11 device, a WiFi device, a WiMax device, a cellular communication device, and / or the like.

[0082] The computing unit 901 can be various general-purpose and / or special-purpose processing components with processing and computing capabilities. Some examples of the computing unit 901 include, but are not limited to, a central processing unit (CPU), a graphics processing unit (GPU), various dedicated artificial intelligence (AI) computing chips, various computing units running machine learning model algorithms, a digital signal processor (DSP), and any suitable processor, controller, microcontroller, etc. The computing unit 901 executes the various methods and processes described above, such as method 700. For example, in some embodiments, method 700 may be implemented as a computer software program tangibly embodied in a machine-readable medium, such as the storage unit 908. In some embodiments, part or all of the computer program may be loaded and / or installed onto the electronic device 900 via the ROM 902 and / or the communication unit 909. When the computer program is loaded into the RAM 903 and executed by the computing unit 901, one or more steps of the method 700 described above can be executed. Alternatively, in other embodiments, the computing unit 901 may be configured to execute method 700 in any other suitable manner (e.g., by means of firmware).

[0083] Various embodiments of the systems and techniques described above in this document can be implemented in digital electronic circuitry, integrated circuit systems, field-programmable gate arrays (FPGA), application-specific integrated circuits (ASIC), application-specific standard products (ASSP), systems-on-a-chip (SOC), complex programmable logic devices (CPLD), computer hardware, firmware, software, and / or combinations thereof. These various embodiments can include: implemented in one or more computer programs that can be executed and / or interpreted on a programmable system including at least one programmable processor, which can be a special or general-purpose programmable processor, that can receive data and instructions from a storage system, at least one input device, and at least one output device, and transmit the data and instructions to the storage system, the at least one input device, and the at least one output device.

[0084] The program code for implementing the methods of the present disclosure can be written in any combination of one or more programming languages. These program codes can be provided to the processor or controller of a general-purpose computer, a special-purpose computer, or other programmable data processing device, such that when the program codes are executed by the processor or controller, the functions / operations specified in the flowcharts and / or block diagrams are implemented. The program code can be executed entirely on the machine, partially on the machine, as a stand-alone software package partially on the machine and partially on a remote machine, or entirely on a remote machine or server.

[0085] In the context of this disclosure, a machine-readable medium can be a tangible medium that can contain or store a program for use by or in connection with an instruction execution system, apparatus, or device. A machine-readable medium can be a machine-readable signal medium or a machine-readable storage medium. A machine-readable medium can include, but is not limited to, electronic, magnetic, optical, electromagnetic, infrared, or semiconductor systems, apparatus, or devices, or any suitable combination of the foregoing. More specific examples of a machine-readable storage medium would include an electrical connection based on one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

[0086] In order to provide interaction with a user, the systems and techniques described herein can be implemented on a computer having: a display device (e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor) for displaying information to the user; and a keyboard and a pointing device (e.g., a mouse or a trackball) by which the user can provide input to the computer. Other kinds of devices can also be used to provide interaction with the user; for example, the feedback provided to the user can be any form of sensory feedback (e.g., visual feedback, auditory feedback, or tactile feedback); and input from the user can be received in any form (including acoustic input, speech input, or tactile input).

[0087] The systems and techniques described herein can be implemented in a computing system including backend components (e.g., as a data server), or a computing system including middleware components (e.g., an application server), or a computing system including frontend components (e.g., a user computer having a graphical user interface or a web browser through which the user can interact with an implementation of the systems and techniques described herein), or a computing system including any combination of such backend components, middleware components, or frontend components. The components of the system can be interconnected by any form or medium of digital data communication (e.g., a communication network). Examples of communication networks include: a local area network (LAN), a wide area network (WAN), and the Internet.

[0088] A computer system can include a client and a server. The client and the server are generally remote from each other and typically interact through a communication network. The client-server relationship is generated by computer programs running on the respective computers and having a client-server relationship with each other. The server can be a cloud server, or a server of a distributed system, or a server incorporating a blockchain.

[0089] It should be understood that the various forms of processes shown above can be used, with steps reordered, added or deleted. For example, the steps recited in this disclosure can be executed in parallel, sequentially or in a different order, as long as the desired results of the technical solutions disclosed in this disclosure can be achieved, and no limitation is imposed herein.

[0090] Although embodiments or examples of the present disclosure have been described with reference to the accompanying drawings, it should be understood that the above methods, systems and devices are merely exemplary embodiments or examples, and the scope of the present invention is not limited by these embodiments or examples, but is only defined by the authorized claims and their equivalent scope. Various elements in the embodiments or examples can be omitted or replaced by their equivalent elements. In addition, the steps can be executed in an order different from that described in the present disclosure. Further, the various elements in the embodiments or examples can be combined in various ways. Importantly, with the evolution of technology, many of the elements described herein can be replaced by equivalent elements that emerge after the present disclosure.

Claims

1. A quantum circuit, comprising: qubits, a readout resonator; and a tunable coupler configured to couple with the qubit and the readout resonator, wherein when initializing the qubit based on the readout resonator, the modulation frequency of the tunable coupler is configured to be equal to the difference between the frequencies of the readout resonator and the qubit, where the modulation frequency refers to the frequency of periodically adjusting the coupling strength.

2. The quantum circuit according to claim 1, wherein when reading the qubit based on the readout resonator, the coupling strength of the tunable coupler is configured to be within a preset range; and when the qubit is in an invalid operation, the coupling between the readout resonator and the qubit is turned off.

3. The quantum circuit according to claim 1 or 2, wherein the tunable coupler includes: a first capacitor, a second capacitor, a third capacitor, a fourth capacitor, and a superconducting quantum interference device, wherein a first end of the first capacitor is connected to the qubit, and a second end of the first capacitor is connected to the readout resonator; a first end of the second capacitor is connected to the first end of the first capacitor, and a second end of the second capacitor is connected to a first end of the third capacitor; a second end of the third capacitor is connected to the second end of the first capacitor; and a first end of the fourth capacitor and a first end of the superconducting quantum interference device are both connected to the second end of the second capacitor.

4. The quantum circuit according to claim 3, wherein the capacitance value of the first capacitor is less than the capacitance values of the second capacitor and the third capacitor, and the capacitance values of the second capacitor and the third capacitor are less than the capacitance value of the fourth capacitor.

5. The quantum circuit according to claim 3, wherein the superconducting quantum interference device includes at least two Josephson junctions connected in parallel.

6. A method for controlling a quantum circuit, the quantum circuit including qubits, a readout resonator, and a tunable coupler, wherein the method comprises: when initializing the qubit based on the readout resonator, adjusting the modulation frequency of the tunable coupler so that the modulation frequency is equal to the difference between the frequencies of the readout resonator and the qubit, where the modulation frequency refers to the frequency of periodically adjusting the coupling strength.

7. The method according to claim 6, wherein the method further comprises: when reading the qubit based on the readout resonator, the coupling strength of the tunable coupler is configured to be within a preset range; and when the qubit is in an invalid operation, turning off the coupling between the readout resonator and the qubit.

8. An electronic device, comprising: at least one processor; and a memory communicatively connected to the at least one processor; wherein the memory stores instructions executable by the at least one processor, and the instructions are executed by the at least one processor to enable the at least one processor to execute the method according to any one of claims 6 - 7.

9. A non-transitory computer-readable storage medium storing computer instructions, wherein, The computer instructions are used to cause the computer to execute the method according to any one of claims 6-7.

10. A computer program product, comprising a computer program, wherein, the computer program, when executed by a processor, implements the method according to any one of claims 6-7.

11. A superconducting quantum chip, comprising the quantum circuit according to any one of claims 1 to 5.

12. A superconducting quantum computer, comprising the superconducting quantum chip according to claim 11.

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