Quantum bit state reading system and method

Abstract

The invention provides a quantum bit state reading system and method, and relates to the technical field of quantum computing. The system comprises a radio frequency reading circuit, a quantum computer and an interconnection module, the radio frequency readout circuit is used for generating a detection signal and injecting the detection signal into the readout resonant cavity through the interconnection module; the read-out resonant cavity is used for modulating the detection signal based on dispersion frequency shift generated by coupling of the current quantum state of the quantum bit and the read-out resonant cavity to generate a modulation signal, and reflecting the modulation signal to the radio frequency read-out circuit through the interconnection module; and the radio frequency reading circuit is also used for demodulating the modulation signal when the frequency of the detection signal is stable so as to obtain the current quantum state of the quantum bit, and the second output end of the radio frequency reading circuit outputs the current quantum state of the quantum bit, so that high-fidelity reading can be realized with lower performance requirements.

Description

Technical Field

[0001] This application relates to the field of quantum computing technology, and more specifically, to a quantum bit state readout system and method. Background Technology

[0002] With the rapid development of quantum computing technology, achieving high-fidelity, fast, and scalable qubit state readout has become one of the key challenges in building practical quantum computers. In superconducting quantum computing, ion trap quantum computing, and other mainstream quantum computing platforms, the state of a qubit (usually represented as the ground state |0|) is crucial. and excited state|1 It requires a specific measurement mechanism for efficient and accurate discrimination in order to support core functions such as subsequent quantum error correction, feedback control, and algorithm execution.

[0003] Currently, radio frequency dispersive readout is the mainstream measurement technique. This technique utilizes the coupling between the readout resonant cavity and the qubit, causing the eigenfrequency of the resonant cavity to change due to the qubit being in its ground state (|0, 1 / 2)... ) or excited state (|1 This results in a tiny, state-dependent dispersion shift. By transmitting a radio frequency probe signal into the resonant cavity and detecting the changes in the amplitude and phase of the signal's reflection, the state information of the qubit can be indirectly obtained.

[0004] However, in order to obtain a sufficiently high signal-to-noise ratio (SNR) to ensure readout fidelity, traditional RF readout solutions typically employ an open-loop architecture. Under this architecture, improving SNR mainly relies on optimizing hardware performance, which leads to a sharp increase in the design complexity and cost of RF readout circuits. Summary of the Invention

[0005] The purpose of this application is to provide a quantum bit state readout system and method to address the shortcomings of the prior art, thereby solving the technical problems existing in the prior art.

[0006] To achieve the above objectives, the technical solutions adopted in the embodiments of this application are as follows: In a first aspect, embodiments of this application provide a quantum bit state readout system, the quantum bit state readout system comprising: a radio frequency readout circuit, a quantum computer, and an interconnection module; The temperature of the environment in which the radio frequency readout circuit is located is higher than the temperature of the environment in which the quantum computer is located; the quantum computer includes: qubits and a readout resonant cavity; the qubits are coupled to one end of the readout resonant cavity; An input end of the radio frequency readout circuit is connected with an output end of the interconnection module, a first output end of the radio frequency readout circuit is connected with a first end of the interconnection module, and a second end of the interconnection module is connected with the other end of the readout resonant cavity; The radio frequency readout circuit is configured to generate a probe signal and inject the probe signal into the readout resonant cavity via the interconnection module. The readout resonant cavity is configured to modulate the probe signal based on a dispersion frequency shift generated by coupling of a current quantum state of the quantum bit with the readout resonant cavity, generate a modulated signal, and reflect the modulated signal to the radio frequency readout circuit via the interconnection module, wherein the modulated signal carries the current quantum state of the quantum bit. The radio frequency readout circuit is further configured to demodulate the modulated signal to obtain the current quantum state of the quantum bit when a frequency of the probe signal is stable, and output the current quantum state of the quantum bit from a second output end of the radio frequency readout circuit.

[0007] Optionally, the radio frequency readout circuit includes a state discriminator, a low-noise amplifier, and a signal generator, an output end of the signal generator is connected with the first end of the interconnection module, and an input end of the low-noise amplifier is connected with the output end of the interconnection module. The signal generator is configured to generate a probe signal and transmit the probe signal to the readout resonant cavity via the interconnection module. The low-noise amplifier is configured to receive a modulated signal returned via the interconnection module, amplify the modulated signal to obtain an amplified modulated signal, and transmit the amplified modulated signal to the signal generator. The signal generator is further configured to demodulate the amplified modulated signal to obtain a signal feature corresponding to the modulated signal when the frequency of the probe signal is stable, and transmit the signal feature corresponding to the modulated signal to the state discriminator, wherein the signal feature corresponding to the modulated signal at least includes an amplitude, a phase, and a frequency. The state discriminator is configured to obtain the current quantum state of the quantum bit based on the signal feature corresponding to the modulated signal.

[0008] Optionally, the frequency of the probe signal being stable indicates that the frequency of the probe signal no longer changes.

[0009] Optionally, obtaining the current quantum state of the quantum bit based on the signal feature corresponding to the modulated signal includes: comparing the signal feature corresponding to the modulated signal with a signal feature of a preset reference signal to obtain a comparison result. According to the comparison result, a current quantum state of the quantum bit is determined.

[0010] Optionally, the comparison of the signal feature corresponding to the modulation signal with the signal feature of the preset reference signal includes: determining a first difference between an amplitude of the modulation signal and an amplitude of the reference signal, and taking the first difference as an amplitude difference; determining a second difference between a phase of the modulation signal and a phase of the reference signal, and taking the second difference as a phase difference.

[0011] Optionally, the determination of the current quantum state of the quantum bit according to the comparison result includes: if the amplitude difference or the phase difference is greater than a preset threshold, determining that the current quantum state of the quantum bit is a ground state; if the amplitude difference or the phase difference is less than a preset threshold, determining that the current quantum state of the quantum bit is an excited state.

[0012] Optionally, the radio frequency readout circuit further includes a controller. The controller is configured to receive an externally input readout operation instruction, generate a control signal according to the readout operation instruction, and initialize the state discriminator, the low-noise amplifier, and the signal generator based on the control signal.

[0013] Optionally, the interconnection module includes an attenuator, a circulator, and an isolator; an output end of the signal generator is connected to an input end of the attenuator; an output end of the attenuator is connected to a first end of the circulator; a second end of the circulator is coupled to the readout resonant cavity; a third end of the circulator is connected to an input end of the isolator; and an output end of the isolator is connected to an input end of the low-noise amplifier. The attenuator is configured to attenuate the power of the probe signal so that the current quantum state of the quantum bit will not be excited to a higher state, and transmit the attenuated probe signal to the first end of the circulator. The circulator is configured to transmit the attenuated probe signal to the quantum computer via the second end of the circulator, receive a modulation signal reflected by the quantum computer, and transmit the modulation signal to the isolator via the third end of the circulator. The isolator is configured to transmit the modulation signal to the radio frequency readout circuit.

[0014] In a second aspect, the embodiments of the present application further provide a quantum bit state readout method applied to the system of the first aspect, and the method includes: The radio frequency readout circuit generates a probe signal and injects the probe signal into the readout resonator; The readout resonator modulates the probe signal based on a dispersion frequency shift generated by coupling of the current quantum state of the quantum bit and the readout resonator, generates a modulated signal, and reflects the modulated signal to the radio frequency readout circuit, wherein the modulated signal carries state information of the current quantum state; The radio frequency readout circuit obtains the current quantum state of the quantum bit based on the modulated signal when the frequency of the probe signal is stable, and the quantum state includes a ground state or an excited state.

[0015] Optionally, the determining the current quantum state of the quantum bit based on the modulated signal comprises: comparing a signal feature corresponding to the modulated signal with a signal feature of a preset reference signal to obtain a comparison result; determining the current quantum state of the quantum bit according to the comparison result.

[0016] The beneficial effects of the present application are: The present application provides a quantum bit state readout system and method. In the present application, a closed-loop quantum bit state readout system is designed. The radio frequency readout circuit transmits a probe signal and injects the probe signal into the readout resonator through the interconnection module. The quantum bit is coupled with the readout resonator to generate a dispersion frequency shift related to the quantum state of the quantum bit. The dispersion frequency shift modulates the probe signal incident to the readout resonator, i.e., the dispersion frequency shift encodes the quantum state of the quantum bit into the amplitude and phase of the probe signal to generate a modulated signal. The modulated signal is reflected back and received by the interconnection module. The radio frequency readout circuit receives the reflected modulated signal and demodulates the modulated signal after a period of time to obtain a signal feature corresponding to the modulated signal which has been significantly amplified and distinguished. Based on the modulated signal, it is determined that the current quantum state of the quantum bit is a ground state |0 or an excited state |1 Thus, the readout of the quantum bit state is realized. The present application uses the "injection locking" effect to realize the continuous interaction of the probe signal and the returned modulated signal of the quantum bit. The signal separation degree Δ² between different quantum states is enhanced through the time domain accumulation of the amplitude and phase information, so that the system can realize high-fidelity readout with lower performance requirements of the LNA and ADC module. That is, under the premise of relaxing the strict requirements for the Cryo-LNA noise coefficient and the sampling rate of the ADC module, high-fidelity readout is still achieved, thereby solving the technical problems existing in the prior art.

[0017] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the following preferred embodiments are described in detail below, and the accompanying drawings are described as follows. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A schematic diagram of a quantum bit state readout system provided in this application embodiment; Figure 2 A schematic diagram of the radio frequency readout circuit in a quantum bit state readout system provided in this application embodiment; Figure 3 This application provides an embodiment of the change in the characteristics of the detection signal during the readout of the quantum bit state. Figures 4(a) and 4(b) are schematic diagrams of the modulation logic of the detection signal based on dispersive frequency shift provided in the embodiments of this application; Figure 5 A schematic diagram of the radio frequency readout circuit in another quantum bit state readout system provided in this application embodiment; Figure 6 A schematic diagram of the interconnect module in a quantum bit state readout system provided in this application embodiment; Figure 7 This is a flowchart illustrating a quantum bit state readout method provided in an embodiment of this application.

[0020] Icons: 100-Qubit state readout system; 1-RF readout circuit; 2-Quantum computer; 3-Interconnect module; 11-State discriminator; 12-Low noise amplifier; 13-Signal generator; 14-Controller; 21-Qubit; 22-Readout resonant cavity; 31-Attenuator; 32-Circulator; 33-Isolator. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0022] Therefore, the following detailed description of the embodiments of the application provided in the accompanying drawings is not intended to limit the scope of the application as claimed, but merely represents selected embodiments of the application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the application without creative labor are within the scope of protection of the application.

[0023] It should be noted that similar reference numbers and letters in the following drawings represent similar items, and therefore, once an item is defined in one drawing, it need not be further defined and explained in subsequent drawings. Meanwhile, in the description of the application, the terms "first", "second", and the like are merely used to distinguish descriptions, and cannot be understood as indicating or implying relative importance.

[0024] It should be noted that, in this document, the relationship terms such as first and second are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between the entities or operations. Moreover, the terms "include", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or device. Without more limitations, the element defined by the statement "including a" does not exclude the presence of additional identical elements in the process, method, article or device including the element.

[0025] In the description of the application, it should be noted that the terms "upper", "lower", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the application is usually placed, only for the convenience of describing the application and simplifying the description, and cannot be understood as indicating or implying that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the application.

[0026] In the description of the application, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set", "connect" should be understood broadly, for example, it can be fixedly connected, or detachably connected, or integrally connected; it can be mechanically connected, or electrically connected; it can be directly connected, or indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For those of ordinary skill in the art, the specific meaning of the above terms in the application can be understood according to the specific circumstances.

[0027] First, the background art related to the application is introduced.

[0028] In order to obtain a high enough signal-to-noise ratio (SNR) to ensure readout fidelity, the traditional radio frequency readout scheme usually adopts an open-loop architecture: first, the signal generator in the radio frequency readout circuit transmits a probe pulse, and the reflected signal returned by the quantum computer is amplified by a cryogenic low-noise amplifier (Cryo-LNA), and then digitized, filtered, integrated and discriminated by a high-speed analog-to-digital converter (ADC) and a digital signal processor (DSP) in the radio frequency readout circuit in sequence. Under this architecture, improving SNR mainly depends on optimizing hardware performance, especially reducing the noise figure of the front-end Cryo-LNA and improving the sampling rate and resolution of the back-end ADC.

[0029] However, with the exponential growth of the number of qubits in the quantum computer, the above traditional scheme faces severe challenges. In order to reduce the number of cables and thermal load at the cryogenic end, frequency division multiplexing (FDM) technology is widely used, and the readout signals of multiple qubits share the same set of room temperature electronics link. However, the implementation of FDM technology inevitably expands the total noise bandwidth of the entire readout system. In order to compensate for the deterioration of signal-to-noise ratio caused thereby, the system is forced to impose more stringent performance indicators on the Cryo-LNA and ADC modules - requiring the low-noise amplifier to have a lower noise temperature, while requiring the ADC module to have a higher sampling rate to capture finer signal features and achieve more effective digital filtering.

[0030] This continuous squeezing of the performance of the front-end and back-end hardware has led to a sharp rise in the design complexity and cost of the radio frequency readout circuit, which has become one of the main bottlenecks restricting the expansion of quantum computing platforms such as superconducting and semiconductor spin to hundreds of bits or even thousands of bits.

[0031] Therefore, in view of the technical problems existing in the prior art, there is an urgent need for a technical solution that can achieve high-fidelity and fast readout of qubits without significantly increasing the noise suppression capability of the cryogenic low-noise amplifier and the sampling rate dependence of the high-speed analog-to-digital converter.

[0032] The specific structure and beneficial effects of the quantum bit state readout system provided by the present application will be introduced through the following embodiments.

[0033] Optionally, as shown in Figure 1 , the structure diagram of the quantum bit state readout system provided by the present application, as shown in Figure 1 , the quantum bit state readout system 100 comprises a radio frequency readout circuit 1, a quantum computer 2 and an interconnection module 3. The temperature of the environment in which the radio frequency readout circuit 1 is located is higher than the temperature of the environment in which the quantum computer 2 is located; for example, considering that complex electronic devices such as radio frequency readout circuits cannot be directly placed in a 10 mK low-temperature environment due to power consumption and heat dissipation problems, the radio frequency readout circuit 1 is placed in a higher temperature level, i.e., the radio frequency readout circuit 1 is located in a relatively high temperature layer (such as 10 mK or above); and the quantum computer 2 must be in an environment close to absolute zero in order to maintain its quantum coherence, so the quantum computer 2 is located in an extremely low-temperature environment (such as 10 mK).

[0034] The quantum computer 2 includes a quantum bit 21 and a readout resonant cavity 22; the quantum bit 21 is coupled to one end of the readout resonant cavity 22; for example, the readout resonant cavity 22 is usually a type of resonant circuit, such as a coplanar waveguide resonator, a transmission line resonator, and an LC resonator, and the intrinsic resonant frequency of the readout resonant cavity 22 is . Wherein, the quantum bit 21 is coupled to the readout resonant cavity 22, and this coupling will cause the intrinsic resonant frequency of the readout resonant cavity 22 to be According to the slight shift of the quantum state (ground state |0 or excited state |1 ) of the quantum bit 21, i.e., the "dispersion frequency shift", the current quantum state of the quantum bit 21 is non-destructively measured. For example, the capacitive coupling between the coplanar waveguide resonator and the superconducting quantum bit 21 produces a shift in the resonant frequency caused by detuning, and the direct connection between the transmission line resonator and the semiconductor spin quantum bit 21 causes a shift in the resonant frequency caused by quantum capacitance variation.

[0035] The input end of the radio frequency readout circuit 1 is connected to the output end of the interconnection module 3, the first output end of the radio frequency readout circuit 1 is connected to the first end of the interconnection module 3, and the second end of the interconnection module 3 is connected to the other end of the readout resonant cavity 22; wherein, the "input end of the radio frequency readout circuit 1" is a port for receiving the reflected modulated signal, and the "first output end of the radio frequency readout circuit 1" is a port for transmitting the original probe signal, i.e., the flow direction of the whole signal is: the first output end of the radio frequency readout circuit 1 → the first end of the interconnection module 3 → the second end of the interconnection module 3 → the readout resonant cavity 22 → the second end of the interconnection module 3 → the output end of the interconnection module 3 → the input end of the radio frequency readout circuit 1, so as to ensure that the signal flows in one direction, thereby decoupling the transmission path and the receiving path. That is, in this embodiment, the state readout system of the quantum bit provided by the present application is a closed-loop radio frequency readout architecture, i.e., the radio frequency readout circuit 1 transmits a probe signal, the probe signal is injected into the readout resonant cavity 22 through the interconnection module 3, and the reflected modulated signal is received through the interconnection module 3, thereby realizing closed-loop feedback to read the current quantum state of the quantum bit 21.

[0036] Continuing to refer to Figure 1As shown, the radio frequency readout circuit 1 is configured to generate a probe signal and inject the probe signal into the readout resonator 22 via the interconnection module 3; wherein the probe signal can be a continuous wave or a pulsed radio frequency signal with a specific frequency (usually close to the resonant frequency of the readout resonator 22 in a certain state) and is sent to the readout resonator 22 at the low temperature end.

[0037] The readout resonator 22 is configured to modulate the probe signal based on the dispersive frequency shift generated by the coupling between the quantum bit 21 and the readout resonator 22, to generate a modulated signal, and reflect the modulated signal to the radio frequency readout circuit 1 via the interconnection module 3, wherein the modulated signal carries the current quantum state of the quantum bit 21; in this embodiment, the reflection characteristics (amplitude and phase) of the probe signal will change when the probe signal enters the readout resonator 22, because the equivalent resonant frequency of the readout resonator 22 changes with the state of the quantum bit 21. Wherein the quantum bit 21 is coupled to the readout resonator 22 to generate a dispersive frequency shift related to the quantum state of the quantum bit 21, and the dispersive frequency shift is used to modulate the probe signal, that is, the dispersive frequency shift can encode the quantum state of the quantum bit 21 into the amplitude and phase of the "probe signal incident on the readout resonator". For example, when the quantum bit 21 is in the |0 state, the frequency of the dispersive frequency shift generated by the coupling can be slightly higher, resulting in a leading phase of the modulated signal; when the quantum bit 21 is in the |1 state, the frequency of the dispersive frequency shift generated by the coupling is slightly lower, resulting in a lagging phase of the modulated signal. Therefore, the modulated signal finally reflected to the radio frequency readout circuit 1 carries the current quantum state of the quantum bit 21.

[0038] The radio frequency readout circuit 1 is further configured to demodulate the modulated signal when the frequency of the probe signal is stable, to obtain the current quantum state of the quantum bit 21, and output the current quantum state of the quantum bit 21 from the second output end of the radio frequency readout circuit 1. It is continued to refer to Figure 1 As shown, the modulated signal reflected back is received by the radio frequency readout circuit 1, and after a period of time, the modulated signal is demodulated to obtain the signal characteristics (such as the accumulated phase shift or amplitude change) corresponding to the modulated signal, which have been significantly amplified and distinguished, and the signal characteristics corresponding to the modulated signal are compared with the preset reference value, so as to determine with high confidence whether the current quantum state of the quantum bit 21 is the ground state |0 or the excited state |1 , and output the current quantum state of the quantum bit 21 through the second output end of the radio frequency readout circuit 1, thus realizing the readout of the state of the quantum bit 21.

[0039] Optionally, this application is based on a closed-loop architecture for reading out qubit states. It utilizes the "injection-locking" effect to achieve continuous interaction between the probe signal and the modulation signal returned by the qubit. By accumulating amplitude and phase information in the time domain, the signal separation Δ² between different quantum states is enhanced. It is this amplified separation that enables the system to achieve high-fidelity readout with LNA and ADC modules that have lower performance requirements. That is, under the premise of relaxing the stringent requirements on the noise figure of the Cryo-LNA and the sampling rate of the ADC module, the goal of high-fidelity readout is still achieved, thereby solving the technical problems existing in the prior art.

[0040] In summary, this application provides a quantum bit state readout system. A closed-loop quantum bit state readout system is designed, in which a radio frequency (RF) readout circuit emits a detection signal, which is injected into the readout resonant cavity via an interconnect module. The quantum bit is coupled to the readout resonant cavity, generating a dispersive frequency shift related to the quantum state of the quantum bit. This dispersive frequency shift is used to modulate the detection signal incident on the readout resonant cavity; that is, the dispersive frequency shift can encode the quantum state of the quantum bit into the amplitude and phase of the detection signal, generating a modulated signal. The reflected modulated signal is then received via the interconnect module and the RF readout circuit. After a period of time, the modulated signal is demodulated, revealing that the signal characteristics corresponding to the modulated signal have been significantly amplified and distinguished. Based on the modulated signal, the current quantum state of the quantum bit is determined to be the ground state |0. Still in excited state | 1 Thus, the readout of the quantum bit state is achieved. This application utilizes the "injection-locking" effect to achieve continuous interaction between the probe signal and the modulation signal returned by the quantum bit. By accumulating amplitude and phase information in the time domain, the signal separation Δ² between different quantum states is enhanced, enabling the system to achieve high-fidelity readout with LNA and ADC modules that have lower performance requirements. That is, under the premise of relaxing the stringent requirements on the noise figure of the Cryo-LNA and the sampling rate of the ADC module, the goal of high-fidelity readout is still achieved, thereby solving the technical problems existing in the prior art.

[0041] Optionally, refer to Figure 2 As shown above, Figure 1 The radio frequency readout circuit 1 includes a state discriminator 11, a low noise amplifier 12, and a signal generator 13; wherein the output terminal of the signal generator 13 is connected to the first terminal of the interconnect module 3, and the input terminal of the low noise amplifier 12 is connected to the output terminal of the interconnect module 3.

[0042] The signal generator 13 is used to generate a detection signal and transmit the detection signal to the readout resonant cavity 22 via the interconnect module 3. In this embodiment, the signal generator 13 transmits a "radio frequency detection signal" for the readout of the state of the quantum bit 21. The signal characteristics of the radio frequency detection signal typically include frequency, amplitude, phase, pulse duration and other related characteristics. At the same time, the signal generator 13 also receives and processes the returned modulated signal carrying the state information of the quantum bit 21.

[0043] The low-noise amplifier 12 is used to receive the modulation signal returned via the interconnect module 3, amplify the modulation signal to obtain the amplified modulation signal, and transmit the amplified modulation signal to the signal generator 13. The returned modulation signal is very weak (often below -120dBm). Therefore, without introducing too much noise, the returned modulation signal can be amplified with low noise by a low-noise amplifier 12. Its parameters mainly include power gain, bandwidth and noise figure, thereby improving the signal-to-noise ratio.

[0044] The signal generator 13 is also used to demodulate the amplified modulation signal when the frequency of the detection signal is stable, to obtain the signal characteristics corresponding to the modulation signal, and to transmit the signal characteristics corresponding to the modulation signal to the state discriminator 11. The signal characteristics corresponding to the modulation signal include at least: amplitude, frequency and phase. In this embodiment, the signal generator 13 can generate probe pulse signals of a specific frequency and form, and can also receive and demodulate the returned modulated signals (such as frequency conversion, digital sampling, or feature extraction). The signal generator 13 has the functions of transmitting, receiving, and demodulating, which reduces external dependence, improves system compactness and synchronization accuracy, and reduces the design complexity and cost of the radio frequency readout circuit.

[0045] In one feasible approach, in order to improve the accuracy of the reading results and avoid misjudgment caused by transient interference, this embodiment proposes to demodulate the received amplified modulation signal when the signal generator 13 determines that the received amplified modulation signal is in a stable state, obtain the signal characteristics corresponding to the modulation signal, such as amplitude, frequency and phase, and transmit the signal characteristics corresponding to the modulation signal to the state discriminator 11.

[0046] The state discriminator 11 is used to obtain the current quantum state of the qubit 21 based on the signal characteristics corresponding to the modulation signal. For example, the state discriminator 11 can be a phase detector, a frequency detector, etc.

[0047] In this embodiment, the state discriminator 11 can compare the signal features (such as amplitude and phase) corresponding to the received modulation signal with preset signal features to obtain the current quantum state of the quantum bit 21, such as the ground state |0. 、or stimulate |1 This ensures rapid reading of the qubit state and achieves high-fidelity readout of the qubit state.

[0048] Optionally, from the above Figure 2 Furthermore, it can be seen that in the RF readout circuit 1, the input terminal of the low-noise amplifier 12 is connected to the output terminal of the interconnect module 3, and the low-noise amplifier 12 receives the returned modulation signal. That is, the low-noise amplifier 12 is positioned close to the detection terminal of the RF readout circuit 1 to maximize the protection of the integrity of the modulation signal and avoid introducing too much system noise.

[0049] Optionally, the frequency of the probe signal is stabilized to indicate that the frequency of the probe signal no longer changes.

[0050] In this embodiment, the radio frequency readout circuit 1 maintains the above operating state for a period of time. This means continuously transmitting detection signals over a period of time (e.g., nanoseconds). Therefore, within a preset duration... If the frequency of the detection signal emitted by the internal signal generator no longer changes, then the frequency of the detection signal is determined to be in a stable state.

[0051] Optionally, based on the signal characteristics corresponding to the modulation signal, the current quantum state of qubit 21 is obtained, including: The signal characteristics corresponding to the modulation signal are compared with the signal characteristics of the preset reference signal to obtain the comparison result; based on the comparison result, the current quantum state of qubit 21 is determined.

[0052] The reference signal refers to the response signal corresponding to a known quantum state (such as the ground state) under ideal conditions.

[0053] In one feasible approach, since the amplitude and phase of the modulation signal change depending on the state of qubit 21, the extracted signal features corresponding to the modulation signal can be compared with the signal features of the reference signal to obtain the comparison result. Based on the comparison result, the current quantum state of qubit 21 can be further determined. That is, by analyzing the differences in amplitude, frequency, or phase between the modulation signal and the reference signal, it can be deduced that qubit 21 is currently in the ground state (|0||). ) or excited state (|1 ).

[0054] Therefore, in this embodiment, a discrimination method based on signal feature comparison is proposed to obtain the current quantum state of qubit 21, which is applicable to multi-level or multi-qubit 21 crosstalk scenarios.

[0055] Optionally, the signal features corresponding to the modulated signal are compared with the signal features of a preset reference signal to obtain a comparison result, including: Determine a first difference between the amplitude of the modulated signal and the amplitude of the reference signal, and use the first difference as the amplitude difference; determine a second difference between the phase of the modulated signal and the phase of the reference signal, and use the second difference as the phase difference.

[0056] In this embodiment, based on the signal characteristics corresponding to the modulated signal and the signal characteristics of a preset reference signal, the amplitude difference ΔA, phase difference Δφ, and frequency difference Δf are extracted. On the polar coordinate plane, the positional offset of the modulated signal relative to the reference signal can be determined based on the amplitude difference ΔA, phase difference Δφ, or frequency difference Δf. This application proposes a single-dimensional signal feature discrimination method, which has stronger noise resistance and improves the robustness and accuracy of the reading results.

[0057] Optionally, based on the comparison results, the current quantum state of qubit 21 is determined, including: If the amplitude difference or phase difference is greater than a preset threshold, the current quantum state of qubit 21 is determined to be the ground state; if the amplitude difference or phase difference is less than the preset threshold, the current quantum state of qubit 21 is determined to be the excited state.

[0058] In one feasible way, continue to refer to Figure 3 As shown, a process for identifying the state of a qubit 21 based on the characteristics of the modulation signal is described, which is a vector representation of the modulation signal in polar coordinate spacetime.

[0059] in, and These represent amplitude and phase characteristics, respectively; counterclockwise indicates positive phase, radial direction represents positive amplitude, and the direction of the dashed arrow represents positive time.

[0060] After initialization is complete, the signal generator 13 emits the following characteristics: The detection signal (long black arrow). At any moment during operation, signal generator 13 and the returned signal carrying the ground state |0 Information (short blue arrow) or excited state |1 The modulation signal of the information (short red arrow) takes effect, causing the amplitude and phase characteristics of the probe signal to change based on the state of qubit 21 (long blue and long red arrows, respectively).

[0061] Over time After repeated measurements, carrying the ground state |0 The modulated signal characteristics of the information constitute the mean value The distribution (blue scattering) carries the ground state |1 The modulated signal characteristics of the information constitute the mean value Distribution (red scatter).

[0062] The state discriminator 11 identifies the state of the qubit 21 by detecting the frequency, amplitude, or phase characteristics of the modulation signal and comparing them with the frequency, amplitude (black dashed line), or phase (gray dashed line) of the reference signal. A single readout result of a signal with a frequency greater than the frequency reference, an amplitude greater than the amplitude reference, or a phase greater than the phase reference is identified as the ground state |0. The single readout result of a signal with a frequency lower than the frequency reference, an amplitude lower than the amplitude reference, or a phase lower than the phase reference is identified as an excited state |1 .

[0063] Optionally, referring to Figures 4(a) and 4(b), a modulation logic based on the "dispersion shift pair of the probe signal" is described in the process of identifying the state of qubit 21.

[0064] Based on the above-described qubit 21 state readout method, the characteristic changes of the probe signal will continue during the operating time of the RF readout circuit 1. As shown in Figure 4(a), this change is manifested in the frequency domain as based on the loaded ground state |0. Positive frequency shift of information, based on the loaded excited state |1 The negative frequency shift of the information; as shown in Figure 4(b), in the time domain it is represented by the loading-based ground state |0. The negative amplitude and negative phase changes of the information are based on the loaded excited state |1 The information produces positive amplitude and positive phase changes.

[0065] Optionally, refer to Figure 5 As shown, the radio frequency readout circuit 1 also includes: a controller 14; The controller 14 is used to receive read operation instructions from external input, generate control signals according to the read operation instructions, and initialize the state discriminator 11, the cryogenic amplifier, and the signal generator 13 based on the control signals.

[0066] In one feasible implementation, the RF readout circuit 1 also incorporates a controller 14 to achieve coordinated control of the entire readout link. The controller 14 receives readout operation commands from external input (such as a "start readout" command from the host computer), generates control signals based on these commands, and initializes the low-noise amplifier 12, signal generator 13, and state discriminator 11 based on these control signals. This includes controlling the power gain of the low-noise amplifier 12, controlling the operating state of the signal generator 13 and setting the initial characteristics of the detection signal, controlling the operating state of the state detector, and setting the reference signal, thereby improving the automation and repeatability of the RF readout circuit 1.

[0067] Optionally, refer toFigure 6 As shown, the interconnect module 3 includes an attenuator 31, a circulator 32, and an isolator 33. The output terminal of the signal generator 13 is connected to the input terminal of the attenuator 31, the output terminal of the attenuator 31 is connected to the first terminal of the circulator 32, the second terminal of the circulator 32 is coupled to the readout resonant cavity 22, the third terminal of the circulator 32 is connected to the input terminal of the isolator 33, and the output terminal of the isolator 33 is connected to the input terminal of the low-noise amplifier 12.

[0068] Attenuator 31 is used to attenuate the power of the probe signal so as not to excite the current quantum state of qubit 21 to a higher state, and to transmit the attenuated probe signal to the first end of circulator 32. Circulator 32 is used to transmit the attenuated detection signal to quantum computer 2 via the second terminal of circulator 32, and to receive the modulated signal reflected by quantum computer 2, and to transmit the modulated signal to isolator 33 via the third terminal of circulator 32; wherein the signals of the three ports in circulator 32 propagate in the same direction, decoupling the incident and reflected detection signals and improving the signal SNR.

[0069] Isolator 33 is used to transmit the modulation signal to the radio frequency readout circuit 1. Isolator 33 enables the probe signal to propagate in one direction, isolating the back-propagation noise of the radio frequency readout circuit 1 so that it does not affect the coherence of the quantum bit 21.

[0070] In this embodiment, the detection signal emitted by the radio frequency readout circuit 1 is injected into the quantum computer 2, which is located in a low-temperature environment, through the interconnection module 3, and the reflected modulation signal is returned to the radio frequency readout circuit 1, thereby achieving high-speed, low-noise signal transmission.

[0071] Optionally, the interconnect module 3 includes multiple microwave transmission lines, each corresponding to a readout channel of a different quantum bit 21. At the same time, the signal generator 13 may also include multiple units, and the modulation signals corresponding to different quantum bits 21 are transmitted to the corresponding signal generator 13 for demodulation via the low-noise amplifier 12.

[0072] Optionally, refer to Figure 7 The diagram shown is a flowchart illustrating a quantum bit state readout method provided in an embodiment of this application. Applied to the quantum bit state readout system provided in the above embodiment, the method includes: S101, the radio frequency readout circuit generates a detection signal and injects the detection signal into the readout resonant cavity.

[0073] S102. The dispersive frequency shift generated by the coupling between the current quantum state of the qubit and the readout resonant cavity modulates the detection signal, generates a modulated signal, and reflects the modulated signal to the radio frequency readout circuit.

[0074] The modulation signal carries the state information of the current quantum state; S103. When the frequency of the detection signal is stable, the radio frequency readout circuit obtains the current quantum state of the qubit based on the modulation signal.

[0075] Quantum states include: ground state or excited state.

[0076] In this embodiment, a detection signal is emitted by the radio frequency (RF) readout circuit and injected into the readout resonant cavity. The readout resonant cavity modulates the detection signal based on the dispersion frequency shift generated by the coupling between the current quantum state of the qubit and the readout resonant cavity, generating a modulated signal. The dispersion frequency shift is determined based on the current quantum state of the qubit. The modulated signal is then reflected back to the RF readout circuit. When the frequency of the detection signal is stable, the RF readout circuit demodulates the modulated signal to obtain the current quantum state of the qubit, achieving high-fidelity readout of the qubit state. During the qubit state readout process, the RF readout circuit reduces the performance requirements of the qubit state readout system for noise and sampling rate by improving the separation degree of qubit state information in the detection signal.

[0077] Optionally, this application adopts a closed-loop architecture to reduce the noise performance requirements of the front-end low-temperature low-noise amplifier and the speed performance requirements of the back-end analog-to-digital converter and digital signal processor, while ensuring fast and high-fidelity readout of quantum bits. The signal generator also performs multiple functions such as signal transmission, reception, analysis and processing, thereby reducing the design complexity and cost of the RF readout circuit.

[0078] Optionally, step S103 above includes: The signal characteristics corresponding to the modulation signal are compared with the signal characteristics of the preset reference signal to obtain the comparison result; based on the comparison result, the current quantum state of the qubit is determined.

[0079] In one feasible approach, a discrimination method based on signal feature comparison is proposed to obtain the current quantum state of a qubit.

[0080] Optionally, in classical radio frequency dispersion readout techniques, the state readout fidelity of a qubit is related to the SNR of the quantum state information in the probe signal, as described by the following formula (1): (1) The SNR of quantum state information and the characteristic separation degree of the probe signal based on the dispersion frequency shift effect. And the noise of the entire RF readout circuit. The relevant information is shown in the following formula (2): (2) in, , , and These are the characteristics of the detection signals received by the radio frequency receiving circuit; Boltzmann's constant; The equivalent noise temperature of the entire system is mainly determined by the low-noise amplifier; The system bandwidth is proportional to the number of qubits. The readout time is primarily determined by the lifetime of the qubit; The sampling rate of the radio frequency receiving circuit is mainly determined by the analog-to-digital converter.

[0081] In this application, the amplitude and phase of the received modulated signal are accumulated to enhance the ground state |0 carried therein. Information and excited states | 1 Information separation Therefore, this application can reduce the noise performance requirements of the low-noise amplifier and the sampling rate performance requirements of the analog-to-digital converter while keeping the quantum state information SNR in the modulated signal, thereby reducing the design complexity and cost of the RF readout circuit. In addition, this invention can also reduce the readout time while keeping the system noise figure and sampling rate constant. This improves the operating speed of the readout system.

[0082] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

[0083] It will be apparent to those skilled in the art that this application is not limited to the details of the exemplary embodiments described above, and that this application can be implemented in other specific forms without departing from the spirit or essential characteristics of this application. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of this application is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within this application. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A quantum bit state readout system, characterized in that, The quantum bit state readout system includes: a radio frequency readout circuit, a quantum computer, and an interconnection module; The temperature of the environment in which the radio frequency readout circuit is located is higher than the temperature of the environment in which the quantum computer is located; the quantum computer includes: qubits and a readout resonant cavity; the qubits are coupled to one end of the readout resonant cavity; The input terminal of the radio frequency readout circuit is connected to the output terminal of the interconnect module, the first output terminal of the radio frequency readout circuit is connected to the first terminal of the interconnect module, and the second terminal of the interconnect module is connected to the other terminal of the readout resonant cavity. The radio frequency readout circuit is used to generate a detection signal and inject it into the readout resonant cavity via the interconnect module; The readout resonant cavity is used to modulate the detection signal based on the dispersion frequency shift generated by the coupling between the current quantum state of the qubit and the readout resonant cavity, to generate a modulated signal, and to reflect the modulated signal to the radio frequency readout circuit via the interconnect module, wherein the modulated signal carries the current quantum state of the qubit; The radio frequency readout circuit is further configured to demodulate the modulation signal when the frequency of the detection signal is stable, so as to obtain the current quantum state of the qubit, and output the current quantum state of the qubit from the second output terminal of the radio frequency readout circuit.

2. The system according to claim 1, characterized in that, The radio frequency readout circuit includes: a state discriminator, a low-noise amplifier, and a signal generator; the output terminal of the signal generator is connected to the first terminal of the interconnect module, and the input terminal of the low-noise amplifier is connected to the output terminal of the interconnect module. The signal generator is used to generate a detection signal and transmit the detection signal to the readout resonant cavity via the interconnect module; The low-noise amplifier is used to receive the modulation signal returned via the interconnect module, amplify the modulation signal to obtain an amplified modulation signal, and transmit the amplified modulation signal to the signal generator. The signal generator is further configured to demodulate the amplified modulation signal when the frequency of the detection signal is stable, obtain the signal features corresponding to the modulation signal, and transmit the signal features corresponding to the modulation signal to the state discriminator. The signal features corresponding to the modulation signal include at least: amplitude, frequency, and phase. The state discriminator is used to obtain the current quantum state of the qubit based on the signal characteristics corresponding to the modulation signal.

3. The system according to claim 1, characterized in that, The frequency of the detection signal is stable to indicate that the frequency of the detection signal no longer changes.

4. The system according to claim 2, characterized in that, The step of obtaining the current quantum state of the qubit based on the signal characteristics corresponding to the modulation signal includes: The signal features corresponding to the modulation signal are compared with the signal features of a preset reference signal to obtain the comparison result; Based on the comparison results, the current quantum state of the qubit is determined.

5. The system according to claim 4, characterized in that, The step of comparing the signal features corresponding to the modulated signal with the signal features of a preset reference signal to obtain a comparison result includes: Determine a first difference between the amplitude of the modulated signal and the amplitude of the reference signal, and use the first difference as the amplitude difference; The second difference between the phase of the modulated signal and the phase of the reference signal is used as the phase difference.

6. The system according to claim 5, characterized in that, Determining the current quantum state of the qubit based on the comparison result includes: If the amplitude difference or the phase difference is greater than a preset threshold, then the current quantum state of the quantum bit is determined to be the ground state; If the amplitude difference or the phase difference is less than a preset threshold, then the current quantum state of the qubit is determined to be an excited state.

7. The system according to claim 2, characterized in that, The radio frequency readout circuit also includes: a controller; The controller is configured to receive readout operation instructions from external input, generate control signals according to the readout operation instructions, and initialize the state discriminator, the low-noise amplifier, and the signal generator based on the control signals.

8. The system according to claim 2, characterized in that, The interconnect module includes an attenuator, a circulator, and an isolator; the output terminal of the signal generator is connected to the input terminal of the attenuator, the output terminal of the attenuator is connected to the first terminal of the circulator, the second terminal of the circulator is coupled to the readout resonant cavity, the third terminal of the circulator is connected to the input terminal of the isolator, and the output terminal of the isolator is connected to the input terminal of the low-noise amplifier. The attenuator is used to attenuate the power of the probe signal so as not to excite the current quantum state of the qubit to a higher state, and to transmit the attenuated probe signal to the first end of the circulator. The circulator is used to transmit the attenuated detection signal to the quantum computer via a second terminal on the circulator, receive the modulated signal reflected by the quantum computer, and transmit the modulated signal to the isolator via a third terminal on the circulator. The isolator is used to transmit the modulated signal to the radio frequency readout circuit.

9. A method for reading out the state of a quantum bit, characterized in that, The method, applied to the system according to any one of claims 1-8, comprises: The radio frequency readout circuit generates a detection signal and injects the detection signal into the readout resonant cavity; The readout resonant cavity modulates the detection signal based on the dispersion frequency shift generated by the coupling between the current quantum state of the qubit and the readout resonant cavity, generating a modulated signal, and reflects the modulated signal to the radio frequency readout circuit, wherein the modulated signal carries the state information of the current quantum state; When the frequency of the detection signal is stable, the radio frequency readout circuit obtains the current quantum state of the qubit based on the modulation signal. The quantum state includes either the ground state or the excited state.

10. The method according to claim 9, characterized in that, Determining the current quantum state of the qubit based on the modulation signal includes: The signal features corresponding to the modulation signal are compared with the signal features of a preset reference signal to obtain the comparison result; Based on the comparison results, the current quantum state of the qubit is determined.

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