Microwave single photon detection method and detector
By introducing a coupling structure between a resonator and a superconducting quantum bit in a microwave single-photon detector, and using the reflection signals of the first and second microwave signals to determine the quantum state change, the problem of low detection rate in the prior art is solved, and a higher detection rate and information processing speed are achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-02-10
- Publication Date
- 2026-04-14
AI Technical Summary
The existing microwave single-photon detectors have low detection rates, mainly due to the need to reset the state of the superconducting qubits in each detection cycle, which limits the speed at which the detector processes information.
By coupling a resonator to a superconducting quantum bit during the detection period, a first microwave signal resonating with the frequency of the superconducting quantum bit and a single-photon microwave signal to be measured are first input. The single-photon state is mapped to a quantum state transition event. After waiting for a set time, a second microwave signal is input. The quantum state is determined by the amplitude and phase of the reflected signal, thus enabling the detection of quantum state changes by comparing adjacent periods.
Without needing to reset the superconducting qubits, the detection rate is significantly improved, the time of a single detection cycle is reduced, and the information processing speed of the detector is increased.
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Figure CN121855686A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of single-photon detection technology, and in particular to a microwave single-photon detection method and detector. Background Technology
[0002] Existing microwave single-photon detectors based on superconducting qubits typically involve the following processes within a detection cycle: the superconducting qubit interacts with a microwave single photon, causing a change in the quantum state of the superconducting qubit; the quantum state of the superconducting qubit is read out by inputting a microwave readout signal; and the state of the superconducting qubit is reset. This detection cycle is repeated cyclically, enabling continuous detection of microwave single photons. The detection rate of a microwave single-photon detector depends on the duration of a single detection cycle. The detection rate is a crucial performance indicator of a microwave single-photon detector, reflecting the upper limit of the detector's information processing speed, determining the fidelity of the detector at high photon flux rates, and also affecting the detector's time resolution. Existing microwave single-photon detectors typically have detection rates not exceeding 100 kHz, meaning a single detection cycle is greater than 10 μs. Their main bottleneck lies in the fact that the detector needs to be reset in each detection cycle, consuming additional time, which limits the detection rate to some extent. Summary of the Invention
[0003] Purpose of the invention: The purpose of this invention is to provide a microwave single-photon detection method and detector, which solves the problem that the state of the superconducting quantum bit needs to be reset in each detection cycle, resulting in a low detection rate of the detector.
[0004] Technical solution: To achieve the above objectives, on the one hand, a microwave single-photon detection method is provided, including:
[0005] During the current detection period, a first microwave signal resonating with the frequency of the superconducting quantum bit and a single-photon signal of the microwave to be tested that is close to the frequency of the resonator are first input to the resonator; wherein the resonator and the superconducting quantum bit have different frequencies and are coupled together; the first microwave signal is transmitted to the superconducting quantum bit by the resonator; the single-photon state of the single-photon signal of the microwave to be tested is mapped to the quantum state transition event of the superconducting quantum bit under the action of the first microwave signal;
[0006] Wait for a set time, then input a second microwave signal that is close to the resonant frequency to the resonator;
[0007] The reflection signal of the resonator to the second microwave signal is obtained, and the quantum state of the superconducting quantum bit in the current detection period is determined based on the amplitude and phase of the reflection signal.
[0008] If the quantum state of the current detection cycle is different from that of the previous detection cycle, then it is determined that a microwave single photon has been detected from the microwave single photon signal to be tested; otherwise, it is determined that no microwave single photon has been detected.
[0009] Preferably, determining the quantum state of the superconducting qubit in the current detection period based on the amplitude and phase of the reflected signal includes:
[0010] By using a predetermined function and threshold to distinguish whether the superconducting quantum bit is in the ground state or the excited state, and the amplitude and phase of the reflected signal, the function value is calculated and compared with the threshold to determine whether the superconducting quantum bit is currently in the ground state or the excited state.
[0011] The function and threshold are predetermined based on the distribution characteristics of the ground state and excited state of the superconducting qubit in a two-dimensional parameter space consisting of amplitude and phase.
[0012] Preferably, the near-resonance frequency means that the detuning between the frequency of the microwave single-photon signal under test or the frequency of the second microwave signal and the natural frequency of the resonator is less than 10 MHz.
[0013] Preferably, the set time is not less than 10 ns.
[0014] Preferably, the resonator is an LC resonator, a distributed resonator, or a transmission line resonator formed by parallel connection of capacitor and inductor.
[0015] On the other hand, a microwave single-photon detector is provided, including a resonator and a superconducting quantum bit, wherein the resonator and the superconducting quantum bit have different frequencies and are coupled together.
[0016] The resonator is used to receive a first microwave signal that resonates with the frequency of the superconducting quantum bit and a single-photon microwave signal to be tested that is close to the frequency of the resonator, and to transmit the first microwave signal to the superconducting quantum bit.
[0017] The superconducting quantum bit is used to receive the first microwave signal and, under the action of the first microwave signal, to map the single-photon state of the microwave single-photon signal to be measured onto the two quantum state transition events of the quantum bit: the ground state or the excited state.
[0018] The resonator is also used to receive a second microwave signal that is close to the resonator frequency after waiting for a set time, and to reflect the second microwave signal to output a reflected signal; the reflected signal is used to determine the quantum state of the superconducting quantum bit, and the quantum state is used to compare with the quantum state of the previous detection cycle to determine whether a microwave single photon is detected from the microwave single photon signal to be tested.
[0019] Preferably, the device further includes a transmission line coupled to the resonator, the transmission line being used for signal transmission with the resonator, the transmission line being a coaxial cable, waveguide, microstrip line, or coplanar waveguide.
[0020] Preferably, the structure for achieving the coupling connection includes a capacitor, mutual inductance, equivalent capacitance, or equivalent mutual inductance.
[0021] Preferably, the superconducting quantum bit is a superconducting circuit, which includes one or more Josephson junctions, or includes one or more Josephson junctions and circuit elements. The circuit elements include one or more of capacitors, inductors, equivalent capacitance, and equivalent inductance. The circuit elements are connected in series, in parallel, or a combination of series and parallel connections with the Josephson junctions.
[0022] Preferably, the resonator is an LC resonator, a distributed resonator, or a transmission line resonator formed by parallel connection of capacitor and inductor.
[0023] Beneficial Effects: This invention has the following advantages: It couples resonators of different frequencies to a superconducting quantum bit. Within the current detection cycle, a first microwave signal resonating with the frequency of the superconducting quantum bit and a single-photon signal to be measured that is close to the frequency of the resonator are first input to the resonator. Then, a second microwave signal that is close to the frequency of the resonator is input to the resonator. The first microwave signal is used to map the single-photon state in the single-photon signal to be measured onto the quantum state transition event of the superconducting quantum bit. Furthermore, the reflected signal of the second microwave signal is used to determine the quantum state of the superconducting quantum bit in the current detection cycle. During the continuous detection of microwave single-photon signals across multiple detection cycles, the detection of microwave single photons is achieved by comparing the quantum state transitions of adjacent detection cycles. Therefore, the state of the superconducting quantum bit does not need to be reset within each detection cycle, significantly reducing the time of a single detection cycle and improving the detection rate.
[0024] In other words, by introducing a coupling structure between a resonator and a superconducting quantum bit, as well as a first microwave signal and a second microwave signal, the detection rate can be improved by detecting whether a microwave single photon exists in the microwave single photon signal. Attached Figure Description
[0025] Figure 1 This is a flowchart of the microwave single-photon detector detection process.
[0026] Figure 2 This is a schematic diagram of a microwave single-photon detector.
[0027] Figure 3 A schematic diagram of the two-dimensional parameter space defined for amplitude and phase;
[0028] Figure 4This is a prototype circuit diagram of a microwave single-photon detector.
[0029] Figure 5 The circuit structure for a microwave single-photon detector;
[0030] Figure 6 This is a timing diagram of the microwave signal input to a microwave single-photon detector.
[0031] Figure 7 This is a record of the measured response of a microwave single-photon detector under different average input photon numbers. Detailed Implementation
[0032] 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, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0033] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in the description of this application is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms "comprising" and "having," and any variations thereof, in the description, claims, and accompanying drawings of this application are intended to cover non-exclusive inclusion. The terms "first," "second," etc., in the description, claims, or accompanying drawings of this application are used to distinguish different objects, not to describe a specific order or hierarchy.
[0034] In this application, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this application can be combined with other embodiments.
[0035] Example 1
[0036] like Figure 1 As shown, this embodiment provides a microwave single-photon detection method, specifically including:
[0037] S1. During the current detection period, a first microwave signal resonating with the frequency of the superconducting quantum bit and a single-photon signal of the microwave to be tested that is close to the frequency of the resonator are first input to the resonator; wherein, the resonator and the superconducting quantum bit have different frequencies and are coupled together; the first microwave signal is transmitted to the superconducting quantum bit by the resonator; the single-photon state of the single-photon signal of the microwave to be tested is mapped to the quantum state transition event of the superconducting quantum bit under the action of the first microwave signal;
[0038] S2. Wait for a set time, then input a second microwave signal that is close to the resonant frequency to the resonator;
[0039] S3. Obtain the reflection signal of the resonator to the second microwave signal, and determine the quantum state of the superconducting quantum bit in the current detection period based on the amplitude and phase of the reflection signal;
[0040] S4. If the quantum state of the current detection cycle is different from the quantum state of the previous detection cycle, then it is determined that a microwave single photon has been detected from the microwave single photon signal to be tested; otherwise, it is determined that no microwave single photon has been detected.
[0041] Within a single detection cycle, the set waiting time is no less than 10 ns. Preferably, the first microwave signal and the microwave single-photon signal to be measured can be allowed to dissipate naturally within the resonator before a second microwave signal with a frequency close to the resonance is input into the resonator. Allowing the second microwave signal to dissipate naturally within the resonator for a period of time reduces the likelihood of residual microwave signals causing false responses in the microwave single-photon detector, thus improving detection accuracy. This allows the next detection cycle to be entered, thereby obtaining the detection record of the microwave single-photon detector in each detection cycle.
[0042] The frequency resonance refers to the situation where the microwave signal frequency is the same as the inherent frequency of the resonator or superconducting quantum bit. The near-frequency resonance refers to the frequency of the microwave single-photon signal under test or the second microwave signal frequency, which is detuned by less than 10 MHz from the inherent frequency of the resonator or superconducting quantum bit.
[0043] In this embodiment, the resonator is coupled to the superconducting quantum bit, and the frequency of the resonator is different from the frequency of the superconducting quantum bit. The resonator is used to receive a first microwave signal, a second microwave signal, and a single-photon microwave signal to be measured. The resonator also reflects the second microwave signal and the single-photon microwave signal to be measured, which are input to the resonator at a frequency close to its own, and transmits the first microwave signal, which is input to the superconducting quantum bit at its frequency, to the superconducting quantum bit. The reflected signal is reflected back to the transmission line of the input resonator.
[0044] Superconducting qubits are used to receive the first microwave signal. The single-photon state of the microwave single-photon signal to be measured is mapped onto the quantum state transition event of the superconducting qubit under the influence of the first microwave signal. Specifically: if there are no single photons in the microwave single-photon signal to be measured during the current detection period, the quantum state of the superconducting qubit remains unchanged compared to the previous detection period; if there are single photons in the microwave single-photon signal to be measured during the current detection period, the quantum state of the superconducting qubit changes compared to the previous detection period. The quantum state transition event is specifically understood as an event of transitioning from the ground state to an excited state, or vice versa.
[0045] One mechanism for achieving the above mapping is as follows: Since the frequency of the resonator differs from the frequency of the superconducting quantum bit (QB), and the two are coupled, this coupling causes an interaction between the resonator and the QB. This interaction shifts the inherent frequency of the QB, and the amount of this shift is influenced by the number of photons within the resonator. Therefore, the number of photons within the resonator affects the frequency of the QB. Because the single-photon microwave signal to be measured is close to the resonator frequency, it is coupled into the resonator, generating a photon-number state within it. If the number of photons within the resonator is one, the frequency of the QB corresponds to that photon-number, and the first microwave signal resonates with this frequency, directly acting on the QB and transforming its quantum state from the ground state to an excited state, or vice versa. If the number of photons within the resonator is zero, the frequency of the QB is the frequency corresponding to the zero photon-number, and the first microwave signal does not resonate with this frequency, thus failing to act on the QB and not changing its quantum state.
[0046] Based on the above mapping mechanism, the quantum state of the superconducting quantum bit in the current detection period is determined according to the reflection signal of the second microwave signal from the resonator. Specifically, this can be understood as follows: Since the frequency of the resonator is different from the frequency of the superconducting quantum bit, and the two are coupled, this coupling causes an interaction between the resonator and the superconducting quantum bit. This interaction also causes a shift in the resonator's intrinsic frequency, and the shift is affected by the quantum state of the quantum bit. Therefore, the quantum state of the superconducting quantum bit affects the frequency of the resonator. Specifically, the ground state and excited state of the superconducting quantum bit correspond to two frequencies of the resonator, and these two frequencies are close to resonance. When a second microwave signal, which is close to both resonant frequencies of the resonator, is input alone, it indicates that the second microwave signal has a different frequency from the superconducting quantum bit. That is, the second microwave signal is coupled into the resonator but does not interact with the superconducting quantum bit, and therefore does not affect the quantum state of the superconducting quantum bit. After the second microwave signal reaches the resonator, it is reflected and output. The frequency of the reflected signal is the same as that of the second microwave signal. Therefore, the reflected signal is close to the two resonant frequencies of the resonator. Under the condition of near resonance, the amplitude and phase of the reflected signal will produce two different results because the resonator frequency is at one of the two resonant frequencies. Therefore, the quantum state of the superconducting quantum bit can be determined by detecting the amplitude and phase of the reflected signal of the second microwave signal.
[0047] In this embodiment, within a single detection cycle, a first microwave signal is used to map the single-photon information in the microwave single-photon signal to be measured onto the quantum state transition event of the superconducting qubit, and a second microwave signal is used to determine the quantum state of the superconducting qubit in the current detection cycle. During the continuous detection of microwave single-photon signals across multiple detection cycles, the detection of microwave single photons is achieved by comparing the quantum state transitions of adjacent detection cycles. The state of the superconducting qubit does not need to be reset within each detection cycle, significantly reducing the time of a single detection cycle and improving the detection rate.
[0048] Example 2
[0049] like Figure 2 As shown, this embodiment provides a microwave single-photon detector, which utilizes the method provided in Embodiment 1 to achieve microwave single-photon detection. The microwave single-photon detector includes a resonator and a superconducting quantum bit, wherein the resonator and the superconducting quantum bit have different frequencies and are coupled together.
[0050] The resonator is used to receive a first microwave signal that resonates with the frequency of the superconducting quantum bit and a single-photon microwave signal to be tested that is close to the frequency of the resonator, and to transmit the first microwave signal to the superconducting quantum bit.
[0051] The superconducting quantum bit is used to receive the first microwave signal and, under the action of the first microwave signal, to map the single-photon state of the microwave single-photon signal to be measured onto the two quantum state transition events of the quantum bit: the ground state or the excited state.
[0052] The resonator is also used to receive a second microwave signal that is close to the resonator frequency after waiting for a set time, and to reflect the second microwave signal to output a reflected signal; the reflected signal is used to determine the quantum state of the superconducting quantum bit, and the quantum state is used to compare with the quantum state of the previous detection cycle to determine whether a microwave single photon is detected from the microwave single photon signal to be tested.
[0053] In one specific embodiment, the microwave single-photon detector further includes a signal transmission line coupled to a resonator. The transmission line is used to input a first microwave signal, a second microwave signal, and a microwave single-photon signal to be tested into the resonator, and to output the microwave signal reflected by the resonator from the second microwave signal.
[0054] Transmission lines can be structured in various ways, including but not limited to coaxial cables, waveguides, microstrip lines, and coplanar waveguides.
[0055] Resonators can be implemented in various ways, including but not limited to LC resonators formed by parallel connection of lumped capacitors and inductors, distributed resonators, and transmission line resonators.
[0056] The transmission line and the resonator are connected via a first coupler. The resonator and the superconducting quantum bit are connected via a second coupler. The first and second couplers can be implemented using capacitance, mutual inductance, equivalent capacitance, or equivalent mutual inductance structures. The capacitance, mutual inductance, equivalent capacitance, or equivalent mutual inductance values determine the coupling strength provided by the first and second couplers, and the coupling strength determines the signal exchange rate between the structures at both ends of the first and second couplers.
[0057] Superconducting qubits can map the state of a single-photon microwave signal to be measured. Taking the first microwave signal as a quantum gate signal as an example, under the action of the quantum gate signal, the superconducting qubit can map the two states of having and not having a single photon to two events: the quantum state of the superconducting qubit remains unchanged and the quantum state changes. The structure of a superconducting qubit is a superconducting circuit containing one or more Josephson junctions. In addition to the Josephson junction, the superconducting circuit may also contain capacitors, inductors, equivalent capacitance, equivalent inductance, bias voltage and bias current across the Josephson junction, etc. The energy level structure of the superconducting circuit exhibits a nonharmonic quantized multi-level structure. Selecting two energy levels from multiple energy levels defines a superconducting qubit.
[0058] The resonator's frequency differs from the superconducting qubit's frequency. The second coupler causes the resonator and the superconducting qubit to interact. Under this interaction, the quantum state of the superconducting qubit affects the resonator's frequency. Specifically, the superconducting qubit's ground state and excited state correspond to the two frequencies of the resonator, and these two frequencies are close to resonance. When a microwave signal that is close to both resonant frequencies is input to the resonator, taking the second microwave signal as an example, the amplitude and phase of the reflected signal will produce two different results because the resonator's resonant frequency is at one of the two resonant frequencies. Therefore, the quantum state of the superconducting qubit can be determined by detecting the amplitude and phase of the reflected signal.
[0059] The method for determining the quantum state of a superconducting qubit is as follows: the amplitude and phase of the reflected signal can define a two-dimensional parameter space. First, two different quantum states are deterministically prepared through preliminary experiments and their corresponding amplitudes and phases are measured. Then, a function and threshold that can distinguish between the two different amplitudes and phases are searched in the parameter space. After obtaining the amplitude and phase in subsequent measurements, the function value is calculated and compared with the threshold to determine whether the measured quantum state is the ground state or the excited state.
[0060] In this embodiment, an oscilloscope, spectrum analyzer, or other equipment can be used to detect the amplitude and phase of the reflected signal of the second microwave signal.
[0061] In this embodiment, based on the coupling connection between the resonator and the superconducting quantum bit, the superconducting quantum bit, under the action of the first microwave signal, can map the single-photon information in the microwave single-photon signal to be measured into two quantum state transition events: the ground state and the excited state. Simultaneously, the quantum state of the superconducting quantum bit further affects the frequency of the resonator. Therefore, within a single detection cycle, by inputting a second microwave signal to the resonator and determining the current quantum state of the superconducting quantum bit based on the resonator's reflection of the second microwave signal, the detection of microwave single photons is achieved by comparing whether the quantum states of adjacent detection cycles are the same. Since the state of the superconducting quantum bit does not need to be reset within each detection cycle, the time of a single detection cycle is significantly reduced, and the detection rate is improved.
[0062] Example 3
[0063] This embodiment provides a method for determining the quantum state of a superconducting qubit based on amplitude and phase. The amplitude and phase of the reflected signal define a two-dimensional parameter space. First, two different quantum states of the superconducting qubit—the ground state and the excited state—are deterministically prepared through preliminary experiments, and the amplitude and phase corresponding to these two quantum states are measured. A function and a threshold that can distinguish the amplitude and phase corresponding to these two quantum states are searched in the parameter space. In each subsequent detection cycle, the function value is calculated based on the measured amplitude and phase of the second microwave signal reflection. The calculated function value is compared with the threshold to determine whether the quantum state of the superconducting qubit in the current detection cycle is the ground state or the excited state.
[0064] like Figure 3 As shown, the two-dimensional plane in the figure is a two-dimensional parameter space defined by amplitude and phase. The position of each scatter point in the plane is determined by amplitude and phase. The distance between the scatter point and the origin is equal to the amplitude, and the radian value of the angle between the line connecting the scatter point and the origin and the horizontal axis is equal to the phase. After each detection of the amplitude and phase of the reflected signal of the second microwave signal, the corresponding scatter points can be plotted in the plane. The black scatter points correspond to the superconducting qubits being prepared in the ground state, and the gray scatter points correspond to the superconducting qubits being prepared in the excited state. After repeated measurements, the two types of scatter points form a specific distribution. The dashed line in the two-dimensional plane can separate the distribution formed by the two types of scatter points. The left side of the dashed line corresponds to the superconducting qubits in the ground state, and the right side corresponds to the superconducting qubits in the excited state. By calculating the analytical expression of the dashed line in the two-dimensional parameter space, the function and threshold for distinguishing the two quantum states can be determined.
[0065] In this embodiment, a two-dimensional parameter space is constructed using the amplitude and phase of the reflected signal. The distribution characteristics and separation function of the ground state and excited state are determined through preliminary experimental calibration. Thus, whether the superconducting quantum bit is in the ground state or excited state is simplified to numerical comparison for determination, which can effectively eliminate noise interference, reduce misjudgment, and improve the quantum state determination rate.
[0066] Example 4
[0067] like Figure 4 As shown, this embodiment provides a prototype circuit diagram of a microwave single-photon detector, including a port, four capacitors (C1-C4), an inductor (L1), and a Josephson junction (J1). L1 and C3 are connected in parallel to form a resonator; C4 and J1 are connected in parallel to form a superconducting quantum bit; C2 acts as a second coupler, providing coupling between the resonator and the superconducting quantum bit; the port connects to an external transmission line, and the resonator and port are connected by C1, which acts as a first coupler, providing coupling between the resonator and the external transmission line.
[0068] The microwave single-photon detector prototype circuit provided in this embodiment offers a highly flexible and reliable transformation basis for practical circuit design. As an abstract ideal model, this prototype circuit enables designers to clearly understand and analyze the detector's working mechanism from a principle perspective, offering strong design universality and guidance. Based on this prototype, various specific physical implementations can be derived using mature equivalent circuit transformation principles. For example, the resonator can be implemented using a lumped-parameter LC circuit or a distributed-parameter transmission line resonator; the coupler can use a parallel-plate capacitor, interdigitated capacitor, or mutual inductance structure, etc. This greatly expands the design freedom of the circuit, allowing designers to choose the most suitable and optimized implementation method according to different process platforms (such as integrated circuits or three-dimensional waveguides), frequency requirements, and integration requirements, thereby overcoming the performance limitations or process compatibility issues that may exist with a single fixed structure.
[0069] Example 5
[0070] like Figure 5 As shown in the diagram, this embodiment provides a circuit structure diagram of a microwave single-photon detector, including a transmission line, a resonator, a superconducting quantum bit, a first coupler, and a second coupler. The transmission line is a coaxial cable, and the resonator is a rectangular resonant cavity. The superconducting quantum bit is a floating superconducting quantum bit with a Josephson junction at its center and two plates connected to its ends, forming a planar capacitor structure. The first coupler is a probe extending from the central conductor of the coaxial cable into the resonant cavity, and is an equivalent capacitor structure. The second coupler is the interaction between the electric field within the resonant cavity and the planar capacitor, and is also an equivalent capacitor structure.
[0071] This embodiment employs a rectangular resonant cavity as the resonator, which typically has a higher quality factor compared to lumped-parameter or miniaturized resonant structures, enabling more efficient storage of microwave photons and thus improving the detector's sensitivity and signal-to-noise ratio. A floating superconducting quantum bit is used; this structure effectively suppresses noise caused by environmental charge fluctuations and extends the quantum bit's coherence time, resulting in higher fidelity and accuracy when detecting weak single-photon signals. The coupling strength is easy to design and adjust: the first coupler uses capacitive coupling via a coaxial probe inserted deep into the cavity, and its coupling strength can be precisely optimized in situ by adjusting the probe's insertion depth and position, facilitating tuning to the optimal operating point after manufacturing. The coaxial interface and rectangular cavity are standard components in microwave engineering, facilitating reliable connection to external measurement systems and enabling future modular systems containing multiple detectors or more complex quantum circuits.
[0072] Example 6
[0073] This embodiment inputs the following to the microwave single-photon detector structure of Embodiment 2: Figure 6The microwave signal timing diagram is shown, and the single-photon response test is performed using the detection method of Example 1.
[0074] Figure 6 The diagram shows the envelope waveforms and corresponding timing of the first microwave signal, the second microwave signal, and the microwave single-photon signal under test during one detection cycle. Within one detection cycle, the three microwave signals are executed according to the timing sequence, and the response of the microwave single-photon detector under different average input photon counts is measured. The average photon count refers to the average number of photons contained in the microwave single-photon signal under test.
[0075] like Figure 7 The figure shows the measured response records of a microwave single-photon detector under different average input photon numbers. The detector completed 1000 detections within 1.85 ms, achieving a detection rate of 540 kHz. The test results reflect the response events of the microwave single-photon detector in 1000 detections under three different average input photon numbers (N=0, 0.21, and 0.86). In the figure, 0 on the vertical axis represents no response from the detector, and 1 represents a response from the detector. The results show that as the average input photon number increases, the number of responses from the detector in 1000 detections also increases. It should be noted that... Figure 6 The microwave single-photon detector also responds when the number of input photons is 0. This is a false response caused by system noise, i.e., dark counting.
[0076] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.
[0077] The various embodiments in this specification are described in a progressive manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.
[0078] The above description is merely an embodiment of this application and is not intended to limit the scope of 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 scope of the claims of this application.
[0079] Although embodiments of this application have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of this application, and such modifications and variations all fall within the scope defined by the appended claims.
Claims
1. A microwave single-photon detection method, characterized in that, include: During the current detection period, a first microwave signal resonating with the frequency of the superconducting quantum bit and a single-photon signal of the microwave to be tested that is close to the frequency of the resonator are first input to the resonator; wherein the resonator and the superconducting quantum bit have different frequencies and are coupled together; the first microwave signal is transmitted to the superconducting quantum bit by the resonator; the single-photon state of the single-photon signal of the microwave to be tested is mapped to the quantum state transition event of the superconducting quantum bit under the action of the first microwave signal; Wait for a set time, then input a second microwave signal that is close to the resonant frequency to the resonator; The reflection signal of the resonator to the second microwave signal is obtained, and the quantum state of the superconducting quantum bit in the current detection period is determined based on the amplitude and phase of the reflection signal. If the quantum state of the current detection cycle is different from that of the previous detection cycle, then it is determined that a microwave single photon has been detected from the microwave single photon signal to be tested; otherwise, it is determined that no microwave single photon has been detected.
2. The microwave single-photon detection method according to claim 1, characterized in that, Determining the quantum state of the superconducting qubit in the current detection period based on the amplitude and phase of the reflected signal includes: By using a predetermined function and threshold to distinguish whether the superconducting quantum bit is in the ground state or the excited state, and the amplitude and phase of the reflected signal, the function value is calculated and compared with the threshold to determine whether the superconducting quantum bit is currently in the ground state or the excited state. The function and threshold are predetermined based on the distribution characteristics of the ground state and excited state of the superconducting qubit in a two-dimensional parameter space consisting of amplitude and phase.
3. The microwave single-photon detection method according to claim 1, characterized in that, The near-resonance frequency is defined as a detuning of less than 10MHz between the frequency of the microwave single-photon signal under test or the frequency of the second microwave signal and the natural frequency of the resonator.
4. The microwave single-photon detection method according to claim 1, characterized in that, The set time is no less than 10 ns.
5. The microwave single-photon detection method according to claim 1, characterized in that, The resonator is an LC resonator, a distributed resonator, or a transmission line resonator formed by parallel connection of capacitor and inductor.
6. A microwave single-photon detector, characterized in that, It includes a resonator and a superconducting quantum bit, wherein the resonator and the superconducting quantum bit have different frequencies and are coupled together; The resonator is used to receive a first microwave signal that resonates with the frequency of the superconducting quantum bit and a single-photon microwave signal to be tested that is close to the frequency of the resonator, and to transmit the first microwave signal to the superconducting quantum bit. The superconducting quantum bit is used to receive the first microwave signal and, under the action of the first microwave signal, to map the single-photon state of the microwave single-photon signal to be measured onto the two quantum state transition events of the quantum bit: the ground state or the excited state. The resonator is also used to receive a second microwave signal that is close to the resonator frequency after waiting for a set time, and to reflect the second microwave signal to output a reflected signal; the reflected signal is used to determine the quantum state of the superconducting quantum bit, and the quantum state is used to compare with the quantum state of the previous detection cycle to determine whether a microwave single photon is detected from the microwave single photon signal to be tested.
7. The microwave single-photon detector according to claim 6, characterized in that, It also includes a transmission line coupled to the resonator, the transmission line being used for signal transmission with the resonator.
8. The microwave single-photon detector according to claim 6 or 7, characterized in that, The structures that realize the coupling connection include capacitors, mutual inductance, equivalent capacitance, or equivalent mutual inductance.
9. The microwave single-photon detector according to claim 6, characterized in that, The superconducting quantum bit is a superconducting circuit, which includes one or more Josephson junctions, or includes one or more Josephson junctions and circuit elements. The circuit elements include one or more of capacitors, inductors, equivalent capacitance, and equivalent inductance. The circuit elements are connected in series, in parallel, or a combination of series and parallel with the Josephson junctions.
10. The microwave single-photon detector according to claim 6, characterized in that, The resonator is an LC resonator, a distributed resonator, or a transmission line resonator formed by parallel connection of capacitor and inductor.