Detection of individual microwave photons using a non-destructive quantum photon detector

Abstract

Microwave detection unit (205) which features: a non-destructive quantum microwave photon detector (100); a quadrature microwave hybrid coupler (210) connected to the non-destructive quantum microwave photon detector; a dispersive nonlinear element connected to the quadrature microwave hybrid coupler; and a measuring unit (255) configured to detect a microwave photon by measuring a non-zero voltage across the dispersive nonlinear element.

Description

BACKGROUND

[0001] The present invention relates to superconducting electronic units and in particular to a high-precision threshold detection of individual microwave photons using a non-destructive quantum photon detector.

[0002] In the optical frequency range, reliable single-photon detectors such as photomultipliers, kinetic microwave inductance detectors, and superconducting nanowire single-photon detectors are widely used in various experiments and applications. However, a disadvantage of these devices is that they destroy (i.e., absorb) the photons they detect. In contrast, reliable and practical single-photon detectors are currently being researched and developed in the microwave range, i.e., the gigahertz (GHz) range.

[0003] The publication "Quantum non-demolition measurement of a superconducting two-level system" refers to experiments in which two consecutive measurements are performed on a quantum mechanical two-level system, a superconducting flux qubit, by investigating the hysteresis behavior of a coupled nonlinear resonator. The strong correlation between the results of the two measurements demonstrates the quantum non-demolition (QND) nature of the readout method. The fact that a QND measurement is possible for superconducting qubits reinforces the view that these fabricated mesoscopic systems should be considered fundamental quantum objects. The results presented in the disclosure are also relevant for quantum information processing for protocols such as state preparation and error correction (LUPASCU, A. [et al.]: Quantum non-demolition measurement of a superconducting two-level system. In: Nature physics, Vol.3, 2007, No. 2, S. 119-123. ISSN 1745-2473).

[0004] The publication “A Balanced Radio Frequency Amplifier Based on a Niobium de SQUID with Microstrip Input Coupling” describes a high-frequency amplifier that uses a DC SQUID in a novel configuration, where the SQUID's input coil is used as a microstrip resonator. By using a coil of suitable length, resonant frequencies from 200 MHz to 1.3 GHz and gains of approximately 20 dB were achieved. At a frequency of 438 MHz and a bath temperature of 0.5 K, a minimum noise temperature of 0.12 ± 0.1 K was measured. By loading the otherwise open end of the microstrip resonator with a varactor, the frequency of maximum gain of such an amplifier could be reduced by about 40%. The use of two SQUID amplifiers in a balanced configuration significantly improves the input impedance matching (MÜCK, Michael [et al.]: A balanced radio frequency amplifier based on a niobium de SQUID with microstrip input coupling. In: Advances in superconductivity XII, proceedings of the 12th international symposium on superconductivity (ISS '99), October 17-19, 1999, Morioka. Springer, 1999. S. 1021-1026).

[0005] US Patent 5,493,719 A describes a radio frequency receiver that detects and converts radio frequency signals in the 50 to 1,000 GHz range using a receiver consisting of a lens and planar antenna, a preamplifier, a mixer, a local oscillator, and an IF amplifier. The insulating dielectric lens is used to focus terahertz radio frequency signals onto the thin-film antenna. The preamplifier amplifies these weak signals so they can be down-converted to an intermediate frequency by the mixer and local oscillator. The mixer is a dual-port device that provides isolation between the local oscillator and the input signal to prevent saturation of the preamplifier. The IF amplifier increases the amplitude of the down-converted IF signal produced by the mixer. SUMMARY

[0006] The invention relates to a microwave detection unit, a method for forming a microwave detection unit, a method for detecting a microwave photon, and a method for detecting the absence of a microwave photon, the features of which are specified in the corresponding patent claims. Embodiments of the invention are specified in the dependent claims.

[0007] A microwave detection unit is provided. The microwave detection unit includes a non-destructive microwave quantum photon detector, a quadrature microwave hybrid coupler connected to the non-destructive microwave quantum photon detector, and a dispersive nonlinear element coupled to the quadrature microwave hybrid coupler.

[0008] A method for constructing a microwave detection unit is provided. The method includes providing a non-destructive quantum microwave photon detector, providing a quadrature microwave hybrid coupler connected to the non-destructive microwave quantum photon detector, and providing a dispersive nonlinear element coupled to the quadrature microwave hybrid coupler.

[0009] Furthermore, a method for detecting a microwave photon is provided. This method involves receiving a reflected microwave signal from a non-destructive quantum microwave unit using a quadrature microwave hybrid coupler. The method also includes detecting the presence of the microwave photon based on a dispersive nonlinear element in a voltage state. The dispersive nonlinear element is connected to the quadrature microwave hybrid coupler.

[0010] Furthermore, a method for detecting the absence of a microwave photon is provided. This method involves receiving a reflected microwave signal from a non-destructive quantum microwave unit using a quadrature microwave hybrid coupler. The method also includes detecting the absence of the microwave photon based on a dispersive nonlinear element in a zero-voltage state. The dispersive nonlinear element is connected to the quadrature microwave hybrid coupler.

[0011] In embodiments, the quadrature microwave hybrid coupler has four terminals, one of which is connected to the non-destructive quantum microwave photon detector and another of which is connected to the non-linear dispersive element. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1A is a schematic representation of a microwave unit according to one or more embodiments. Fig. Figure 1B is a schematic representation of the pump equivalent circuit of the microwave unit as seen (or influenced by) the pump connection according to one or more embodiments. Fig. Figure 1C is a schematic representation of the signal equivalent circuit of the microwave unit as seen from the quantum signal port according to one or more embodiments. Fig. Figure 2 is a schematic representation of a system according to one or more embodiments. Fig. Figure 3 is a schematic representation of a system that shows its operation when there is no input signal of photons according to one or more embodiments. Fig. Figure 4 is a schematic representation of the system, showing its operation when an input signal of photons is present according to one or more embodiments. Fig. Figure 5 is a schematic representation of the system, showing the detection of a single microwave photon in situ according to one or more embodiments. Fig. Figure 6 is a schematic representation of the system, showing the detection of a single microwave photon in situ according to one or more embodiments. Fig. Figure 7 is a schematic representation of the system according to one or more embodiments in which no input signal photon is present. Fig. Figure 8 is a graph showing the effect of the non-destructive photon detector of Fig. 7 characterizes one or more embodiments. Fig. Figure 9 is a schematic representation of the system, showing the reception of an input signal photon according to one or more embodiments. Fig. Figure 10 is a graph showing the effect of the non-destructive photon detector of Fig. 9 characterizes one or more embodiments. Fig. Figure 11 is a schematic representation of the system, showing the use of wave interference in the quadrature microwave hybrid coupler according to one or more embodiments. Fig. Figure 12 is a schematic representation of the system, showing the use of wave interference in the quadrature microwave hybrid coupler according to one or more embodiments. Fig. 13 is a flowchart of a method for forming a microwave detection unit for threshold detection according to one or more embodiments. Fig. Figure 14 is a flowchart of a method for detecting a microwave photon according to one or more embodiments. Fig. 15 is a flowchart of a method for detecting the absence of a microwave photon according to one or more embodiments. DETAILED DESCRIPTION

[0012] Various embodiments are described here with reference to the accompanying drawings. Alternative embodiments may be developed without exceeding the scope of this document. It is noted that various connections and positional relationships (e.g., above, below, next to, etc.) between elements are specified in the following description and in the drawings. These connections and / or positional relationships may be direct or indirect unless otherwise specified and are not intended to be restrictive in this respect. Accordingly, a connection between entities may refer to a direct or an indirect connection, and a positional relationship between entities may be a direct or an indirect positional relationship. As an example of an indirect positional relationship, references to forming layer "A" above layer "B" include situations where one or more intermediate layers (e.g.,a layer “C”) lies between a layer “A” and a layer “B”, as long as the relevant properties and functionalities of layer “A” and layer “B” are not significantly changed by the intermediate layer(s).

[0013] A photon is an elementary particle that, along with all other forms of electromagnetic radiation, constitutes a quantum of light. A photon carries energy proportional to its radiation frequency and has a rest mass of zero. One reason why detecting individual microwave photons is challenging is that the energy of a single microwave photon is very low. The energy of a photon in the microwave range, for example in the range of 1 to 10 gigahertz (GHz), is at least 10 4 -times less than the energy of a photon of visible light.

[0014] Circuit quantum electrodynamics (cQED) is one of the leading architectures for realizing a quantum computer based on superconducting microwave circuits. It uses artificial atoms made of nonlinear superconducting units called qubits, which are dispersively coupled to microwave resonators, meaning the frequencies of the qubits and the resonators are detuned. For example, each superconducting qubit can contain one or more Josephson junctions, with capacitors connected in parallel across the junctions. The qubits are capacitively coupled to planar two-dimensional (2D) waveguide resonators or three-dimensional (3D) microwave cavities. The electromagnetic energy associated with the qubit is stored in the Josephson junctions and in the capacitive and inductive elements that comprise the qubit.Currently, a major focus is on improving the lifetime of qubits so that computations (i.e., manipulation and readout) can take place before the information is lost due to qubit decoherence.

[0015] Dispersive coupling of a superconducting qubit to a microwave resonator in a cQED architecture loads the resonator and makes its resonant frequency dependent on the quantum state of the qubit (i.e., the resonator's resonant frequency differs depending on whether the qubit is in the ground state or the excited state). This property enables non-destructive quantum measurement of the qubit state by sending a microwave signal with a few photons close to the resonator frequency to the cQED and measuring the amplitude and / or phase of the output microwave field, which carries information about the qubit state. A possible application of a functional and reliable single-photon detector in the microwave range is thus to measure this weak output signal in the dilution cooler (i.e.,qubit state detection) without requiring the use of high-gain, low-noise, and high-isolation output chains typically used to perform such measurements according to the state of the art.

[0016] One or more embodiments provide a non-destructive threshold detection scheme for detecting the absence or presence of photons. A photon detector / threshold detection system comprises a non-destructive single-microwave photon detector based on the cross-Kerr effect, a symmetrical 3 dB coupler (a 90-degree hybrid coupler), an isolator / attenuator, a matching network / network, and a DC-biased Josephson junction. The photon detector / threshold detection system is a microwave unit for detecting single microwave photons. Embodiments are configured to 1) detect individual photons within a specific bandwidth in the microwave range (i.e., in the gigahertz (GHz) range, e.g., 1 to 20 GHz), and 2) perform the photon detection non-destructively, i.e., without destroying (or absorbing) the detected photons.

[0017] Now to aspects of the present invention, Fig. Figure 1A is a schematic representation of a microwave detection unit 100 according to one or more embodiments. The microwave detection unit 100 includes a quarter-wavelength resonator 102 for pump control and a quarter-wavelength resonator 104 for the quantum signals. One end of the pump resonator 102 is connected to a coupling capacitor 106A, and the coupling capacitor 106A is connected to a pump feed / transmission line. The pump feed is connected to the pump terminal 111 and / or the pump terminal 111 is located on the pump feed. The pump feed receives a microwave pump signal 305 (i.e., a strong microwave tone) from a microwave generator or pump source. The other end of the pump resonator 102 is connected to a dispersive nonlinear element, e.g. the Josephson junction (JJ) 110, and to a half-wavelength stub line 120A at the pump frequency.The connection between the pump resonator 102, the JJ 110, and the spur line 120A can be referred to as node A. The spur line 120A is terminated opposite node A with an open circuit (OC).

[0018] In the microwave device 100, one end of the quarter-wavelength signal resonator 104 is connected to a coupling capacitor 106B, and the coupling capacitor 106B is connected to a signal feed / transmission line. The signal feed is connected to a signal terminal 113, and / or the signal terminal 113 is located on the signal feed. The signal feed is configured to receive a microwave quantum signal 405, i.e., a measured / verified microwave signal, from a quantum unit. The quantum unit can be a qubit, a cavity / resonator connected to a qubit, a photon source, a qubit resonator system, etc. The other end of the signal resonator 104 is connected to the JJ 110 and to a half-wavelength stub line 120B at the pump frequency. The connection between signal resonator 104, JJ 110 and spur line 120B can be called node B.The spur line 120B is terminated as a short circuit with respect to node B, since the ground acts like a short circuit with respect to the application of a microwave signal. The signal feed line can be connected to the quantum unit.

[0019] The pump resonator 102 has a basic mode, which can be referred to as the pump operating mode or the pump resonant operating mode. The pump mode of the pump resonator 102 has a resonant frequency, which is referred to as the pump resonant frequency f. P The pump mode of the pump resonator 102 has a wavelength λ. P , where λ P = c' / f P, and c' is the speed of light in the transmission line or waveguide used in the implementation of the pump resonator 102. The microwave pump signal 305 applied to the pump resonator 102 is a strong coherent resonant tone (i.e., its frequency matches the resonant frequency of the pump resonator 102). The pump resonator 102 is designed to have a length λ p / 4 corresponds to one quarter of the wavelength of the pump signal. The branch lines 120A and 120B are each designed to have a length that λ p / 2 corresponds to half the wavelength of the pump signal 305.

[0020] The signal resonator 104 has a basic mode, which can be called the signal mode or signal resonance mode. The signal mode of the signal resonator 104 has a resonant frequency, which is called the signal resonance frequency f. SThe quantum microwave signal 405, which is fed into the signal resonator, is a weak resonant tone with a few individual photons, whose frequency f S coincides with the resonant frequency of the signal mode. The signal mode of the signal resonator 104 has a wavelength λ. S , where λ S = c' / f S , and c' is the speed of light in the transmission line or waveguide used in the implementation of the unit. The signal resonator 104 is designed to have a length such that λ S / 4 corresponds to one quarter of the wavelength of the quantum signal 405.

[0021] In the microwave unit 100, there is a frequency condition between the (pump) resonance frequency of the pump resonator 102 and the (signal) resonance frequency of the signal resonator 104. The frequency condition consists of the fact that the pump resonance frequency f Pof the pump resonator 102 equal to twice the signal resonance frequency f S of signal resonator 104. In other words, the frequency condition is f P = 2·fs. Accordingly, the applied pump signal 305 has a frequency f P , which is twice the frequency f S of the quantum signal 405.

[0022] The microwave unit 100 (and / or the operation via the pump signal 305 and the quantum signal 405) is configured so that it can be operated by the effective Hamiltonian operator (without the control cables and supply lines) Heff=ℏω˜PNP+ℏω˜SNS+ℏKNP2+ℏK'NPNS can be described, where ħω̃ P N P represents the pump resonance mode term (modeled as a harmonic oscillator with ω̃). P as the trained resonant frequency of the pump resonant mode), ħω̃ S N S represents the signal resonance mode term (modeled as a harmonic oscillator with ω̃) S(as the trained resonant frequency of the signal resonance mode), ℏKNP2 represents the Kerr self-nonlinearity of the unit and ħK'N P N S represents the unit Kerr-cross nonlinearity. Furthermore, K is the Kerr self-constant (i.e., the Kerr frequency shift per photon) and K' is the Kerr-cross constant (i.e., the Kerr-cross frequency shift per photon). Additionally, N P the photon number operator of the pump mode (whose eigenvalue is the number of photons in the pump resonance mode), where NP=aP†aP, and N S is the photon number operator of the signal mode, (whose eigenvalue is the number of photons around the signal resonance mode), where NS=aS†aS, and ℏ=h2π, where h is Planck's constant. Furthermore, a P and a SQuantum operators (i.e., annihilation operators associated with the pump and signal resonance modes). It should be noted that the symbols N occasionally appear in this document. P , N S These terms can be used to represent the eigenvalues ​​of the numerical operators, and not the numerical operators themselves. It is also noted that any person skilled in the art can easily make this distinction from the context.

[0023] Fig. Figure 1B is a schematic representation of the pump equivalent circuit of the microwave unit 100 from the perspective of (or influenced by) the pump connection 111 according to one or more embodiments. In addition to showing what the pump connection 111 sees, it illustrates Fig. 1B simultaneously the circuit as it is determined by the incoming pump signal 305 at the pump resonance frequency f P is seen. Accordingly, the explanation regarding pump connection 111 applies to the incoming pump signal 305.

[0024] The pump equivalent circuit shows Fig. 1B is the pump supply line (with pump terminal 111), which is connected via the coupling capacitor 106A to the transmission line part of the pump resonator 102, and the other end of the transmission line part of the pump resonator is connected to ground via the Josephson junction 110. To explain this equivalent circuit, it is noted that 1) the stub line 120A, which acts as an impedance converter, is closed as an open circuit and its length corresponds to half the wavelength of the pump signal 305, so that node A sees an open circuit at the pump frequency, and 2) the stub line 120B, which acts as an impedance converter, is closed as a short circuit and its length corresponds to half the wavelength of the pump signal 305, thus node B sees a short circuit at the pump frequency.

[0025] An advantageous result of this pump equivalent circuit is that it shows that the pump resonant mode does not "see" (i.e., is not affected by) the signal resonator 104. In other words, the pump resonator 102 is isolated from the signal resonator 104. Another advantageous result is that the RF current I P , which belongs to the pump resonance mode, has an anti-node at the location of Josephson junction 110.

[0026] Fig. Figure 1C is a schematic representation of the signal equivalent circuit of the microwave unit 100, as seen by the quantum signal port 113 according to one or more embodiments. In addition to illustrating what the signal port 113 sees, Figure 1C shows Fig. 1C simultaneously the equivalent circuit as it is from the incoming quantum signal 405 at the signal resonance frequency f Sis seen. Accordingly, the explanation regarding signal connection 113 applies to the incoming quantum signal 405.

[0027] In the equivalent circuit of the microwave unit 100, as seen from the signal connection, it shows Fig. 1C the signal input line (with signal terminal 113), which is connected to the transmission line part of the signal resonator 104 via the coupling capacitor 106B, and the other end of the transmission line part of the signal resonator 104, which is connected to ground via the Josephson junction 110. Since the frequency condition for the pump frequency f P = 2 · f S is (the fundamental resonance mode of the pump resonator 102 corresponds to the pump frequency f P , while the fundamental resonance mode of the signal resonator 104 of the signal frequency f S (corresponds to), the signal connection 113 (quantum signal 405 with the signal resonance frequency f) S ) the counterpart of the pump connection 111.

[0028] In this case (i.e., in the case of the signal connection), the stub line 120B, which serves as an impedance converter, is terminated by a short circuit, and its length corresponds to a quarter of the wavelength of the quantum signal 405, so that node B forms an open circuit with the signal resonant frequency f S Similarly, the spur line 120A, which serves as an impedance converter, is terminated by an open circuit, and its length corresponds to a quarter of the wavelength of the signal, so that node A at the signal frequency f S a short circuit is seen.

[0029] An advantageous result of this signal equivalent circuit is that it shows that the signal resonance mode does not see the pump resonator 102. In other words, the signal resonator 104 is isolated from the pump resonator 102. Another advantageous result is that the RF current I S, which belongs to the signal resonance mode, has an anti-node at the location of Josephson junction 110.

[0030] At this point, it is necessary to consider the following: Fig. 1 to 3 of the clarification that 1) the pump resonator 102 (without considering the coupling capacitor and the supply line) contains the quarter-wavelength transmission line which at the pump frequency f P 1) is connected to ground via the Josephson junction 110, and 2) the signal resonator 104 (without considering the coupling capacitor and the feed line) contains the quarter-wavelength transmission line, which at the signal frequency f S is connected to mass via the Josephson junction 110.

[0031] The microwave unit 100 is configured to couple two microwave resonance modes (i.e., the pump resonance mode and the signal resonance mode) to a common dispersive nonlinear element, i.e., the Josephson junction 110. The microwave unit 100 is configured to operate in one mode, i.e., the pump mode, at the pump resonance frequency f. P , used as a photon number detector for the photons present in the second mode, i.e., the quantum signal mode with the signal resonance frequency fs. In the microwave unit 100, the signal resonance frequency f corresponds to S of the signal mode of the microwave frequency of the microwave photons to be detected and / or counted.

[0032] By driving the pump mode (of the pump resonator 102) using a strong coherent microwave tone (i.e., the pump signal 305) with the pump resonant frequency f PThe microwave unit 100 is configured to induce a nonlinear cross-Kerr effect in the Josephson junction 110, leading to a nonlinear interaction between the pump and signal modes (and consequently between the pump signal 305 with the pump resonance frequency f). P and the quantum signal 405 at the signal resonance frequency f S ). As a result of this cross-Kerr effect, the microwave unit 100 is configured such that the pump resonance frequency f P of the pump mode of the number of photons in signal resonance mode with frequency f S and vice versa.

[0033] The microwave unit 100 is configured such that by monitoring the phase of the reflected pump signal 305' at the frequency f Pa measurement / analysis unit (not shown) can detect the presence or absence of signal photons in signal mode during a non-destructive quantum measurement (i.e., the presence or absence of signal photons in the quantum signal 405 with frequency f S (based on the magnitude of the phase shift in the reflected pump signal 305'). Therefore, the microwave unit 100 can serve as a non-destructive microwave photon detector and counter. By introducing a frequency shift at the resonant frequency of the pump mode, the microwave unit 100 does not absorb or destroy the signal photons in the quantum signal 405. Instead, the quantum signal 405' is reflected by the microwave unit 100 at the signal feed line after interacting with the pump signal 305 in the unit 100 via the Josephson junction 110.

[0034] It is noted that, in addition to the pump and signal modes measured in reflection mode and explained in detail above, the Microwave Unit 100 also has two common resonance modes that can be measured between the pump and signal terminals in transmission mode. However, these common resonance modes do not play a role in the signal-pump interaction described above and have frequencies significantly detuned from the pump and signal resonance modes (and can therefore be filtered out if necessary). For example, for a unit with a pump resonance frequency of approximately 16 GHz and a signal resonance frequency of approximately 8 GHz, the unit's common modes are expected to oscillate at approximately 3 GHz and 13 GHz, respectively.

[0035] Two useful advantages of the microwave unit 100, which can be readily deduced from the unit's description, are: 1) The strong pump drive (i.e., the pump signal 305), which enables the detection of the signal photons, is injected via a connection different from the one used to detect the weak signal (e.g., the quantum signal 405); and 2) the pump and signal modes are completely isolated from each other (due to the use of the spur lines 120A and 120B). They interact only via the JJ 110, which connects their respective resonators 102 and 104. Therefore, by design, there should be no direct current leakage between the pump and signal connections 111 and 113.

[0036] To provide an overview of a non-destructive threshold detection scheme for detecting the absence or presence of photons, a general view of the unit's circuitry is explained (as in Fig. 2 shown). The photon detector / threshold detection system includes a non-destructive single-microwave photon detector based on the cross-Kerr effect, a symmetrical 3 dB coupler (a 90-degree hybrid coupler), an isolator / damping element, a matching circuit / network, and a DC-biased Josephson junction.

[0037] The main requirements for a non-destructive microwave photon detector are: 1) a strong cross-Kerr effect at the single-photon level, 2) spatial and spectral separation between signal and pump modes, 3) sufficient isolation between pump and signal terminals. An example of such a unit is the non-destructive single-microwave photon detector 100 in the Fig. 1A, Fig. 1B and Fig. 1C. The scheme relies on wave interference between pump signals reflected at the hybrid coupler inputs (i.e., the pump terminal of the non-destructive microwave photon detector 100 and the second output of the hybrid coupler, which is terminated by an open circuit) to generate a large microwave signal at one output of the hybrid coupler by constructive interference, depending on the presence or absence of signal photons. When no input signal photon is present, the pump resonant frequency coincides with the pump drive frequency. Consequently, the pump signal reflected from the pump terminal of the non-destructive photon detector 100 interferes destructively with the pump signal reflected from the open side of the hybrid coupler at the hybrid input connected to the JJ, thus leaving the JJ in a zero-voltage state.However, when a signal photon is present (entering the signal port of the non-destructive photon detector 100), the pump resonant frequency is shifted by a larger value than the bandwidth. This causes the phase of the pump drive signal reflected from the pump port to undergo a phase shift of ±180 degrees. This, in turn, generates a large reflected pump signal through constructive interference at the hybrid input connected to the JJ, thereby driving the JJ into the voltage state (which can be reliably measured and detected). The shift by a larger value than the bandwidth refers to the bandwidth of the pump resonator at the unit's operating point.

[0038] Now for a more detailed view, Fig. Figure 2 is a schematic representation of a system 200 according to one or more embodiments. Fig. 2 contains a photon detector / threshold detection system 205, which is delimited by the dashed lines. The photon detector / threshold detection system 205 is a microwave unit configured to provide high-accuracy threshold detection of individual microwave photons using the non-destructive microwave photon detector 100. The photon detector / threshold detection system 205 is located in a cryogenic unit, such as a solution chiller. The entire system 200 can also be located in the cryogenic unit.

[0039] The effective Hamiltonian of the non-destructive microwave photon detector 100, which is based on cross-Kerr nonlinearity, is given by H off = ℏ(ω̃ P +KN P + K'N S )N P + ℏω̃ S N SGiven. Although the details of the non-destructive quantum microwave photon detector 100 are omitted for brevity in Fig. Since paragraph 2 has been omitted, it is clear that the non-destructive quantum microwave photon detector 100 contains the details described herein. In one implementation, the non-destructive quantum microwave photon detector 100 can be replaced by another non-destructive microwave photon detector configured to operate as described herein, as understood by a person skilled in the art.

[0040] The photon detector for threshold detection 205 also includes a quadrature microwave hybrid coupler 210, which is connected to the non-destructive quantum microwave photon detector 100 and a matching circuit / network 220. In one implementation, an isolator 215A can optionally be inserted between the quadrature microwave hybrid coupler 210 and the matching network 220. In another implementation, the isolator 215A can be replaced by a resistive attenuator to prevent multiple reflections. Furthermore, the system / unit 205 can be implemented on a single chip, particularly if the isolator 215A is replaced by an attenuator on the chip. In another implementation, there is no insulator 215A, and in this case, the photon detector for threshold detection 205 can rely on deriving the large microwave signal in the JJ 250 to prevent multiple reflections.Furthermore, in the photon detector for threshold detection 205, the matching network 220 is connected to a JJ 250. The matching network 220 can be an impedance converter configured to match the impedance of the insulator 215A and / or the quadrature microwave hybrid coupler 210 to the impedance of the JJ 250. A measuring unit 255 is connected in parallel to the JJ 250 to measure a voltage drop across the JJ 250. In one implementation, the measuring unit 255 can be part of or integrated into the photon detector for threshold detection 205. In another implementation, the measuring unit 255 is separate from the photon detector for threshold detection 205.

[0041] System 200 includes a photon source 235 connected to another insulator 215B, and the insulator 215B is connected to the non-destructive quantum microwave photon detector 100 of the threshold detection photon detector 205. Additionally, System 205 includes a microwave pump signal 230 connected to an insulator 215C, and the insulator 215C is connected to a terminal of the quadrature microwave hybrid coupler 210. The quadrature microwave hybrid coupler 210 is a four-terminal unit. For explanatory purposes, and not as a limitation, the four terminals are designated as terminals 1, 2, 3, and 4.

[0042] The 215B and 215C isolators are optional. In one implementation, the 215B and 215C isolators can be replaced by circulators.

[0043] System 200 can include a controller 280. The controller 280 is connected via a feedback loop to the measuring unit 255 and the photon source (qubit resonator) 235. The controller 280 can be an electronic circuit that replaces and / or integrates the measuring unit 255, such that the controller 280 is configured to provide a voltage measurement. The controller 280 can include a processing unit, memory, and computer-executable instructions in memory. If the photon source 235 is a qubit resonator system, the controller 280 can control microwave sources and / or be integrated with them to cause microwave signals to be sent to the qubit resonator based on the voltage measurement of the JJ 250.

[0044] Fig. Figure 3 is a schematic representation of a system 200, illustrating its operation when there is no input signal of photons according to one or more embodiments. Fig. Figure 3 illustrates in particular a scenario in which the non-destructive photon detector 100 is biased to resonance by the microwave pump signal 305 and no quantum signal 405 is input into the non-destructive photon detector 100.

[0045] In Fig. 3 the microwave pump signal 305 with the pump frequency f PThe signal from pump source 230 is transmitted via insulator 215C to terminal 2 of quadrature microwave hybrid coupler 210. Quadrature microwave hybrid coupler 210 is a 90-degree hybrid coupler. Accordingly, half of the microwave pump signal 305 is split at quadrature microwave hybrid coupler 210 and output via terminals 1 and 3. Terminal 3 of quadrature microwave hybrid coupler 210 is connected to an open circuit and half of the microwave pump signal. Signal 305 is transmitted from quadrature microwave hybrid coupler 210 to the open circuit. The microwave pump signal 305 is then reflected back as a pump signal 305' from the open circuit to terminal 3 of the quadrature microwave hybrid coupler 210.

[0046] With respect to terminal 1, half of the microwave pump signal 305 is transmitted from the quadrature microwave hybrid coupler 210 to the pump terminal 111 on the non-destructive quantum microwave photon detector 100. The non-destructive quantum microwave photon detector 100 is configured to reflect half of the microwave pump signal 305 back as half of the reflected microwave pump signal 305'.

[0047] If in Fig. 3 no quantum (microwave) signal 405 (i.e. no photons, therefore N S When a signal of 0 is applied to the signal terminal 113 of the non-destructive quantum microwave photon detector 100, the pump resonator 102 remains in resonance, which means that the pump resonance frequency f P is and the pump signal 305 with the frequency f P was transmitted. If no quantum (microwave) signal 405 with the signal frequency f SThe effective Hamiltonian operator of the non-destructive quantum microwave photon detector 100, which is based on cross-Kerr nonlinearity, is transferred from the photon source 235 to the non-destructive quantum microwave photon detector 100 by H eff = ℏ(ω̃ P +KN P )N P given, since the term N S zero.

[0048] The quadrature microwave hybrid coupler 210 is configured to receive the microwave pump signal 305' transmitted by the non-destructive quantum microwave photon detector 100 at terminal 1, and the microwave pump signal 305' transmitted by the open circuit at terminal 3. Since the pump signal 305 remains in resonance with the pump resonator 102 (i.e., the frequency of the pump signal 305 is the same as the pump resonant frequency f), PFrom the pump resonator 102, a strong reflected pump signal 305' (indicated by the large arrow) is transmitted back to the pump source 230 via terminal 2 of the quadrature microwave hybrid coupler 210. However, no reflected pump signal is transmitted to the isolator 215A via terminal 4 of the quadrature microwave hybrid coupler 210. It should be noted that even if a negligible amount of the reflected pump signal is output from terminal 4 of the quadrature microwave hybrid coupler 210 to the isolator 215A, no additional energy (or an insignificant amount of energy) is transferred to the JJ 250. A negligible amount of reflected pump signal or power that could reach the JJ 250 could be less than 0.1 femtowatt (fW), or a small amount of power. This is an example of low power / low current that does not cause a voltage drop.Since no reflected pump signal (or no negligible amount) is transmitted from terminal 4 of the microwave hybrid coupler 210 to the JJ 250, the JJ 250 remains in the supercurrent state, also known as the zero-voltage state. A person skilled in the art understands that if the current flowing in the JJ 250 is less than the critical current I. C The voltage drop across the JJ is zero. In one implementation, the JJ 250 can be powered by a DC current I. BIAS , for example, biased by a low-noise DC source. During biasing, the DC current I BIAS smaller than the overcurrent / critical current I C .

[0049] The measuring unit 255 is used to measure whether there is a voltage drop across the JJ 250. If the measuring unit 255 detects that the voltage drop is zero (V = 0), V = 0 indicates that no signal photons have entered the unit, and the JJ 250 is in the zero-voltage / overcurrent state as detected by the controller 280. If the measuring unit 255 detects that the voltage drop is not zero (V ≠ 0), V ≠ 0 indicates that the JJ 250 is in the voltage state (as detected by the controller 280) because an additional RF current (other than I) is present. BIAS ) flows through the JJ 250, as shown in Fig. Section 7 will be explained further.

[0050] Accordingly, the photon detector / threshold detection system 205 is configured to detect when the threshold has been met to cause a voltage drop across the JJ 250, where no voltage drop means that no microwave photons are present (i.e., no photons have been transferred from the photon source 235 or have entered the signal resonator 104) and where a voltage drop means that microwave photons are present (i.e., photons are being transferred from the photon source 235 via the quantum microwave signal 405 and are entering the signal resonator 104).

[0051] Fig. Figure 4 is a schematic representation of System 200, illustrating its operation when an input signal of photons is present according to one or more embodiments. In this case, it illustrates Fig. 4 a scenario in which the input signal photon (of the quantum signal 405) is the pump resonance frequency f P The pump resonator 102 in the non-destructive photon detector 100 is shifted, causing the pump control / pump signal 305 to go out of resonance (if the same pump signal 305 was previously in resonance). The shift of the pump resonance frequency f P The pump resonator 102 in the non-destructive photon detector 100 is activated by the quantum signal 405 with frequency f S caused by the input into the non-destructive photon detector 100. Due to the shift in the pump resonance frequency f PThe quantum signal 405, generated by the pump resonator 102, causes a phase shift in the reflected pump signal 305', which is transmitted from the non-destructive photon detector 100 to the quadrature microwave hybrid coupler 210. As a result, the quadrature microwave hybrid coupler 210 is configured to output a strong reflected pump signal 405 from terminal 4. Further explanation regarding the phase shift is given in the Fig. 9, Fig. 10 and Fig. 12. This strong reflected pump signal 305' is transmitted to the JJ 250 via the insulator 215A and the matching network 220. Due to the strong reflected pump signal 305 received by the JJ 250, the JJ 250 is shifted from the zero-voltage / overcurrent state to the voltage state, and the measuring unit 255 (and / or the controller 280) determines that the voltage drop is not zero (V ≠ 0), thus indicating that photons are present (i.e., one or more photons have been transmitted from the photon source 235).

[0052] The following is an example scenario to illustrate the difference between Fig. 4 (in which at least one microwave photon in the quantum signal 405 is transferred from the photon source 235 to the non-destructive photon detector 100) compared to Fig. 3 to illustrate (in which no quantum signal 405 is transmitted from the photon source 235 to the non-destructive photon detector 100). In Fig. 4 the microwave pump signal 305 with the pump frequency f PThe microwave pump signal 305 is transmitted from the pump source 230 via the insulator 215C to terminal 2 of the quadrature microwave hybrid coupler 210. Since the quadrature microwave hybrid coupler 210 is a 90-degree hybrid coupler, half of the microwave pump signal 305 is split and output via terminals 1 and 3. Terminal 3 of the quadrature microwave hybrid coupler 210 is connected to the open circuit, so the microwave pump signal 305 is transmitted to the open circuit and reflected back by the open circuit as a reflected pump signal 305'. With respect to terminal 1, the microwave pump signal 305 is transmitted from the quadrature microwave hybrid coupler 210 to the pump terminal 111 of the non-destructive quantum microwave photon detector 100. The non-destructive microwave photon detector 100 is configured to reflect the microwave pump signal 305 back as reflected microwave pump signal 305'.

[0053] When the quantum (microwave) signal 405 is applied to the signal terminal 113 of the non-destructive quantum microwave photon detector 100, indicating that photons are present (N S > 0), the pump resonator 102 has a shift in its fundamental resonant frequency, which means that the pump resonant frequency f P is shifted to a different value and the frequency of the pump signal 305 does not correspond to the pump resonance frequency f P of the pump resonator 102 matches (or no longer matches).

[0054] Since the quantum (microwave) signal 405 has the signal frequency f S from the photon source 235 to the non-destructive quantum microwave photon detector 100, the effective Hamiltonian operator of the non-destructive quantum microwave photon detector 100, which is based on the cross-Kerr nonlinearity, is replaced by H eff = ℏ(ω̃ P +KN P+ K'N S )N P +ℏω̃ S N S , specified, since N S greater than zero.

[0055] The quadrature microwave hybrid coupler 210 is configured to receive at terminal 1 the reflected microwave pump signal 305' transmitted by the non-destructive quantum microwave photon detector 100, and at terminal 3 the reflected microwave pump signal 305' transmitted by the open circuit. Since the frequency of the pump signal 305 does not correspond to, or no longer corresponds to, the pump resonant frequency f PWhen the pump resonator 102 is in resonance, a strong reflected pump signal 305' (indicated by the long arrow) is transmitted from terminal 4 of the quadrature microwave hybrid coupler 210, via the insulator 215A and the matching network 220, to the JJ 250. The strong reflected pump signal 305' received at the JJ 250 causes a relatively large amount of energy at the JJ 250, which puts the JJ 250 into the voltage state in which a voltage drop (i.e., a voltage value) is detected by the measuring unit 255. In one implementation, an example of the power or current that can be used to exceed the threshold for switching the JJ 250 into the voltage state could be 0.1 nW (nanowatt). In another implementation, an exemplary value of power or current as the threshold for switching the JJ 250 to the voltage state can be 10 pW (picowatts).

[0056] The switch from the zero-voltage / overcurrent state to the voltage state indicates the detection of a photon via the measuring unit 255. Therefore, the photon detector / threshold detection system 205 is configured to determine that microwave photons are present (i.e., photons are being transmitted from the photon source 235 via the quantum microwave signal 405) by detecting that the threshold has been reached to cause a voltage drop across the JJ 250.

[0057] In Fig. However, no reflected microwave pump signal 305' is transmitted from terminal 2 of the quadrature microwave hybrid coupler 210 back to the pump source 230. This is because the pump signal 305, which was previously transmitted to the pump resonator 102, was out of resonance.

[0058] Fig. Figure 5 is a schematic representation of the system 200, which depicts the detection of a single microwave photon in situ according to one or more embodiments. Fig. Figure 6 is a schematic representation of System 200, which depicts the in situ detection of a single microwave photon according to one or more embodiments. Fig. 5 and Fig. Six are partial views of System 200, illustrating the detection process. Fig. 5 is an example of the case "no photons present", where N S = 0, while Fig. 6 is an example of the case “signal photons are present”, where N S > 0. By measuring a peak in the voltage across the JJ 250 in Fig. 6 or the absence of a peak in Fig. 5. An operator (or a controller 280) can determine on-site (in a cryogenic facility, such as a solution cooler) the presence (voltage spike) or absence (no voltage spike) of input signal photons.

[0059] Fig. Figure 7 is a schematic representation of system 200, which, according to one or more embodiments, represents the case "no input signal photon". Fig. Figure 7 is merely a partial view of the system 200, illustrating the signals on the transmission lines connected to the non-destructive photon detector 100. Fig. Figure 8 is a graph 800, which shows the effect of the non-destructive photon detector 100 of Fig. 7 according to one or more embodiments. In this case, the non-destructive photon detector 100 is in Fig. 7 is biased to resonance, and there is no input signal photon to set the pump resonance frequency f. Pto shift. Accordingly, the microwave pump signal 305 is transmitted at a frequency corresponding to the pump resonant frequency f. P This corresponds to the fact that the reflected pump signal 305' (transmitted by the non-destructive photon detector 100) experiences no phase shift relative to the phase of the incident pump signal 305 (0° phase shift) under the condition that no quantum signal 405 is input into the non-destructive photon detector. This is also shown by the pump resonance curve 805 for the pump resonator 102 in graph 800 in Fig. 8 shown.

[0060] Fig. Figure 9 is a schematic representation of the system 200, which depicts the reception of an input signal photon via the quantum microwave signal 405 according to one or more embodiments. Fig. Figure 9 is merely a partial view of the system 200, illustrating the signals on the transmission lines connected to the non-destructive photon detector 100. Fig. 10 is a graph 1000, which shows the effect of the non-destructive photon detector 100 of Fig. 9 according to one or more embodiments. In this case, the non-destructive photon detector 100 is biased to resonance, but the presence of input signal photons (via the microwave signal 405) shifts the pump resonance frequency f. P of the pump resonator 102 around Δf p = K'N s / 2π, where Δf p the shift in the pump resonance frequency f P designated.

[0061] Accordingly, the microwave pump signal 305 is transmitted at a frequency that does not correspond to the pump resonance frequency f. PThis coincides, such that the pump signal 305 has a 0-degree phase and the reflected pump signal 305' (transmitted by the non-destructive photon detector 100) has a -180-degree phase under the condition that the quantum signal 405 is input into the non-destructive photon detector 100. This is also shown by the -180-degree shift in the phase of the reflected pump signal 305' between the original pump resonance curve 805 and the shifted resonance curve 905 of the pump resonator 102 in graph 1000 of Fig. Figure 10 shows the frequency of the pump signal. This phase shift of -180 degrees in the reflected pump signal 305' causes the JJ 250 to shift the voltage state (i.e., V ≠ 0).

[0062] Further details on how the phase of the reflected pump signal 305' is used in the photon detector / threshold detection system 205 are given in the Fig. 11 and Fig. 12 explained. Fig. Figure 11 is a schematic representation of the system 200, showing the use of the phase of the reflected pump signal in the quadrature microwave hybrid coupler 210 according to one or more embodiments. Fig. Figure 11 is merely a partial view of the system 200, illustrating the relationship of the phase shift at the signals 305, 305' with respect to the quadrature microwave hybrid coupler 210, and to further distinguish the signals 305, 305', signal designations 305_1, 305_1', 305_2, 305_2' are used for explanatory purposes. Fig. Figure 11 illustrates the case where the absence of input signal photons causes the strong pump drive to reflect the non-destructive quantum detector towards the pump source 230.

[0063] Now for the details of Fig. 11, a strong pump signal 305 with phase 0° and frequency f PThe signal is input at terminal 2 of the quadrature microwave hybrid coupler 210, and the broad arrow represents a strong signal. Since the quadrature microwave hybrid coupler 210 is a 90-degree hybrid coupler, it is configured to split the large pump signal 305 into two halves, so that the microwave pump signal 305_1 is output through terminal 1 at phase 0°, and the microwave pump signal 305_2 at phase 90° is output through terminal 3. Because terminal 3 of the quadrature microwave hybrid coupler 210 is connected to an open circuit, the microwave pump signal 305_2 at phase 90° (from the open circuit) is reflected back to terminal 3 as the reflected pump signal 305_2' at phase 270°.With respect to port 1, the microwave pump signal 305_1 with phase 0° is transmitted from terminal 1 of the quadrature microwave hybrid coupler 210 to the non-destructive quantum microwave photon detector 100 at pump terminal 111. Since the pump signal 305_1 has the frequency that corresponds to the pump resonant frequency f. P where the pump resonator 102 matches, the non-destructive microwave photon detector 100 is configured to reflect the microwave pump signal 305_1 with phase 0° to terminal 1 of the quadrature microwave hybrid coupler 210 as a reflected microwave pump signal 305_1' with phase 0°.

[0064] At this point, the quadrature microwave hybrid coupler 210 receives the reflected microwave pump signal 305_1' with phase 0° at terminal 1 and the reflected microwave pump signal 305_2' with phase 270° at terminal 3. Since the quadrature microwave hybrid coupler 210 is designed to split the incoming signal in half to increase the phase by 90° when outputting the incoming signal in the transverse direction, the quadrature microwave hybrid coupler 210 is configured to output one half of the reflected microwave pump signal 305_1' (which previously had phase 0° and was input at terminal 1) at terminal 4 with a phase increased by 90° (i.e., 0° + 90° = 90°) (due to the transverse direction), so that the reflected microwave pump signal 305_1' leaves terminal 4 with a phase of 90°.Additionally, the quadrature microwave hybrid coupler 210 is configured to output one half of the reflected microwave pump signal 305_1' (which previously had phase 0° and was input at terminal 1) with a phase change of 0° (i.e. 0° + 0°= 0°) at terminal 2 in a horizontal direction, so that the reflected microwave pump signal 305_1' with phase 0° leaves terminal 2.

[0065] With respect to the reflected pump signal 305_2' with phase 270° entering at terminal 3, the quadrature microwave hybrid coupler 210 is configured to output one half of the reflected microwave pump signal 305_2' (which previously had phase 270° and was entered at terminal 3) at terminal 2 with a phase increased by 90° (i.e., 270° + 90° = 360°) due to the transverse direction, so that the reflected microwave pump signal 305_2' with phase 360° leaves terminal 2 of the quadrature microwave hybrid coupler 210. Furthermore, the quadrature microwave hybrid coupler 210 is configured to receive one half of the reflected microwave pump signal 305_2' (which previously had a phase of 270° and was input at terminal 3) at terminal 4 with a phase change of 0° (i.e.0° + 270° = 270°) because of the horizontal direction, so that the reflected microwave pump signal 305_2' with phase 270° leaves terminal 4 of the quadrature microwave hybrid coupler 210.

[0066] On the transmission line towards pump source 230 are the microwave pump signal 305_1' with phase 0° and the reflected microwave pump signal 305_2' with phase 360°. Due to wave interference, the signal 305_1' with phase 0° and the signal 305_2' with phase 360° are added / combined to generate the strong reflected pump signal 1105 with phase 0°. The strong reflected pump signal 1105 is the same as the strong reflected pump signal 305' in Fig. 3.

[0067] However, on the transmission line towards the JJ 250, the reflected microwave pump signal 305_1' with a phase of 90° and the reflected microwave pump signal 305_2' with a phase of 270° are present. Due to wave interference, the signal 305_1' with a phase of 90° and the signal 305_2' with a phase of 270° are combined destructively to produce no signal. Accordingly, there is no shift in the JJ 250 from the zero-voltage state to the voltage state, and therefore the controller 280 determines that no microwave photon is detected. Fig. Figure 11 illustrates the operation of the photon detector / threshold detection system 205, as shown in the Fig. 2, Fig. 3, Fig. 5, Fig. 7 and Fig. 8 explained.

[0068] Fig. Figure 12 is a schematic representation of the system 200, showing the use of the phase of the reflected pump signal in the quadrature microwave hybrid coupler 210 according to one or more embodiments. Fig. Figure 12 is merely a partial view of the system 200, illustrating the relationship of the phase in the signals 305, 305', 405 with respect to the quadrature microwave hybrid coupler 210, and to further distinguish the signals 305, 305', the signal designations 305_1, 305_1', 305_2, 305_2' are used. Fig. Figure 12 illustrates the case where the presence of input signal photons causes the strong pumping action to reflect the non-destructive quantum detector towards the JJ 250, thereby causing a photon to be detected.

[0069] Now for the details in Fig. 12, a strong pump signal 305 with phase 0° and frequency f PThe signal is input to terminal 2 of the quadrature microwave hybrid coupler 210, and the broad arrow represents a strong signal. Since the quadrature microwave hybrid coupler 210 is a 90-degree hybrid coupler, it is configured to split the large pump signal 305 into two halves, so that the microwave pump signal 305_1 is output through terminal 1 at phase 0°, and the microwave pump signal 305_2 at phase 90° is output through terminal 3. Because terminal 3 of the quadrature microwave hybrid coupler 210 is open-circuited, the microwave pump signal 305_2 at phase 90° (from the open circuit) is reflected back to terminal 3 as the pump signal 305_2' at phase 270°. Up to this point, the explanation of Fig. 12 identical to Fig. 11. With respect to terminal 1, the microwave pump signal 305_1 with phase 0° is transmitted from terminal 1 of the quadrature microwave hybrid coupler 210 to the pump terminal 111 of the non-destructive quantum microwave photon detector 100. Simultaneously, the quantum microwave signal 405 is input to the signal terminal 113 of the non-destructive quantum microwave photon detector 100. The quantum microwave signal 405 shifts the pump resonance frequency f P Thus, the pump signal 305_1 no longer has a frequency that corresponds to the pump resonance frequency f. Pof the pump resonator 102, and due to this mismatch, the non-destructive quantum microwave photon detector 100 is configured to reflect the microwave pump signal 305_1 (which is transmitted with phase 0°) back to terminal 1 as a reflected microwave pump signal 305_1' with a phase of -180°. In this case, there is a phase shift of -180° in the reflected microwave pump signal 305_1' that is sent back to terminal 1 of the quadrature microwave hybrid coupler 210, as shown in the Fig. 9 and Fig. 10 explained.

[0070] At this point, the quadrature microwave hybrid coupler 210 receives the reflected microwave pump signal 305_1' with a phase of -180° at terminal 1 and the reflected pump signal 305_2' with a phase of 270° at terminal 3. Since the quadrature microwave hybrid coupler 210 is designed to split the incoming signal in half to increase the phase by 90° for the transverse direction and to output the incoming signal in both the transverse and horizontal directions, the quadrature microwave hybrid coupler 210 is configured to output half of the microwave pump signal 305_1' (which previously had a phase of -180° and was input at terminal 1) at terminal 4 with a phase increased by 90° (i.e., -180° + 90° = -90°), so that the reflected microwave pump signal 305_1' leaves the terminal 4 of the quadrature microwave hybrid coupler 210 with a phase of -90°.Furthermore, the quadrature microwave hybrid coupler 210 is configured to output one half of the reflected microwave pump signal 305_1' (which previously had the phase -180° and was input at terminal 1) with a phase change of 0° (i.e. -180° + 0° = -180°) at terminal 2, so that the reflected microwave pump signal 305_1' with the phase -180° leaves terminal 2.

[0071] With respect to the reflected pump signal 305_2' with phase 270° entering at terminal 3, the quadrature microwave hybrid coupler 210 is configured to output one half of the reflected microwave pump signal 305_2' (which previously had phase 270° and was entered at terminal 3) at terminal 2 with a phase increased by 90° (i.e. 270° + 90° = 360°), so that the reflected microwave pump signal 305_2' with phase 360° leaves terminal 2. Furthermore, the quadrature microwave hybrid coupler 210 is configured to output one half of the reflected microwave pump signal 305_2' (which previously had phase 270° and was input at terminal 3) at terminal 4 with a phase change of 0° (i.e., 0° + 270° = 270°), so that the reflected microwave pump signal 305_2' with phase 270° leaves terminal 4.

[0072] On the transmission line towards the pump source 230 are the reflected microwave pump signal 305_1' with phase -180° and the reflected microwave pump signal 305_2' with phase 360° and, due to destructive wave interference, the signal 305_1' with phase -180° and the signal 305_2' with phase 360°, which are added / combined to produce no signal.

[0073] However, on the transmission line towards the JJ 250 are the reflected microwave pump signal 305_1' with a phase of -90° and the reflected microwave pump signal 305_2' with a phase of 270°, and due to constructive wave interference, the signal 305_1' with a phase of -90° and the signal 305_2' with a phase of 270°, which are added / combined to generate a strong reflected microwave signal 1205 (which is the same as the strong reflected microwave signal 305' in the Fig. 4 and Fig. 6) Due to the strong reflected microwave signal 1205 received by the JJ 250 (e.g. 1 nW or more), there is a shift in the JJ 250 from the zero voltage state to the voltage state, and thus the controller 280 determines that a microwave photon is detected. Fig. Figure 12 illustrates the operation of the photon detector / threshold detection system 205 as shown in the Fig. 2, Fig. 4, Fig. 6, Fig. 9 and Fig. 10 explained.

[0074] The non-destructive quantum microwave photon detector 100 with the capacitors (except for the dielectric material in the capacitors), the transmission lines, the Josephson junctions 110, 250 (except for the thin insulating material), the resonators 102, 104, and the matching network 220 are made of superconducting material. Furthermore, the quadrature microwave 90-degree hybrid coupler 210 is made of ordinary low-loss metals or may be made of superconducting material. The qubit resonator system is also made of superconducting material. Examples of superconducting materials (at low temperatures, such as about 10 to 100 millikelvin (mK) or about 4 K) include niobium, aluminum, tantalum, etc.

[0075] The photon detector / threshold detection system 205 and / or the system 200 are configured to support scalability by reducing the number of output lines from a mixing cryostat containing a superconducting quantum processor. This scheme can also be easily extended to a large number of qubits. The photon detector / threshold detection system 205 and / or the system 200 add an extra layer of protection to the quantum system because, by omitting the output line, the system 200, 205 is configured to prevent thermal and electromagnetic noise (originating outside the cryostat or generated by active units) from propagating to the quantum system and affecting its coherence.

[0076] The photon detector / threshold detection system 205 and / or the system 200 enable in-situ measurement of the quantum state without leaving the cryostat. The photon detector / threshold detection system 205 and / or the system 200 close the feedback loop within the cryostat by integrating a decision-making mechanism (i.e., the controller 280) into the cryostat. The controller 280 makes decisions based on these measurements and applies feedback driver signals (e.g., via fast single-flux quantum (RSFQ) / field-programmable gate array (FPGA) circuits). Closing the feedback loop within the cryostat significantly reduces the electrical length of the loop, thus enabling faster feedback cycles.

[0077] Furthermore, in the system 200 the quantum system can be probed or measured relatively easily at room temperature by connecting an output line to the third terminal of a cryogenic circulator, which is inserted between the quantum system (e.g. photon source 235) and the non-destructive microwave photon detector 100 (on the signal side).

[0078] Fig. Figure 13 is a flowchart 1300 of a method for forming a microwave detection unit for threshold detection 205 according to one or more embodiments. In block 1305, a non-destructive quantum microwave photon detector 100 is provided. In block 1310, a quadrature microwave hybrid coupler 210 is connected to the non-destructive quantum microwave photon detector 100. A dispersive nonlinear element 250 is connected to the quadrature microwave hybrid coupler 210.

[0079] The dispersive nonlinear element 250 is configured to switch to a voltage state indicating the detection of a microwave photon. This voltage state corresponds to the presence of a voltage drop across the dispersive nonlinear element 250. The dispersive nonlinear element 250 is also configured to be in a zero-voltage state, indicating that no microwave photon is detected. This zero-voltage state corresponds to the absence of a voltage drop across the dispersive nonlinear element 250. The dispersive nonlinear element 250 is a Josephson junction. It is a superconducting direct current (DC) quantum interference device (SQUID).

[0080] An insulator 215A is connected between the quadrature microwave hybrid coupler 210 and the dispersive nonlinear element 250. The non-destructive quantum microwave photon detector 100 is configured to receive a microwave signal 405 and a pump signal 305 such that the dispersive nonlinear element 250 enters a voltage state, thereby detecting a microwave photon in the microwave signal 405. A measuring unit 255 is configured to detect a microwave photon by measuring a non-zero voltage across the dispersive nonlinear element 250.

[0081] Fig. Figure 14 is a flowchart 1400 of a method for detecting a microwave photon according to one or more embodiments. In block 1405, a quadrature microwave hybrid coupler 210 is configured to receive a reflected microwave pump signal 305' from a non-destructive quantum microwave unit 100 (e.g., at terminal 1). In block 1410, a measuring unit 255 and / or a controller 280 are configured to determine the presence of the microwave photon, which is in a voltage state, based on a dispersive nonlinear element 250, the dispersive nonlinear element 250 being connected to the quadrature microwave hybrid coupler 210.

[0082] The microwave photon was introduced into the non-destructive quantum microwave unit 100 in a quantum microwave signal 405. The quadrature microwave hybrid coupler 210 outputs a portion of the reflected microwave signal 305' to the dispersive nonlinear element 250.

[0083] Fig.Figure 15 is a flowchart 1500 of a method for detecting the absence of a microwave photon according to one or more embodiments. In block 1505, a quadrature microwave hybrid coupler 210 is configured to receive a reflected microwave pump signal 305' from a non-destructive quantum microwave unit 100. In block 1510, a measuring unit 255 and / or a controller 280 are configured to detect the absence of the microwave photon, based on a dispersive nonlinear element 250, which is in a zero-voltage state and is connected to the quadrature microwave hybrid coupler 210.

[0084] The dispersive nonlinear element 250, which is in the zero-voltage state, indicates that no microwave photon from a quantum source 235 is input into the non-destructive quantum microwave unit 100.

[0085] Among the technical advantages is a non-destructive photon detector / threshold detection system. Another advantage is that the output voltage signal, indicating the presence or absence of signal photons, can be measured in situ within a solution cooler using single-flux fast (RSFQ) or semiconductor-based electronics. Therefore, it is not necessary to measure the quantum signal output signal outside the cryostat using equipment at room temperature. Furthermore, such voltage measurements would allow feedback signals to be applied back to the quantum signal source without leaving the cryostat. In other words, the quantum feedback loop can be closed within the cryostat.This capability reduces the number of output lines and the amount of hardware, such as quantum-limited amplifiers, circulators, high-electron mobility transistors (HEMTs), and coaxial cables, required to implement a scalable superconducting quantum processor. Reducing the number of output lines and the amount of hardware also shortens the quantum loop duration (i.e., accelerating the feedback rate or allowing more time for computation and decision-making) and eliminates potential sources of noise propagating along the output lines or generated by the active components (i.e., amplifiers) within them. The quantum signal emanating from the quantum system and reflected at the signal port of the non-destructive photon detector can be amplified using quantum-limited amplifiers and measured using standard room-temperature equipment.This ability can be helpful for testing or monitoring purposes.

[0086] The term "approximately" and variations thereof are intended to include the degree of error in measuring the specified quantity based on the equipment available at the time the application was filed. For example, "approximately" may include a range of ± 8%, 5%, or 2% of a given quantity.

[0087] Aspects of the present invention are described herein with reference to flowcharts and / or block diagrams or charts of methods, devices (systems), and computer program products according to embodiments of the invention. It is pointed out that each block of the flowcharts and / or block diagrams or charts, as well as combinations of blocks in the flowcharts and / or block diagrams or charts, can be executed by means of computer-readable program instructions.

[0088] The flowcharts and block diagrams or charts in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this context, each block in the flowcharts or block diagrams or charts can represent a module, segment, or part of instructions that includes one or more executable instructions for performing the specific logical function(s). In some alternative embodiments, the functions specified in the block may occur in a different order than shown in the figures. For example, two blocks shown consecutively may in reality be executed essentially simultaneously, or the blocks may sometimes be executed in reverse order depending on the corresponding functionality.It should also be noted that each block of the block diagrams or charts and / or flowcharts, as well as combinations of blocks in the block diagrams or charts and / or flowcharts, can be implemented by special hardware-based systems that perform the specified functions or steps, or execute combinations of special hardware and computer instructions.

[0089] The descriptions of the various embodiments of the present invention are provided for illustrative purposes; however, they are not intended to be exhaustive or limiting for the embodiments described herein. Many modifications and variations will be obvious to a person skilled in the art without deviating from the scope and inventive concept of the described embodiments. The terminology used here has been chosen to best explain the basic ideas of the embodiments, their practical application, or technical improvements over technologies available on the market, or to enable other persons skilled in the art to understand the embodiments described herein.

Claims

[1] Microwave detection unit (205) comprising: a non-destructive quantum microwave photon detector (100); a quadrature microwave hybrid coupler (210) connected to the non-destructive quantum microwave photon detector; a dispersive nonlinear element connected to the quadrature microwave hybrid coupler; and a measuring unit (255) configured to detect a microwave photon by measuring a non-zero voltage across the dispersive nonlinear element. [2] Microwave detection unit according to claim 1, wherein the dispersive nonlinear element is configured to switch to a voltage state that indicates the detection of a microwave photon. [3] Microwave detection unit according to claim 2, wherein the voltage state means that a voltage drop occurs across the dispersive nonlinear element. [4] Microwave detection unit according to claim 1, wherein the dispersive nonlinear element is configured in a zero-voltage state which indicates no detection of a microwave photon. [5] Microwave detection unit according to claim 4, wherein the zero voltage state means that there is no voltage drop across the dispersive nonlinear element. [6] Microwave detection unit according to claim 1, wherein the dispersive nonlinear element is a Josephson junction (110, 250). [7] Microwave detection unit according to claim 1, wherein the dispersive nonlinear element is a superconducting DC quantum interference unit. [8] Microwave detection unit according to claim 1, wherein an insulator is connected between the quadrature microwave hybrid coupler and the nonlinear dispersive element. [9] Microwave detection unit according to claim 7, wherein the non-destructive quantum microwave photon detector receives a microwave signal and a pump signal, such that the dispersive nonlinear element switches into a voltage state, thereby detecting a microwave photon in the microwave signal. [10] Method for forming a microwave detection unit (205), wherein the method comprises: Providing (1305) a non-destructive quantum microwave photon detector (100); Providing (1310) a quadrature microwave hybrid coupler (210) connected to the non-destructive quantum microwave photon detector; and Providing (1315) a dispersive nonlinear element connected to the quadrature microwave hybrid coupler, wherein a measuring unit (255) is configured to detect a microwave photon by measuring a non-zero voltage across the dispersive nonlinear element. [11] Method according to claim 10, wherein the dispersive nonlinear element is configured to switch to a voltage state that indicates the detection of a microwave photon. [12] Method according to claim 11, wherein the stress state means that a stress drop occurs across the dispersive nonlinear element. [13] Method according to claim 10, wherein the dispersive nonlinear element is configured such that a zero-voltage state indicates that no microwave photon is detected. [14] Method according to claim 13, wherein the zero stress state means that there is no stress drop across the dispersive nonlinear element. [15] Method according to claim 10, wherein the dispersive nonlinear element is a Josephson transition (110; 250). [16] Method according to claim 10, wherein the dispersive nonlinear element is a superconducting direct current quantum interference unit. [17] Method according to claim 10, wherein an insulator is connected between the quadrature microwave hybrid coupler and the dispersive nonlinear element. [18] Method according to claim 16, wherein the non-destructive quantum microwave photon detector is configured to receive a microwave signal and a pump signal, such that the non-linear dispersive element switches into a voltage state, thereby detecting a microwave photon in the microwave signal. [19] Method for detecting a microwave photon using the microwave detection unit (205) according to any one of claims 1 to 9, wherein the method comprises: Receiving (1405) a reflected microwave signal from the non-destructive quantum microwave unit (100) by the quadrature microwave hybrid coupler (210); and Determining (1410) the presence of the microwave photon based on the dispersive nonlinear element being in a state of stress, wherein the dispersive nonlinear element is connected to the quadrature microwave hybrid coupler. [20] Method according to claim 19, wherein the microwave photon was introduced into a quantum microwave signal in the non-destructive quantum microwave unit. [21] Method according to claim 19, wherein the quadrature microwave hybrid coupler outputs a portion of the reflected microwave signal to the dispersive nonlinear element. [22] Method for detecting the absence of a microwave photon using the microwave detection unit (205) according to any one of claims 1 to 9, wherein the method comprises: Receiving (1505) a reflected microwave signal from the non-destructive quantum microwave unit (100) by the quadrature microwave hybrid coupler (210); and Determining (1510) the absence of the microwave photon based on the dispersive nonlinear element being in a zero-voltage state, wherein the dispersive nonlinear element is connected to the quadrature microwave hybrid coupler. [23] Method according to claim 22, wherein the dispersive nonlinear element, which is in the zero-voltage state, indicates that no microwave photon is being input from a quantum source into the non-destructive quantum microwave unit.

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