CN110277969B - Quantum parametric amplifier - Google Patents

Abstract

The invention discloses a quantum parametric amplifier, which comprises a first capacitance module, a transmission type microwave resonant cavity, a second capacitance module and a superconducting quantum interference device with adjustable inductance, wherein the first capacitance module, the transmission type microwave resonant cavity and the second capacitance module are used for forming an oscillation amplifying circuit; the oscillation amplifying circuit amplifies a signal to be amplified under the action of a pumping signal and generates a plurality of idle frequency signals, and the oscillation amplifying circuit further comprises a voltage modulation circuit, a transmission type microwave resonant cavity and a transmission type microwave resonant cavity, wherein the voltage modulation circuit is arranged at one end of the superconducting quantum interference device, which is close to the transmission type microwave resonant cavity; the superconducting quantum interference device can release at least one idler frequency signal generated in the oscillation amplifying circuit under the action of the bias voltage provided by the voltage modulating circuit, and the frequency of the pumping signal of the quantum parametric amplifier in the optimal working mode does not need to be selected as the frequency multiplication of the frequency of the signal to be amplified.

Description

A quantum parametric amplifier

Technical Field

The invention belongs to the field of signal amplifiers, in particular to a quantum parameter amplifier.

Background technique

In the field of quantum computing, in order to obtain the calculation results of quantum chips, we need to collect and analyze the signals output by quantum chips, namely the quantum bit reading signals. Usually, the quantum bit reading signals are very weak, and it is generally necessary to add multiple amplifiers to the output line of the quantum bit reading signals to increase the signal strength. Usually, the front-stage amplifier uses a quantum parametric amplifier. When the quantum parametric amplifier is working, the incidental noise is as low as close to the quantum limit, which is the origin of its name.

Existing quantum parametric amplifiers work based on the principle of nonlinear mixing. In order to effectively amplify the quantum bit reading signal and make the quantum parametric amplifier work in the optimal mode, it is necessary to additionally apply a pump signal with a frequency close to the frequency of the signal to be amplified or its multiple. For example, the applied pump signal close to the signal to be amplified corresponds to the four-wave mixing working mode, and the applied pump signal close to twice the frequency of the signal to be amplified corresponds to the three-wave mixing working mode.

The current problem is that in the optimal working mode of the existing quantum parametric amplifier, that is, the frequency of the pump signal must be selected as a multiple of the frequency of the signal to be amplified, there are irrelevant signals in the output signal whose frequencies are extremely close to the frequency of the signal to be amplified. These irrelevant signals are difficult to eliminate through filters because their frequencies are too close to the signals to be amplified. They will interfere with the demodulation process of the quantum bit reading signal, thereby greatly reducing the demodulation fidelity and demodulation efficiency of the quantum chip calculation results.

Summary of the invention

The object of the present invention is to provide a quantum parametric amplifier to solve the deficiencies in the prior art, so that the frequency of the pump signal of the quantum parametric amplifier in the optimal working mode does not need to be selected as a multiple of the frequency of the signal to be amplified.

The technical solution adopted by the present invention is as follows:

A quantum parametric amplifier, comprising a first capacitor module, a transmission microwave resonant cavity, a second capacitor module and a superconducting quantum interference device with adjustable inductance for constituting an oscillation amplifier circuit; the first capacitor module, the transmission microwave resonant cavity and the second capacitor module are connected in sequence, one end of the superconducting quantum interference device with adjustable inductance is connected to the middle of the transmission microwave resonant cavity and the other end is grounded; and the resonant frequency of the transmission microwave resonant cavity can be made equal to the frequency of a signal to be amplified by adjusting the inductance of the superconducting quantum interference device with adjustable inductance, wherein: the signal to be amplified is coupled into the oscillation amplifier circuit from the first capacitor module, and the oscillation amplifier circuit amplifies the signal to be amplified under the action of a pump signal and generates a plurality of idle frequency signals;

The quantum parametric amplifier also includes a voltage modulation circuit;

The voltage modulation circuit is arranged at one end of the superconducting quantum interference device of the adjustable inductance close to the transmission type microwave resonant cavity;

The superconducting quantum interference device with adjustable inductance can release at least one idle frequency signal generated in the oscillation amplification circuit under the action of the bias voltage provided by the voltage modulation circuit.

Furthermore, the superconducting quantum interference device with adjustable inductance includes a superconducting quantum interference device and a magnetic flux modulation circuit connected by mutual inductance coupling;

The superconducting quantum interference device is a closed-loop device consisting of a number of Josephson junctions connected in parallel;

The magnetic flux modulation circuit is used to adjust the inductance of the superconducting quantum interference device by adjusting the magnetic flux of the closed-loop device.

Furthermore, the superconducting quantum interference device is a closed-loop device consisting of two Josephson junctions connected in parallel.

Further, the flux modulation circuit includes a flux modulation line and a current device for generating a bias current connected in sequence;

Wherein: the magnetic flux modulation line is used to transmit the bias current and make the bias current mutually coupled with the superconducting quantum interference device.

Furthermore, the magnetic flux modulation line is a coplanar waveguide microstrip transmission line.

Furthermore, the current device is a current source, or a voltage source and a resistor connected in sequence and capable of providing the bias current.

Furthermore, a pump signal for amplifying the signal to be amplified is coupled from the first capacitor module or the magnetic flux modulation circuit into the oscillation amplification circuit.

Furthermore, the first capacitor module and the second capacitor module are respectively one of an interdigital capacitor, a distributed capacitor and a parallel capacitor.

Furthermore, the transmission type microwave resonant cavity is a coplanar waveguide microwave resonant cavity having a length that is half of the wavelength of the signal to be amplified.

Furthermore, the transmission type microwave resonant cavity is formed by a pair of coplanar waveguide microwave resonant cavities connected in series, each having a length of one quarter of the wavelength of the signal to be amplified.

Furthermore, the quantum parametric amplifier also includes a filter;

The filter is arranged at an end of the second capacitor module away from the transmission microwave resonant cavity.

Compared with the prior art, the present invention provides a quantum parametric amplifier, comprising a first capacitor module, a transmission microwave resonant cavity, a second capacitor module, a superconducting quantum interference device with adjustable inductance, and a voltage modulation circuit for constituting an oscillation amplifier circuit; the first capacitor module, the transmission microwave resonant cavity, and the second capacitor module are connected in sequence, one end of the superconducting quantum interference device with adjustable inductance is connected to the middle of the transmission microwave resonant cavity, and the other end is grounded, and the voltage modulation circuit is arranged at one end of the superconducting quantum interference device with adjustable inductance close to the transmission microwave resonant cavity; by adjusting the inductance of the superconducting quantum interference device with adjustable inductance, the resonant frequency of the transmission microwave resonant cavity is equal to the frequency of the signal to be amplified, so that the signal to be amplified and the pump signal interact nonlinearly in the transmission microwave resonant cavity to amplify the signal to be amplified, after the signal to be amplified and the pump signal interact nonlinearly, the output signal includes not only the signal to be amplified but also various idle frequency signals _fi , and when a bias voltage is applied, the pump signal frequency _fp that makes the quantum parametric amplifier in the optimal working mode does not need to be selected as the frequency of the signal to be amplified f _s , so that when the selected pump signal frequency and the frequency of the signal to be amplified have a distance that can be separated by the filter, each of the output idler signals _fi also has a distance that can be separated by the filter from the signal to be amplified _fs . The present invention sets a voltage modulation circuit so that the operation mode adjustment of the quantum parametric amplifier is no longer only subject to the pump signal, but is adjusted together with the voltage bias and pump signal provided by the voltage modulation circuit. When appropriate bias voltage and pump signal are selected, each irrelevant signal generated in the quantum parametric amplifier can maintain a distance that can be separated by the filter from the signal to be amplified in the spectrum, thereby eliminating these irrelevant signals and improving the reading fidelity of the quantum parametric amplifier for the quantum bit reading signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of a quantum parametric amplifier provided by an embodiment of the present invention;

FIG2 is a circuit diagram of a quantum parametric amplifier provided by an embodiment of the present invention;

FIG3 is a circuit diagram of a quantum parametric amplifier provided by another embodiment of the present invention.

Detailed ways

The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, but should not be construed as limiting the present invention.

Referring to FIG. 1 , an embodiment of the present invention provides a quantum parametric amplifier, which includes a first capacitor module 100, a transmission type microwave resonant cavity 200, a second capacitor module 300 and a superconducting quantum interference device 400 with adjustable inductance for constituting an oscillation amplifier circuit; the first capacitor module 100, the transmission type microwave resonant cavity 200 and the second capacitor module 300 are connected in sequence, one end of the superconducting quantum interference device 400 with adjustable inductance is connected to the middle of the transmission type microwave resonant cavity 200 and the other end is grounded; and the inductance of the superconducting quantum interference device 400 with adjustable inductance can be adjusted to adjust the resonant capacity of the transmission type microwave resonant cavity 200. The oscillation frequency is equal to the frequency of the signal to be amplified, wherein: the signal to be amplified is coupled into the oscillation amplifier circuit from the first capacitor module 100, and the oscillation amplifier circuit amplifies the signal to be amplified under the action of the pump signal and generates several idle frequency signals; the quantum parameter amplifier also includes a voltage modulation circuit 500; the voltage modulation circuit 500 is arranged at one end of the superconducting quantum interference device 500 with adjustable inductance close to the transmission microwave resonant cavity 200; the superconducting quantum interference device 400 with adjustable inductance can release at least one of the idle frequency signals generated in the oscillation amplifier circuit under the action of the bias voltage provided by the voltage modulation circuit 500.

Compared with the prior art, the present invention provides a quantum parametric amplifier, comprising a first capacitor module 100, a transmission type microwave resonant cavity 200, a second capacitor module 300, a superconducting quantum interference device 400 with adjustable inductance, and a voltage modulation circuit 500 for constituting an oscillation amplifier circuit; the first capacitor module 100, the transmission type microwave resonant cavity 200, and the second capacitor module 300 are connected in sequence, one end of the superconducting quantum interference device 400 with adjustable inductance is connected to the middle of the transmission type microwave resonant cavity 200, and the other end is grounded, and the voltage modulation circuit 500 is arranged at one end of the superconducting quantum interference device 400 with adjustable inductance close to the transmission type microwave resonant cavity 200; by adjusting the inductance of the superconducting quantum interference device 400 with adjustable inductance, the resonant frequency of the transmission type microwave resonant cavity 200 is equal to the frequency of the signal to be amplified, so that the signal to be amplified and the pump signal interact nonlinearly in the transmission type microwave resonant cavity 200 to amplify the signal to be amplified. After the signal to be amplified and the pump signal interact nonlinearly, the output signal includes not only the signal to be amplified but also various idle frequency signals f _i . When the bias voltage is applied, the pump signal frequency _fp that makes the quantum parametric amplifier in the optimal working mode does not need to be selected as the multiple of the signal to be amplified _fs . Therefore, when the selected pump signal frequency and the signal frequency to be amplified have a distance that can be separated by the filter, each output idler signal _fi also has a distance that can be separated by the filter from the signal to be amplified _fs . The present invention sets a voltage modulation circuit 500 so that the working mode adjustment of the quantum parametric amplifier is no longer only subject to the pump signal, but is adjusted together with the voltage bias provided by the voltage modulation circuit and the pump signal. When the appropriate bias voltage and pump signal are selected, each irrelevant signal generated in the quantum parametric amplifier can maintain a distance that can be separated by the filter from the signal to be amplified in the spectrum, thereby eliminating these irrelevant signals and improving the reading fidelity of the quantum parametric amplifier for the quantum bit reading signal.

It should be noted that in the field of quantum computing, in order to obtain the calculation results of the quantum chip, we need to collect and analyze the signal output by the quantum chip, that is, the quantum bit read signal. Taking the superconducting quantum bit system as an example, the quantum bit read detection signal is usually in the 4-8GHz frequency band, and the power is as low as below -140dBm, or even below -150dBm. Considering the coupling efficiency between the quantum bit detection signal and the quantum bit read detector, the power of -150dBm to -140dBm corresponds to about 1-10 photons inside the detector. Such a weak detection signal will suffer additional losses after passing through the detector and being transmitted again. Therefore, one of the core issues that need to be solved in the application of quantum chips is how to extract effective quantum state information from such a weak quantum bit read signal.

Assume that the qubit read signal that eventually leaves the qubit read detector has 10 effective photons, which will enter the subsequent circuit and mix with thermal noise, electrical noise, etc. Among them, the standard thermal noise satisfies the thermodynamic distribution and can be used Converted into the number of photons n, k _B is the Boltzmann constant, T is the ambient noise temperature at frequency f, and h is Planck's constant. Assuming that the quantum chip is in a 10mK temperature environment, according to the above formula, n is less than 0.1 and can be ignored. However, the receiving system for the quantum bit reading signal is at room temperature, and n is about 1000. If the quantum bit reading signal is transmitted directly, it will be submerged in the noise. Therefore, the use of a parametric amplifier is necessary.

Any amplifier will introduce additional noise while amplifying the original signal. We usually measure it by the equivalent temperature of noise, that is, the noise. The larger the index, the worse the noise. The amplifier will definitely deteriorate the signal-to-noise ratio. Therefore, the setting of the amplifier should be to raise the gain of the amplifier as much as possible while controlling the noise temperature of the amplifier.

The noise temperature also satisfies Therefore, we can convert the noise temperature into the number of noise photons with frequency f. The signal-to-noise ratio can be described as the ratio of the number of signal photons to the number of noise photons.

The best commercial amplifier currently is the low-noise amplifier produced by the Swedish company LNF, which can amplify signals in the 4-8GHz frequency band and has a noise temperature of about 3K. By this measure, the number of noise photons is about 10, so the maximum signal-to-noise ratio that can be obtained using a commercial amplifier is about 1. The best quantum parameter amplifier can reach the standard quantum limit noise level, that is, n = 0.5. Usually, n fluctuates within 0.5-2. Therefore, the use of quantum parameter amplifiers can increase the system's signal-to-noise ratio by about 5-20 times.

Although quantum parametric amplifiers have solved the problem of extracting effective quantum state information from such weak quantum bit read signals by significantly improving the signal-to-noise ratio, they have brought new problems. Existing quantum parametric amplifiers work based on the principle of nonlinear mixing. In order to effectively amplify the quantum bit read signal and make the quantum parametric amplifier work in the optimal mode, it is necessary to apply an additional pump signal with a frequency close to the frequency of the signal to be amplified or its multiple frequency. For example, the corresponding applied pump signal close to the signal to be amplified corresponds to the four-wave mixing working mode, and the applied pump signal close to twice the frequency of the signal to be amplified corresponds to the three-wave mixing working mode.

In the operation process of the existing quantum parametric amplifier, the signal to be amplified _fs and the pump signal _fp are input, the signal to be amplified _fs is amplified under the action of the pump signal _fp , and the signal to be amplified is output. At the same time, based on the principle of nonlinear mixing, the output signal also includes various idle signals _fi . The signal to be amplified _fs , the pump signal _fp and the idle signal _fi will satisfy the formula: _mfs + _nfi = _lfp , where: m, n and l are all integers. When m, n and l take different values, different idle signals _fi are obtained. At the same time, the signal to be amplified _fs or the pump signal _fp will also generate some signals inside the quantum parametric amplifier based on the nonlinear principle. These signals are included in the output signal, which may also affect the accurate acquisition of the signal to be amplified in the output signal. When an existing quantum parametric amplifier is working, the pump signal frequency must be selected as a multiple of the amplified signal frequency to obtain the best amplification effect. For example, when the quantum parametric amplifier is in a four-wave mixing working mode, that is, the pump signal _fp frequency is selected to be close to the frequency of the signal to be amplified _fs , and _fp , _2fp - _fs , and _2fs - _fp in the idle signal affect the acquisition of the signal to be amplified because they are close to the signal to be amplified _fs ; when the quantum parametric amplifier is in a three-wave mixing working mode, that is, the pump signal _fp frequency is selected to be close to twice the frequency of the signal to be amplified _fs , and 1/ _2fp and _fp - _fs in the idle signal affect the acquisition of the signal to be amplified because they are close to the signal to be amplified _fs .

Specifically, referring to Figures 1 and 2, Embodiment 1 of the present invention provides a quantum parametric amplifier, which includes a first capacitor module 100, a transmission microwave resonant cavity 200, a second capacitor module 300 and a superconducting quantum interference device 400 with adjustable inductance, which are connected in sequence to form an oscillation amplifier circuit, one end of the superconducting quantum interference device 400 with adjustable inductance is connected to the middle of the transmission microwave resonant cavity 200, and the other end is grounded; and the frequency of the transmission microwave resonant cavity 200 can be made equal to the frequency of the signal to be amplified by adjusting the inductance of the superconducting quantum interference device 400 with adjustable inductance, wherein: the signal to be amplified is coupled into the oscillation amplifier circuit from the first capacitor module 100, and the oscillation amplifier circuit amplifies the signal to be amplified under the action of a pump signal, and generates several idle frequency signals.

It should be noted that each of the idler signals satisfies the following formula:

mf _s + nf _i = lf _p

Wherein: m, n, l are integers, _fs is the frequency of the signal to be amplified, _fp is the frequency of the pump signal, and _fi is the frequency of the idle signal. It should be noted that the above formula is based on the principle of nonlinear mixing. When the signal to be amplified _fs and the pump signal _fp are determined, different values of m, n and l will result in various idle signals _fi .

Among them, the first capacitor module 100 is used to couple the signal to be amplified into the transmission microwave resonant cavity 200, and the second capacitor module 300 is used to output the signal. It should be noted that, usually, the microwave resonant cavity must be connected to an external circuit to form a microwave system to work, and the microwave signal in the external circuit must be excited to establish oscillation in the cavity, and the oscillation in the cavity must be output to the external load through coupling. Usually, a capacitor module is used to establish coupling with the microwave resonant cavity. In this embodiment, the first capacitor module 100 and the second capacitor module 300 can respectively select interdigital capacitors, distributed capacitors or parallel capacitors. The present invention does not limit the specific forms of the first capacitor module 100 and the second capacitor module 300.

It should be noted that the oscillation amplifier circuit is a commonly used structure in the field of signal amplification and is a key component of many electronic devices. The oscillation amplifier circuit is usually in the form of an LC oscillation circuit, including interconnected capacitors and inductors. It can be used to generate signals of specific frequencies and to separate signals of specific frequencies from more complex signals. In the field of quantum computing, in order to obtain the calculation results of the quantum chip, we need to collect and analyze the signal output by the quantum chip, that is, the quantum bit reading signal. Usually, the quantum bit reading signal is very weak and needs to be amplified. Since the quantum bit reading signal is a high-frequency signal, its wavelength is very short, and the capacitor and inductor devices used in the lumped LC oscillation circuit are large in size, and the energy of the LC oscillation circuit is dispersed in the surrounding space and dissipated very quickly, we must use a quantum parametric amplifier used in the quantum field.

Typically, a quantum parametric amplifier includes a capacitor, a microwave resonant cavity, a superconducting quantum interferometer, and a flux bias adjustment circuit for modulating the superconducting quantum interferometer connected in sequence. The end of the superconducting quantum interferometer away from the resonant cavity is grounded. The basic principle is as follows: the alternating current generated in the superconducting quantum interferometer is used to form an inductor, which forms an LC oscillation circuit with the capacitor, thereby constructing a single-mode light field in the microwave resonant cavity. At this time, the weak signal to be amplified and the pump signal enter the device together, and the signal to be amplified is amplified in the microwave resonant cavity. At the same time, the whole process is in a superconducting state with almost no dissipation.

Among them: It should be noted that the superconducting quantum interference device is a closed-loop device composed of several Josephson junctions connected in parallel, wherein: a Josephson junction is generally composed of two superconductors sandwiched by a very thin barrier layer, such as an S (superconductor)-I (semiconductor or insulator)-S (superconductor) structure, referred to as SIS. In SIS, superconducting electrons can tunnel from one side of a superconductor through a semiconductor or insulator to the superconductor on the other side, or called the Josephson effect. The current generated is called the Josephson current. When multiple Josephson junctions are connected together to form a closed-loop device, a Josephson interferometer, or a superconducting quantum interference device, is formed.

The quantum parametric amplifier also includes a voltage modulation circuit 500; the voltage modulation circuit 500 is arranged at one end of the superconducting quantum interference device 400 with adjustable inductance close to the transmission microwave resonant cavity 200; the superconducting quantum interference device 400 with adjustable inductance can release one of the idle frequency signals generated in the oscillation amplification circuit under the action of the bias voltage provided by the voltage modulation circuit 500.

It should be noted that when a voltage bias is applied to both ends of the superconducting quantum interference device, the current passing through the Josephson junction is an alternating oscillating superconducting current, and the oscillation frequency (or Josephson frequency) will be proportional to the bias voltage, which makes the Josephson junction have the ability to radiate or absorb electromagnetic waves, which satisfies the following relationship:

2eV＝hf

Where: h is Planck's constant.

Since the superconducting quantum interference device composed of a number of Josephson junctions connected in parallel has the ability to absorb electromagnetic waves, when a voltage bias is applied to the superconducting quantum interference device 400 with adjustable inductance, the current Cooper pairs on the Josephson junction will absorb the energy of the microwave signal and tunnel through the Josephson junction to the ground and flow out. When a suitable voltage bias is selected, f in the relationship 2eV=hf is equal to the frequency of one of the idle signals generated by the oscillation amplifier circuit, and the idle signal generated in the oscillation amplifier circuit will be completely absorbed, which manifests as the idle signal being released.

It should be noted that the working process of the present invention is as follows: by adjusting the inductance of the superconducting quantum interference device 400 with adjustable inductance, the working resonant frequency of the transmission microwave resonant cavity 200 is made consistent with the frequency of the signal to be amplified, so that the signal to be amplified has the best resonance amplification effect in the transmission microwave resonant cavity 200, and the signal to be amplified and the pump signal are coupled into the transmission microwave resonant cavity 200, and the signal to be amplified will be amplified under the action of the pump signal. It should be noted that the output signal includes not only the amplified signal, but also the pump signal, the half-frequency pump signal, the doubled frequency pump signal and various idle signals. At this time, when a suitable voltage bias is applied, the relationship 2eV=hf is satisfied, where f is equal to the frequency of one of the idle signals, and the idle signal generated in the oscillation amplifier circuit will be completely absorbed, which is manifested as the idle signal being released.

It should be noted that before the quantum parametric amplifier of the present invention works, various parameters need to be designed, including the voltage bias size and the frequency of the pump signal. One of the ultimate purposes of the present invention is to ensure that the irrelevant signals outputted will not interfere with the signal to be amplified, that is, they can be separated by the filter. Here is a specific example. When the frequency of the signal to be amplified is 4 GHz, one of the idler signals can be designed to be 2 GHz first, and the voltage bias is calculated by the relationship 2eV=hf, and then one of the possible pump signal frequencies is calculated according to the formula _mfs + _nfi = _lfp . For example, when m, n, and l are all 1, the pump signal frequency is selected to be 6 GHz. At this time, when other possible idler signals are considered according to the formula _mfs + _nfi = _lfp , it can be proved that when m, n, and l take different values, the idler signal _fi will not interfere with the signal to be amplified _fs . The following table shows 8 idler signals fi that are closest to the frequency of the signal to be amplified _fs when the frequency of the signal to be amplified is 4 GHz and the frequency of the pump signal is 6 _GHz .

Table 1: 8 idler signals _fi

m 1 1 1 1 -1 -1 -1 -1 l 1 1 -1 -1 1 1 -1 -1 n 1 -1 1 -1 1 -1 1 -1 f _i 2GHz 10GHz -10Ghz -2GHz -2GHz 10GHz -10GHz 2GHz

It can be seen from the above table that the 8 idler signals _fi that are closest in frequency to the signal to be amplified _fs all maintain a certain distance from the signal to be amplified _fs , so the other idler signals _fi generated will not interfere with the signal to be amplified _fs .

There is another problem with traditional quantum parametric amplifiers. When the actual quantum chip is working, we need to read out a large number of quantum bit signals at the same time. The quantum state information of each quantum bit is carried and transmitted by an independent signal, and its frequency is different from the frequency of the quantum state information carrying signal of other quantum bits. Reading multiple quantum bits at the same time means that there are multiple signals to be amplified that carry information and need to pass through the quantum parametric amplifier. Each of these signals will generate a large number of irrelevant signals while obtaining the amplification effect, and at least one of the irrelevant signals is close to the signal to be amplified. In addition, the irrelevant signal generated by a certain signal to be amplified is likely to be close to the frequency of another signal to be amplified.

Specifically, for example: the frequencies of the signals to be amplified _fs input into the traditional quantum parametric amplifier are 6.4 GHz and 6.58 GHz respectively (0.18 GHz apart, and the filter can be split), and the frequency of the traditional quantum parametric amplifier pump signal _fp can be designed to be 6.5 GHz, corresponding to the four-wave mixing working mode. Then, according to the formula _mfs + _nfi = _lfp , one of the idler signals fo of the 6.4 GHz amplified signal _{fs is} 6.6 GHz, which will affect the 6.58 GHz signal (0.02 GHz apart, which is difficult to split).

When the quantum parametric amplifier of the present invention is used, by designing an idle frequency signal, for example 4 GHz, _{and designing the pump signal f p to} be 5.2 GHz and the bias voltage based on the 4 GHz signal and the 6.4 GHz amplified signal f s, it can be known that according to the mixing effect of the 5.2 GHz pump signal f _p with the 6.4 GHz and 6.58 GHz amplified signals f _s signals respectively, all the idle frequency signals _fi are kept at a separable distance from the 6.4 GHz and 6.58 GHz amplified signals f _s .

Furthermore, the superconducting quantum interference device 400 with adjustable inductance includes a superconducting quantum interference device 410 and a flux modulation circuit 420 connected by mutual inductance coupling, as shown in Figure 2; the superconducting quantum interference device 410 is a closed-loop device composed of a plurality of Josephson junctions in parallel; the flux modulation circuit 420 is used to adjust the inductance of the superconducting quantum interference device 410 by adjusting the magnetic flux of the closed-loop device.

The flux modulation circuit 420 includes a flux modulation line and a current device for generating a bias current which are connected in sequence; wherein: the flux modulation line is used to transmit the bias current and make the bias current mutually inductively coupled with the superconducting quantum interference device 410.

It should be noted that the current device for generating the bias current may be a current source, or a voltage source and a resistor connected in sequence that can provide the bias current. The present invention does not limit the specific form of the current source.

Furthermore, the transmission type microwave resonant cavity 200 is a coplanar waveguide microwave resonant cavity having a length of half the wavelength of the signal to be amplified. A coplanar waveguide microwave resonant cavity having a length of half the wavelength of the signal to be amplified is used. Since the highest points of the electric field strength of the half-wavelength coplanar waveguide microwave resonant cavity are respectively located at one end close to the first capacitor module 100 and one end close to the second capacitor module 300, and the electric field at the middle position is almost 0, introducing a DC voltage bias here will not affect the microwaves in the transmission type microwave resonant cavity 200, and the output signal will be output from the end close to the second capacitor module 300. Unlike the use of a quarter-wavelength reflective resonant cavity in which the signal to be amplified is amplified at the strongest suppression point, the use of a half-wavelength coplanar waveguide microwave resonant cavity will amplify the signal to be amplified at the weakest signal absorption point in the transmission type resonant cavity 200, thereby improving the signal amplification gain.

Preferably, as shown in FIG. 2 , the transmission type microwave resonant cavity 200 may be formed by a pair of coplanar waveguide microwave resonant cavities 210 connected in series, each having a length of one quarter of the wavelength of the signal to be amplified.

It should be noted that in the microwave field, a coplanar waveguide is three parallel metal film guide strip layers prepared on the surface of a dielectric layer, wherein the guide strip layer in the center is used to transmit microwave signals, and the guide strip layers on both sides are connected to the ground plane. The biggest difference from a general circuit is that a coplanar waveguide is a distributed circuit element, and its capacitance/inductance/conductance/impedance are evenly distributed along the propagation direction of the coplanar waveguide signal. The coplanar waveguide propagates TEM waves, and along the signal propagation direction, the impedance of the waveguide is equal everywhere, so there is no signal reflection, and the signal can pass almost losslessly; in addition, the coplanar waveguide has no cutoff frequency, while common lumped circuits all have cutoff frequencies. For a uniform coplanar waveguide, microwave signals of most frequency bands can be transmitted unimpeded, so it is also called a transmission line, that is, a coplanar waveguide transmission line. When the designed coplanar waveguide transmission line has a certain length, and a capacitance node is constructed at both ends of the coplanar waveguide transmission line, the microwave signal will reflect after encountering the node, and resonance will be formed in this transmission line, forming a resonant cavity.

Preferably, the flux modulation line for transmitting the bias current may also use a coplanar waveguide transmission line.

Furthermore, referring to FIG. 3 , in order to filter out irrelevant signals other than the amplified signal in the output signal, a filter 600 is further provided at the signal output end of the second capacitor module 300 , wherein the irrelevant signals mainly refer to pump signals, half-frequency pump signals, double-frequency pump signals, and various idle frequency signals.

It should be noted that the existing quantum parametric amplifier can only achieve the maximum amplification effect when the pump signal frequency is equal to an integer multiple of the frequency of the signal to be amplified. In the corresponding three-wave mixing working mode, the pump signal frequency is equal to the signal frequency to be amplified. In the four-wave mixing working mode, the pump signal frequency is equal to twice the signal frequency to be amplified. In the three-wave mixing working mode, the pump signal in the output signal is difficult to distinguish from the amplified signal. In the four-wave mixing working mode, the half-frequency pump signal in the output signal is difficult to distinguish from the amplified signal. Using a quantum parametric amplifier of the present invention, the working mode adjustment of the quantum parametric amplifier is no longer only subject to the pump signal, but is adjusted together with the pump signal through the voltage modulation circuit. When the appropriate bias voltage and pump signal are selected, each irrelevant signal generated in the quantum parametric amplifier can be kept at a distance that can be separated by the filter on the spectrum from the signal to be amplified, and then the post-stage filter can be used to conveniently eliminate these irrelevant signals, thereby improving the reading fidelity of the quantum parametric amplifier for the quantum bit reading signal.

The above describes in detail the structure, features and effects of the present invention based on the embodiments shown in the drawings. The above is only a preferred embodiment of the present invention, but the present invention is not limited to the scope of implementation shown in the drawings. Any changes made according to the concept of the present invention, or modified into equivalent embodiments with equivalent changes, which still do not exceed the spirit covered by the description and the drawings, should be within the protection scope of the present invention.

Claims (11)

1. A quantum parametric amplifier, characterized in that the quantum parametric amplifier comprises a first capacitor module, a transmission microwave resonant cavity, a second capacitor module and a superconducting quantum interference device with adjustable inductance for constituting an oscillation amplifier circuit; the first capacitor module, the transmission microwave resonant cavity and the second capacitor module are connected in sequence, one end of the superconducting quantum interference device with adjustable inductance is connected to the middle of the transmission microwave resonant cavity and the other end is grounded; and the resonant frequency of the transmission microwave resonant cavity can be made equal to the frequency of a signal to be amplified by adjusting the inductance of the superconducting quantum interference device with adjustable inductance, wherein: the signal to be amplified is coupled into the oscillation amplifier circuit from the first capacitor module; The quantum parametric amplifier also includes a voltage modulation circuit; The voltage modulation circuit is arranged at one end of the superconducting quantum interference device of the adjustable inductance close to the transmission type microwave resonant cavity; The superconducting quantum interference device with adjustable inductance can make the oscillation frequency of the superconducting quantum interference device equal to the frequency of one of the idle frequency signals under the action of the bias voltage provided by the voltage modulation circuit to release at least one idle frequency signal generated in the oscillation amplification circuit. 2. The quantum parametric amplifier according to claim 1, characterized in that the superconducting quantum interference device with adjustable inductance comprises a superconducting quantum interference instrument and a magnetic flux modulation circuit connected by mutual inductance coupling; The superconducting quantum interference device is a closed-loop device consisting of a number of Josephson junctions connected in parallel; The magnetic flux modulation circuit is used to adjust the inductance of the superconducting quantum interference device by adjusting the magnetic flux of the closed-loop device. 3. The quantum parametric amplifier according to claim 2 is characterized in that the superconducting quantum interference device is a closed-loop device composed of two Josephson junctions connected in parallel. 4. The quantum parametric amplifier according to claim 2, characterized in that the flux modulation circuit comprises a flux modulation line and a current device for generating a bias current connected in sequence; Wherein: the magnetic flux modulation line is used to transmit the bias current and make the bias current mutually coupled with the superconducting quantum interference device. 5. The quantum parametric amplifier according to claim 4 is characterized in that the flux modulation line is a coplanar waveguide microstrip transmission line. 6. The quantum parametric amplifier according to claim 4 is characterized in that the current device is a current source, or a voltage source and a resistor connected in sequence that can provide the bias current. 7. The quantum parametric amplifier according to claim 2, characterized in that a pump signal for amplifying the signal to be amplified is coupled from the first capacitor module or the flux modulation circuit into the oscillation amplifier circuit. 8 . The quantum parametric amplifier according to claim 1 , wherein the first capacitor module and the second capacitor module are respectively one of an interdigital capacitor, a distributed capacitor and a parallel capacitor. 9. The quantum parametric amplifier according to claim 1 is characterized in that the transmission-type microwave resonant cavity is a coplanar waveguide microwave resonant cavity having a length of half the wavelength of the signal to be amplified. 10. The quantum parametric amplifier according to claim 1 is characterized in that the transmission-type microwave resonant cavity is formed by a pair of coplanar waveguide microwave resonant cavities connected in series and having a length of one quarter of the wavelength of the signal to be amplified. 11. The quantum parametric amplifier according to any one of claims 1 to 10, characterized in that the quantum parametric amplifier further comprises a filter; The filter is arranged at an end of the second capacitor module away from the transmission microwave resonant cavity.