Multi-superconducting quantum bit chip

Abstract

The invention provides a multi-superconducting quantum bit chip, which comprises an input microwave transmission line, an input coupling capacitor or inductor, a filter, an output coupling capacitor or inductor, an output microwave transmission line, a plurality of reading resonant cavities and a plurality of reading coupling capacitors or inductors, and is characterized in that the input microwave transmission line is coupled to the filter through the input coupling capacitor or inductor; the output microwave transmission line is coupled to the filter through an output coupling capacitor or inductor, and each reading resonant cavity is coupled to a coupling point on the filter through a corresponding reading coupling capacitor or inductor so as to enable the filter to filter microwaves except for the reading frequency of the reading resonant cavity, the coupling strength of all the reading resonant cavities and the filter is the same, and the reading coupling capacitance or inductance of each coupling point is configured according to the coupling strength of the reading resonant cavities and the filter, the effective ground capacitance or effective ground inductance, obtained through simulation, of the filter at the coupling point, and the effective ground capacitance or effective ground inductance of the reading resonant cavities.

Description

Technical Field

[0001] The present invention relates to the field of superconducting quantum computing, specifically, to an integrated design technology for designing superconducting quantum bit chips, and more specifically, to a multi-superconducting quantum bit chip. Background Art

[0002] The Purcell effect exists in superconducting qubits. This effect accelerates decoherence due to coupling between the superconducting qubit and the external environment through the resonant cavity, thereby reducing the qubit's lifetime. Purcell filters are widely used at the readout end of superconducting qubits to suppress decoherence caused by the Purcell effect, thereby extending the bit decoherence time (T1) and improving readout fidelity.

[0003] Currently, the most common method used in superconducting qubits is to use a linear bandpass filter to achieve the above functions. There are two common structures: the first is to add a half-wavelength or quarter-wavelength geometric resonant cavity between the read signal transmission line and the read cavity, whose resonant frequency is equal to that of the read cavity. The second structure is to add multiple geometric resonant cavities with specific resonant frequencies between the read signal transmission line and the read cavity. At specific node locations, they are coupled with specific capacitance or inductance to form specific frequency band amplitude variations, such as a strong signal passband near the read cavity resonant frequency and a strong signal forbidden band near the qubit resonant frequency. Alternatively, in the case of multiple read cavities, a flat and wide passband is formed in the read frequency range.

[0004] In addition to the decoherence and readout discussed above, dephasing of qubits due to photon noise at the readout end is also a concern, as it can lead to a decrease in quantum gate fidelity. However, linear bandpass filters cannot address dephasing. Nonlinear bandpass filters, on the other hand, not only suppress the Purcell effect and improve readout fidelity, but also suppress the impact of photon noise on the qubit, thereby increasing the dephasing time (T2) of the qubit. The nonlinearity of nonlinear filters is introduced by the Josephson junction. Because their passband frequency varies with flux bias and readout power, they offer many advantages over linear filters. For example, DC control can flexibly adjust the effective leakage rate of the readout cavity during measurement and control, maximizing the signal-to-noise ratio and reducing readout parameter mismatches caused by process errors during sample preparation.

[0005] Unfortunately, however, the development of nonlinear filters in the existing technology is limited to single-bit samples. The existing nonlinear filters for single quantum bits are difficult to be directly used on multi-qubit chips for the following reasons: 1. The reading cavities of multiple quantum bits need to be coupled to the filter at different positions of the filter, but the existing solutions do not provide how to calculate the coupling strength between the filter and the reading cavity at different coupling points; 2. The multi-qubit chip needs to control each structure on the chip separately, and the existing solutions do not have an integrated flux bias line to individually control the filter frequency; 3. The existing solutions only use a quarter-wavelength geometric cavity structure with a short geometric length, and the positions that can be provided for reading cavity coupling are limited.

[0006] Therefore, there is an urgent need for a filtering solution that can meet the application requirements of multi-superconducting quantum bit chips.

[0007] It should be noted that this background information is provided solely to introduce relevant information of the present invention to facilitate understanding of the technical solution of the present invention. It does not necessarily constitute prior art. In the absence of evidence demonstrating that the relevant information was disclosed prior to the filing date of the present invention, the relevant information should not be considered prior art. Summary of the Invention

[0008] Therefore, the purpose of the present invention is to overcome the above-mentioned defects of the prior art and provide a multi-superconducting quantum chip based on a new nonlinear tunable filter solution.

[0009] The present invention provides a multi-superconducting qubit chip, comprising: an input microwave transmission line, an input coupling capacitor or inductor, a filter, an output coupling capacitor or inductor, an output microwave transmission line, multiple readout resonant cavities, and multiple readout coupling capacitors or inductors, each readout resonant cavity corresponding to a coupled readout coupling capacitor or inductor. The input microwave transmission line is coupled to the filter via the input coupling capacitor or inductor to input microwave signals into the filter. The output microwave transmission line is coupled to the filter via the output coupling capacitor or inductor to output microwaves transmitted by the filter. The filter includes a first coplanar waveguide cavity, a superconducting quantum interference device (SQID), a second coplanar waveguide cavity, and a DC control line. The first coplanar waveguide cavity is coupled to a microwave input transmission line at one end, and connected to one end of the SQID at the other end. The SQID is connected to the first and second coplanar waveguide cavities at both ends, respectively. The second coplanar waveguide cavity is connected to the SQID at one end, and coupled to a microwave output transmission line at the other end. The DC control line is used to apply DC control of magnetic flux to adjust the filter frequency. The first and second coplanar waveguide cavities each have multiple coupling points. Each read resonant cavity is coupled to a coupling point on the filter via its corresponding read coupling capacitor or inductor, so that the filter filters out microwaves outside the read frequency of the read resonant cavity. All read resonant cavities have the same coupling strength with the filter. The read coupling capacitor or inductor at each coupling point is configured based on the coupling strength between the read resonant cavity and the filter, the effective capacitance or inductance of the filter at the coupling point obtained by simulation, and the effective capacitance or inductance of the read resonant cavity.

[0010] Preferably, the geometric lengths of the first coplanar waveguide cavity and the second coplanar waveguide cavity are determined according to the frequency and number of the read resonant cavities, so that the number of coupling points on the first coplanar waveguide cavity and the second coplanar waveguide cavity matches the number of the read resonant cavities.

[0011] Preferably, the length ratio of the first coplanar waveguide cavity and the second coplanar waveguide cavity can be arbitrarily adjusted based on the following rules: when the superconducting quantum interference device is short-circuited, the first coplanar waveguide cavity and the second coplanar waveguide cavity can be formed into a half-wavelength cavity with both ends open, a half-wavelength cavity with both ends closed, or a quarter-wavelength cavity with one end open and the other closed.

[0012] Preferably, the superconducting quantum interference device includes two parallel Josephson junctions, the size of each Josephson junction is determined according to the expected adjustment range of the filter frequency, and the superconducting quantum interference device is connected to the two coplanar waveguide cavities in series or bypass parallel.

[0013] Preferably, the DC control of the magnetic flux is applied on the DC control line by applying the magnetic flux through an integrated control line, through an external coil or through any external device that provides the magnetic flux.

[0014] Preferably, the effective capacitance to ground or effective inductance to ground of the filter at each coupling point in the multi-superconducting quantum bit chip and the effective capacitance to ground or effective inductance to ground of the reading resonant cavity are obtained through the Y11 parameters of microwave simulation; and the coupling strength between the reading resonant cavity and the filter is obtained through finite element simulation.

[0015] Preferably, the size of the output coupling capacitor is determined according to the following constraint: the effective spectral linewidth of the read resonant cavity is equal to twice the dispersion shift of the read resonant cavity.

[0016] Preferably, the size of the input coupling capacitor is one tenth of the size of the output coupling capacitor.

[0017] Preferably, the read coupling capacitor of each coupling point is configured as follows:

[0018]

[0019] in, is the coupling strength, The read coupling capacitance to be solved, is the filter frequency, is the frequency of the reading resonant cavity, is the effective capacitance of the filter to ground at the coupling point, is the effective capacitance of the resonant cavity to ground; the reading coupling inductance of each coupling point is set as follows:

[0020]

[0021] in, is the coupling strength, The coupled inductance to be solved is read, is the filter frequency, is the frequency of the reading resonant cavity, is the effective inductance to ground of the filter at the coupling point, is the effective capacitance to ground of the reading resonant cavity.

[0022] Preferably, the multi-superconducting quantum bit chip is further configured to adjust the filter frequency by adjusting the amplitude of the input microwave.

[0023] Compared with the prior art, the present invention has the following advantages: its frequency tunability allows for adjustable effective cavity leakage in the reading cavity, improving the performance of reading quantum bits while suppressing the effects of photon noise on the bits. When the filter passband peak frequency and the reading cavity frequency are perfectly aligned, the effective cavity leakage is maximized, and when they deviate, the effective cavity leakage gradually decreases. This characteristic greatly improves the fault tolerance of the reading design and preparation. The maximum cavity leakage can be designed to be much larger than the difference in dispersion shift of the reading cavity when the quantum bit is in the 0 and 1 states, and then the filter frequency can be adjusted to achieve the optimal reading point where the dispersion shift difference matches the cavity leakage. In addition, the dispersion shift of the reading cavity caused by different quantum bit states varies in magnitude. Considering that actual quantum systems have higher energy levels occupied, the measurement resolution of these high energy levels requires different reading cavity leakage rates to achieve the maximum signal-to-noise ratio. The tunable filter can achieve the function of flexibly adjusting the reading cavity leakage rate, which improves the reading performance of actual superconducting quantum samples. Furthermore, tuning the filter frequency during non-reading time causes dispersion shift and cavity leakage to mismatch, hindering qubit dephasing caused by photon noise, thereby improving the qubit's T2. This also allows the filter's idle time to be at a position more detuned from the qubit, no longer constrained by the read cavity frequency, further suppressing qubit decoherence due to the Purcell effect. Each filter in the present invention can be coupled to multiple read resonant cavities, allowing integration into multi-bit devices, enabling multiple qubit reads with one filter. The filters in the present invention occupy minimal wiring space, nearly equivalent to current mainstream multi-bit linear filters. Furthermore, the DC signal used for frequency modulation is applied via an integrated control line, enabling precise frequency modulation of the target filter. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The embodiments of the present invention are further described below with reference to the accompanying drawings, in which:

[0025] Figure 1 Schematic diagram of the structure of a multi-superconducting quantum bit chip according to an embodiment of the present invention;

[0026] Figure 2 Schematic diagram of the relationship between the filter frequency, the reading cavity frequency and the dephasing rate according to an embodiment of the present invention;

[0027] Figure 3 Schematic diagram of an integrated multi-superconducting quantum chip circuit according to an embodiment of the present invention;

[0028] Figure 4 Schematic diagram showing the change of the peak frequency of the filter passband according to an embodiment of the present invention with the input power at the reading end;

[0029] Figure 5 Schematic diagram of the change of the filter passband peak frequency with the DC amplitude in the control line according to an embodiment of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with specific examples in combination with the drawings. It should be understood that the specific examples described herein are only used to explain the present application and should not be used to limit the present application.

[0031] As mentioned in the background section, the existing single-qubit-oriented nonlinear filter is difficult to be directly used on a multi-qubit chip, because: 1. The readout cavities of multiple qubits need to be coupled with the filter at different positions of the filter, but the existing scheme does not provide how to calculate the coupling strength of the filter and the readout cavity at different coupling sites; 2. The multi-qubit chip needs to control each structure on the chip separately, and the existing scheme does not have an integrated magnetic flux bias line to control the filter frequency separately; 3. The existing scheme only uses a quarter-wave geometry cavity structure, which has a short geometric length and can provide limited positions for coupling with the readout cavity. In view of this, the present application provides a filter scheme that can integrate multi-bit superconducting qubits and is frequency adjustable.

[0032] In order to better understand the present application, first introduce the design idea of the present application. In general, the design idea of the present application comes from the research on the following aspects:

[0033] First, how to improve the readout fidelity of the qubit. The readout of the superconducting qubit generally uses the method of dispersive readout, each bit is coupled with a readout resonant cavity, this coupling makes the frequency of the readout resonant cavity shift related to the state of the qubit, so that the state of the bit can be indirectly obtained by measuring the resonant cavity. The present application realizes this by designing a tunable nonlinear filter. The passband frequency of the tunable nonlinear filter is adjustable, and then the effective leakage rate (linewidth) of the readout cavity is adjustable, which ensures the matching of the dispersive shift and the leakage rate.

[0034] Second, how to improve the decoherence time T1 and relaxation time T2 of the bit. The coupling of a larger readout resonant cavity and a qubit can bring high readout fidelity, but at the same time it will speed up the relaxation and decoherence of the bit. The nonlinear filter designed by the present application can suppress the decay of the qubit excitation from the readout line by selecting the signal frequency to pass through, thereby improving T1, and can suppress the influence of photon noise on the bit by dynamically adjusting the matching of the dispersive shift and the leakage rate, thereby improving T2.

[0035] Third, how to assist in the rapid reset of the qubit. Rapid reset is of great significance for complex quantum information processing, such as implementing error correction schemes such as surface codes. The tunable filter designed by the present application can selectively directly or indirectly eliminate the excitation of the bit through resonant exchange.

[0036] Fourth, how to optimize nonlinear filters into a scalable and integrable architecture for application to superconducting quantum devices with hundreds of qubits. The solution designed in this paper utilizes filter structural design to enable integration of multiple qubits. By adjusting the filter frequency and increasing the decoherence time of superconducting qubits, large-scale integration is achieved.

[0037] In summary, if Figure 1 As shown, the present invention provides a multi-superconducting quantum bit chip, which includes: an input microwave transmission line 101, an input coupling capacitor 102, a filter, an output coupling capacitor 106, an output microwave transmission line 107, a plurality of read resonant cavities ( Figure 1 referred to as the reading cavity), multiple reading coupling capacitors ( Figure 1 Indicated as 108-1, 108-2, ..., 108-n in the figure), each read resonant cavity corresponds to a coupled read coupling capacitor or inductor, and multiple read cavity-qubits can be coupled to the filter cavity. Figure 1 The ellipsis in the figure indicates this scalability. Figure 1 It can be seen that the input microwave transmission line 101 is coupled to the filter through the input coupling capacitor 102 to input the microwave signal into the filter; the output microwave transmission line 107 is coupled to the filter through the output coupling capacitor 106 to output the microwave transmitted by the filter; the filter includes a first coplanar waveguide cavity 103, a superconducting quantum interference device 104, a second coplanar waveguide cavity 105, and a DC control line 109, wherein: one end of the first coplanar waveguide cavity 103 is used to couple to the microwave input transmission line 101, and the other end is connected to one end of the superconducting quantum interference device 104; the two ends of the superconducting quantum interference device 104 are respectively connected to the first coplanar waveguide cavity 103 and the second coplanar waveguide cavity 105; one end of the second coplanar waveguide cavity 105 is connected to the superconducting quantum interference device 104 , and the other end is used to couple to the microwave output transmission line 107; the DC control line 109 is used to apply DC control of the magnetic flux to adjust the frequency of the filter; wherein, the first coplanar waveguide cavity 103 and the second coplanar waveguide cavity 105 respectively have multiple coupling points; wherein, each reading resonant cavity is coupled to a coupling point on the filter through its corresponding reading coupling capacitor so that the filter filters out microwaves other than the reading frequency of the reading resonant cavity, wherein the coupling strength between all reading resonant cavities and the filter is the same, and the reading coupling capacitor or inductor of each coupling point is configured according to the coupling strength between the reading resonant cavity and the filter, the effective capacitance to ground or effective inductance to ground of the filter at the coupling point obtained by simulation, and the effective capacitance to ground or effective inductance to ground of the reading resonant cavity.

[0038] In addition, it should be noted that Figure 1All coupling capacitors shown in the figure can be replaced with coupling inductors, and the embodiments of the present invention are described using coupling capacitors as an example.

[0039] According to one embodiment of the present invention, the geometric lengths of the first coplanar waveguide cavity 103 and the second coplanar waveguide cavity 105 are determined according to the frequency and number of the read resonant cavities, so that the number of coupling points on the first coplanar waveguide cavity and the second coplanar waveguide cavity matches the number of the read resonant cavities. According to one embodiment of the present invention, the length ratio of the first coplanar waveguide cavity 103 and the second coplanar waveguide cavity 105 can be arbitrarily adjusted based on the following rules: when the superconducting quantum interference device is short-circuited, the first coplanar waveguide cavity and the second coplanar waveguide cavity can be formed into a half-wavelength cavity with both ends open, a half-wavelength cavity with both ends closed, or a quarter-wavelength cavity with one end open and the other closed. The two coplanar waveguide cavities constitute the main structure of the filter, and the connection method between the superconducting quantum interference device and the two coplanar waveguide cavities can be series or bypass parallel. Figure 1 In the embodiment, a superconducting quantum interference device consisting of two Josephson junctions in parallel is inserted at the midpoint of two coplanar waveguide cavities.

[0040] In the solution of the present invention, the size of the superconducting inductor of the superconducting quantum interference device will change with the magnetic flux passing through the loop. The sensitivity of the superconducting inductor to the change in magnetic flux can be adjusted by changing the ratio of the resistance sizes of the two Josephson junctions. The size of each Josephson junction is determined according to the desired adjustment range of the filter frequency. The specific sizes of the two Josephson junctions can be adjusted according to the application scenario requirements of the specific superconducting quantum chip. This is a technology known to those skilled in the art, and the present invention does not provide a detailed description of the specific adjustment operations.

[0041] According to one embodiment of the present invention, the DC control of applying the magnetic flux on the DC control line is applied through an integrated control line, through an external coil, or through any external device that provides magnetic flux. Figure 1 In the embodiment shown, the DC control line 109 is in the form of an integrated control line.

[0042] from Figure 1 It can be seen that the two ends of the filter are coupled to the microwave transmission line (101, 107) through capacitors (102, 106), so that the microwave signal can be input into the filter at one end and output from the filter at the other end. Among them, the size of the output coupling capacitor is determined according to the dispersion shift of the reading resonant cavity. The output capacitor determines the spectral linewidth of the filter, and the spectral linewidth of the filter affects the spectral linewidth of the reading cavity. The ultimate goal of setting the size of the output capacitor is to make the spectral linewidth of the reading cavity equal to twice the dispersion shift to ensure matching with the dispersion shift of the reading cavity, thereby improving the reading fidelity. In addition, based on the determination of the output coupling capacitor, the size of the input coupling capacitor is set to one tenth of the size of the output coupling capacitor, that is, the output end capacitor for input capacitor times of the value, thereby ensuring high amplitude of the band-pass signal. Similarly, the output end and the input end can also be coupled by inductance, and the principle is similar to the capacitor, which is not described here.

[0043] As described in the foregoing embodiments, the coupling strength of all read resonant cavities and filters is the same, and the read coupling capacitor or inductor at each coupling point is configured according to the coupling strength of the read resonant cavity and the filter, the effective ground capacitor or effective ground inductor of the filter at the coupling point obtained by simulation, and the effective ground capacitor or effective ground inductor of the read resonant cavity. Among them, the effective ground capacitor or effective ground inductor of the filter at each coupling point in the plurality of superconducting quantum bit chips and the effective ground capacitor or effective ground inductor of the read resonant cavity are obtained by microwave simulation Y11 parameter; the coupling strength of the read resonant cavity and the filter is obtained by finite element simulation. Among them, the coupling strength of the read cavity and the filter of the capacitor coupling can be calculated by the following formula:

[0044]

[0045] Among them, is the coupling strength of the read cavity and the filter at the coupling point, which can be obtained by finite element simulation; is the mutual capacitance value of the filter and the read cavity, that is, Figure 1 the capacitance value of the 108 series shown in the formula (1); respectively, the frequency of the filter and the read cavity; is the effective ground capacitor of the filter at the coupling point, is the effective ground capacitor of the read cavity at the coupling point, since the filter and the read cavity have a length of millimeter, the present application obtains the Y11 parameter at the coupling position by Y11 simulation, and the effective ground capacitor at the coupling position is solved by , is an imaginary number, is the coplanar waveguide geometric cavity fundamental frequency without superconducting quantum interference device, is the resonant fundamental frequency of the filter, and C is the effective ground capacitor of the filter at the coupling point. Thus, after the coupling strength g, and , and are known, the read coupling capacitor at each coupling point can be solved.

[0046] Among them, the resonant fundamental frequency of the filter is:

[0047]

[0048] Among them, is the fundamental frequency of the coplanar waveguide geometry cavity without superconducting quantum interference device, is the inductance introduced by the superconducting quantum interference device, is the geometric unit inductance of the coplanar waveguide cavity, is the geometric length of the cavity. By changing the geometric unit of the coplanar waveguide cavity, the inductance of the superconducting quantum interference device, etc., the resonant fundamental frequency of the filter can be changed. Among them, the inductance value introduced by the superconducting quantum interference device will change with the magnetic flux of the Josephson junction loop, and the magnetic flux will change with the DC size of the control line. This is one of the reasons why the filter frequency of the present invention is adjustable. In addition, the nonlinearity introduced by the Josephson junction makes the filter frequency change with the power of the input microwave signal, that is, Kerr nonlinearity. As the current value of the input microwave signal increases, the passband frequency of the filter will gradually decrease, which can be explained by the Josephson critical current. To characterize, or use anharmonic (name the lowest three energy levels of the filter resonator are 0, 1, 2, anharmonic is defined as ) and the number of photons. In the solution of the present invention, by inputting current into the integrated DC control line 109, the equivalent inductance introduced by the Josephson junction can be changed, thereby changing the resonant (passband) frequency of the filter. The change in the filter passband frequency will lead to a change in the effective leakage rate of the reading cavity, specifically as follows Figure 2 As shown, with the reading cavity frequency as a reference, after adjusting the filter frequency, the effective leakage rate at a specific position is equal to the dispersion shift, the dephasing rate is maximized at this time, and the read signal-to-noise ratio takes a maximum value. During non-reading time, the filter frequency can be located at other positions to keep the dephasing rate at a lower point. In short, the adjustable frequency range of the filter depends on: the geometric length of the cavity, the junction resistance of the two Josephson junctions, the size of the Josephson junction loop magnetic flux, the input microwave signal power, etc. It can be seen that based on the solution of the present invention, the filter frequency can be effectively adjusted by designing the geometric length of the filter, the size of the Josephson junction in the superconducting quantum interference device, the size of the input signal, the magnetic flux control of the DC control line, etc., thereby changing the effective leakage rate of the reading cavity, thereby adjusting the dephasing rate, etc. Among them, the preparation method of the Josephson junction is not limited, including but not limited to the Dolan bridge method.

[0049] According to one embodiment of the present invention, the resonant frequency utilized in filtering is not limited to the fundamental frequency, but also includes higher harmonic frequencies such as double frequency and triple frequency. The number of faces of the superconducting quantum chip in which the filter is integrated is not limited, including but not limited to single plane, flip-chip double-sided, perforated double-sided, and modular multi-sided. The number of reading cavities coupled to a single filter is greater than or equal to 2, and the specific number is not limited. It is only necessary to design the cavity geometry and coupling devices according to the design scheme of the present invention. The number of microwave transmission lines coupled to the filter is also not limited. The structure of the quantum bits coupled to the filter includes but is not limited to grounded transmission bits, floating bits, charge bits, and flux bits.

[0050] Similarly, if a read coupling inductor is used at the coupling point, the coupling strength between the capacitively coupled read cavity and the filter can be calculated using the following formula:

[0051]

[0052] in, is the mutual inductance between the filter and the reading cavity, i.e. Figure 1 The 108 series position shown in the figure requires a coupled inductor to be configured. is the effective inductance of the filter to ground at the coupling point, obtained through Y11 simulation. In addition, a hybrid coupling method of capacitance and inductance can also be used, that is, inductive coupling is used in some places and capacitive coupling is used in some places. The principle of inductive coupling is similar to that of capacitive coupling, and will not be described in detail in the embodiment of the present invention.

[0053] In summary, the present invention integrates a frequency-tunable nonlinear filter on a multi-superconducting qubit chip. Each filter is coupled to multiple readout resonant cavities. Whether the frequency is tunable by altering magnetic flux through DC control or by varying the input microwave power, the filter frequency can be adjusted. By utilizing a frequency-tunable nonlinear filter design that can be integrated into a multi-superconducting qubit chip, the present invention achieves adjustable passband frequency for signals at the integrated multi-qubit readout end.

[0054] Compared to existing linear filters used in multi-bit devices, the filter proposed in the present invention offers the following advantages: Its frequency tunability allows for adjustable effective cavity leakage in the readout cavity, improving the performance of reading qubits while suppressing the impact of photon noise on the bits. When the filter passband peak frequency and the readout cavity frequency are perfectly aligned, the effective cavity leakage is maximized, and when these frequencies deviate, the effective cavity leakage gradually decreases. This characteristic significantly improves the fault tolerance of the readout design and fabrication. The maximum cavity leakage can be designed to be significantly greater than the difference in dispersion shifts between the readout cavity when the qubit is in the 0 and 1 states, and the filter frequency can then be adjusted to achieve the optimal readout point where the dispersion shift difference matches the cavity leakage. Furthermore, the dispersion shifts in the readout cavity vary in magnitude for different qubit states. Given that actual quantum systems often occupy higher energy levels, measuring these higher energy levels requires different readout cavity leakage rates to achieve the maximum signal-to-noise ratio. Tunable filters enable flexible adjustment of the readout cavity leakage rate, improving the readout performance of actual superconducting quantum samples. Furthermore, tuning the filter frequency during non-reading time causes dispersion shift and cavity leakage to mismatch, hindering qubit dephasing caused by photon noise, thereby improving the qubit's T2. This also allows the filter's idle time to be at a position more detuned from the qubit, no longer constrained by the read cavity frequency, further suppressing qubit decoherence due to the Purcell effect. Each filter in the present invention can be coupled to multiple read resonant cavities, allowing integration into multi-bit devices, enabling multiple qubit reads with one filter. The filters in the present invention occupy minimal wiring space, nearly equivalent to current mainstream multi-bit linear filters. Furthermore, the DC signal used for frequency modulation is applied via an integrated control line, enabling precise frequency modulation of the target filter.

[0055] In addition, based on the frequency tunability of the design of the scheme of the present invention, the filter in the scheme of the present invention can be used for the rapid reset of quantum bits. The filter has a leakage rate of more than 100MHz and is a good channel for the rapid dissipation of photons. During reset, the filter can be adjusted to a frequency that is offset from the reading cavity, so that after the photon enters the filter, it can be guaranteed to dissipate to the environment instead of entering the reading cavity. There are two ideas for reset. One is to adjust the filter to resonate with the quantum bit, so as to use the coupling between the filter and the quantum bit to take away the excitation of the quantum bit; the other idea is to use the coupler as an intermediary, move the quantum bit excitation to the coupler through the swap gate, and then transfer the excitation to the filter by adjusting the resonance of the filter and the coupler. Due to the large leakage rate of the filter, the excitation will be eliminated in a very short time. The good performance of the filter in the scheme of the present invention in reading, resetting, and protecting bits is helpful for the implementation of quantum error correction schemes such as surface codes, thereby providing a beneficial design idea for the hardware development of fault-tolerant quantum computers.

[0056] In order to understand the present invention more intuitively, the present invention is described below with reference to a specific example. In the following example, capacitive coupling is used as the coupling means.

[0057] like Figure 3As shown, a tunable nonlinear filter with three coupling points based on the present invention is integrated into a multi-qubit chip. The filter is coupled to the input transmission line via input coupling capacitor 201 and to the output coupling transmission line via output coupling capacitor 206. The filter has three coupling points, with capacitors 202, 203, and 204 coupling to the readout cavity, respectively. The superconducting quantum interference device 206 employs an asymmetric structure. The quantum chip circuit utilizes a coplanar waveguide with a 100-nanometer-thick aluminum film grown on a 430-micron-thick sapphire substrate. It features 24 qubits (with an O1 energy level transition frequency adjustable between 3 GHz and 5 GHz) and 38 couplers (with an O1 energy level transition frequency adjustable between 4 GHz and 9 GHz). The chip is a flip-chip, double-layer sample using indium bonding. The transmission lines, readout cavity, and filter cavity are located on the bottom layer. The qubits, couplers, and quantum interference device are located on the top layer. This layout allows for a single Josephson junction process on the top layer. The filter consists of a double-Josephson junction quantum interference device (QID) inserted at the geometric midpoint of an open-ended half-wavelength resonant cavity. The filter has a geometric length of 5600 microns and is based on a coplanar waveguide structure. The intermediate conduction band is 4 microns wide and 2 microns above ground. The frequency is tunable between 6 GHz and 7 GHz. The filter is connected to the input and output transmission lines via capacitive coupling, primarily 201 (136 fF) and 205 (14 fF), respectively. This allows microwave-band signals to enter the filter from the transmission lines. Typically, the larger capacitor serves as the output port, while the smaller capacitor serves as the input port. This results in a high-amplitude S21 signal and sufficient readout efficiency. The filter is connected to the read cavity (with intrinsic fundamental frequencies of 6.45 GHz, 6.5 GHz, and 6.55 GHz) via capacitive coupling via capacitors 202 (capacitor value 3.6 fF), 203 (capacitor value 5.4 fF), and 204 (capacitor value 2.3 fF). This creates a channel coupling the read cavity to the transmission line through the filter. The filter can control the effective cavity leakage of the read cavity, which is adjustable from 0.1 MHz to 10 MHz. The filter's frequency tunability is primarily achieved by the superconducting quantum interference device (SQID) shown in 206. The SQID is connected in series to the underlying geometric cavity via a 40 micron by 40 micron cross-section and 7 micron height indium bond on the top layer. A DC control line is integrated opposite the Josephson junction region 206. By varying the DC amplitude, the magnetic flux in the Josephson junction loop is altered, thereby varying the frequency. The Josephson junction shown in 206 employs an asymmetric structure, with the two junction resistances being 900 ohms and 3200 ohms, respectively, to adjust the frequency tunability. In addition, due to the Kerr nonlinearity of the Josephson junction, changes in the intensity of the microwave signal passing through the junction will also cause the frequency to change. That is, in the present invention, the passband frequency of the filter can be jointly controlled by the amplitude of the control line DC signal and the amplitude of the filter microwave signal. The passband frequency changes with the signal. Figure 4 and Figure 5 As shown, Figure 4 This is the experimentally measured S21 signal amplitude image. The black dots represent the filter frequencies obtained by fitting at different reading powers. Figure 5 This is also the S21 signal amplitude image measured experimentally. The black dotted line represents the relationship between the fitted filter frequency and the DC signal amplitude. Figure 4 and Figure 5 As can be seen, the present invention's solution can adjust the filter passband frequency in real time by controlling the amplitude of the DC signal on the control line and the amplitude of the microwave signal on the filter to achieve optimal read matching conditions. During reading, the microwave signal enters the circuit board via a coaxial cable, is wire-bonded to the coplanar waveguide, and enters the left end of the structure 201. It then enters the filter via capacitive coupling, which is the right end of the structure 201. The output terminal is 205, with the filter at the top. It is capacitively coupled to the transmission line at the bottom. The read signal then flows out from this terminal and enters a differential analysis device, such as an analog-to-digital converter, through a method similar to the input terminal. A superconducting quantum processor incorporating the present invention was placed in a BlueFors dilution refrigerator and cooled to approximately 10 mK. Characterization using a standard microwave measurement and control system revealed that 10 measurements, each containing 30,000 single-shot data points, achieved an average quantum bit read fidelity of 99.3%, and the signal-to-noise ratio of the orthogonal component raw data (IQ raw) reached 6.4.

[0058] In addition, in order to verify the dephasing rate of the present invention, the present invention uses a coupler 207 to couple with the filter to complete a quantum bit fast reset experiment with a reset error rate of less than 1% within 75ns, and by artificially injecting Gaussian white noise from the read line and measuring the quantum bit T2 at different filter frequencies, it is observed that the filter frequency modulation effectively suppresses the photon noise, and the dephasing rate change is consistent with Figure 2 The law shown is the ratio of dephasing rate to photon number Adjust the filter frequency to read the cavity effective leak rate (spectral line width) and twice the dispersion shift It reaches its maximum value when the phase degeneration rate is shifted away from this point.

[0059] While various embodiments of the present invention have been described above, the above descriptions are intended to be illustrative, non-exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is selected to best explain the principles of the embodiments, their practical applications, or technological improvements in the marketplace, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A multi-superconducting quantum bit chip, characterized in that: The multi-superconducting quantum bit chip includes: an input microwave transmission line, an input coupling capacitor or inductor, a filter, an output coupling capacitor or inductor, an output microwave transmission line, a plurality of read resonant cavities, and a plurality of read coupling capacitors or inductors, wherein each read resonant cavity corresponds to a coupled read coupling capacitor or inductor, wherein: The input microwave transmission line is coupled to the filter through an input coupling capacitor or inductor to input the microwave signal into the filter; The output microwave transmission line is coupled to the filter through an output coupling capacitor or inductor to output the microwave transmitted by the filter; The filter includes a first coplanar waveguide cavity, a superconducting quantum interference device, a second coplanar waveguide cavity, and a DC control line, wherein: one end of the first coplanar waveguide cavity is used to couple to a microwave input transmission line, and the other end is connected to one end of the superconducting quantum interference device; the two ends of the superconducting quantum interference device are respectively connected to the first coplanar waveguide cavity and the second coplanar waveguide cavity; one end of the second coplanar waveguide cavity is connected to the superconducting quantum interference device, and the other end is used to couple to a microwave output transmission line; the DC control line is used to apply DC control of magnetic flux to adjust the frequency of the filter; wherein the first coplanar waveguide cavity and the second coplanar waveguide cavity each have multiple coupling points; In which, each reading resonant cavity is coupled to a coupling point on the filter through its corresponding reading coupling capacitor or inductor so that the filter filters out microwaves other than the reading frequency of the reading resonant cavity. In which, the coupling strength between all reading resonant cavities and the filter is the same, and the reading coupling capacitor or inductor of each coupling point is configured according to the coupling strength between the reading resonant cavity and the filter, the effective capacitance to ground or effective inductance to ground of the filter at the coupling point obtained by simulation, and the effective capacitance to ground or effective inductance to ground of the reading resonant cavity.

2. The multi-superconducting quantum bit chip according to claim 1, characterized in that: The geometric lengths of the first coplanar waveguide cavity and the second coplanar waveguide cavity are determined according to the frequency and number of the read resonant cavities, so that the number of coupling points on the first coplanar waveguide cavity and the second coplanar waveguide cavity matches the number of the read resonant cavities.

3. The multi-superconducting quantum bit chip according to claim 2, characterized in that: The length ratio of the first coplanar waveguide cavity and the second coplanar waveguide cavity can be arbitrarily adjusted based on the following rule: when the superconducting quantum interference device is short-circuited, the first coplanar waveguide cavity and the second coplanar waveguide cavity can form a half-wavelength cavity with both ends open, a half-wavelength cavity with both ends closed, or a quarter-wavelength cavity with one end open and the other closed.

4. The multi-superconducting quantum bit chip according to claim 1, characterized in that: The superconducting quantum interference device includes two parallel Josephson junctions. The size of each Josephson junction is determined according to the expected adjustment range of the filter frequency. The superconducting quantum interference device is connected to the two coplanar waveguide cavities in series or in bypass parallel.

5. The multi-superconducting quantum bit chip according to claim 4, characterized in that: The DC control method of applying the magnetic flux on the DC control line is to apply the magnetic flux through an integrated control line, through an external coil, or through any external device that provides the magnetic flux.

6. The multi-superconducting quantum bit chip according to claim 1, characterized in that: The effective capacitance to ground or effective inductance to ground of the filter at each coupling point in the multi-superconducting quantum bit chip and the effective capacitance to ground or effective inductance to ground of the reading resonant cavity are obtained by the Y11 parameter of microwave simulation; the coupling strength between the reading resonant cavity and the filter is obtained by finite element simulation.

7. The multi-superconducting quantum bit chip according to claim 1, characterized in that: The size of the output coupling capacitor is determined according to the following constraints: The effective spectral linewidth of the read resonant cavity is equal to twice the dispersion shift of the read resonant cavity.

8. The multi-superconducting quantum bit chip according to claim 7, characterized in that: The size of the input coupling capacitor is one tenth of the size of the output coupling capacitor.

9. The multi-superconducting quantum bit chip according to claim 6, characterized in that: The read coupling capacitors for each coupling point are configured as follows: in, is the coupling strength, The read coupling capacitance to be solved, is the filter frequency, is the frequency of the reading resonant cavity, is the effective capacitance of the filter to ground at the coupling point, is the effective capacitance to ground of the reading resonant cavity; The read coupling inductance of each coupling point is set as follows: in, is the coupling strength, The coupled inductance to be solved is read, is the filter frequency, is the frequency of the reading resonant cavity, is the effective inductance of the filter to ground at the coupling point, is the effective capacitance to ground of the reading resonant cavity.

10. The multi-superconducting quantum bit chip according to claim 1, characterized in that: The multi-superconducting quantum bit chip is further configured to adjust the filter frequency by adjusting the amplitude of the input microwave.

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