CN116579434A - Double-quantum bit large-range adjustable coupling structure, method and storage medium - Google Patents
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Abstract
The invention relates to a double-quantum bit large-range adjustable coupling structure, a method and a storage medium. The invention connects two tunable qubits through a multimode coupled resonant link, wherein: the multimode coupling resonant link comprises a set number of resonant cavities with the same resonant frequency, the resonant cavities are connected in series, and two quantum bits are respectively coupled with the resonant cavities at two ends of the multimode coupling resonant link; connecting two quantum bits through any non-turn-off mode of the multimode coupling resonant link, and realizing the information transfer and quantum logic gate operation of the quantum bits comprises the following steps: fixing the energy spectrum of one quantum bit, then coupling the quantum bit with one mode of the multimode coupling resonant link by the fixed energy spectrum to generate exchange, then scanning the change of the energy spectrum of the other quantum bit along with the magnetic flux, and exchanging the information temporarily stored in the mode of the multimode coupling resonant link to the other quantum bit to finish the two-bit gate operation.
Description
Technical Field
The invention relates to the technical field of double-quantum bit large-range adjustable coupling, in particular to a double-quantum bit large-range adjustable coupling structure, a double-quantum bit large-range adjustable coupling method and a storage medium.
Background
In the last decade, the implementation technology of quantum computers has advanced significantly, and quantum computers have shown capabilities beyond classical computers in dealing with complex problems such as chemical molecular potential analysis, material science, and bulk decomposition. This capability is mainly manifested in a great reduction in both consumption of resources and time.
At present, the number of quantum bits contained in a chip of a superconducting quantum computer can reach hundreds, and some simple algorithm demonstration and quantum simulation can be performed. Superconducting quantum computers want to achieve these operations accurately, requiring single and double qubit operations in the chip to achieve higher fidelity. The fidelity of single bit gate operation can reach 99.99% experimentally at present, but for a two-qubit gate this level is difficult to achieve. Therefore, the fidelity of the double qubit gate operation is a major limitation of superconducting quantum computers. Currently, there are many experimental groups that have proposed using tunable couplers or a single Bus cavity to achieve a two-qubit gate operation with greater than 99% fidelity. The door operation realized by the single Bus cavity is limited by the coupling strength, so that the door operation time is too long, and the calculation efficiency is reduced. The application of the adjustable coupler to improve the operation fidelity of the double-quantum bit gate requires a complex circuit to control the adjustable coupler between the double-quantum bits, and the introduction of the complex control circuit is easy to cause mutual interference due to limited space when the large-scale circuit is applied, so that the realization of the large-scale circuit is not facilitated. Therefore, a coupling scheme is required to be provided, and the characteristics of circuit structure and simple and convenient waveform control are achieved while the fidelity is ensured.
Disclosure of Invention
In order to solve the above technical problems or at least partially solve the above technical problems, the present invention provides a dual-qubit widely adjustable coupling structure, a method and a storage medium.
In a first aspect, the present invention provides a double-qubit widely tunable coupling structure comprising: two tunable qubits, two of which are connected by a multimode coupled resonant link, wherein:
the multimode coupling resonant link comprises a set number of resonant cavities, the resonant cavities are connected in series, the resonant frequency of each resonant cavity is the same, and the two qubits are respectively coupled with the resonant cavities at two ends of the multimode coupling resonant link; connecting two quantum bits through any non-turn-off mode of the multimode coupling resonant link, and realizing the information transfer and quantum logic gate operation of the quantum bits comprises the following steps: fixing the energy spectrum of one quantum bit, then coupling the quantum bit with one mode of the multimode coupling resonant link by the fixed energy spectrum to generate exchange, then scanning the change of the energy spectrum of the other quantum bit along with the magnetic flux, and exchanging the information temporarily stored in the mode of the multimode coupling resonant link to the other quantum bit to finish the two-bit gate operation.
Furthermore, the resonant cavity comprises a central superconductor arranged on the chip substrate and a half infinite ground plane at two sides of the central superconductor, the distances between the central superconductor and the ground planes at two sides are equal, and vacuum is arranged above the central superconductor and the ground plane.
Furthermore, the width of the central superconductor is designed to be far larger than the thicknesses of the central superconductor and the ground plane, the distance between the central superconductor and the ground planes at two sides is far smaller than the thickness of the substrate, so that the capacitance and inductance of unit length are known, and the resonant cavity with the set resonant frequency is realized by designing the set length.
Still further, the qubit that is tunable includes: two parallel-connected Josephson junctions are integrally connected in parallel with a capacitor, the qubit is coupled with an XY control line through a coupling capacitor, and the qubit is coupled with a Z control line.
Still further, the Z control line comprises an adjustable current source which is connected in series with a protection resistor and a mutual inductance coil which is mutually inductance with two parallel Josephson junctions in the qubit.
Still further, the XY control line comprises an adjustable microwave source connected to a resistor coupled to the coupling capacitor.
Still further, the qubit is coupled to the quantum read cavity and a read feed line that transmits a read signal through a read capacitance.
In a second aspect, the present invention provides a method for coupling a double-quantum bit with a large-scale tunable coupling, which is applied to the double-quantum bit with a large-scale tunable coupling structure, and includes:
and fixing the energy spectrum of one quantum bit in the double-quantum bit large-range adjustable coupling structure, then coupling the quantum bit with one mode of a multimode coupling resonant link in the double-quantum bit large-range adjustable coupling structure by the fixed energy spectrum to generate exchange, then scanning the energy spectrum of the other quantum bit along with the change of magnetic flux, and exchanging the information temporarily stored in the multimode coupling resonant link mode to the other quantum bit to finish the two-bit gate operation.
Further, the mode of the multimode coupled resonant link is selected based on the requirement of coupling strength, wherein high fidelity of the qubit gate is achieved through preset non-unitary coupling off.
In a third aspect, the present invention provides a computer readable storage medium storing a computer program which, when executed by a control circuit in a two-qubit widely tunable coupling architecture, implements the two-qubit widely tunable coupling method.
Compared with the prior art, the technical scheme provided by the embodiment of the invention has the following advantages:
the invention connects two tunable qubits through a multimode coupled resonant link, wherein: the multimode coupling resonant link comprises a set number of resonant cavities, the resonant cavities are connected in series, the resonant frequency of each resonant cavity is the same, and the two qubits are respectively coupled with the resonant cavities at two ends of the multimode coupling resonant link; connecting two quantum bits through any non-turn-off mode of the multimode coupling resonant link, and realizing the information transfer and quantum logic gate operation of the quantum bits comprises the following steps: fixing the energy spectrum of one quantum bit, then coupling the quantum bit with one mode of the multimode coupling resonant link by the fixed energy spectrum to generate exchange, then scanning the change of the energy spectrum of the other quantum bit along with the magnetic flux, and exchanging the information temporarily stored in the mode of the multimode coupling resonant link to the other quantum bit to finish the two-bit gate operation. The multi-mode coupling resonant link does not need a control circuit, and the formed coupling scheme has simple and convenient advantages for circuit structure and experimental waveform control, and is beneficial to the realization of a large-scale circuit. In addition, the formed scheme can realize non-single coupling Guan Duandian and can improve the fidelity of single-quantum bit gate operation. By limiting the amount of cavity of the multimode coupled resonant link in terms of delay, delay can be effectively avoided, and a balance is sought between delay and amount of off-point, delay and tunable range.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and together with the description, serve to explain the principles of the invention.
In order to more clearly illustrate the embodiments of the invention or the technical solutions of the prior art, the drawings which are used in the description of the embodiments or the prior art will be briefly described, and it will be obvious to a person skilled in the art that other drawings can be obtained from these drawings without inventive effort.
FIG. 1 is a schematic diagram of a large-scale adjustable coupling structure of double quantum bits according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a tunable qubit according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of an XY control line provided by an embodiment of the present invention;
FIG. 4 is a schematic diagram of a Z control line provided by an embodiment of the present invention;
FIG. 5 is a schematic diagram of a resonant cavity in a multimode-coupled resonant link according to an embodiment of the present invention;
FIG. 6 is a schematic diagram of a read chamber and a read feed line provided by an embodiment of the present invention.
The reference numerals and meanings in the figures are as follows:
1. a resonant cavity, 11, a chip substrate, 12, a central superconductor, 13 and a ground plane;
2. qubits, 21, josephson junctions, 22, capacitance.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
It should be noted that, in this document, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.
In the research of the fidelity of the double-quantum bit gate operation, at present, many experimental groups have proposed to realize the double-quantum bit gate operation with the fidelity of more than 99% by using an adjustable coupler or a single Bus cavity, and the gate operation realized by using the single Bus cavity is limited by the coupling strength, so that the gate operation time is too long, and the calculation efficiency is reduced.
In the research of the operation fidelity of the double-quantum bit gate, the fidelity is improved by relying on an adjustable coupler, the main tunable couplers coupled between the double-quantum bit are single-trans-mon couplers at present, and the single-trans-mon couplers can not only realize the rapid double-quantum bit gate, but also close the energy exchange interaction called transverse or XY coupling to reduce the residual coupling errors. In single-fransmon couplers, there is unavoidable ZZ coupling for highly detuned qubits. Thus, ZZ coupling in single-fransmon couplers results in qubit frequency crowding or crosstalk between qubits. To address the shortcomings of single-fransmon couplers, toshiba theoretically proposed a tunable coupler-dual transmission gate coupler: the dual transmission gate coupler is formed by coupling two fixed frequency transmission qubits through a common loop with an additional josephson junction. The coupling between the two transmission qubits is controlled by controlling the magnetic flux in the loop, thereby adjusting the coupling strength between the computation qubits. The double transmission gate coupler can realize high double quantum bit gating fidelity exceeding 99.99% in a short gating time of 24ns, and has no residual ZZ coupling in an idle time of high-detuning fixed-frequency transmission of detuning 0.7 GHz. However, the coupling is realized by controlling the coupling strength between two calculation qubits to be completely zero through the tuning of magnetic flux of an external magnetic field, so that the coupling strength is improved to tens of megahertz through the increasing of the magnetic flux, and the rapid double-quantum bit operation is realized. Complexity control circuits are introduced, which is not beneficial to large-scale circuit implementation. In order to solve the above technical problems or at least partially solve the above technical problems, the present invention provides a dual-qubit widely adjustable coupling structure, a method and a storage medium.
Example 1
Referring to fig. 1, an embodiment of the present invention provides a two-qubit large-scale tunable coupling structure, two qubits 2 with tunable flux, and two qubits 2 are connected through a multimode coupled resonant link. Wherein:
the multimode coupling resonant link comprises a set number of resonant cavities 1 connected in series, the resonant frequency of each resonant cavity 1 is the same, and two qubits 2 are respectively coupled with the resonant cavities 1 at two ends of the multimode coupling resonant link.
In the specific implementation process, the superconducting quantum computer is described by taking the superconducting quantum bit as an example:
in superconducting quantum computers, two tunable qubits 2 are implemented based on josephson junctions. The structure of the josephson junction includes superconductor, insulator and superconductor configuration, superconductor, normal conductor and superconductor configuration, normal conductor, insulator and superconductor configuration. The voltage across the josephson junction is related to the phase difference between the flux quanta and the junction, and the nonlinear equivalent inductance is formed by the correlation of the superconducting current of the josephson junction with the critical current of the josephson junction and the phase difference across the junction:wherein (1)>Phase difference at both ends of junction, phi 0 Is a magnetic flux quantum, phi (t) is a magnetic flux, I c Is the critical current. The josephson junction forms qubits by introducing a nonlinear inductance such that the energy levels of the josephson junction forming the LC resonance are no longer equidistant.
As a preferred embodiment, referring to fig. 2, the adjustable qubit 2 uses two parallel josephson junctions 21, the josephson junctions 21 connected in parallel are connected in parallel to form an overall parallel capacitor 22, the qubit 2 is coupled to an XY control line through a coupling capacitor, and the qubit 2 is coupled to a Z control line. A control circuit of a double-qubit large-range adjustable coupling structure inputs microwave signals corresponding to energy levels of the qubits through the XY control line to carry out resonance driving or detuning driving on the qubits; a control circuit of a double-quantum bit large-range adjustable coupling structure induces magnetic flux in a SQUID loop formed by a Josephson junction through a Z control line input current signal, and changes phases at two ends of the Josephson junction.
In a specific implementation process, referring to fig. 3, the XY control line includes an adjustable microwave source, where the adjustable microwave source is connected to a resistor, and the resistor is coupled to the coupling capacitor.
When the adjustable microwave source is not driven by voltage, the Hamiltonian quantity of the circuit is as follows:
C Σ ＝C+C O ，C O for the capacitance value of the coupling capacitance, +.>
The lagrangian quantity of the line is:
when the adjustable microwave source is driven by voltage, the Lagrange quantity of the circuit is as follows:
the hamiltonian of the line is calculated as:
wherein (1)>
The Hamiltonian amount is converted into a lifting operator in the form of:
under the drive of the adjustable microwave source, the ratio oscillation frequency of the qubit 2 is as follows:
referring to FIG. 4, the Z control line comprises an adjustable current source and an electrically adjustable current source stringAnd the protection resistor and the mutual inductance coil are connected, and the mutual inductance coil is mutually inducted with two parallel Josephson junctions in the qubit. When current is present in the Z control line, magnetic flux Φ (t) =L generated in SQUID formed by Josephson junction m I (t), wherein I (t) is the drive current, L m Is mutual inductance.
The adjustable current source and the adjustable microwave source are controlled by a control circuit in the double-quantum bit large-range adjustable coupling structure, and the state of the quantum bit 2 is adjusted.
Each of the resonant cavities in the multimode coupled resonant link is an LC resonator formed by a capacitive element and an inductive element in a quantum circuit.
The hamiltonian of the LC resonator is:
wherein, the liquid crystal display device comprises a liquid crystal display device,is the angular frequency of the LC resonator, magnetic flux operator +.>Charge arithmetic symbol To raise and lower the arithmetic, the charge arithmetic and the magnetic flux arithmetic satisfy the relationship +.>
The hamiltonian quantization of the LC resonator is:
the eigenstates of the hamiltonian amount of the LC resonator satisfy:
representing the quantum states represented by the energy levels of the LC harmonic oscillator, the energy of each energy levelThe interval between adjacent energy levels is->
In a specific implementation, referring to fig. 5, a preferred structure of the resonant cavity 1 includes: the central superconductor 12 is arranged on the chip substrate 11, and the two sides of the central superconductor 12 are provided with a semi-infinite ground plane 13, the distances between the central superconductor 12 and the ground planes 13 at the two sides are equal, and vacuum is arranged above the central superconductor 12 and the ground planes 13. The width W of the center superconductor 12 is much larger than the thickness H of the center superconductor 12 and the ground plane 13, and the distance L between the center superconductor 12 and the ground planes 13 on both sides is much smaller than the thickness H of the substrate 11.
Capacitance per unit length
Inductance per unit length
The equivalent dielectric constant of the epsilon resonant cavity is epsilon relative to the relative dielectric constant r Is provided with a plurality of the chip substrates,the resonant cavity is designed to set a resonant frequency by designing a set length.
The qubit is coupled to the quantum read cavity and a read feed line that transmits a read signal. In the specific implementation process, referring to fig. 6, the quantum bit is coupled to the quantum reading cavity formed by the equivalent capacitor C4 and the equivalent inductor L2 through the readout coupling capacitor C3, the quantum reading cavity is a quarter-wavelength LC resonant cavity, the quantum reading cavity is coupled to the reading feeder line formed by the equivalent inductor L3 and the equivalent capacitor C5 through mutual inductance, the reading feeder line adopts a half-wavelength LC filter, and the reading feeder line is connected to the signal receiving device through the output coupling capacitor C2.
Connecting two qubits 2 through any non-turn-off mode of the multimode coupling resonant link, and realizing the information transfer and quantum logic gate operation of the qubits 2 comprises the following steps: fixing the energy spectrum of one quantum bit, then coupling the quantum bit with the fixed energy spectrum with any non-turn-off mode of the multimode coupling resonant link to generate exchange, then scanning the change of the energy spectrum of the other quantum bit along with the change of magnetic flux, and exchanging the information temporarily stored in the multimode coupling resonant link mode to the other quantum bit to finish the two-bit gate operation. When the multimode coupling resonant link comprising five resonant cavities is applied, due to the mutual coupling between energy spectrums, an adjustable range of four times of coupling strength can be realized, and six non-crossing can be found by scanning the magnetic flux energy spectrum of the quantum bit. The multimode coupling resonant link realizes large-scale adjustable coupling between the double quantum bit gates, and the formed coupling scheme has simple and convenient advantages for circuit structure and experimental waveform control, and is beneficial to the realization of a large-scale circuit. In addition, the formed coupling scheme can realize non-single coupling Guan Duandian and can improve the fidelity of single-quantum bit gate operation.
Example 2
The embodiment of the invention provides a double-quantum bit large-range adjustable coupling method which is applied to the double-quantum bit large-range adjustable coupling structure and comprises the following steps:
selecting a mode of the multimode coupling resonant link based on the requirement of coupling strength, and fixing the energy spectrum of one quantum bit in the double-quantum bit large-range adjustable coupling structure capable of coupling the set mode of the multimode coupling resonant link; then the quantum bit with fixed energy spectrum is coupled with one mode of the multimode coupling resonance link in the double-quantum bit large-range adjustable coupling structure to generate exchange, then the energy spectrum of another quantum bit is scanned along with the change of magnetic flux, and the information temporarily stored in the multimode coupling resonance link mode is exchanged to another quantum bit to finish the two-bit gate operation. The high-fidelity of the single-quantum bit gate is realized through preset non-single coupling turn-off.
Example 3
The embodiment of the invention provides a computer readable storage medium, which stores a computer program, and when the computer program is executed, the method for realizing the double-quantum-bit large-range adjustable coupling comprises the following steps: selecting a mode of the multimode coupling resonant link based on the requirement of coupling strength, and fixing the energy spectrum of one quantum bit in the double-quantum bit large-range adjustable coupling structure capable of coupling the set mode of the multimode coupling resonant link; then the quantum bit with fixed energy spectrum is coupled with one mode of the multimode coupling resonance link in the double-quantum bit large-range adjustable coupling structure to generate exchange, then the energy spectrum of another quantum bit is scanned along with the change of magnetic flux, and the information temporarily stored in the multimode coupling resonance link mode is exchanged to another quantum bit to finish the two-bit gate operation. The high-fidelity of the single-quantum bit gate is realized through preset non-single coupling turn-off.
In the embodiments provided in the present invention, it should be understood that the disclosed structures and methods may be implemented in other manners. For example, the structural embodiments described above are merely illustrative, and for example, the division of the units is merely a logical function division, and there may be other manners of division in actual implementation, for example, multiple units or components may be combined or integrated into another system, or some features may be omitted, or not performed. Alternatively, the coupling or direct coupling or communication connection shown or discussed with each other may be an indirect coupling or communication connection via interfaces, structures or units, which may be in electrical, mechanical or other forms.
The units described as separate units may or may not be physically separate, and units shown as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.
In addition, each functional unit in the embodiments of the present invention may be integrated in one processing unit, or each unit may exist alone physically, or two or more units may be integrated in one unit. The integrated units may be implemented in hardware or in software functional units.
The foregoing is only a specific embodiment of the invention to enable those skilled in the art to understand or practice the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (10)
1. A double-qubit widely tunable coupling structure, comprising: two tunable qubits, two of which are connected by a multimode coupled resonant link, wherein:
the multimode coupling resonant link comprises a set number of resonant cavities, the resonant cavities are connected in series, the resonant frequency of each resonant cavity is the same, and the two qubits are respectively coupled with the resonant cavities at two ends of the multimode coupling resonant link; connecting two quantum bits through any non-turn-off mode of the multimode coupling resonant link, and realizing the information transfer and quantum logic gate operation of the quantum bits comprises the following steps: fixing the energy spectrum of one quantum bit, then coupling the quantum bit with one mode of the multimode coupling resonant link by the fixed energy spectrum to generate exchange, then scanning the change of the energy spectrum of the other quantum bit along with the magnetic flux, and exchanging the information temporarily stored in the mode of the multimode coupling resonant link to the other quantum bit to finish the two-bit gate operation.
2. The dual qubit widely tunable coupling structure of claim 1 wherein the resonant cavity comprises a center superconductor disposed on a chip substrate and a semi-infinite ground plane on both sides of the center superconductor, the center superconductor being equidistant from the ground planes on both sides, and a vacuum above the center superconductor and the ground plane.
3. The double qubit widely tunable coupling structure according to claim 2, wherein the width of the center superconductor is designed to be much larger than the thickness of the center superconductor and the ground plane, and the distance between the center superconductor and the ground planes on both sides is much smaller than the thickness of the substrate, so that the capacitance and inductance per unit length are known, and the resonant cavity with the set resonant frequency is realized by designing the set length.
4. The dual qubit widely tunable coupling structure of claim 1, wherein the tunable qubit comprises: and the two parallel-connected Josephson junctions are integrally connected with a capacitor in parallel, the qubit is coupled with the XY control line through a coupling capacitor, and the qubit inductor is coupled with the Z control line.
5. The dual qubit widely tunable coupling structure of claim 4, wherein the Z control line comprises an adjustable current source that concatenates a protection resistor and a mutual inductance coil that is mutually inductive with two parallel josephson junctions in the qubit.
6. The dual qubit widely tunable coupling structure of claim 4, wherein the XY control line comprises a tunable microwave source connected to a resistor, the resistor coupled to the coupling capacitor.
7. The dual qubit widely tunable coupling structure of claim 1, wherein the qubit couples the quantum read cavity and a read feed line transmitting a read signal through a read capacitance.
8. A double-quantum bit large-scale adjustable coupling method applied to the double-quantum bit large-scale adjustable coupling structure as claimed in any one of claims 1 to 8, comprising:
and fixing the energy spectrum of one quantum bit in the double-quantum bit large-range adjustable coupling structure, then coupling the quantum bit with one mode of a multimode coupling resonant link in the double-quantum bit large-range adjustable coupling structure by the fixed energy spectrum to generate exchange, then scanning the energy spectrum of the other quantum bit along with the change of magnetic flux, and exchanging the information temporarily stored in the multimode coupling resonant link mode to the other quantum bit to finish the two-bit gate operation.
9. The method of claim 8, wherein the mode of the multimode coupled resonant link is selected based on a requirement of coupling strength, wherein high fidelity of the qubit gate is achieved by a preset non-unitary coupling off.
10. A computer readable storage medium storing a computer program which, when executed by a control circuit in a double-qubit widely tunable coupling architecture, implements the double-qubit widely tunable coupling method according to claim 8 or 9.
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