Adjustable coupler with coupling extension

Abstract

A tunable coupler is provided for generating a controllable coupling to at least a first qubit (801). It comprises a first fixed coupling element (803) and a tunable coupling element (804). The first fixed coupling element (803) forms a non-galvanic coupling interface to at least the first qubit (801) at a first end remote from the tunable coupling element (804). The tunable coupling element (804) is positioned adjacent to a non-galvanic coupling interface formed between the first fixed coupling element (803) and a further circuit element (802) at its second end.

Description

Technical Field

[0001] The present invention generally relates to the technology of quantum computing. In particular, the present invention relates to hardware used to create qubits and form couplings between two or more qubits.

Background Art

[0002] In quantum computing, the term qubit is generally used to designate not only the basic unit of information but also the information storage element used to store the information of one qubit. As an example, a superconducting memory circuit having one or more qubits (i.e., an information storage element of qubit size) can be considered. In such an example, the qubit is a non-harmonic oscillator such as a transmon, and the non-harmonic oscillator can be coupled to a nearby readout resonator to facilitate the readout of the state of the qubit stored therein.

[0003] To implement a quantum gate, it is essential that there is a controllable coupling between qubits so that the states of the qubits can interact with each other in a controllable manner. In the case of an electrical qubit having a characteristic resonance frequency, a relatively simple way to control the coupling between adjacent qubits involves frequency tuning, whereby the qubit is tuned (or brought close) to resonance for strong coupling (on position) and detuned for weak coupling (off position). Such an arrangement places an upper limit on the on-off ratio of the gate for a given gate speed. There is no known scalable method to cancel out the unwanted entanglement of the idling qubits due to weak always-on interactions.

[0004] A more general approach is to use an adjustable coupling element between two qubits, as described, for example, in Non-Patent Document 1. However, known methods using adjustable coupling elements have drawbacks in terms of distance and dimensions. Sufficient capacitance is required not only between each individual qubit and the adjustable coupling element, but also between the qubits themselves, which implies keeping the qubits relatively close to each other. At the same time, short distances between qubits increase coupling between undesirable pairs of qubits and between qubits and control leads, causing crosstalk. Furthermore, short distances between qubits also limit the amount of space available for other necessary components, such as readout resonators.

[0005] A structural and functional solution is needed that eliminates undesirable crosstalk, giving greater freedom in circuit hardware design and implementation, while simultaneously enabling a sufficiently strong but controllable coupling between qubits. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] F. Yan et al., "Tunable Coupling Scheme for Implementing High-Fidelity Two-Qubit Gates," Phys. Rev. Applied, Vol. 10, No. 5, p. 54062, November 2018. [Overview of the project] [Problems that the invention aims to solve]

[0007] The objective is to eliminate undesirable crosstalk, provide greater freedom in circuit hardware design and implementation, and simultaneously offer a configuration that enables strong yet controllable coupling between qubits. [Means for solving the problem]

[0008] The object of the present invention is achieved by utilizing a tunable coupler equipped with a separate coupling extender to implement coupling between qubits that can be generated to implement gates in quantum computing.

[0009] According to a first embodiment, an adjustable coupler is provided for generating a controllable coupling to at least a first qubit, comprising a first fixed coupling element and an adjustable coupling element. The first fixed coupling element forms a non-galvanic coupling interface to at least the first qubit at a first tip of the first fixed coupling element, away from the adjustable coupling element. The adjustable coupling element is positioned adjacent to a non-galvanic coupling interface formed to a further circuit element at a second tip of the first fixed coupling element.

[0010] According to one embodiment, the first fixed coupling element is a waveguide. This has the advantage that the length of the first fixed coupling element can be used to create a sufficiently long distance between other circuit elements.

[0011] According to one embodiment, the first fixed coupling element is a waveguide resonator. This has the additional advantage, in addition to the advantages described above, that the resonant characteristics of the first fixed coupling element can be used when setting the strength of each electromagnetic coupling in which the first fixed coupling element is involved.

[0012] According to one embodiment, the first fixed coupling element is a lumped element resonator. This has the advantage that the characteristic impedance of the first fixed coupling element may be selected over a very wide range, which allows for mediating a very strong coupling between the qubits.

[0013] According to one embodiment, the first fixed coupling element is a conductive island. This has the advantage that its dimensions can be utilized very effectively, particularly in conjunction with quantum dot qubits.

[0014] According to one embodiment, the adjustable coupler comprises a second fixed coupling element that forms a non-galvanic coupling interface to a second qubit at the tip of the second fixed coupling element, away from the adjustable coupling element. The adjustable coupling element may then be positioned adjacent to the non-galvanic coupling interface formed between the first and second fixed coupling elements. This has the advantage that a coupling may exist between the first and second fixed coupling elements, and the adjustable coupling element may be used to influence the strength of that coupling.

[0015] According to one embodiment, the second fixed coupling element is one of a waveguide, a waveguide resonator, a lumped element resonator, or a conductor island. Each of these options offers the same advantages as those already described above with respect to the first fixed coupling element.

[0016] According to one embodiment, the first fixed coupling element and the second fixed coupling element are waveguides, each of which comprises a coupling region at each end adjacent to an adjustable coupling element. Both the coupling regions of the first fixed coupling element and the second fixed coupling element comprise a first edge adjacent to the first edge of the other coupling region, and a second edge adjacent to each edge of the adjustable coupling element. This has the advantage that coupling between various elements can be designed with high precision and repeatability.

[0017] According to one embodiment, an adjustable coupling element occupies a first sector of an annular two-dimensional region, and each of the coupling regions of the first fixed coupling element and the second fixed coupling element occupies a further sector of the annular two-dimensional region. The further sector may then be an adjacent sector of the annular two-dimensional region. The first sector and the further sector together may cover the entire annular two-dimensional region. This has the advantage that desired characteristics of the element can be realized in a very small size and shape.

[0018] According to one embodiment, the adjustable coupler comprises a chain of successive fixed coupling elements, one of which is the first fixed coupling element, and a non-galvanic coupling interface is formed between successive fixed coupling elements in the chain. The adjustable coupler can then comprise at least two adjustable coupling elements, each of the at least two adjustable coupling elements being adjacent to one respective one of the non-galvanic coupling interfaces formed between successive fixed coupling elements in the chain. This has the advantage that even very large quantum computing circuits can be designed using the principles described above.

[0019] According to a second aspect, there is provided a quantum computing circuit comprising an adjustable coupler of the kind described above and at least one qubit, the adjustable coupler forming a controllable coupling to the at least one qubit.

[0020] According to one embodiment, the quantum computing circuit comprises two qubits, and the adjustable coupler forms a controllable coupling between the two qubits. This has the advantage that an accurately controllable coupling can be formed between the two qubits while minimizing crosstalk and other adverse effects typical of prior art means.

Brief Description of the Drawings

[0021] The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

[0022] [Figure 1] Shows a fixed coupling between two qubits. [Figure 2] Shows the use of an adjustable coupling element between two qubits. [Figure 3] Illustrates the principle of FIG. 2 in a different diagrammatic representation. [Figure 4] Shows an example of a qubit and a capacitive coupler. [Figure 5] Shows the arrangement of an X-shaped qubit and an adjustable coupling element. [Figure 6] Shows the arrangement of an X-shaped qubit and a slab-shaped adjustable coupling element. [Figure 7] Shows two X-shaped qubits having a non-adjustable coupling element therebetween. [Figure 8] Shows an example using both an adjustable coupling element and a non-adjustable coupling element. [Figure 9] Shows an example of a possible implementation of the principle of FIG. 8. [Figure 10] Shows another example of a possible implementation of the principle of FIG. 8. [Figure 11] Shows another example using both an adjustable coupling element and a non-adjustable coupling element. [Figure 12] Shows an example of a possible implementation of the principle of FIG. 11. [Figure 13] Shows an example of a possible implementation of the principle of FIG. 8. [Figure 14] Shows one possible modification of a part of the arrangement shown in FIG. 13. [Figure 15] Shows a generalization of the principle of FIG. 8 for an arrangement having more qubits.

Best Mode for Carrying Out the Invention

[0023] FIG. 1 is an example of how the coupling between two qubits 101 and 102 can be affected by adjusting the qubits. Assuming that qubits 101 and 102 form a standard two-qubit gate, their on and off positions correspond to strong and weak couplings between the qubits. In FIG. 1, each of qubits 101 and 102 has a dedicated adjustment input, which can be used to change the resonance frequency of the corresponding qubit. In the off position, the qubits are detuned. Such an arrangement shows the inconvenient features mentioned above in the description of the prior art.

[0024] Figure 2 shows an example using an adjustable coupling element 203 to affect the coupling between two qubits 201 and 202. Figure 3 shows this in the form of a schematic circuit diagram. Capacitive couplings 1C, C2, and 12 in Figure 2 appear as capacitors C1C, C2C, and C12 drawn in Figure 3, respectively. The adjustment inputs shown in Figure 2 are omitted from Figure 3 for clarity. Detuning between the first qubit 201 and the adjustable coupling element 203 affects coupling 1C. Similarly, detuning between the second qubit 202 and the adjustable coupling element 203 affects coupling C2. Mutual adjustment of the two qubits 201 and 202 affects coupling 12. In addition to adjustment, the physical distance between each pair of circuit elements also has a significant effect on their coupling.

[0025] For proper operation of the arrangement shown in Figure 2, it is important to understand that qubits 201 and 202, as well as the adjustable element 203, must be relatively close to each other so that capacitance C12, in addition to capacitances 1C and C2, is also sufficiently large. This, in turn, prevents the coupling between undesirable pairs of qubits and the control lead from becoming too large in systems containing more qubits.

[0026] In many of the following diagrams, a simplified plus sign is used for qubits (and in some cases for tunable coupling elements). Figure 4 is provided to show and illustrate what such a plus-shaped circuit element might actually look like. Figure 4 is a top view of a piece of a quantum computing circuit, where a substrate (e.g., silicon or sapphire) has layers and patterns of conductive and / or superconducting material deposited on its surface. The cross-hatched areas in Figure 4 represent the exposed parts of the substrate surface, while the white-painted areas represent the conductive and / or superconducting material.

[0027] The majority of the substrate surface is filled with a ground surface 401 patterned with a matrix of small openings, fabricated from a superconducting material to reduce the effects of undesirable eddy currents. A plus-shaped region 402 of the superconducting material constitutes the capacitive portion of the qubit, while a detailed pattern of 403 provides a Josephson junction. Two examples are shown of how other circuit elements within a quantum computing circuit may form a non-galvanic coupling interface to a qubit. At the top, a fork-shaped region 404 implements capacitive coupling via the upper branch of the plus-shaped region 402. At the bottom, the end of a transmission line 405 forms another type of non-galvanic coupling interface to that portion of the qubit where a Josephson junction is located.

[0028] Figure 5 illustrates how the principle of using adjustable coupling elements for coupling between qubits, as described above with reference to Figures 2 and 3, can be utilized in an array of qubits. Here, the plus signs in the crosshatch represent qubits, and the plus signs in the simple hatches between them represent adjustable coupling elements. Figure 6 shows another example where the adjustable coupling elements are simple linear shapes rather than plus shapes, and are displayed adjacent to the mutually facing branches of plus-shaped qubits that affect the coupling. Regardless of the shape of the adjustable coupling elements, the distance between qubits cannot be increased because this would reduce the direct capacitance between qubits (shown as C12 in Figure 3). As a result, in both Figures 5 and 6, the qubits must be relatively close to each other, which causes crosstalk between qubits. The narrow spacing also limits the amount of space available on the substrate surface for other necessary circuit elements, such as readout resonators, which are not shown in the schematic diagrams of Figures 5 and 6.

[0029] The distance between adjacent qubits is also related to the size and shape of the adjustable coupling element. When qubits are close together, the direct capacitance between the two qubits becomes sufficiently large, even if the adjustable coupling element is positive-shaped (the same shape as the qubit itself). As shown in Figure 6, using linear or slab-shaped adjustable coupling elements allows the qubits to be brought even closer together. This means increased coupling between qubits, resulting in faster gates, but at the cost of increased crosstalk between qubits.

[0030] Figure 7 illustrates the concept of a quantum bus, which is essentially an extended conductor 701, with each end of the extended conductor 701 forming a non-galvanic coupling interface to its respective qubit. Such an extended conductor 701 is not tunable and is therefore sometimes called a fixed coupling element. In the diagram, different kinds of simple hatches (sparse and slanted to the left) are used to highlight the difference from the tunable coupling elements shown in Figures 5 and 6. The left end of the extended conductor 701 forms a non-galvanic coupling interface to the first qubit 702, and the right end of the extended conductor 701 forms a non-galvanic coupling interface to the second qubit 703. The quantum bus is a means of increasing the distance between two qubits, thereby mitigating the problems of crosstalk and congestion on the substrate surface while maintaining sufficient capacitive coupling between qubits.

[0031] Figure 8 illustrates the principle of an adjustable coupler for controllable coupling to (or between) two qubits 801 and 802. The adjustable coupler comprises a first fixed coupling element 803 and an adjustable coupling element 804. In the embodiment of Figure 8, the adjustable coupler also comprises a second fixed coupling element 805.

[0032] The first fixed coupling element 803 forms a non-galvanic coupling interface to the first qubit 801. The schematic diagram in Figure 8 does not take any stance on the physical outlines of the various elements, but it can be assumed that the non-galvanic coupling interface to the first qubit 801 is formed at the first tip of the first fixed coupling element, away from the adjustable coupling element 804.

[0033] The adjustable coupling element 804 is positioned adjacent to the non-galvanic coupling interface formed at the second tip of the first fixed coupling element 803 to a further circuit element (here, the second fixed coupling element 805). This feature can be considered in more detail by comparing it with Figures 9 and 10.

[0034] In Figure 9, the two qubits 801 and 802 are plus-shaped, as is the intermediate adjustable coupling element 804. The fixed coupling elements 803 and 805 are linear or slab-shaped. The relative arrangement of the elements in Figure 9 is otherwise the same as described above with reference to Figure 8, except that the adjustable coupling element 804 is not positioned adjacent to the non-galvanic coupling interface formed at the second tip of the first fixed coupling element 803 to further circuit elements. Rather, in Figure 9, the adjustable coupling element 804 is positioned between the "second tip" (i.e., the rightmost end) of the first fixed coupling element 803 and the nearest part of the second fixed coupling element 805, filling the gap between them. As a result, in Figure 9, there is a relatively weak direct coupling between the fixed coupling elements 803 and 805.

[0035] In Figure 10, the adjustable coupling element 804 is linear or slab-shaped and is positioned adjacent to a non-galvanic coupling interface formed for a further circuit element at the second tip of the first fixed coupling element 803. That is, the first tip of the first fixed coupling element 803 is its left end, and the non-galvanic coupling interface is formed for the first qubit 801. The second tip of the first fixed coupling element 803 is its right end, and in Figure 10, the non-galvanic coupling interface is formed for the second fixed coupling element 805.

[0036] In the embodiment shown in Figure 10, the second fixed coupling element 805 forms a non-galvanic coupling interface with the second qubit 802 at its tip, away from the adjustable coupling element 804. The adjustable coupling element 804 is positioned in the middle adjacent to the non-galvanic coupling interface formed between the first fixed coupling element 803 and the second fixed coupling element 805.

[0037] The couplings between various elements are schematically shown at the top of Figure 8, where they can be particularly compared with Figure 10. A direct qubit-to-qubit coupling 810 exists, but due to the relatively large distance between qubits 801 and 802 in Figure 10, this is relatively weak. Quite the opposite, the proximity of each pair of elements means that there are multiple couplings that can be relatively strong: coupling 811 between the first qubit 801 and the first fixed coupling element 803, coupling 812 between the first fixed coupling element 803 and the second fixed coupling element 805, and coupling 813 between the second coupling element 805 and the second qubit 802. The strength of couplings 814 and 815, respectively, between the fixed coupling elements 803 and 805 and the tunable coupling element 804 depends on how the tunable coupling element 804 is tuned. If the fixed coupling elements 803 and 805 are resonators, they can also be tuned, further affecting the strength of couplings 814 and 815. If the fixed coupling elements 803 and 805 are coupler islands, short waveguides, or other such elements that cannot be tuned themselves, the effective coupling 810 resulting from all other couplings depends on how the qubits 801 and 802 and the tuneable coupling elements 804 are tuned.

[0038] Figures 11 and 12 show another embodiment. Compared to Figures 8 and 10, the second fixed coupling element 805 is missing. Rather, in Figures 11 and 12, the further circuit element that forms a non-galvanic coupling interface at the second tip of the first fixed coupling element 803 is the second qubit 802. In the embodiments of Figures 11 and 12, there are two couplings that can be relatively strong due to the proximity of each pair of elements: coupling 1101 between the first qubit 801 and the first fixed coupling element 803, and coupling 1102 between the first fixed coupling element 803 and the second qubit 802. The strength of couplings 1103 and 1104 between the first fixed coupling element 803 and the tunable coupling element 804, and between the tunable coupling element 804 and the second qubit 802, depends on how the first qubit 801, the tunable coupling element 804, and the second qubit 802 are tuned.

[0039] The effect of a fixed coupling element (such as element 803 in Figure 12) or a chain of fixed coupling elements (such as elements 803 and 805 in Figure 10) is that a sufficiently effective coupling between qubits, comparable to C12 in Figure 3, can be achieved even when qubits 801 and 802 are relatively far apart. The adjustable coupling element 804 does not directly mediate coupling between qubits (as done in embodiments of Figures 2, 3, 5, and 6). Instead, it mediates coupling between two fixed coupling elements (as in Figure 10), between a fixed coupling element and one of the qubits (as in Figure 12), or between a fixed coupling element and some other type of further circuit element.

[0040] With respect to Figures 8-12, the types of fixed coupling elements described above can also be called coupling extensions. They allow qubits to be positioned further apart than previously known means, then provide space for larger grounding surfaces, vias, or bump couplings to other grounding layers between qubits. More grounding between qubits means reduced crosstalk.

[0041] Any of the above fixed coupling elements may be waveguide resonators, meaning that the coupling element has a length comparable to the natural wavelength at a given frequency of interest. Waveguides are particularly convenient to use as fixed coupling elements for transmon qubits. This is because the typical natural dimension of a transmon qubit is about 1 / 20th of the wavelength at the resonant frequency, while the minimum distance between two transmon qubits recommended for low crosstalk is about 10 times the natural dimension of the transmon qubit.

[0042] The coupling strength between a waveguide (used as a fixed coupling element, or in other words, a coupling extension) and a qubit is enhanced when the waveguide length is close to an integer half-wavelength at the desired frequency. In this case, the waveguide (or fixed coupling element of the dimensions to become a waveguide) is a waveguide resonator. Such higher coupling strengths enable faster 2-qubit gates, but the coupling is enhanced only with respect to the resonant frequency. This phenomenon, called frequency dispersion, makes circuit design more sensitive to dimensional and manufacturing inaccuracies.

[0043] In another embodiment, any of the fixed coupling elements may be lumped resonators. In addition to the enhanced coupling at the resonant frequency, as with waveguide resonators, lumped resonators allow for a much wider range of characteristic impedance design and can further strengthen the coupling between qubits. However, in addition to strong frequency dispersion, the self-resonant frequency of a lumped resonator can be very sensitive to the geometry of other nearby circuit elements, which can make the design of quantum computing circuits considerably more difficult.

[0044] In yet another embodiment, any of the fixed coupling elements may be a conducting island. A conducting island is a circuit element having little self-inductance and coupling to ground. Conducting islands are particularly useful as fixed coupling elements for quantum dot qubits because the actual distance between them may be much smaller than the wavelength at a typical resonant frequency of the qubit for a realistic coupling element shape.

[0045] Figure 13 shows an adjustable coupler according to one embodiment, as well as two qubits 801 and 802. The adjustable coupler comprises a first fixed coupling element 803, an adjustable coupling element 804, and a second fixed coupling element 805. The first fixed coupling element 803 forms a non-galvanic coupling interface with the first qubit 801 at its first tip, which is away from the adjustable coupling element 804. The second fixed coupling element 805 forms a non-galvanic coupling interface with the second qubit 802 at its tip, which is away from the adjustable coupling element 804. The adjustable coupling element 804 is positioned adjacent to the non-galvanic coupling interface formed between the first fixed coupling element 803 and the second fixed coupling element 805. In other words, the non-galvanic coupling interface mentioned above is located between the first fixed coupling element 803 and the tip of the second fixed coupling element 805, which are closest to the adjustable coupling element 804. The ground plane and adjustment connections are not shown in Figure 13 for clarity of the diagram. The capacitors shown by dashed lines indicate capacitive coupling between elements having the same reference identifier as in Figure 8 above.

[0046] In the embodiment shown in Figure 13, each of the first fixed coupling element 803 and the second fixed coupling element 805 is a waveguide. Each of them has a coupling region at its tip, and an adjustable coupling element 804 is positioned adjacent to its tip. These coupling regions are indicated by reference identifiers 1301 and 1302 in Figure 13. Their shapes are suitable for generating capacitive coupling between the two fixed coupling elements 803 and 805 on the one hand, and between each of them and the adjustable coupling element 804 on the other hand. In particular, both the coupling regions 1301 and 1302 of the first and second fixed coupling elements have a first edge adjacent to the first edge of the other coupling region, and a second edge adjacent to each edge of the adjustable coupling element 804.

[0047] A substantially annular shape is used for the adjustable coupling element 804 and coupling regions 1301 and 1302 in Figure 13. The adjustable coupling element 804 occupies a first sector of the annular two-dimensional region. Each of the coupling regions 1301 and 1302 of the first fixed coupling element 803 and the second fixed coupling element 805 occupies a further sector of the annular two-dimensional region. These further sectors are adjacent sectors of the annular two-dimensional region. Together, the first sector and the further sectors cover the entire annular two-dimensional region. Capacitance enhancement forms, such as the interleaved finger in Figure 13, may be used at any of the edges of adjacent sectors.

[0048] The nearly annular shape, when used, does not necessarily mean a circular annular shape, and various polygons may be used. Furthermore, the intertwined finger shape (which in itself is merely an example of a shape that enhances capacitance) is not a mandatory feature, even though it is used between the adjustable coupling element 804 and the coupling regions 1301 and 1302, respectively, in Figure 13. Figure 4 shows an alternative embodiment in which annular hexagonal shapes are used for the adjustable coupling element 804 and the coupling regions 1301 and 1302. Also, in the embodiment of Figure 14, the intertwined finger shape is used between the coupling regions 1301 and 1302, in addition to its use between the adjustable coupling element 804 and each of the individual coupling regions 1301 and 1302. The nearly annular shape may be used between any of the regions involved without using the intertwined finger shape.

[0049] Figure 15 shows an example of how a tunable coupler conforming to the above principle may be used in a quantum computing circuit with multiple qubits. As an example, the rightmost column of qubits 1501-1505 can be considered. The tunable coupler to their right comprises a chain of consecutive fixed coupling elements. The one closest to the top qubit 1501 can be called the first fixed coupling element 1506. Non-galvanic coupling interfaces are formed between consecutive fixed coupling elements in the chain. For example, according to the chain, non-galvanic coupling interfaces can be jumped from the first fixed coupling element 1506 to a vertically oriented fixed coupling element 1507 and further to a fixed coupling element 1508. The tunable coupler comprises at least two tunable coupling elements, tunable coupling elements 1509 and 1510 being examples. Each of these tunable coupling elements is adjacent to each of the non-galvanic coupling interfaces formed between consecutive fixed coupling elements in the chain.

[0050] As shown in the example in Figure 15, at least some of the fixed coupling elements in an adjustable coupler can form a shared bus through which connections to and from multiple different qubits can be made.

[0051] Modifications and alterations to the above embodiments are possible without departing from the scope of the appended claims. For example, the qubit may be any kind of electrical qubit having sufficient voltage support for a given coupler impedance, and these include, but are not limited to, transmon and quantum dot qubits. Various capacitive and other non-galvanic coupling methods are known to those skilled in the art and can be used instead of or in addition to those described above.

Claims

1. An adjustable coupler for generating a controllable coupling to at least a first qubit (801, 1501), The first fixed coupling element (803, 1506) and An adjustable coupling element (804, 1509) wherein a coupling is formed between the first fixed coupling element (803, 1506) and the adjustable coupling element (804, 1509), Equipped with, The first fixed coupling elements (803, 1506) form a first non-galvanic coupling interface with at least the first qubits (801, 1501) at the first tip of the first fixed coupling elements (803, 1506) away from the adjustable coupling elements (804, 1509), and a strong coupling is obtained by having the first fixed coupling elements (803, 1506) and the first qubits (801, 1501) adjacent to each other in the first non-galvanic coupling interface. The adjustable coupling elements (804, 1509) are positioned adjacent to a second non-galvanic coupling interface formed at the second tip of the first fixed coupling elements (803, 1506) to further circuit elements (802, 805, 1507), and a coupling is formed between the adjustable coupling elements (804, 1509) and the further circuit elements (802, 805, 1507) at the second non-galvanic coupling interface, and a strong coupling is obtained because the first fixed coupling elements (803, 1506) and the further circuit elements (802, 805, 1507) are positioned adjacent to each other. The adjustable coupling elements (804, 1509) are configured to be adjustable for adjusting the coupling strength of the coupling between the adjustable coupling elements (804, 1509) and the first fixed coupling elements (803, 1506), and the coupling strength of the coupling between the adjustable coupling elements (804, 1509) and the further circuit elements (802, 805, 1507), wherein the further circuit elements (802, 805, 1507) are second fixed coupling elements (805, 1507) or second qubits, wherein the adjustable coupler.

2. The adjustable coupler according to claim 1, wherein the first fixed coupling element (803, 1506) is a waveguide.

3. The adjustable coupler according to claim 2, wherein the first fixed coupling element (803, 1506) is a waveguide resonator.

4. The adjustable coupler according to claim 1, wherein the first fixed coupling element (803, 1506) is a lumped element resonator.

5. The adjustable coupler according to claim 1, wherein the first fixed coupling elements (803, 1506) are conductive islands.

6. The adjustable coupler comprises the further circuit elements, The further circuit element comprises the second fixed coupling element (805), The second fixed coupling element (805) forms a third non-galvanic coupling interface with respect to the second qubit (802) at the tip of the second fixed coupling element (805) away from the adjustable coupling element (804), and a strong coupling is obtained because the second fixed coupling element (805) and the second qubit (802) are arranged adjacent to each other in the third non-galvanic coupling interface. The adjustable coupling element (804) is positioned adjacent to the second non-galvanic coupling interface formed between the first fixed coupling element (803) and the second fixed coupling element (805). An adjustable coupler according to any one of claims 1 to 5.

7. The adjustable coupler according to claim 6, wherein the second fixed coupling element (805) is one of a waveguide, a waveguide resonator, a lumped element resonator, and a conductor island.

8. The first fixed coupling element (803) and the second fixed coupling element (805) are a waveguide or a waveguide resonator. Each of the first fixed coupling element (803) and the second fixed coupling element (805) has its respective coupling region (1301, 1302) at its respective tip where an adjustable coupling element (804) is located adjacent to it. Both the coupling regions (1301, 1302) of the first fixed coupling element (803) and the second fixed coupling element (805) each have a first edge adjacent to the first edge of the other coupling region, and a second edge adjacent to each edge of the adjustable coupling element (804). The adjustable coupler according to claim 6, as dependent on claim 1 or claim 2.

9. The adjustable coupling element (804) occupies a first sector of the annular two-dimensional region, Each of the coupling regions (1301, 1302) of the first fixed coupling element (803) and the second fixed coupling element (805) occupies a further sector of the annular two-dimensional region. The further sectors are adjacent sectors of the annular two-dimensional region, The first sector and the further sectors together cover the entire annular two-dimensional region. The adjustable coupler according to claim 8.

10. The adjustable coupler comprises a chain of continuous fixed coupling elements (1506, 1507, 1508), one of which is the first fixed coupling element (1506), and a non-galvanic coupling interface is formed between the continuous fixed coupling elements (1506, 1507, 1508) in the chain. The adjustable coupler comprises at least two adjustable coupling elements (1509, 1510), each of which is adjacent to one of the non-galvanic coupling interfaces formed between the continuous fixed coupling elements (1506, 1507, 1508) in the chain. An adjustable coupler according to any one of claims 1 to 5.

11. The adjustable coupler according to any one of claims 1 to 10, At least the first qubit (801, 1501) or the second qubit (802), A quantum computing circuit equipped with the following features.

12. The quantum computing circuit according to claim 11, comprising the first qubit (801, 1501) and the second qubit (802).

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