A microwave filter arrangement comprising a frequency selective surface

Abstract

A microwave filter arrangement (100) for quantum computing applications, the filter arrangement (100) comprising a first port (113) and a second port (114). The microwave filter arrangement (100) comprises a main waveguiding structure (110) arranged between the first (113) and second (114) port, where the main waveguiding structure (110) comprises an inner conductor (111) having a circular cross-section that is surrounded by a hollow outer conductor (112) having a rectangular cross-section with two opposed long sides and two opposed short sides. The waveguiding structure (110) comprises a plurality of hollow waveguides (115) extending away from one or both of the long sides of the outer conductor, the hollow waveguides (115) comprising a dielectric material (122) and being terminated by an electromagnetically absorptive material (124).

Description

[0001] TITLE

[0002] A MICROWAVE FILTER ARRANGEMENT COMPRISING A FREQUENCY SELECTIVE SURFACE

[0003] TECHNICAL FIELD

[0004] The present disclosure refers to a microwave low-pass filter arrangement for use with superconducting devices, in particular for quantum processors. However, the disclosed filter arrangements can be used in any electrical application.

[0005] BACKGROUND

[0006] Quantum processors made of superconducting materials are typically operated in the 1-10 GHz microwave regime, where control signals at these frequencies are used to manipulate the circuits comprising the quantum processor. The quantum processors are highly sensitive to radiation close to the band of operation as well as to higher frequencies exceeding the superconducting gap of the superconductor, which is around 100 GHz for most devices. Therefore, it is important to filter the control signals for a wide band of frequencies.

[0007] Microwave filters based on resonant structures, such as multi-stage LC filters, offer a steep rolloff at the edge of the band, but leak at higher frequencies (for example, at 20 GHz and above) due to parasitic components. By contrast, filters based on absorptive materials effectively filter radiation over a wide frequency band; however, they do so at the expense of introducing significant, frequency-dependent in-band losses, which is undesired. Hence, it is difficult to construct a microwave filter that filters signals effectively both near and far out of the band of operations, while still maintaining low in-band losses. A recently introduced design for a microwave filter, called High Energy Radiation Drain (HERD), uses a frequency-selective surface to separate the absorbing region from the main waveguiding path, thereby breaking the tradeoff between low in-band loss and high attenuation at high frequencies. However, it is not clear to what extent the HERD design can be miniaturized to meet the increasing demand of the quantum computing industry for high-density cabling solutions. In addition, HERD does not feature a steep rolloff at the edge of the operating band, and therefore needs to be supplemented with a conventional resonant filter for optimal performance.

[0008] JP 2007158713 A discloses a filter comprising a coaxial waveguide.

[0009] Still, there is an unresolved need for a microwave filter featuring low in-band losses, steep rolloff, and high-frequency protection, in a small form factor. SUMMARY

[0010] The object of the present disclosure is to provide improved filter arrangements for quantum processors. Compared to HERD, these arrangements provide improved attenuation in the stopband and larger miniaturization, while preserving good in-band performance and protection at high frequencies.

[0011] This object is at least in part obtained by a microwave filter arrangement for a quantum processor. The microwave filter arrangement comprises a main waveguiding structure with a first and second port. The main waveguiding structure comprises an inner conductor having a circular cross-section and a hollow outer conductor with a rectangular cross-section having two opposed long sides and two opposed short sides. The main waveguiding structure further comprises a plurality of hollow waveguides extending away from one or both of the long sides of the outer conductor. The hollow waveguides comprise a dielectric material and are terminated by an electromagnetically absorptive material.

[0012] The hollow waveguides are connected to the interior of the outer conductor via apertures in the outer conductor walls, making the inner surface of the outer conductor a frequency selective surface. The frequency selective surface guides the electromagnetic waves away from the main waveguiding structure above certain cutoff frequency, for example, 20 GHz. Hence, according to this example, frequencies referred to as being far out of the band of operation or being high-frequency signals could be interpreted as being above 20 GHz while frequencies below 20 GHz, but above 10 GHz, can be considered to be near the band of operation.

[0013] According to aspects, the main waveguiding structure is designed to support a single electromagnetic mode in the band of operations supporting frequencies down to DC.

[0014] According to aspects, the inner conductor of the main waveguiding structure is arranged so that the minimum distance between the inner conductor and the long side of the outer conductor is smaller than the minimum distance between the inner conductor and the short side of the outer conductor. In addition, in combination with the circular shape of the inner conductor, this arrangement ensures that most of the electromagnetic field is concentrated towards the frequency selective surface. This leads to more high-frequency signals entering the hollow waveguides rather than propagating along the main waveguide structure, which is an advantage.

[0015] Optionally, the angle between the hollow waveguides and a surface of the outer conductor is between 85 and 95 degrees, or preferably between 89 and 91 degrees. The angle may also be substantially 90 degrees, i.e. the hollow waveguides may be perpendicular to the direction of propagation in the main waveguiding structure. This results in an efficient filtering of high-frequency signals, which is an advantage.

[0016] According to some examples, the inner conductor comprises at least one first inner conductor section and at least one second inner conductor section extending along the main waveguiding structure. The diameter of the inner conductor is smaller in the first inner conductor section than in the second inner conductor section.

[0017] The outer conductor may also comprise at least one first outer conductor section and at least one second outer conductor section extending along the main waveguiding structure. A width and / or a height of the outer conductor is then smaller in the first outer conductor section than in the second outer conductor section.

[0018] Advantageously, a variation in the diameter of the inner conductor and / or in the width and height of the outer conductor can be used to vary properties of the filter arrangement, such as the equivalent electrical reactance, along the main waveguiding structure.

[0019] According to a preferred alternative, the inner conductor comprises a plurality of first inner conductor sections and second inner conductor sections arranged alternating along the main waveguiding structure, while the outer conductor likewise comprises a plurality of first outer conductor sections and second outer conductor sections arranged alternating along the main waveguiding structure. The inner conductor and outer conductor are then arranged so that a first inner conductor section is surrounded by a second outer conductor section and a second inner conductor section is surrounded by a first outer conductor section. That is, a section with a narrow inner conductor coincides with a wider and taller outer conductor section and vice versa.

[0020] In such a structure, sections with a narrow inner conductor and a wide outer conductor have a higher characteristic impedance compared to sections with a wide inner conductor and a narrower outer conductor. This creates a resonant filter structure acting as a low-pass filter with a cutoff frequency that is determined by the dimensions of the conductors in various sections as well as the length of the sections. The cutoff frequency may for example be 10 GHz.

[0021] Optionally, a length of any of the first outer conductor sections, second outer conductor sections, first inner conductor sections and second inner conductor sections is selected to achieve a desired equivalent electrical reactance of the section, and / or a desired lower cutoff frequency of the filter arrangement. According to some examples, the desired lower cutoff frequency is between 8 and 12 GHz.

[0022] According to some aspects, an aperture connecting a hollow waveguide in the plurality of hollow waveguides to the main waveguiding structure may be arranged in a second outer conductor section where the width and / or height of the outer conductor is larger than in a first outer conductor section. That the hollow waveguides are implemented in the sections with the large width and height of the outer conductor effectively combines suppression of signals close to and far out of the operational band of the filter arrangement, resulting in an efficient filtering of electromagnetic signals negatively affecting the quantum processor.

[0023] According to some alternatives, a width of at least one hollow waveguide in the plurality of hollow waveguides is larger than the width of the aperture forming the opening between the main waveguiding structure and the hollow waveguide. That is, the aperture exposing the inner conductor to the hollow waveguides can be made smaller that the cross section perpendicular to the propagation of electromagnetic fields guided by the hollow waveguides. This allows for high attenuation far out of the band of operation while reducing the size of the main waveguiding structure. This also reduces the in-band losses caused by unwanted leakage of the in-band electromagnetic fields into the hollow waveguides.

[0024] According to aspects, the distance between the inner conductor and the dielectric material comprised in the plurality of hollow waveguides is arranged to achieve a specific characteristic impedance which results in good in-band performance. In addition, the distance between the short side of the outer conductor and the hollow waveguide aperture is minimized, ensuring that the electromagnetic field is confined in close proximity to the hollow waveguide apertures. This improves attenuation at frequencies far out of the band of operation, which is an advantage.

[0025] There is also herein disclosed a quantum computing setup comprising at least one quantum computing chip and a filter arrangement according to any previous claim, wherein the filter arrangement is configured to transmit a control signal to the quantum computing chip.

[0026] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present disclosure will now be described in more detail with reference to the appended drawings, where:

[0028] Figures 1A and IB show different views and aspects of a microwave filter arrangement;

[0029] Figures 2A and 2B aspects of a microwave filter arrangement;

[0030] Figures 3A and 3B aspects of a microwave filter arrangement;

[0031] Figure 4A and 4B aspects of a microwave filter arrangement;

[0032] DETAILED DESCRIPTION

[0033] Aspects of the present disclosure will now be described more fully with reference to the accompanying drawings. The different devices and methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

[0034] The terminology used herein is for describing aspects of the disclosure only and is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0035] A quantum computing setup comprises one or more quantum bits, which may be housed on a quantum computing chip. Control of the qubits, as well as readout of calculation results, is frequently achieved via microwave signals that are transmitted to and from the quantum computing chips via a series of waveguides. The microwave signals are often in the 1-10 GHz frequency band. The quantum computing chip can be sensitive to signals that are just above this frequency band, e.g. from 10 to 20 GHz, which makes it very important to filter out unwanted or parasitic signals in this frequency range.

[0036] In order for the qubits to remain in a desired quantum state, the quantum computing chips are kept at cryogenic temperatures, and preferably at only a few degrees Kelvin above absolute zero, by use of e.g. liquid helium. At these temperatures, some materials such as aluminium become superconducting. A superconducting material is characterized, among other parameters, by superconducting gap or energy gap. If it is exposed to a signal with a photon energy larger than the energy gap, electrons within the superconducting material may enter a state in which they encounter resistance, i.e. the superconducting effect is diminished. Such signals could for example have frequencies around 100 GHz or higher. Diminishing of the superconducting effect is highly undesirable, and it is therefore also important to filter out higher-frequency signals as well. There is thus a need to combine near and far out of band frequency filtering. There is also a need to miniaturize the filter device footprint and, at the same time, maintain low in-band losses in a microwave filter arrangement for a quantum processor.

[0037] Therefore, there is disclosed herein a filter arrangement 100 for a quantum processor as shown in Figures 1A and IB. The filter arrangement comprises a main waveguiding structure 110 arranged to guide electromagnetic waves between a first port 113 and a second port 114. The main waveguiding structure 110 comprises an inner conductor 111 having a circular cross-section, that is surrounded by a hollow outer conductor 112 with a rectangular cross-section, the cross-section having two opposed long sides and two opposed short sides. In addition, the main waveguiding structure comprises a plurality of hollow waveguides 115 extending away from one or both of the long sides of the outer conductor. The hollow waveguides 115 comprise a dielectric material 125 and are terminated by an absorptive material 124. Figure 1A shows an example of such a filter arrangement while Figure IB shows a sectional view of such an arrangement.

[0038] Herein, a conductor is taken to be a component that conducts electric current, or that has high electric conductivity. A high electric conductivity is considered to be an electric conductivity such as that of a metal or a semiconducting material. According to an example, a conductor may comprise a material with an electric conductivity of more than 100 (Qm)1. A conductor may preferably be formed at least in part from a metal.

[0039] The disclosed microwave filter arrangement 100 is suitable for quantum processors. However, the microwave filter arrangement can be useful to protect any superconducting circuit from unwanted generation of quasiparticles caused by the impact of high frequency radiation, while ensuring good performance in the band of operation. More generally, the filter arrangement can be used to block very high-frequency radiation while still transmitting low to moderately high frequency waves in a transmission path. Even more generally, the disclosed filter arrangements can be used in any electrical application.

[0040] A main waveguiding structure 110 herein refers to a structure guiding electromagnetic waves along a path in between the inner and outer conductor between a first 113 and second 114 port. A port is an interface to the main waveguiding path that can be interfaced with another electrical component through a connector. In the Figure 1A, such connectors consisting of a SubMiniature version A (SMA) can be seen, making it possible to interface with for example a coaxial cable or other device. However, the filter arrangement can comprise any suitable connector standard.

[0041] The inner conductor 111 and outer conductor 112 in the main waveguiding structure 110 both generally extend in a substantially straight line between the first port 113 and second port 114. The cross-section of each conductor is herein taken perpendicular to the straight line between the first and second ports 113, 114, or equivalently perpendicular to the direction of wave propagation in the main waveguiding structure 110.

[0042] That the inner conductor 111 and outer conductor 112 have a circular and rectangular cross-section, respectively, should be understood to mean that the conductors have substantially circular and rectangular cross-sections. That is, the inner conductor 111 may be somewhat uneven or elliptical in shape within the tolerances of the method used for producing the conductor. Similarly, the outer conductor 112 may deviate from a perfectly rectangular shape in the same manner.

[0043] According to some examples, the length of the short sides and the long sides of the outer conductor 112 are selected to produce a desired value of a property of the filter arrangement 100. In particular, the dimensions of the outer conductor 112 affect the characteristic impedance of the main waveguiding structure 110 and may therefore be selected to match a characteristic impedance of an external circuit or device to which the filter arrangement 100 may be connected. Impedance matching mitigates signal reflection and losses at the connection point between devices, which is an advantage. According to other examples, the short sides of the outer conductor 112 may have a length that is between half and two thirds of the length of the long sides.

[0044] Herein, the length of the long sides may be referred to as the width of the outer conductor 112, while the length of the short side can be referred to as the height of the outer conductor 112. The length of the outer conductor along the main waveguiding structure 110, perpendicular to the crosssection, may be referred to as the length or extent of the outer conductor 112.

[0045] That the inner conductor 111 is surrounded by the outer conductor 112 should herein be taken to mean that the inner conductor 111 is arranged inside the hollow outer conductor 112, with a crosssection of both conductors together showing the circular cross-section of the inner conductor 111 inside the rectangle formed by the outer conductor 112. According to some examples, the width of the outer conductor 112 may be between 1 and 3 mm, while the diameter of the inner conductor 111 may be between 0.4 and 0.8 mm.

[0046] According to one alternative, the waveguiding structure 110 is arranged so that a minimum distance between the inner conductor and a long side of the outer conductor is smaller than a minimum distance between the inner conductor and the short side of the outer conductor. Such an arrangement is shown in Figure 2A, where the circular inner conductor 111 can be seen to be positioned close to the long sides of the outer conductor 112 and the dielectric material 125 extending out from the hollow waveguides 115. In order to maximize signal transmission, the distance between the inner conductor and the dielectric material 125 must be chosen carefully in order for the impedance to match the impedance of external circuitry. It is also seen that the long side of the outer conductor 112 is sufficiently wide in comparison to the short side. As an example, the long side of the outer conductor 112 may be at least as wide as the hollow waveguide, or up to 0.35 mm wider. This arrangement confines the electric field to the long side of the outer conductor where the hollow waveguides 115 reside, improving attenuation of high frequency fields in the stopband of the filter. Figure 2B shows a sectional sideview of the microwave filter arrangement 100.

[0047] The inner conductor 111 may also be centred in the outer conductor 112. That is, the centre of the circular cross-section of the inner conductor 111 may coincide with the middle point of the rectangular cross-section of the outer conductor 112 along most or all of the waveguiding structure 110.

[0048] The volume separating the inner 111 and outer 112 conductors may be filled with air or vacuum. It could also be filled with another dielectric material, for example a non-conducting polymer or polymer foam. Herein, the term dielectric material is given the definition common in the field, i.e. the term refers to a material that is electrically insulating or has an electrical conductivity several orders of magnitude lower than that of e.g. a typical metal or semiconductor.

[0049] Each of the hollow waveguides 115 is a waveguide comprising a single conductor and hence supporting electromagnetic wave propagation only above a certain cutoff frequency determined by the size and shape by the hollow waveguide cross section. The hollow waveguides 115 are arranged so that one aperture at a first end of the hollow waveguide 115 opens into the interior of the outer conductor 112. Put another way, at least one side of the outer conductor 112 comprises a series of apertures positioned along the length of the main waveguiding structure 110 and leading into hollow waveguides 115.

[0050] As mentioned above, these apertures are confined to one or both of what is referred to as the long sides of the outer conductor 112. As discussed above, the outer conductor 112 has a rectangular cross-section perpendicular to the direction of propagation in the main waveguiding structure 110, with the cross-section having two opposed longer sides and two opposed shorter sides. The apertures of the hollow waveguides 115 are positioned on the walls of the outer conductor 115 associated with the longer sides in the cross-section.

[0051] The hollow waveguides 115 extend away from the outer surface of the outer conductor 112. Preferably, an angle between the hollow waveguides 115 and a surface of the outer conductor 112 is between 85 and 95 degrees, or preferably between 89 and 91 degrees. Even more preferably, the angle is substantially 90 degrees. The hollow waveguides 115 may be shaped like hollow cylinders, rectangular cuboids, or any other shape suitable for conducting electromagnetic waves.

[0052] In the context of the microwave filter arrangement 100 the hollow waveguides are used to filter frequencies far above the band of operations, which is also at frequencies above the cutoff frequency of the hollow waveguides. That is, a wave or signal with a frequency above the cutoff frequency is selectively enabled to leave the main waveguiding structure 110 and propagate into one of the hollow waveguides 115. Due to this frequency selectivity, the inner surface of the outer conductor 112 where the apertures are placed can be referred to as a frequency selective surface.

[0053] The band of operation of the filter arrangement, i.e. the frequency range of signals intended to travel between the first port 113 and second port 114, is generally between 1 and 10 GHz. The cutoff frequency of the hollow waveguides 115 and the frequency selective surface is selected to be far out of the band of operation of the filter arrangement. According to some examples, the cutoff frequency is around 20 GHz.

[0054] The hollow waveguides 115 comprise a dielectric material 125, which fills the interior of the hollow waveguides 115 either partially or completely. If the space between the outer conductor 112 and the inner conductor 111 comprises a dielectric material, the dielectric material 125 filling the hollow waveguides 115 is a material with higher relative permittivity than the dielectric material inside the main waveguiding structure.

[0055] The dielectric material 125 comprised in the hollow waveguides 115 may also extend into the interior of the outer conductor 112. The extent to which the dielectric material 125 extends into the main waveguiding structure 110, and thereby also the distance between the inner conductor and the dielectric material 125 comprised in the plurality of hollow waveguides 115, can be arranged to achieve a desired characteristic impedance.

[0056] The hollow waveguides 115 are generally terminated with an electrically absorptive material. That is, an electrically absorptive material is arranged at the end of the hollow waveguides 115 that is furthest from the main waveguiding path 110. Any signal propagating to the end of a hollow waveguide 115 will therefore be partially or fully absorbed in the absorptive material, which is an advantage.

[0057] The microwave filter arrangement 100 can be arranged to incorporate a resonant filter structure, shown in Figure 3A and 3B. To this end, the inner conductor 111 may comprise at least one first inner conductor section 121b and at least one second inner conductor section 121a extending along the main waveguiding structure 110, wherein the diameter of the inner conductor 111 is smaller in the first inner conductor section 121 than in the second inner conductor section 121. In addition, the outer conductor 112 may comprise at least one first outer conductor section 122a and at least one second outer conductor section 122b extending along the main waveguiding path, wherein a width and / or a height of the outer conductor 112 is smaller in the first outer conductor section 122a than in the second outer conductor section 122b.

[0058] Preferably, the inner conductor 111 comprises a plurality of first inner conductor sections 121b and second inner conductor sections 121a arranged alternating along the direction of propagation of the main waveguiding structure 110, while the outer conductor 112 also comprises a plurality of first outer conductor sections 122a and second outer conductor sections 122b arranged alternating along the main waveguiding structure 110. The inner conductor 111 and outer conductor 112 are arranged so that a first inner conductor section 121b is surrounded by a second outer conductor section 122b and a second inner conductor section 121a is surrounded by a first outer conductor section 122a. Note that the inner and outer conductor sections occupying the same position along the main waveguiding structure 110 should preferably have similar lengths, i.e. a first inner conductor section 121b should have the same length as a surrounding second outer conductor section 122b. However, that length may be different compared to a pair of outer and inner conductor sections further along the main waveguiding structure 110.

[0059] Put another way, according to this alternative the main waveguiding structure 110 comprises a number of alternating first and second waveguide sections, where the first waveguide sections are characterized by the inner conductor 111 having a larger diameter and the outer conductor 112 having a smaller width and / or height. The second sections are characterized by the inner conductor 111 being narrower in diameter and the outer conductor 112 having a larger width and / or height. The sections with a large inner conductor diameter and small width and height of the outer conductor, i.e. the first waveguide sections, will be of low characteristic impedance, while the sections with small inner conductor diameter and large width and height of the outer conductor, or second waveguide sections, will be of high characteristic impedance.

[0060] According to an example, the width of the outer conductor 112 may be between 1 and 1.5 mm in the first outer conductor sections 122a and at least 3 mm in the second outer conductor sections 112b. The inner conductor 111 may be between 0.4 and 0.8 mm in diameter.

[0061] Each first and second waveguide sections will have a length extending along the main waveguiding structure 110. The length of each first and second waveguide section can be different as shown e.g. in Figure 3A. Preferably, the lengths of each first and second waveguide section is chosen to achieve a desired equivalent electrical reactance in that particular section. That is, a length of any of the first outer conductor sections 122a, second outer conductor sections 122b, first inner conductor sections 121b and second inner conductor sections 121a is selected to achieve a desired equivalent electrical reactance of the section. The lengths of the sections comprised in the main waveguiding structure 110 can also be selected to achieve a desired lower cutoff frequency of the filter arrangement 100. According to one example, the desired lower cutoff frequency is between 8 and 12 GHz.

[0062] Here, the desired lower cut-off frequency refers to the lowest frequency at which the filter impedes signal propagation.

[0063] As previously described, the hollow waveguides 115 open into the main waveguiding structure via apertures in the wall of the outer conductor 112. According to some alternatives, such an aperture 117 connecting a hollow waveguide 115 in the plurality of hollow waveguides to the main waveguiding structure 110 is arranged in a second outer conductor section 122b, i.e. in a section where the width and / or height of the outer conductor is larger than in a first outer conductor section 122a.

[0064] At the point in the main waveguide structure 110 where the aperture 117 is arranged, a high parasitic impedance will arise due to the gap in the outer conductor 112. By implementing the hollow waveguides 115 in the sections with the small inner conductor 111 and large width and height of the outer conductor 112, the high impedance parasitics coming from the absence of the outer conductor at the position of the hollow waveguide can be incorporated into the high impedance section. This integration makes sure that electromagnetic fields with frequencies near the band of operation are filtered and do not reach the second port 114. Hence, the resonant filter takes care of the near band filtration while the frequency selective surface takes care of filtering far out of band fields.

[0065] In Figure 2A, the distance between the short side of the outer conductor and the aperture 117 of the hollow waveguide is small. Small, in this context, is taken to mean that the distance is small enough that the aperture 117 takes up most of the long side of the outer conductor 112. This is important at higher frequencies when the main waveguiding structure 110 is highly overmoded and the field may no longer be confined towards the long sides of the outer conductor where the hollow waveguides 115 reside. Due to the hollow waveguide aperture taking up most of the long side of the outer conductor 112, the propagating electromagnetic fields are confined in close proximity to the hollow waveguides resulting in improved electromagnetic coupling to the hollow waveguides at higher frequencies. This in turns leads to higher attenuation of frequencies far out of the band of operation. According to some examples, the distance between the short side of the outer conductor 112 and the near edge of the aperture 117 is 0.35 mm or less. According to one alternative, shown in Figure 4A and 4B, a width 120 of at least one hollow waveguide 115 in the plurality of hollow waveguides is larger than the width of the aperture 117 forming the opening between the main waveguiding structure 110 and the hollow waveguide 115. That is, the aperture 117 through which the inner conductor 111 is exposed to the hollow waveguide 115 is smaller than the horizontal cross section 120 of the hollow waveguide. This allows for a decreased size of the main waveguiding structure while maintaining high suppression of frequencies far away from the band of operation. In addition, the small aperture coupling to the hollow waveguides allow for the hollow waveguide cross section to be made larger since the small aperture suppresses in-band tunnelling of electromagnetic fields into the waveguide in the band of operation. According to some examples, the distance between the short side of the outer conductor and the hollow waveguide aperture is minimized.

[0066] There is also herein disclosed a quantum computing setup comprising at least one quantum computing chip and a filter arrangement 100 according to any previous claim, wherein the filter arrangement 100 is configured to transmit a control signal to the quantum computing chip.

Claims

1. CLAIMS1. A microwave filter arrangement (100) for superconducting quantum computing applications, the filter arrangement (100) comprising a first port (113) and a second port (114); the microwave filter arrangement (100) comprising a main waveguiding structure (110) arranged between the first (113) and second (114) port, the main waveguiding structure (110) comprising an inner conductor (111) having a circular cross-section and surrounded by a hollow outer conductor (112) having a rectangular cross-section with two opposed long sides and two opposed short sides; the filter arrangement (100) comprising a plurality of hollow waveguides (115) extending away from one or both of the long sides of the outer conductor, the hollow waveguides (115) comprising a dielectric material (125) and being terminated by an electromagnetically absorptive material (124), wherein the waveguiding structure (110) is arranged so that a minimum distance between the inner conductor and a long side of the outer conductor is smaller than a minimum distance between the inner conductor and the short side of the outer conductor.

2. The filter arrangement (100) according to claim 1, wherein an angle between the hollow waveguides (115) and a surface of the outer conductor (112) is between 85 and 95 degrees, or preferably between 89 and 91 degrees.

3. The filter arrangement (100) according to any previous claim, wherein the inner conductor(111) comprises at least one first inner conductor section (121b) and at least one second inner conductor section (121a) extending along the main waveguiding structure (110), wherein the diameter of the inner conductor (111) is smaller in the first inner conductor section (121) than in the second inner conductor section (121).

4. The filter arrangement (100) according to any previous claim, wherein the outer conductor(112) comprises at least one first outer conductor section (122a) and at least one second outer conductor section (122b) extending along the main waveguiding structure (110), wherein a width and / or a height of the outer conductor (112) is smaller in the first outer conductor section (122a) than in the second outer conductor section (122b).

5. The filter arrangement (100) according to claim 3 or 4, wherein the inner conductor (111) comprises a plurality of first inner conductor sections (121b) and second inner conductor sections (121a) arranged alternating along the main waveguiding structure (110), wherein the outer conductor (112) comprises a plurality of first outer conductor sections (122a) and second outer conductor sections (122b) arranged alternating along the main waveguiding structure (110), and wherein the inner conductor (111) and outer conductor (112) are arranged so that a first innerconductor section (121b) is surrounded by a second outer conductor section (122b) and a second inner conductor section (121a) is surrounded by a first outer conductor section (122a).

6. The filter arrangement (100) according to claim 4 or 5, where a length of any of the first outer conductor sections (122a), second outer conductor sections (122b), first inner conductor sections (121b) and second inner conductor sections (121a) is selected to achieve a desired equivalent electrical reactance of the section, and / or a desired lower cutoff frequency of the filter arrangement (100).

7. The filter arrangement (100) according to claim 6, wherein the desired lower cutoff frequency is between 8 and 12 GHz.

8. The filter arrangement (100) according to any of claims 4 to 7, where an aperture (117) connecting a hollow waveguide (115) in the plurality of hollow waveguides to the main waveguiding structure (110) is arranged in a second outer conductor section (122b) where the width and / or height of the outer conductor is larger than in a first outer conductor section (122a).

9. The filter arrangement (100) according to any previous claim, wherein a width (120) of at least one hollow waveguide (115) in the plurality of hollow waveguides is larger than the width of the aperture (117) forming the opening between the main waveguiding structure (110) and the hollow waveguide (123).

10. The filter arrangement (100) according to any previous claim, where the distance between the inner conductor and the dielectric material (125) comprised in the plurality of hollow waveguides (115) is arranged to achieve a desired characteristic impedance.

11. The filter arrangement (100) according to any previous claim, where the distance between the short side of the outer conductor and the hollow waveguide aperture is minimized.

12. A quantum computing setup comprising at least one quantum computing chip and a filter arrangement (100) according to any previous claim, wherein the filter arrangement (100) is configured to transmit a control signal to the quantum computing chip.

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