SE543480C2

SE543480C2 - Tunable microwave resonator

Info

Publication number: SE543480C2
Application number: SE1951451A
Authority: SE
Inventors: Andrey Danilov; Sergey Kubatkin; Sumedh Mahashabde
Original assignee: Andrey Danilov; Sergey Kubatkin; Sumedh Mahashabde
Priority date: 2019-12-13
Filing date: 2019-12-13
Publication date: 2021-03-02
Also published as: EP4073880A1; WO2021118445A1; SE1951451A1; EP4073880A4

Abstract

The present invention relates to a tunable microwave resonator device (100, 200) having an associated base wavelength, the microwave resonator device comprising: a dielectric substrate (102); an electrically conductive line pattern (104) supported by the dielectric substrate, the electrically conductive line pattern having an open end (106) and a closed end (108), a tuning arrangement (110) adapted to provide a tuning electric current (IT) into the electrically conductive line pattern for tuning a resonance frequency of the microwave resonator device, the tuning arrangement is adapted to pass the electric current from a first point (112) of the electrically conductive line pattern to a second point (114) of the electrically conductive line pattern, wherein the first point and the second point are located a predetermined distance (d) from the open end, wherein the predetermined distance is based on a length of a quarter base wavelength.

Description

TU NAB LE M ICROWAVE RESONATOR Field of the lnventionThe present invention generally relates to a tunable microwave resonator device.

BackgroundMicrowave resonators are versatile devices that can be used for various applications such as in filter, amplifiers, and in some sensingapplications. With the recent advance in quantum computing architecturesmicrowave resonators have also been used as qubit communication buses.

The base frequency of a microwave resonator is defined by thegeometry of the resonator. Adjusting the base frequency can therefore bemade by varying the geometry of the resonator. However, mechanicalsolutions for adjusting the base frequency are technically complicated andonly provide for relatively slow tuning of the base frequency.

Another approach includes tuning the kinetic inductance of asuperconducting microwave resonator by means of applying a magnetic fieldto the superconducting resonator. This does not require mechanicallyadjusting the resonator geometry but suffers from slow tuning of the basefrequency.

Attempts have been made to integrate so-called superconductingquantum interference devices (SQUlDs) into the resonator design. Thisallows for controlling the nonlinear inductance of Josephson junctions of theSQUID with an external magnetic field to in this way tune the base frequencyof the resonator. However, insertion of the Josephson junctions degrades theresonance quality, often defined by the so-called Q-value.

Accordingly, there appears to be a need for improvements with regards to tunable microwave resonators.

Summaryln view of the above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved tunablemicrowave resonator.

According to a first aspect of the present invention, it is thereforeprovided a tunable microwave resonator device having an associated basewavelength, the microwave resonator device comprising: a die|ectricsubstrate; an electrically conductive line pattern supported by the die|ectricsubstrate, the electrically conductive line pattern having an open end and aclosed end, a tuning arrangement adapted to provide a tuning electric currentinto the electrically conductive line pattern for tuning a resonance frequencyof the microwave resonator device, the tuning arrangement is adapted topass the electric current from a first point of the electrically conductive linepattern to a second point of the electrically conductive line pattern, whereinthe first point and the second point are located a predetermined distance fromthe open end, wherein the predetermined distance is based on a length of aquarter base wavelength.

The present invention is based on the realization that the resonancefrequency of the microwave resonator can be tuned by injecting an electriccurrent at appropriate locations in the microwave resonator. The injectedtuning electrical current flows in the closed part of the electrically conductiveline pattern and effectively tunes the inductance of the closed part of theelectrically conductive line pattern. The inventors realized that it isadvantageous to inject the tuning electric current approximately a quarterbase wavelength from the open end of the electrically conductive line pattern.ln this way, the losses from the electrically conductive line pattern through theinjection point may be reduced since the injection points are located nearvoltage nodes of the microwave mode of the resonator.

More specifically, a section of the electrically conductive line patternfrom the first and second point to the open end acts an impedance converter for providing a low impedance path at the resonance frequency of the tunable microwave resonator. ln other words, a "microwave short" is providedbetween the first point and the second point.

Accordingiy, the quarter base wavelength part of the eiectricallyconductive line pattern, from the first and second point to the open end,provides an impedance converter. The open end of the impedance converterprovides an infinite impedance at the open end, whereas a low impedancepath at least at the resonance frequency is provided between the first pointand the second point. The "path" is provided by that a voltage node is locatedat the first point and the second point, while at the same time a high current isprovided in the corresponding current pattern of the tunable microwaveresonator. ln other words, at the location of the first point and the secondpoint, a low voltage and a high current is present at the resonance frequency,thereby providing a low impedance path.

A length of a quarter base wavelength is the length of a quarter, i.e.25% of the base wavelength.

A base wavelength is defined by the configuration of the microwaveresonator device. Thus, the design of eiectrically conductive line patterndetermines the base wavelength, whereby the eiectrically conductive linepattern is designed to tailor the base wavelength to a given applicationassociated with a predetermined operation frequency.

Generally, the performance of a resonator may be quantified by the so-called quality factor, often denoted Q, i.e. the Q-factor, which is adimensionless factor. ln a microwave resonant circuit, the Q-factor may beprovided by the formula fo faaß' where fo is the center resonance frequency and fsdß is the bandwidth,i.e. frequency span, of the frequency response of the microwave resonantcircuit at the -3dB point, i.e. when the response has dropped by 3dB. Thequality factor is well known in the field of microwave resonant circuits and will not be described in more detail herein. ln view of the above Q-factor discussion, tuning a resonance frequencyof the microwave resonator device relates to shifting or altering fo, i.e. thecenter resonance frequency of the microwave resonator device, from a firstresonance frequency to a second or further resonance frequency by injectingDC electric current into the electrically conductive line pattern.

The tuning of the resonance frequency may be performed independence of a varying tuning electric current. ln other words, theresonance frequency may be a function of the tuning electric current. Forexample, continuously increasing the tuning electric current may continuouslyreduce the resonance frequency.

The electrically conductive line pattern may take any pattern but shouldprovide for a structure that is associated a base wavelength and thus aresonance frequency.

The tuning electric current may be a direct current, i.e. a tuning DCelectric current. Alternatively, the tuning electric current may be an alternatingcurrent, i.e. a tuning AC electric current, preferably with a frequency less thanthe resonance frequency.

The first point may be electrically connected to the second pointthrough the closed end, and further through a capacitive coupling parallel withthe closed end.

Preferably, the microwave resonator may comprise a first electricallyconductive member and a second electrically conductive member, whereinthe first electrically conductive member is electrically connected to the firstpoint for providing the tuning electric current to the first point, and the secondelectrically conductive member is electrically connected to the second pointfor draining the tuning electric current from the second point.

The first electrically conductive member and the second electricallyconductive member may preferably be capacitively coupled to each other.The capacitive coupling may be adapted to provide a near zero, e.g.negligible, impedance at the resonance frequency of the tunable microwaveresonator. This advantageously provides for eliminating any residual voltageat the first and second points on the electrically conductive line pattern that may be present due to finite fabrication tolerances that cause the first andsecond point to be shifted from zero voltage points on the electricallyconductive line pattern. This further means that radiation |osses through thefirst and second points are reduced, or even eliminated.

Accordingly, the capacitive coupling between the first electricallyconductive member and the second electrically conductive member mayadvantageously provide a low impedance path at the resonance frequency ofthe tunable microwave resonator device.

With the inventive concept, fast tuning of the microwave resonator isobtained while at the same time providing high Q-factors. With hereindescribed tunable microwave resonance devices, Q-factors exceeding 100000, even as high as 1000 000 have been obtained with a tuning time below20 ns, and a tuning range of about 2%.

At the closed end of the electrically conductive line pattern theelectrically conductive line pattern is continuous such that an electric currentmay travel therethrough.

The electrically conductive line pattern is not connected at the openend. ln order for a DC electric current to travel from one side of the open endto the other side of the open end, the electric current travels via the closedend.

The predetermined distance may be an electrical distance, in otherwords, the distance that a microwave signal propagates. This is regardless ofthe shape or outline of the conductive line pattern. lf the conductive linepattern from the first point to the open end is a straight line, thepredetermined distance is the length of the conductive line from the first pointto the open end.

The predetermined distance may be the length of the conductive line ofthe electrically conductive line pattern from the first point to the open end.Further, the distance from the second point to the open end is also thepredetermined distance.

That the predetermined distance is based on a length of a quarter basewavelength should be interpreted broadly. The predetermined distance may deviate from a quarter base wavelength by some amount, such as a fewpercent of a quarter base wavelength. The predetermined distance may be adistance the microwave signal propagates across the conductive line patterfor a quarter of the microwave signal period. ln embodiments, the first point and the second point may be at arespective location associated with a voltage node of the tunable microwaveresonator. Accordingly, the first point and the second point may be locatedwhere the residual voltage is negligible. This may advantageously provide forfurther reduced losses and improved quality factor of the microwaveresonator. ln embodiments, the tuning electric current may be injected through thefirst point and drained through the second point. ln embodiments, the electrically conductive pattern may comprise afirst line segment including the first point and a second line segment includingthe second point, wherein the first line segment and the second line segmentare connected at the closed end. Although not strictly required, the first linesegment and the second line segment may be substantially parallel. Forexample, the electrically conductive line pattern may be substantially U-shaped.

A line segment is a segment of electrically conductive material thatmay carry the tuning electric current and that may serve as part of theresonator structure. A line segment is configured to carry microwave signals. ln embodiments, the tunable microwave resonator may compriseelectrically conductive lines supported by the substrate for connecting a DCpower source to the first point and the second point. The electricallyconductive lines, being part of the tuning arrangement, are terminal lines thatprovide an electrically conductive path for injecting the tuning electric currentto the first point and for draining the tuning electric current from the secondpoint. ln embodiments, the tunable microwave resonator device may beadapted to be capacitively or inductively coupled to a read-out transmission line. ln embodiments, the read-out transmission line may be supported bythe substrate. ln embodiments, the electrically conductive line pattern may be planar. ln embodiments, the tunable microwave resonator device may bemanufactured by thin film technology. ln embodiments, the electrically conductive line pattern mayadvantageously be made from a superconductor material. This provides fortuning the kinetic inductance of the superconducting electrically conductiveline pattern using the tuning electric current.

Superconducting materials are per se known to the skilled addressee.Generally, a superconducting material has no electrical resistance whencooled below a material specific transition temperature.

The tunable microwave resonator device may comprise an electricground plane supported by the dielectric substrate, wherein the tuning DCelectric current may be injected into the electrically conductive line pattern viathe electric ground plane.

Further features of, and advantages with, the present invention willbecome apparent when studying the appended claims and the followingdescription. The skilled addressee realizes that different features of thepresent invention may be combined to create embodiments other than thosedescribed in the following, without departing from the scope of the present invenﬁon.

Brief Description of the Drawinqs These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showing an exampleembodiment of the invention, wherein: Fig. 1A conceptually illustrates a tunable microwave resonator deviceaccording to embodiments of the invention; Fig. 1B schematically illustrates a voltage pattern and a current patternfor a microwave mode of a tunable microwave resonator device according to embodiments of the invention; Fig. 1C conceptually illustrates a tunable microwave resonator deviceaccording to embodiments of the invention; Fig. 1D schematically illustrates a voltage pattern and a current patternfor a microwave mode of a tunable microwave resonator device according toembodiments of the invention; Fig. 2 conceptually illustrates a tunable microwave resonator deviceaccording to embodiments of the invention; Fig. 3A schematically illustrates a tunable microwave resonator deviceaccording to embodiments of the invention; and Fig. 3B schematically illustrates a tunable microwave resonator deviceaccording to embodiments of the invention.

Detailed Description of Example Embodiments ln the present detailed description, various embodiments of amicrowave resonator device according to the present disclosure aredescribed. However, the microwave resonator device may be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided forthoroughness and completeness, and to fully convey the scope of the presentdisclosure to the skilled person. Like reference characters refer to likeelements throughout.

Fig. 1A conceptually illustrates a tunable microwave resonator device100 having an associated base wavelength, Å. The microwave resonatordevice 100 comprises a dielectric substrate 102 and an electrically conductiveline pattern 104 supported by the dielectric substrate 102. The electricallyconductive line pattern 104 having an open end 106 and a closed end 108.Further, the microwave resonator device 100 comprises a tuning arrangement110 adapted to provide a tuning electric current into the electrically conductiveline pattern 104 for tuning a resonance frequency of the microwave resonatordevice 100. The tuning arrangement 110 is adapted to pass the electriccurrent from a first point 112 of the electrically conductive line pattern 104 to a second point 114 of the electrically conductive line pattern 104.The first point 112 and the second point 114 are located a predetermined distance dfromthe open end 106, wherein the predetermined distance (d) is based on alength of a quarter base wavelength, M4.

A section 109 of the electrically conductive line pattern 104 from thefirst point 112 and second point 114 to the open end 106 provides animpedance converter for providing a low impedance path at the resonancefrequency of the tunable microwave resonator. ln fig. 1A, the electricallyconductive line pattern is capacitively coupled, via capacitors 120 to a read-out transmission line 122. The open end 106 is at the capacitors 120, on theside of the capacitors not connected to the read-out transmission line 122.

Generally, a microwave resonator may be represented by a circuitincluding a capacitive component and an inductive component, which bothcontribute to the resonance behavior. lnjecting the tuning electric current in tothe electrically conductive line pattern 104 effectively alters the kineticinductance, i.e. an inductive component of the electrically conductive linepattern 104, whereby the resonance frequency is altered.

Fig. 1B illustrates a voltage 139 (solid line) and current 140 (dashedline) pattern of a 3Å/4 mode of the tunable microwave resonator deviceconceptually shown in fig. 1A. The voltage pattern provides nodes 103, 105,where the node 105 is Å/4 away from the open end 106 of the electricallyconductive line pattern 104, i.e. the same distance as from the first point 112to the open end 106, and as the distance from the second point 114 to theopen end 106. The distance 142 denotes Å/2.

During operation of the microwave resonator, 100, a microwave travelstowards the open end 106 and is reflected in the opposite direction due to thehigh impedance presented at the open end 106. Similar to a mechanicaltuning fork, a standing wave is formed, in this case a standing microwave.Due to the high impedance the electrical current is zero at the open end 106,i.e. a current node 141 is present at the open end. Thus, open end 106 of theimpedance converter presents a high impendence end.

At the first and second points 112, 114, the current wave 140 is non-zero or even at maximum, and the voltage node 105 is located at or near the first and second points 112, 114. Accordingly, at the resonance frequency,the voltage 139 is zero or negligible at the first and second points 112, 114.This presents a low impedance end of the impedance converter 109,effectively providing a microwave short for the microwaves travelling in theelectrically conductive line pattern 104. This provides for reducing oreliminating voltages at the first and second points 112, 114 which therebyreduces radiation losses in the tunable microwave resonator, and thereforeincreases the quality-factor.

Fig. 1C conceptually i||ustrates a tunable microwave resonator 200according to embodiments of the present disclosure. The difference betweenthe tunable microwave resonator 100 i||ustrated in fig. 1A, and the tunablemicrowave resonator 200 i||ustrated in fig. 1C is that the electricallyconductive line pattern of the tunable microwave resonator 200 is inductivelycoupled to the read-out transmission line 122. For this, the closed end 108 isarranged towards the read-out transmission line 122.

Fig. 1D conceptually i||ustrates a voltage pattern of a 3Å/4 mode of thetunable microwave resonator device conceptually shown in fig. 1C. Thevoltage pattern provides nodes 103, 105, where the node 105 is Å/4 awayfrom the open end 106 of the electrically conductive line pattern 104, i.e. thesame distance as from the first point 112 to the open end 106, and as thedistance from the second point 114 to the open end 106. For a discussion ofthe operation of the impedance converter 109, refer to the above descriptionrelated to fig. 1B.

The read-out transmission line 122 may be supported by the substrate102.

Referring now to figs 1A and fig. 1C in conjunction. ln the exampleembodiments, the tuning arrangement 110 includes electrically conductivelines, e.g. terminal lines 126, 127 supported by the substrate 102 to providethe DC tuning electric current to the first point 112 and draining it at thesecond point 114. ln addition, as an example the tuning arrangement 110 here optionallycomprises a first electrically conductive member 128 and a second electrically 11 conductive member 129 which may be provided as e.g. electrically conductivepads, or plates, or electrical connection points. The first electrically conductivemember 128, e.g. pad or plate is electrically connected to the first point 112for providing the tuning electric current, Ir, to the first point 112 via terminalline 126. Furthermore, the second electrically conductive member 129 iselectrically connected to the second point 114 for draining the tuning electriccurrent (lr) from the second point 114 via terminal line 127.

A DC current or power source (not shown) may be connected acrossthe pads 128, 129 for passing the tuning current from the first point 112 to thesecond point 114, via the closed end 108. ln other words, the tuning electriccurrent flows along the closed end 108 of the electrically conductive linepattern 104 to tune the kinetic inductance of the material, preferably asuperconductor, of the electrically conductive line pattern 104. The tuning DCelectric current is adjustable such that the magnitude of the tuning DC electriccurrent may be altered to thereby tune resonance frequency of the microwaveresonator device. DC electric current or power sources are known per se inthe art and will not be described further herein.

The tuning arrangement 110 includes connection pads, leads, or othercomponents needed for connecting an external DC current source to theterminal lines 126, 127.

With the predetermined distance being based on a length of a quarterbase wavelength (M4), terminal lines 126, 127 are connected a quarter basewavelength from the open end 106, close or at a voltage node of theelectrically conductive line pattern 104. This reduces the residual voltage thatmay couple to the terminal lines 126, 127, acting as antennas, from theelectrically conductive line pattern 104. The residual voltage that anyway maycouple to the terminal lines 126, 127, causes radiation losses for the tunablemicrowave resonator device, and thus reduced quality factor, i.e. reducedperformance. The residual voltages may for example be due to fabricationtolerances that causes the impedance converter 109 to be non-ideal.

Turning now to fig. 2 which illustrates the microwave resonator device 200 in fig. 1C but here with the first electrically conductive member 128 and 12 the second electrically conductive member 129 capacitively coupled to eachother as is conceptually illustrated by the capacitor 131 _ The capacitivecoupling may be in parallel with the electrical path between the first point 112and the second point 114 provided through the closed end 108. This reducesthe radiation losses caused by the radiating terminal lines 126, 127. lnparticular, the capacitive coupling should be configured such that thecapacitance between the first electrically conductive member 128 and thesecond electrically conductive member 129 provides a low impedance path atleast at the resonance frequency of the tunable microwave resonator. ln thisway, the residual voltage between the first point 112 and the second point114 at the resonance frequency, is further reduced or even eliminated, sincethe capacitive coupling provides an electrical "microwave short".

Again, the quarter base wavelength (M4) section of the electricallyconductive line pattern 104 serves as an impedance converter 109 whichtranslates an infinite open end impedance at the open end 106 into near zeroimpedance between the first point 112 and the second point 114, thusreducing or even eliminating the voltages at the resonance frequency.

The capacitive coupling 131 lies outside of the electrically conductiveline pattern 104. ln other words, the first electrically conductive member 128and the second electrically conductive member 129 are connected to eachother through the electrically conductive line pattern 104 providing a DCelectrical path, and the first electrically conductive member 128 and thesecond electrically conductive member 129 are also connected to each othervia the capacitive coupling 131. Electrically, the capacitive coupling 131 is inparallel with the electrical connection provided through the electricallyconductive line pattern 104 through line segments 134, 136 and the closedend 108. ln some embodiments, as will be addressed with reference tosubsequent drawings, the first electrically conductive member and the secondelectrically conductive member may be adapted to form a capacitor inthemselves. 13 The electrically conductive line pattern may be provided in variousdesigns and configurations. As illustrated in e.g. fig. 2, the electricallyconductive line pattern 104 comprises a first line segment 134 including thefirst point 112 and a second line segment 136 including the second point 114.The first line segment 134 and the second line segment 136 are connected atthe closed end 108.

Although not strictly required, the first line segment 134 and the secondline segment 136 are closely spaced and substantially parallel. Generally, thismeans that the capacitance between the first segment 134, i.e. from the firstpoint 112 to the closed end 108, and the second segment 136, i.e. from thesecond point 114 to the closed end 108, is higher or significantly higher thana capacitance between any one of the segments and the ground plane of themicrowave resonator device. ln this way, it may be ensured that themicrowave mode is localized in-betvveen the first line segment 134 and thesecond line segment 136, thereby reducing parasitic resonances with e.g. aground plane or other components of the tunable microwave resonator.Further, the capacitance per unit length of the parallel line segments isincreased compared to a plain coplanar line. This further enables a relativelycompact tunable microwave resonator device.

The distance between the first line segment 134 and the second linesegment 136, transversely to the line segments depends on the specificdesign of the resonator, but may be in the range of about 10 micrometer toabout 150 micrometer, such as about 20 micrometer, about 35 micrometer,about 50 micrometer, about 70 micrometer, about 90 micrometer, about 100micrometer, etc. Note that this list is non-exhaustive.

Fig. 3A illustrates a tunable microwave resonator device 300 accordingto embodiments of the present invention where the electrically conductive linepattern is capacitively coupled, via a capacitor 120 to a read-out transmissionline 122, and a capacitor 120 that is connected to ground via a split groundplane part 129. The tunable microwave resonator device 300 includeselements denoted with reference numerals also found in fig. 1A, if notaddressed here again, refer to the above description. 14 ln fig. 3A, the first electrically conductive member 128 is provided as afirst part of a split ground plane and the second electrically conductivemember 129 is provided as a second part of a split ground plane for themicrowave resonator device 300. The first part 128 is electrically connected tothe first point 112 by electrically conductive member 126 for providing thetuning electric current to the first point 112, and the second part 129 iselectrically connected to the second point 114 by electrically conductivemember 127 for draining the tuning electric current from the second point114.

One of the capacitors 120 is connected to the read-out line 122 and theother capacitor is connected to electrical ground, here via the electricallyconductive member 129.

The split ground plane parts 128 and 129 may be both connected to ashared electrical ground 152, 153, provided that the electrically conductivepattern 104, the terminal lines 126, 127, and the split ground plane parts 128and 129 are superconducting, while couplings 150, 151 to ground from theground plane parts 128 and 129 are non-superconducting. However, the splitground plane parts 128 and 129 may equally well be connected to individualelectrical grounds 152, 153.

The first part 128 of the split ground plane is capacitively coupled to thesecond part 129 of the split ground plane via interdigitated capacitorstructures 306. Thus, the first electrically conductive member and the secondelectrically conductive member are here adapted to form a capacitor and arehere provided as parts of a split ground plane.

The interdigitated capacitor structures 306 between the first part of thesplit ground plane and the second part of the split ground plane provides alow impedance path at the resonance frequency of the tunable microwaveresonator.

Fig. 3B illustrates a tunable microwave resonator 400 according toembodiments of the present disclosure. One difference between the tunablemicrowave resonator 300 illustrated in fig. 3A, and the tunable microwaveresonator 400 illustrated in fig. 3B is that the electrically conductive line pattern 104 of the tunable microwave resonator 400 is inductively coupled tothe read-out transmission line 122.

With reference to figs. 3A-B, the sp|it ground p|ane includes the firstpart 128 that is capacitively coupled to the second part 129 of the sp|it groundp|ane via interdigitated, e.g. comb-shaped, capacitor structures 306. Thus,the first part 128 includes one side of each of the capacitor structures 306 andthe second part includes the other side of each of the capacitor structures306. Each of the sides are formed to interdigital match the other side, tothereby form an interdigitated capacitor 306.

The sp|it ground p|ane parts 128, 129 may advantageously be locatedin the same p|ane as the electrically conductive line pattern 104.

However, the sp|it ground p|ane parts 128, 129 may be located in adifferent p|ane than the electrically conductive line pattern 104. ln such case,the sp|it ground p|ane parts 128, 129 may be connected to the first point 112,and the second point 114 by e.g. vertical interconnect access (VIA), throughe.g. layers of the tunable microwave resonator or the substrate 102depending on the specific implementation. ln accordance with embodiments, the electrically conductive linepattern may be planar. ln other words, the electrically conductive line patternmay be made in a single p|ane on the dielectric substrate.

The tunable microwave resonator may be manufactured by thin filmtechnology. Thin film technology includes various techniques known per se tothe skilled person, such as electron beam lithography, photolithographysputtering, chemical vapor deposition, physical vapor deposition, pulsed laserdeposition, etc. A typical film thickness is in the range of 10 nm to 500 nm.

The electrically conductive line pattern and other electrically conductiveelements of the tunable microwave resonator may be made from a metalmaterial.

Preferably, the electrically conductive line pattern is made from asuperconductor material. Further, the terminal lines, the electricallyconductive members 128, 129, any other electrically conductive parts of the tunable microwave resonator device may be made from a superconductor 16 material. The specific superconductor material may be selected based on aspecific implementation.

A wide range of superconductors are known per se to the skilledperson. Purely for example purposes, example superconductors may beniobium-based superconductors (e.g. niobium, niobium-nitride, niobium-tin,Niobium-titanium, niobium-germanium, niobium-aluminum), ceramic orcuprate superconductors (e.g. YBCO, NBCO, BSCO, etc..), iron-basedsuperconductors, or other compound (e.g. covalent superconductors)superconductors or single element superconductors. The above examplesuperconductors represent a non-exhaustive list of superconductors that maybe applicable to embodiments presented herein.

Using a superconducting material for the electrically conductive linepattern provides for efficient tuning of the kinetic inductance of the electricallyconductive line pattern by injecting a tuning DC or AC current. The microwaveresonator device is operated when the superconductor material is in thesuperconducting state.

The tunable microwave resonator device according to embodiments ofthe present invention may be used for various applications, such as filters intelecommunications applications and qubit communication buses for quantumcomputers.

The size of the tunable microwave resonator device depends on thespecific implementation and is adapted to microwave electronics. Thus, theline width of the electrically conductive line pattern may be in the order ofmicrometers such as for example 1, 2, 3, 4, 5, 6, 7, 10, 15 micrometer, orbelow a micrometer, such as a fraction of a micrometer.

The person skilled in the art realizes that the present invention by nomeans is limited to the preferred embodiments described above. On thecontrary, many modifications and variations are possible within the scope ofthe appended claims.

For example, the specific shape of the electrically conductive linepatterns is herein depicted in a U-shape. However, the outline of theelectrically conductive line pattern may take other forms, such as with non- 17 parallel line segments, with turns in the line segments, with sharp cornersresembling an open rectangular shape, etc. ln the claims, the word "comprising" does not exclude other elementsor steps, and the indefinite article "a" or "an" does not exclude a plurality. Asingle processor or other unit may fulfill the functions of several items recitedin the claims. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A tunable microwave resonator device (100, 200, 300, 400) havingan associated base wavelength, the microwave resonator device comprising: a dielectric substrate (102); an electrically conductive line pattern (104) supported by the dielectricsubstrate, the electrically conductive line pattern having an open end (106)and a closed end (108), a tuning arrangement (110) adapted to provide a tuning electric current(lr) into the electrically conductive line pattern for tuning a resonancefrequency of the microwave resonator device, the tuning arrangement isadapted to pass the electric current from a first point (112) ofthe electricallyconductive line pattern to a second point (114) of the electrically conductiveline pattern via the closed end, wherein the first point and the second point are located apredetermined distance (d) from the open end, wherein the predetermineddistance is appro>

2. The tunable microwave resonator according to claim 1, wherein asection (109) of the electrically conductive line pattern from the first andsecond point to the open end is configured as an impedance converter forproviding a low impedance path at the resonance frequency of the tunable microwave resonator.

3. The tunable microwave resonator device according to any one of thepreceding claims, wherein the first point and the second point are at arespective location associated with a voltage node (105) of the tunable microwave resonator.

4. The tunable microwave resonator device according to any one of thepreceding claims, wherein the tuning electric current is injected through the first point and drained through the second point.

5. The tunable microwave resonator device according to any one of thepreceding claims, comprising a first electrically conductive member (128) anda second electrically conductive member (129), wherein the first electricallyconductive member is electrically connected to the first point for providing thetuning electric current to the first point, and the second electrically conductivemember is electrically connected to the second point for draining the tuning electric current from the second point.

6. The tunable microwave resonator device according to claim 5,wherein the first electrically conductive member (128) and the second electrically conductive member (129) are capacitively coupled to each other.

7. The tunable microwave resonator device according to claim 6,wherein the first electrically conductive member and the second electrically conductive member are adapted to form a capacitor.

8. The tunable microwave resonator device according to any one ofclaims 6 and 7, wherein the capacitive coupling between the first electricallyconductive member and the second electrically conductive member providesa low impedance path at the resonance frequency of the tunable microwave resonator.

9. The tunable microwave resonator device according to any one of thepreceding claims, wherein the electrically conductive pattern comprises a firstline segment (134) including the first point and a second line segment (136)including the second point, wherein the first line segment and the second line segment are connected at the closed end.

10. The tunable microwave resonator device according to claim 9wherein the first line segment and the second line segment are substantially parallel.

11. The tunable microwave resonator device according to any one ofthe preceding claims, comprising electrically conductive lines (126, 127)supported by the substrate for connecting a DC current source to the first point and the second point.

12. The tunable microwave resonator device according to any one ofthe preceding claims, adapted to be capacitively or inductively coupled to a read-out transmission line.

13. The tunable microwave resonator device according to claim 12,wherein the read-out transmission line is supported by the dielectric substrate.

14. The tunable microwave resonator device according to any one of the preceding claims, wherein the electrically conductive line pattern is planar.

15. The tunable microwave resonator device according to any one ofthe preceding claims, manufactured by thin film technology.

16. The tunable microwave resonator device according to any one ofthe preceding claims, wherein the electrically conductive line pattern is made from a superconductor material.