Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2512.23119

Efficient flip-chip and on-chip-based modulation of flux-tunable superconducting resonators

Achintya Paradkar, Paul Nicaise, Karim Dakroury, Fabian Resare, Christian Dejaco, Lukas Deeg, Gerhard Kirchmair, Witlef Wieczorek

quant-phcond-mat.supr-conSubmitted Dec 28, 2025• Announced Dec 30, 2025• 0 citations
arXivHTMLPDF

Abstract

We demonstrate the efficient modulation of flux-tunable superconducting resonators (FTRs) using flip-chip or on-chip-based input coils. The FTRs we use are aluminum-based quarter-wave coplanar waveguide resonators terminated with 100um or 200um-wide square loop dc superconducting quantum interference devices (SQUIDs) employing 1um-sized Josephson junctions. We employ SQUIDs with a geometric loop inductance of up to 0.7nH to increase the flux transfer efficiency. The geometric inductance of the SQUID results in a non-zero screening parameter $β_L$, whose branch switching effect is mitigated by using asymmetric junctions. We achieve flux modulation of the FTRs by more than one GHz and flux responsivities of up to tens of GHz/$Φ_0$ with uA-scale on-chip currents. We compare flip-chip with on-chip input-coil-based flux modulation, where the former is realized through galvanically connected and closely spaced chips, while the latter is achieved through superconducting air-bridge connections. We achieve a flux-transfer efficiency from the input coil to the SQUID loop of up to 20%. Our work paves the way for efficient low current flux modulation of FTRs and sensitive measurement of flux signals.

Comments
7+20 pages, 5+8 figures

Related papers

Frequency control of superconducting resonators using integrated interferometers

Irina E. Tarasova, Sergey V. Egorov, Ivan A. Nazhestkin +7 more

Predictable frequency trimming of superconducting resonators at liquid-helium temperatures requires a tuning element whose usable range remains well defined despite screening currents and hysteresis. Here, we realize flux-controlled λ/4 coplanar-waveguide resonators terminated by two-junction DC superconducting quantum interference devices (SQUIDs) fabricated in a standard Nb/Al-AlOx/Nb multilayer process. Integrated on-chip flux-bias lines allow the resonant frequency to be controlled at 4.2 K. RF measurements show periodic frequency modulation with applied flux, with a maximum shift of δfr=8.63 MHz and a single-valued tuning interval of δfr0=2.85 MHz. Near half-integer flux quanta, the response exhibits hysteretic jumps caused by the finite SQUID loop inductance, so that the usable single-valued tuning interval is smaller than the full modulation range. We develop a semi-analytical model of the frequency–flux response, supported by numerical simulations and 3D electromagnetic extraction of the device parameters, which reproduces the measured hysteretic branches and identifies the SQUID loop inductance and junction critical current as the main parameters limiting the usable tuning range. These results provide a quantitative design framework for MHz-scale frequency trimming of Nb superconducting resonators in 4 K cryogenic readout circuits.

Applied Physics Letters•Aug 24, 2026•0 citations

Coupling a Flux-Tunable Resonator to a Magnetically Levitated Superconducting Microparticle

Achintya Paradkar

Magnetically levitated superconducting microparticles provide a promising platform for fundamental quantum experiments and quantum-limited sensing. In the Meissner state, the levitated particle is trapped without mechanical clamping, allowing its center-of-mass motion to exhibit ultralow dissipation. The particle's displacement can be read out dispersively via magnetic flux coupling to superconducting resonators. A challenge for magnetomechanical coupling is to realize a flux-tunable microwave circuit that provides strong flux-to-frequency transduction, low dissipation, and a sufficiently large linear operating range for readout and cooling. This thesis develops and characterizes a chip-based superconducting platform for realizing this transduction.Chip-based levitation of a superconducting microparticle is demonstrated at 40 mK. Passive cryogenic vibration isolation is developed to increase the mechanical quality factors of the levitated particle. For coupling to a microwave circuit, a superconducting flip-chip process based on indium microspheres was developed. The need for Au passivation of Nb or NbN under-bump metallization is demonstrated, and indium-based superconducting interconnects are shown to carry ampere-scale currents. Flux-tunable resonators with 100-200 µm SQUID loops and integrated input coils are designed, fabricated, and characterized. A peak flux responsivity of 20 GHz/Φ₀ is achieved, with one flux quantum modulation requiring only 10-20 µA of input-coil current. Two integrated flux-biasing architectures are developed with flip-chip and on-chip input coils, achieving a total flux transfer efficiency of 1.6%. Junction asymmetry is shown to suppress branch-switching behavior associated with finite SQUID screening.Together, these results establish the superconducting subsystems required for dispersive microwave readout of the center-of-mass motion of a levitated microparticle. Such a magnetomechanical coupling provides a route toward cooling the center-of-mass motion to the ground state, paving the way for the preparation of nonclassical motional states. More broadly, such systems are well-suited for quantum-limited force and acceleration sensing, as well as for probing quantum physics in a previously unexplored mass regime.

•Apr 13, 2026•0 citations

Flux-coupled tunable superconducting resonator

Juliang Li, Peter S. Barry, Tom Cecil +6 more

We present a design and implementation of a frequency-tunable superconducting resonator. The resonance frequency tunability is achieved by flux-coupling a superconducting LC loop to a current-biased feedline; the resulting screening current leads to a change of the kinetic inductance and shift in the resonance frequency. The thin-film aluminum resonator consists of an interdigitated capacitor and thin line inductors forming a closed superconducting loop. The magnetic flux from the nearby niobium current feedline induces Meissner shielding currents in the resonator loop leading to a change in the kinetic part of the total inductance of the resonator. We demonstrate continuous frequency tuning within 160 MHz around the resonant frequency of 2.7 GHz. We show that: (1) frequency up-conversion is achieved when a kilohertz ac modulation signal is superimposed onto the dc bias resulting in sidebands to the resonator tone; (2) three-wave mixing is attained by parametrically pumping the nonlinear kinetic inductance using a strong rf pump signal in the feedline. The simple architecture is amenable to large-array multiplexing and on-chip integration with other circuit components. The concept could be applied in flux magnetometers, up-converters, and parametric amplifiers operating above 4 K when alternative high-critical-temperature material with high kinetic inductance is used.

arXiv:2312.12755•Physical Review Applied•Jul 30, 2024•9 citations

Broad-Band Coupling With Improved Efficiency Between the Nano-SQUID and Coplanar Waveguide

Long Wu, Lei Chen, Linxian Ma +4 more

Besides its well-known magnetic sensitivity, the nano superconducting quantum interference device (SQUID) has an almost instantaneous response time. However, transferring an ultrafast magnetic signal to a nano-SQUID is a technical challenge. We describe a very efficient direct-coupling structure between the SQUID and the coplanar waveguide (CPW). The center conductor of the CPW and its grounding planes are connected by a short line. We fabricated devices having direct and indirect coupling between the short line and nano-SQUIDs. The coupling efficiencies of both schemes are compared quantitatively. By injecting microwaves of frequencies 0.1-18 GHz through the CPW, we found that microwave power of 100 nW and 158 μW is required to suppress the critical current of nano-SQUIDs for the direct and indirect-coupling devices, respectively. By applying a dc current to the short line, we were able to measure the flux-modulation curves of SQUIDs. To achieve a flux quanta in SQUIDs requires currents of 0.46 and 11 mA for the direct- and indirect-coupling devices, respectively. Therefore, the direct-coupling scheme improves the efficiency by a factor of 39 from 0.1 to 18 GHz and 24 at dc, respectively. Furthermore, while sending a pulse-modulated signal of 1 μW power, a SQUID incorporating the direct-coupling scheme produced a working flux modulation.

IEEE Transactions on Applied Superconductivity•Mar 8, 2019•0 citations

HTS YBCO Resonator Configuration With a Coplanar Optimized Flux Concentrator Strongly Coupled to rf SQUID

Fatemeh Qaderi, Faezeh Shanehsazzadeh, Behnam Mazdouri +1 more

We developed a novel magnetic coupling module, formed of a monolayer superconducting flux concentrator that is integrated with a coplanar resonator, strongly coupled to high-temperature superconducting radio frequency superconducting quantum interference device (SQUID). Three types of resonators, including a long stripline resonator between the input loop and the pick-up loop of the flux concentrator, a complementary split ring resonator, and a spiral shape inside the input loop, are explored. The resonance quality factor of different patterns of these three types of the resonators, as well as their coupling to the SQUID, is evaluated using finite-element-method simulations. Several readout methods are tested to couple the electronic system to the resonators by inductive (coil) or capacitive (transmission line) methods, and the optimum readout is reported for each of the resonators. Among the evaluated resonator types, a spiral shape with an optimal behavior at the resonance frequency of 836 MHz, showing the highest quality factor together with the strongest coupling to the SQUID, compared to other types of the resonators, is fabricated using a 200-nm-thick YBCO film on a 1-mm-thick crystalline LaAlO3substrate. The flux concentrator of the module is optimized by the variation of the linewidth of its different parts and also its input loop radius to obtain the maximum flux transformation efficiency.

arXiv:1709.09960•IEEE Transactions on Applied Superconductivity•Feb 16, 2018•2 citations

Microwave photon generation in a doubly tunable superconducting resonator

I-M. Svensson, M. Pierre, Michaël Simoen +7 more

We have created a doubly tunable resonator, with the intention to simulate relativistic motion of the resonator boundaries in real space. Our device is a superconducting coplanar-waveguide microwave resonator, with fundamental resonant frequency ω 1 / (2 π ) ~ 5 GHz. Both of its ends are terminated to ground via dc-SQUIDs, which serve as magnetic-flux-controlled inductances. Applying a flux to either SQUID allows the tuning of ω 1 / (2 π ) by approximately 700 MHz. Using two separate on-chip magnetic-flux lines, we modulate the SQUIDs with two tones of equal frequency, close to 2 ω 1 . We observe photon generation, at ω 1 , above a certain pump amplitude threshold. By varying the relative phase of the two pumps we are able to control this threshold, in good agreement with a theoretical model. At the same time, some of our observations deviate from the theoretical predictions, which we attribute to parasitic couplings resulting in current driving of the SQUIDs.

arXiv:1706.06821•Journal of Physics Conference Series•Mar 1, 2018•15 citations
We demonstrate the efficient modulation of flux-tunable superconducting resonators (FTRs) using flip-chip or on-chip-based input coils. The FTRs we use are aluminum-based quarter-wave coplanar waveguide resonators terminated with 100um or 200um-wide square loop dc superconducting quantum interference devices (SQUIDs) employing 1um-sized Josephson junctions. We employ SQUIDs with a geometric loop inductance of up to 0.7nH to increase the flux transfer efficiency. The geometric inductance of the SQUID results in a non-zero screening parameter βLβ_LβL​, whose branch switching effect is mitigated by using asymmetric junctions. We achieve flux modulation of the FTRs by more than one GHz and flux responsivities of up to tens of GHz/Φ0Φ_0Φ0​ with uA-scale on-chip currents. We compare flip-chip with on-chip input-coil-based flux modulation, where the former is realized through galvanically connected and closely spaced chips, while the latter is achieved through superconducting air-bridge connections. We achieve a flux-transfer efficiency from the input coil to the SQUID loop of up to 20%. Our work paves the way for efficient low current flux modulation of FTRs and sensitive measurement of flux signals.
Predictable frequency trimming of superconducting resonators at liquid-helium temperatures requires a tuning element whose usable range remains well defined despite screening currents and hysteresis. Here, we realize flux-controlled λ/4 coplanar-waveguide resonators terminated by two-junction DC superconducting quantum interference devices (SQUIDs) fabricated in a standard Nb/Al-AlOx/Nb multilayer process. Integrated on-chip flux-bias lines allow the resonant frequency to be controlled at 4.2 K. RF measurements show periodic frequency modulation with applied flux, with a maximum shift of δfr=8.63 MHz and a single-valued tuning interval of δfr0=2.85 MHz. Near half-integer flux quanta, the response exhibits hysteretic jumps caused by the finite SQUID loop inductance, so that the usable single-valued tuning interval is smaller than the full modulation range. We develop a semi-analytical model of the frequency–flux response, supported by numerical simulations and 3D electromagnetic extraction of the device parameters, which reproduces the measured hysteretic branches and identifies the SQUID loop inductance and junction critical current as the main parameters limiting the usable tuning range. These results provide a quantitative design framework for MHz-scale frequency trimming of Nb superconducting resonators in 4 K cryogenic readout circuits.
Magnetically levitated superconducting microparticles provide a promising platform for fundamental quantum experiments and quantum-limited sensing. In the Meissner state, the levitated particle is trapped without mechanical clamping, allowing its center-of-mass motion to exhibit ultralow dissipation. The particle's displacement can be read out dispersively via magnetic flux coupling to superconducting resonators. A challenge for magnetomechanical coupling is to realize a flux-tunable microwave circuit that provides strong flux-to-frequency transduction, low dissipation, and a sufficiently large linear operating range for readout and cooling. This thesis develops and characterizes a chip-based superconducting platform for realizing this transduction.Chip-based levitation of a superconducting microparticle is demonstrated at 40 mK. Passive cryogenic vibration isolation is developed to increase the mechanical quality factors of the levitated particle. For coupling to a microwave circuit, a superconducting flip-chip process based on indium microspheres was developed. The need for Au passivation of Nb or NbN under-bump metallization is demonstrated, and indium-based superconducting interconnects are shown to carry ampere-scale currents. Flux-tunable resonators with 100-200 µm SQUID loops and integrated input coils are designed, fabricated, and characterized. A peak flux responsivity of 20 GHz/Φ₀ is achieved, with one flux quantum modulation requiring only 10-20 µA of input-coil current. Two integrated flux-biasing architectures are developed with flip-chip and on-chip input coils, achieving a total flux transfer efficiency of 1.6%. Junction asymmetry is shown to suppress branch-switching behavior associated with finite SQUID screening.Together, these results establish the superconducting subsystems required for dispersive microwave readout of the center-of-mass motion of a levitated microparticle. Such a magnetomechanical coupling provides a route toward cooling the center-of-mass motion to the ground state, paving the way for the preparation of nonclassical motional states. More broadly, such systems are well-suited for quantum-limited force and acceleration sensing, as well as for probing quantum physics in a previously unexplored mass regime.
We present a design and implementation of a frequency-tunable superconducting resonator. The resonance frequency tunability is achieved by flux-coupling a superconducting LC loop to a current-biased feedline; the resulting screening current leads to a change of the kinetic inductance and shift in the resonance frequency. The thin-film aluminum resonator consists of an interdigitated capacitor and thin line inductors forming a closed superconducting loop. The magnetic flux from the nearby niobium current feedline induces Meissner shielding currents in the resonator loop leading to a change in the kinetic part of the total inductance of the resonator. We demonstrate continuous frequency tuning within 160 MHz around the resonant frequency of 2.7 GHz. We show that: (1) frequency up-conversion is achieved when a kilohertz ac modulation signal is superimposed onto the dc bias resulting in sidebands to the resonator tone; (2) three-wave mixing is attained by parametrically pumping the nonlinear kinetic inductance using a strong rf pump signal in the feedline. The simple architecture is amenable to large-array multiplexing and on-chip integration with other circuit components. The concept could be applied in flux magnetometers, up-converters, and parametric amplifiers operating above 4 K when alternative high-critical-temperature material with high kinetic inductance is used.
Besides its well-known magnetic sensitivity, the nano superconducting quantum interference device (SQUID) has an almost instantaneous response time. However, transferring an ultrafast magnetic signal to a nano-SQUID is a technical challenge. We describe a very efficient direct-coupling structure between the SQUID and the coplanar waveguide (CPW). The center conductor of the CPW and its grounding planes are connected by a short line. We fabricated devices having direct and indirect coupling between the short line and nano-SQUIDs. The coupling efficiencies of both schemes are compared quantitatively. By injecting microwaves of frequencies 0.1-18 GHz through the CPW, we found that microwave power of 100 nW and 158 μW is required to suppress the critical current of nano-SQUIDs for the direct and indirect-coupling devices, respectively. By applying a dc current to the short line, we were able to measure the flux-modulation curves of SQUIDs. To achieve a flux quanta in SQUIDs requires currents of 0.46 and 11 mA for the direct- and indirect-coupling devices, respectively. Therefore, the direct-coupling scheme improves the efficiency by a factor of 39 from 0.1 to 18 GHz and 24 at dc, respectively. Furthermore, while sending a pulse-modulated signal of 1 μW power, a SQUID incorporating the direct-coupling scheme produced a working flux modulation.
We developed a novel magnetic coupling module, formed of a monolayer superconducting flux concentrator that is integrated with a coplanar resonator, strongly coupled to high-temperature superconducting radio frequency superconducting quantum interference device (SQUID). Three types of resonators, including a long stripline resonator between the input loop and the pick-up loop of the flux concentrator, a complementary split ring resonator, and a spiral shape inside the input loop, are explored. The resonance quality factor of different patterns of these three types of the resonators, as well as their coupling to the SQUID, is evaluated using finite-element-method simulations. Several readout methods are tested to couple the electronic system to the resonators by inductive (coil) or capacitive (transmission line) methods, and the optimum readout is reported for each of the resonators. Among the evaluated resonator types, a spiral shape with an optimal behavior at the resonance frequency of 836 MHz, showing the highest quality factor together with the strongest coupling to the SQUID, compared to other types of the resonators, is fabricated using a 200-nm-thick YBCO film on a 1-mm-thick crystalline LaAlO3substrate. The flux concentrator of the module is optimized by the variation of the linewidth of its different parts and also its input loop radius to obtain the maximum flux transformation efficiency.
We have created a doubly tunable resonator, with the intention to simulate relativistic motion of the resonator boundaries in real space. Our device is a superconducting coplanar-waveguide microwave resonator, with fundamental resonant frequency ω 1 / (2 π ) ~ 5 GHz. Both of its ends are terminated to ground via dc-SQUIDs, which serve as magnetic-flux-controlled inductances. Applying a flux to either SQUID allows the tuning of ω 1 / (2 π ) by approximately 700 MHz. Using two separate on-chip magnetic-flux lines, we modulate the SQUIDs with two tones of equal frequency, close to 2 ω 1 . We observe photon generation, at ω 1 , above a certain pump amplitude threshold. By varying the relative phase of the two pumps we are able to control this threshold, in good agreement with a theoretical model. At the same time, some of our observations deviate from the theoretical predictions, which we attribute to parasitic couplings resulting in current driving of the SQUIDs.