A switch is a device that controls signal on/off, serving as a critical component for ensuring efficient and stable operation of electronic systems. In recent years, room-temperature semiconductor switches, MHz-band power switches, and optical-band temperature-controlled switches have been widely applied. However, these devices often fail in cryogenic environments due to carrier de-formation effects, limiting their use in low-temperature applications. To address this issue, we propose a low-temperature RF switch based on a distributed half-wavelength superconducting coplanar waveguide resonator. This device leverages the thermal sensitivity of the dynamic inductance in superconducting thin films to achieve signal switching through temperature modulation. Using time-domain coupled-mode theory, we designed a superconducting Fano device and analyzed how its physical parameters regulate signal transmission. We measured the temperature-dependent characteristics of the Fano resonance peak in the$825-1150 \text{mK}$temperature range and characterized its on/off functionality for specific RF signals. The results demonstrate that the device can achieve on/off control for an RF signal at$\boldsymbol{f}_{\boldsymbol{c}}=5.86832 \text{GHz}$, with a maximum extinction ratio of$\mathbf{2 8. 2 ~ d B}$.
A switch is a device that controls signal on/off, serving as a critical component for ensuring efficient and stable operation of electronic systems. In recent years, room-temperature semiconductor switches, MHz-band power switches, and optical-band temperature-controlled switches have been widely applied. However, these devices often fail in cryogenic environments due to carrier de-formation effects, limiting their use in low-temperature applications. To address this issue, we propose a low-temperature RF switch based on a distributed half-wavelength superconducting coplanar waveguide resonator. This device leverages the thermal sensitivity of the dynamic inductance in superconducting thin films to achieve signal switching through temperature modulation. Using time-domain coupled-mode theory, we designed a superconducting Fano device and analyzed how its physical parameters regulate signal transmission. We measured the temperature-dependent characteristics of the Fano resonance peak in the825−1150mKtemperature range and characterized its on/off functionality for specific RF signals. The results demonstrate that the device can achieve on/off control for an RF signal atfc=5.86832GHz, with a maximum extinction ratio of28.2dB.
Cryogenic systems are essential to many modern scientific and technological applications, including MRI superconducting magnets, particle accelerators, astrophysical detectors and quantum computing platforms, all of which operate at temperatures well below 120 K. In this framework, accurate temperature monitoring is, therefore, critical. However, conventional Resistance Temperature Detectors (RTDs) suffer from significant limitations in cryogenic environments, such as wiring complexity, power dissipation, limited low-temperature operation and large footprint, which restrict their integration in increasingly complex cryogenic infrastructures. Superconducting devices offer a promising alternative due to their high sensitivity at low temperatures, low power dissipation and compatibility with integrated quantum technologies. Inspired by Microwave Kinetic Inductance Detectors (MKIDs), this work investigates superconducting microwave resonators as minimally invasive, frequency-multiplexed cryogenic thermometers operating below 12 K. In particular, strategies to tune and enhance temperature sensitivity are explored by studying the effects of the superconducting thin-film thickness and surface nanostructuring. The first part of the study examines NbTi(N) planar resonators with thicknesses ranging from 20 nm to 150 nm, where temperature sensing is based on the temperature dependence of the kinetic inductance. S-shaped split-ring resonators resonating near 1.1 GHz are inductively coupled to a coplanar transmission line and characterized under cryogenic conditions. A clear trade-off between temperature sensitivity and quality factor is observed: thinner films achieve higher sensitivity, up to approximately 19 MHz/K for 20 nm thick devices, while the quality factor, typically exceeding 10^4, is found to degrade when the thickness is reduced. This trade-off leads to an optimal temperature resolution, with sub-microkelvin noise-equivalent temperature (NET) values. In particular, a minimum NET of (0.53±0.09) µK/Hz^(1/2) at 4.2 K is achieved for a 100 nm thick resonator, which is also exploited to study low-frequency temperature fluctuations in liquid helium. In the second part, a possible approach to enhance the temperature sensitivity, through surface nanostructuring, is investigated. Arrays of nanogaps are patterned in high-current regions of a 1.3 GHz resonator, in order to locally suppress superconductivity, reduce the critical temperature of the film and, therefore, accentuate the slope of the temperature-sensitive curve. These weak-links are realized by partially etching a 100 nm NbTi film, to leave 20-30 nm thick remaining nanogaps. Measurements reveal a significant increase in temperature sensitivity, reaching values as high as up to (62±1) MHz/K, which correspond to one order-of-magnitude improvement over non-nanostructured devices. Although this enhancement comes with increased noise-equivalent temperature, the results demonstrate the feasibility of engineering superconducting weak links to improve RF cryogenic thermometry and provide a solid foundation for future optimization. In reason of the achieved results, such an approach could find successful applications for a sub-µK high-resolution cryogenic temperature monitoring, specifically for integrated quantum architectures, further benefiting from the frequency-multiplexing that naturally arises from the multiple allocation of different superconducting microwave resonators.
A novel design of a radio-frequency (RF) power limiter for protecting sensitive superconductor receiver components from high-power microwave signals is presented. In this approach, two low-Tcsuperconductor microstrip lines, one of which is coupled with an array of RF superconducting quantum interference devices (rf-SQUIDs), are combined with a pair of hybrid couplers to provide the power limiting operation at very low RF power levels. The key concept is built on the fact that a microstrip line coupled to an array of rf-SQUIDs behaves as a power-dependent phase shifter and its phase can be controlled by the input RF power. This phenomenon is exploited to achieve the desired RF power-limiting operation. The device is fabricated using an eight-layer niobium-based superconducting process node SFQ5ee by MIT Lincoln Laboratory (MIT-LL). A monolithically integrated wideband hybrid coupler is used in this design as it dictates the bandwidth of the overall device. The operating frequency of the power limiter is 10 GHz with a 2-GHz bandwidth. The performance of the device is measured for input RF power levels ranging from −30 to +10 dBm, as well as arising intermodulation products are experimentally investigated. The output power increases linearly with the input power up to −15 dBm, and for higher power levels, the device offers an increasing attenuation to limit the output RF power starting above −15 dBm.
We present the RF characterization of 2-µm-wide superconducting coplanar waveguides made of 100-nm-thick niobium (Nb) or niobium nitride (NbN) films fabricated on 200 mm silicon wafers. Measurements of the two-ports S-parameters up to 25 GHz in a 4.7K–14 K temperature range with an added direct current up to 115 mA were done using a cryogenic probe station. With proper RF calibration and de-embedding procedure, both temperature and dc current dependence of the RF superconducting properties are analyzed. We report a kinetic inductance of 0.15 pH/□, a critical current density of 50 MA/cm2, and a critical temperature of 8.9 K for Nb as well as 4.27 pH/□, 3 MA/cm2, and 13.8 K for NbN. A telegraphist transmission line model taking into account the BCS theory has been developed and the comparison with the measurements is discussed.
Abstract The rising complexity of cutting-edge cryogenic systems is currently imposing challenging technical constraints to the monitoring of ultra-cold temperatures through standard commercially available sensors. Among different alternative technologies, superconducting microwave resonators have been recently investigated as ideal candidates for performing on-chip cryogenic thermometry, in reason of their intrinsically low power dissipation, typically large temperature sensitivities and excellent sub- μ K resolution below 10 K. In such a framework, through this study we aim at demonstrating the possibility to enhance the temperature performance of superconducting microwave resonators by means of surface nanostructuring. More specifically, different arrays of nanotrenches are strategically patterned on the inductive line of a 1.3 GHz planar resonator, by partially etching a Nb 50 Ti 50 thin-film, in order to tune the critical transition of the material and, therefore, increase the curvature of the f res ( T ) response. Although the presence of such thinned superconducting regions introduces larger microwave losses, a 1.5 K decrease of T C is recorded, which directly translates into an enhancement of the temperature sensitivity by a factor 10, with respect to a reference non-nanostructured sensor. In particular, a maximum value of d f res / d T ≃ 62 MHz/K , at 4.2 K, is achieved for the device showing the largest nanotrench width of about 350 nm. These results demonstrate that such a surface nanostructuring approach can be exploited as an effective design strategy to tune the response of superconducting microwave resonators and enhance their temperature sensitivity around a target operating condition. Although this technique is found to introduce additional RF losses into the superconducting device, thereby limiting the achievable temperature resolution to the few- μ K/Hz 1 / 2 range (at 1 Hz and 4.2 K), we believe that similar approaches could be further investigated for the development of future cryogenic sensors.
In this paper, we present a novel design of a compact, double-fold half-wave superconducting coplanar waveguide resonator, specifically optimized for quantum readout. The geometry offers advantages in terms of scalability and allows for a more compact integration of qubits per unit area. The superconducting resonators are designed to operate within a fundamental frequency range of 1.9 GHz$-1.95GHz,utilizingbothgapandfingercouplingschemes.Thedevicesarefabricatedusingthinaluminumfilmsdepositedonhigh−resistivitysapphiresubstrates.Theproposedresonatorsexhibitqualityfactor(Q_{L})of2.9× 10^{5}$and 1.58 ×103for gap coupled and finger coupled, respectively, during experimental measurements. The simulated QLof single-folded meander line resonators are found to be 4.65 ×105and 258.42, for gap coupled and finger coupled, respectively, for comparison purpose at similar operating frequency. A comprehensive analysis of the microwave transmission spectra was conducted at cryogenic temperatures near 10 mK, across very high input drive powers ($-30dBmto-10dBm).Experimentalresultsshowagoodagreementwithsimulationsandtheequivalentlumpedcircuitmodel.Notably,theproposeddesignreducesthephysicalfootprintoftheresonatordevicesupto28.2\%comparedtoconventionalsingle−foldmeanderedstructures.TheresonatorsdemonstratestrongpotentialforintegratingqubitswithreadoutandcontrolbusesinquantuminformationprocessingwhichrequireaQ_{L}$between104−105. They also being well-suited for cryogenic detector applications that utilize microwave kinetic inductance detector (MKID) arrays in frequency multiplexing for different coupling schemes.
Abstract Superconducting microwave resonators have recently gained a primary importance in the development of cryogenic applications, such as circuit quantum electrodynamics, electron spin resonance spectroscopy and particles detection for high-energy physics and astrophysics. In this work, we investigate the influence of the film thickness on the temperature response of microfabricated Nb 50 Ti 50 superconducting resonators. S-shaped split ring resonators (S-SRRs), approximately 20 nm to 150 nm thick, are designed to be electromagnetically coupled with standard Cu coplanar waveguides (CPWs) and their microwave properties are characterised at temperatures below 10 K . The combined contributions of the kinetic inductance L K ( T ) increase and the decreasing loaded quality factor Q L , for thinner films, induce an optimum condition on the temperature sensitivity and resolution of the resonators. A noise equivalent temperature as low as ( 0.53 ± 0.09 ) μ K Hz − 1 / 2 , at 1 Hz , is reported for 100 nm thick resonators at 4.2 K . We also asses the possibility of implementing a multiplexed frequency readout, allowing for the simultaneous temperature tracking of several sensors along a single CPW. Such results demonstrate the possibility to perform a distributed cryogenic temperature monitoring, with a sub- μ K resolution. Thus, the application of superconducting S-SRRs, eventually benefiting from an even higher