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Nano Letters

Utilizing Gate-Controlled Supercurrent for All-Metallic Tunable Superconducting Microwave Resonators

Younghun Ryu, Jinhoon Jeong, Junho Suh, Jihwan Kim, Hyoungsoon Choi, Jinwoong Cha

Published Jan 17, 2024• 10 citations
Publisher

Abstract

Hybridizing a microwave mode with a quantum state requires precise frequency matching of a superconducting microwave resonator and the corresponding quantum object. However, fabrication always brings imperfections in geometry and material properties, causing deviations from the desired operating frequencies. An effective and universal strategy for their resonant coupling is to tune the frequency of a resonator, as quantum states like phonons are hardly tunable. Here, we demonstrate gate-tunable, titanium-nitride (TiN)-based superconducting resonators by implementing a nanowire inductor whose kinetic inductance is tuned via the gate-controlled supercurrent (GCS) effect. We investigate their responses for different gate biases and observe 4% (∼150 MHz) frequency tuning with decreasing internal quality factors. We also perform temperature-controlled experiments to support phonon-related mechanisms in the GCS effect and the resonance tuning. The GCS effect-based method proposed in this study provides an effective route for locally tunable resonators that can be employed in various hybrid quantum devices.

DOI
10.1021/acs.nanolett.3c04080

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Hybridizing a microwave mode with a quantum state requires precise frequency matching of a superconducting microwave resonator and the corresponding quantum object. However, fabrication always brings imperfections in geometry and material properties, causing deviations from the desired operating frequencies. An effective and universal strategy for their resonant coupling is to tune the frequency of a resonator, as quantum states like phonons are hardly tunable. Here, we demonstrate gate-tunable, titanium-nitride (TiN)-based superconducting resonators by implementing a nanowire inductor whose kinetic inductance is tuned via the gate-controlled supercurrent (GCS) effect. We investigate their responses for different gate biases and observe 4% (∼150 MHz) frequency tuning with decreasing internal quality factors. We also perform temperature-controlled experiments to support phonon-related mechanisms in the GCS effect and the resonance tuning. The GCS effect-based method proposed in this study provides an effective route for locally tunable resonators that can be employed in various hybrid quantum devices.
Frequency-tunable microwave resonators are in great demand especially in hybrid systems where precise frequency alignment of resonances is required. Here, we present frequency-tunable high-Q superconducting resonators fabricated from thin niobium nitride and niobium titanium nitride films. The resonant frequency is tuned by applying a magnetic field perpendicular to the hole structures in the resonator's inductor wire, whose kinetic inductance is modified by wirelessly induced DC supercurrents. A continuous in situ frequency tuning of over 300 MHz is achieved for a 10 GHz resonator with a moderate magnetic field of 1.2 mT. The planar resonator design and the noncontact tuning scheme greatly ease the fabrication complexity and can be widely applied in many hybrid systems for coupling microwave modes with other forms of excitations such as optical photons, phonons, magnons, and spins.
We have designed, fabricated, and tested a frequency-tunable high-Q superconducting resonator made from a niobium titanium nitride film. The frequency tunability is achieved by injecting a DC through a current-directing circuit into the nonlinear inductor whose kinetic inductance is current-dependent. We have demonstrated continuous tuning of the resonance frequency in a 180 MHz frequency range around 4.5 GHz while maintaining the high internal quality factor Qi > 180 000. This device may serve as a tunable filter and find applications in superconducting quantum computing and measurement. It also provides a useful tool to study the nonlinear response of a superconductor. In addition, it may be developed into techniques for measurement of the complex impedance of a superconductor at its transition temperature and for readout of transition-edge sensors.
We report on a simple yet versatile design for a tunable superconducting microstrip resonator. Niobium nitride is employed as the superconducting material and aluminum oxide, produced by atomic layer deposition, as the dielectric layer. We show that the high quality of the dielectric material allows to reach the internal quality factors in the order of Qi∼104 in the single photon regime. Qi rapidly increases with the number of photons in the resonator N and exceeds 105 for N∼10−50. A straightforward modification of the basic microstrip design allows to pass a current bias through the strip and to control its kinetic inductance. We achieve a frequency tuning δf=62 MHz around f0=2.4 GHz for a fundamental mode and δf=164 MHz for a third harmonic. This translates into a tuning parameter Qiδf/f0=150. The presented design can be incorporated into essentially any superconducting circuitry operating at temperatures below 2.5 K.
Several experiments have recently reported on gate-tunable superconducting properties in metallic devices, holding promise for the realization of cryogenic switches, tunable resonators, and superconducting logic. In particular, the suppression of the critical current as a function of the gate voltage has been widely investigated. However, time domain studies are discussed only in a few cases. In this paper, we present a microwave characterization of a gate-controlled Al-capped InAs nanowire embedded in a λ/4 coplanar waveguide resonator. We observe a shift in the resonator frequency and an increase in its internal losses as a function of the gate voltage, which we relate to a change in the imaginary and real components of the nanowire impedance, respectively. We demonstrate that these changes are described by the Mattis–Bardeen model with an effective temperature. We further study the resonator response to fast-varying gate signals and measure characteristic response times of the order of 40 ns, both in time domain and parametric modulation experiments. Our study elucidates the impact of the gate on the complex impedance of the nanowire in the superconducting state, as well as its dynamic performance, providing a foundation for the design of gate-controlled superconducting devices.
Frequency tuning enables both high-sensitivity and wide-bandwidth detection in the resonator. Potassium tantalate (KTO) is a quantum paraelectric with tunable permittivity and low microwave loss at cryogenic temperature, making it promising for tunable devices. Here, we demonstrate a superconducting microwave resonator on a KTO substrate. The resonant frequency can be tuned by ∼40 MHz using a 9 V bias at 20 mK, with no observable hysteresis. The absence of hysteresis is a key metric for practical devices, addressing one of the major obstacles in other paraelectric-based tunable resonators. More interestingly, we observed that the quality factor oscillates during voltage tuning, arising from the competition between phonon hardening and charge injection. Therefore, our work provides a practical tunable resonator platform and offers a potential approach to study energy-loss characteristics in quantum paraelectrics.
We characterize inductors fabricated from ultra-thin, approximately 100 nm wide strips of niobium (Nb) and niobium nitride (NbN). These nanowires have a large kinetic inductance in the superconducting state. The kinetic inductance scales linearly with the nanowire length, with a typical value of 1 nH µm(-1) for NbN and 44 pH µm(-1) for Nb at a temperature of 2.5 K. We measure the temperature and current dependence of the kinetic inductance and compare our results to theoretical predictions. We also simulate the self-resonant frequencies of these nanowires in a compact meander geometry. These nanowire inductive elements have applications in a variety of microwave frequency superconducting circuits.