Superconducting coplanar waveguide (SCPW) resonators have a wide range of applications due to the combination of their planar geometry and high quality factors relative to normal metals. However, their performance is sensitive to both the details of their geometry and the materials and processes that are used in their fabrication. In this paper, we study the dependence of SCPW resonator performance on materials and geometry as a function of temperature and excitation power. We measure quality factors greater than 2 × 106 at high excitation power and 6 × 105 at a power comparable to that generated by a single microwave photon circulating in the resonator. We examine the limits to the high excitation power performance of the resonators and find it to be consistent with a model of radiation loss. We further observe that while in all cases the quality factors are degraded as the temperature and power are reduced due to dielectric loss, the size of this effect is dependent on resonator materials and geometry. Finally, we demonstrate that the dielectric loss can be controlled in principle using a separate excitation near the resonance frequencies of the resonator.
Superconducting coplanar waveguide (SCPW) resonators have a wide range of applications due to the combination of their planar geometry and high quality factors relative to normal metals. However, their performance is sensitive to both the details of their geometry and the materials and processes that are used in their fabrication. In this paper, we study the dependence of SCPW resonator performance on materials and geometry as a function of temperature and excitation power. We measure quality factors greater than 2 × 106 at high excitation power and 6 × 105 at a power comparable to that generated by a single microwave photon circulating in the resonator. We examine the limits to the high excitation power performance of the resonators and find it to be consistent with a model of radiation loss. We further observe that while in all cases the quality factors are degraded as the temperature and power are reduced due to dielectric loss, the size of this effect is dependent on resonator materials and geometry. Finally, we demonstrate that the dielectric loss can be controlled in principle using a separate excitation near the resonance frequencies of the resonator.
Superconducting coplanar-waveguide (CPW) resonators are one of the key devices in circuit quantum electrodynamics (cQED). Their performance can be limited by dielectric losses in the substrate and in the material interfaces. Reliable modeling is required to aid in the design of low-loss CPW structures for cQED. We analyze the geometric dependence of the dielectric losses in CPW structures using finite-element modeling of the participation ratios of the lossy regions. In a practical scenario, uncertainties in the the dielectric constants and loss tangents of these regions introduce uncertainties in the theoretically predicted participation ratios. We present a method for combining loss simulations with measurements of two-level-system-limited quality factors and resonance frequencies of CPW resonators. Namely, we solve an inverse problem to find model parameters producing the measured values. High quality factors are obtainable by properly designing the cross-sectional geometries of the CPW structures, but more accurate modeling and design methods for low-loss CPW resonators are called for major future improvements. Our nonlinear optimization methodology for solving the aforementioned inverse problem is a step in this direction.
In this master's thesis, I present the design, fabrication, and characterisation of superconducting microwave resonators. These resonators are quarter-wavelength coplanar waveguide resonators, operating at resonance frequencies between 4 GHz and 8 GHz. This kind of resonator is a key component in several applications, ranging from quantum computation devices to parametric amplifiers. Minimising the losses (achieving high internal quality factors) in these resonators is important to maximise performance of the applications they are used for. During the thesis work, we have studied several possible changes to improve the resonator quality.\n\nFabrication of the resonators was done in the Nanofabrication Laboratory at Chalmers University of Technology, and consist of the following steps: cleaning a sapphire or silicon wafer, depositing a layer of superconducting niobium, photolithography, and etching the waveguide. We then measured the resonators in a cryogen-free dilution refrigerator, at a temperature of 10 mK. The measurements consisted of a power sweep, in order to quantify the quality factor at high and low excitation powers. As an example, the first resonator we fabricated had a quality factor of 4.3 x 10^4 at high power (about 10^5 photons in the resonator), and quality factor of 3.7 x 10^4 at the single-photon level.\n\nWe then investigated different loss mechanisms that we expect to negatively affect the quality factor, and how to reduce this loss. Firstly, to reduce radiation losses and magnetic vortex losses, we reduced the lateral dimensions of the resonator. Secondly, as two-level system (TLS) losses are believed to mainly reside at the surfaces of the materials and interfaces between them, we etched into the substrate to selectively move a part of the TLS away from the resonator. We also studied the effect of the meandering of the resonator on the quality factor, as well as using two different etching gasses.\n\nBy gathering all this data, we were then able to compare the different combinations to see the effect of the changing parameters. In conclusion, the reduced lateral dimensions and the removal of part of the substrate indeed reduce the losses, as expected from the theory. We also found interesting behaviour with regards to the meandering, namely that having a meandered structure negatively affects the quality factor.\n\nCombining all results, we can give a guideline for the fabrication of resonators in the future, which gave us a quality factor up to 4.9 x 10^5 at high excitation power, and 3.0 x 10^5 at the single photon level.
The loss in superconducting microwave resonators at low-photon numbers and low temperatures is not well understood but has implications for achievable coherence times in superconducting qubits. We have fabricated single-layer resonators with a high quality factor by patterning a superconducting aluminum film on a sapphire substrate. Four resonator geometries were studied with resonant frequencies ranging from 5 to 7 GHz: a quasi-lumped element resonator, a coplanar strip waveguide resonator, and two hybrid designs that contain both a coplanar strip and a quasi-lumped element. Transmitted power measurements were taken at 30 mK as a function of frequency and probe power. We find that the resonator loss, expressed as the inverse of the internal quality factor, decreases slowly over four decades of photon number in a manner not merely explained by loss from a conventional uniform spatial distribution of two-level systems in an oxide layer on the superconducting surfaces of the resonator.
Measuring the internal quality factor of coplanar waveguide superconducting resonators is an established method of determining small losses in superconducting devices. Often, resonator losses are attributed to two-level system (TLS) defects using a power dependent model for the quality factor. However, excess non-equilibrium quasiparticles can also limit the quality factor of superconducting resonators. Here, a two-temperature, power and temperature dependent model is used to evaluate resonator losses for titanium nitride and aluminum coplanar waveguide resonators with loss tangents of 10−5–10−6. This model enables a set of parameters to describe both the internal quality factor and fractional frequency shift of the resonator across a broad range of average cavity photon numbers, from 1 to 106, and temperatures between those limited by the experimental apparatus near 50 mK and thermal quasiparticle dominated. The presented model also offers potential insight into observing the temperature and power dependence of TLS–TLS interactions in these resonators.
Abstract A quarter wavelength superconducting aluminum film coplanar waveguide resonator with a high-quality factor was designed and fabricated. Furthermore, its resonant frequency and quality as a function of temperatures and exciting powers were investigated. The experimental results showed that the resonance frequency decreases about 1% with the increase of temperature from 50 mk to 1 K, and the load quality factor of the resonator also decreases. The resonant frequency decreases with power from − 120 to − 70 dBm. The quality factor increases with increased excitation power due to the loss decreases and nonlinearity. The results are consistent with the theoretical analysis of the surface impedance model.
Superconducting coplanar waveguide (SCPW) resonators, key components for quantum computing and sensing applications, require a high internal quality factor ( Q i ) for effective qubit readout and quantum sensing applications. Minimizing two-level system (TLS) losses, particularly at material interfaces, is critical for gatemon and topological qubits operating at low temperatures and in high magnetic fields. NbTi, a superconducting alloy with a high upper critical field, enables SCPW resonators resilient to such conditions. We fabricated NbTi SCPW resonators on Si/SiO 2 substrates and systematically characterized their TLS-limited quality factors as functions of temperature and microwave photon number. Our results demonstrate that NbTi-based SCPWs on Si/SiO 2 substrates provide a promising platform for developing next-generation quantum circuits. • Successfully fabricated NbTi SCPW resonators on Si/SiO 2 substrates. • Characterized NbTi SCPW across temperature and photon ranges for quantum apps. • Achieved record TLS-limited quality factor Q TLS,0 = 9.21 × 10 4 for SCPW on Si/SiO 2 . • NbTi SCPW on Si/SiO 2 is promising for gatemon and topological qubits.