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Superconductor Science and Technology

Low-loss superconducting aluminum microwave coplanar waveguide resonators on sapphires for the qubit readouts

Qing He, P. H. Ouyang, Haiyan Gao, Suirong He, Yibing Li, Ying-Ying Wang, Yanru Chen, Xingyun Dai, Yiwen Wang, L. F. Wei

Published Apr 25, 2022• 11 citations
Publisher

Abstract

Abstract The coplanar waveguide (CPW) microwave resonators have been widely applied for solid-state quantum computation and single-photon detection. Based on the physical analysis for the high fidelity readouts of the qubit(s), in this paper we design and then fabricate accordingly the desired aluminum (Al) film quarter-wavelength resonators on sapphire substrates. The ultra-low temperature measurement results show that the linearity of the internal quality factor under the proper driving power is satisfied well for the high fidelity readouts of the qubit(s); and the phase- and amplitude fractional frequency noises are significantly less for avoiding the potential information false alarms of the detected qubit during the readouts. With the demonstrated qubit-resonator experimental parameters, we argue that the designed and fabricated quarter-wavelength CPW resonators, with optimized parameters, can be used to implement the desired weakly perturbing readout measurements of the solid-state qubits on-chip.

DOI
10.1088/1361-6668/ac6a1d

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Abstract The coplanar waveguide (CPW) microwave resonators have been widely applied for solid-state quantum computation and single-photon detection. Based on the physical analysis for the high fidelity readouts of the qubit(s), in this paper we design and then fabricate accordingly the desired aluminum (Al) film quarter-wavelength resonators on sapphire substrates. The ultra-low temperature measurement results show that the linearity of the internal quality factor under the proper driving power is satisfied well for the high fidelity readouts of the qubit(s); and the phase- and amplitude fractional frequency noises are significantly less for avoiding the potential information false alarms of the detected qubit during the readouts. With the demonstrated qubit-resonator experimental parameters, we argue that the designed and fabricated quarter-wavelength CPW resonators, with optimized parameters, can be used to implement the desired weakly perturbing readout measurements of the solid-state qubits on-chip.
We successfully designed and fabricated an absorption-type of superconducting coplanar waveguide (CPW) resonators. The resonators are made from a niobium film (about 160 nm thick) on a high-resistance Si substrate, and each resonator is fabricated as a meandered quarter-wavelength transmission line (one end is short to the ground and another end is capacitively coupled to a through feedline). With a vector network analyzer we measured the transmissions of the applied microwave through the resonators at ultra-low temperature. The obtained loaded quality factors are significantly high, i.e. up to ∼10 6 . When the temperature increases slowly from the base temperature (20 mK), the resonance frequencies of the resonators are blue shifted and the quality factors are lowered slightly. In principle, this type of device can integrate a series of CPW resonators with a common feedline, making it a promising candidate as the data bus for coupling distant solid-state qubits and the sensitive detector of single photons.
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$-1.95GHz,utilizingbothgapandfingercouplingschemes.Thedevicesarefabricatedusingthinaluminumfilmsdepositedonhigh−resistivitysapphiresubstrates.Theproposedresonatorsexhibitqualityfactor(Q1.95 GHz, utilizing both gap and finger coupling schemes. The devices are fabricated using thin aluminum films deposited on high-resistivity sapphire substrates. The proposed resonators exhibit quality factor (Q1.95GHz,utilizingbothgapandfingercouplingschemes.Thedevicesarefabricatedusingthinaluminumfilmsdepositedonhigh−resistivitysapphiresubstrates.Theproposedresonatorsexhibitqualityfactor(Q_{L})of2.9×) of 2.9 ×)of2.9× 10^{5}$and 1.58 ×10310^{3}103for gap coupled and finger coupled, respectively, during experimental measurements. The simulated QL_{L}L​of single-folded meander line resonators are found to be 4.65 ×10510^{5}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 ($-30dBmto30 dBm to30dBmto-10dBm).Experimentalresultsshowagoodagreementwithsimulationsandtheequivalentlumpedcircuitmodel.Notably,theproposeddesignreducesthephysicalfootprintoftheresonatordevicesupto28.210 dBm). Experimental results show a good agreement with simulations and the equivalent lumped circuit model. Notably, the proposed design reduces the physical footprint of the resonator devices up to 28.210dBm).Experimentalresultsshowagoodagreementwithsimulationsandtheequivalentlumpedcircuitmodel.Notably,theproposeddesignreducesthephysicalfootprintoftheresonatordevicesupto28.2\%comparedtoconventionalsingle−foldmeanderedstructures.TheresonatorsdemonstratestrongpotentialforintegratingqubitswithreadoutandcontrolbusesinquantuminformationprocessingwhichrequireaQcompared to conventional single-fold meandered structures. The resonators demonstrate strong potential for integrating qubits with readout and control buses in quantum information processing which require a Qcomparedtoconventionalsingle−foldmeanderedstructures.TheresonatorsdemonstratestrongpotentialforintegratingqubitswithreadoutandcontrolbusesinquantuminformationprocessingwhichrequireaQ_{L}$between10410^{4}104−-−10510^{5}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 Dielectric losses are one of the key factors limiting the coherence of superconducting qubits. The impact of materials and fabrication steps on dielectric losses can be evaluated using coplanar waveguide (CPW) microwave resonators. Here, we report on superconducting CPW microwave resonators with internal quality factors systematically exceeding 5 × 106 at high powers and 2 × 106 (with the best value of 4.4 × 106) at low power. Such performance is demonstrated for 100-nm-thick aluminum resonators with 7–10.5 um center trace on high-resistivity silicon substrates commonly used in Josephson-junction based quantum circuit. We investigate internal quality factors of the resonators with both dry and wet aluminum etching, as well as deep and isotropic reactive ion etching of silicon substrate. Josephson junction compatible CPW resonators fabrication process with both airbridges and silicon substrate etching is proposed. Finally, we demonstrate the effect of airbridges’ positions and extra process steps on the overall dielectric losses. The best quality factors are obtained for the wet etched aluminum resonators and isotropically removed substrate with the proposed ultrasonic metal edge microcutting.
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.
Quantum information processing (QIP) concerns communication between qubits in quantum computer. Coplanar waveguide (CPW) resonators which could easily be fabricated with the micro-techniques in integrated circuits started to show its potentiality in QIP in last few years. So far CPW resonators were used to arrange communication between Josephson q-bits which have relatively short decoherence time. The q-bits based on molecular spins mountain quantum coherence for much longer time, but they have too weak coupling to microwave field to allow effective spin manipulation. In this project we aimed to optimize CPW resonators in order to achieve strong coupling to molecular spins. \nWe present results for standard Coplanar Waveguide (CPW) Nb resonators fabricated on sapphire substrate. To achieve strong coupling between microwave field and molecular spins constrictions of the central conductor at the antinode of current were made to get local enhancement of magnetic field. We also present the simulation results for the key design elements: T-junctions and constrictions, which gave a good guideline for design and fabrication. Low temperature measurements of resonators with (and without) local constrictions were made and verified the accuracy of simulations. Finally, we discuss the temperature and power dependence of resonators transmission spectra.
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.