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IEEE Microwaves, Antennas, and Propagation Conference (MAPCON)

Design of Superconducting Coplanar Waveguide Rat-Race and Branch Line coupler for Circuit Quantum Electrodynamics

Jagmohan, Darshak K Bhatt

Published Dec 11, 2023• 0 citations
Publisher

Abstract

This paper presents the design and simulation of a thin-film on-chip superconducting rat-race and quadrature branch-line coupler for circuit quantum electrodynamics. These couplers are designed at a center frequency of 6GHz using coplanar waveguide technology using a thin film of niobium or aluminum on sapphire substrates. The electrical connections are made between the ground using aluminum air bridges. The insertion losses to the through and coupled ports of rat-race and quadrature coupler are 3.1dB. The quadrature and rat-race couplers offer a wide bandwidth of 2GHz and 3GHz, respectively. A spectral shift of around 400MHz is observed in these couplers when operation is shifted from room temperature to the cryogenic regime. The simulated results have been discussed and analyzed.

DOI
10.1109/mapcon58678.2023.10464011

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This paper presents the design and simulation of a thin-film on-chip superconducting rat-race and quadrature branch-line coupler for circuit quantum electrodynamics. These couplers are designed at a center frequency of 6GHz using coplanar waveguide technology using a thin film of niobium or aluminum on sapphire substrates. The electrical connections are made between the ground using aluminum air bridges. The insertion losses to the through and coupled ports of rat-race and quadrature coupler are 3.1dB. The quadrature and rat-race couplers offer a wide bandwidth of 2GHz and 3GHz, respectively. A spectral shift of around 400MHz is observed in these couplers when operation is shifted from room temperature to the cryogenic regime. The simulated results have been discussed and analyzed.
Superconducting circuit quantum electrodynamics experiments with propagating microwaves require devices acting as beam splitters. Using niobium thin films on silicon and sapphire substrates, we fabricated superconducting 180° microstrip hybrid ring couplers, acting as beam splitters with center frequencies of about 6 GHz. For the magnitude of the coupling and isolation, we find −3.5±0.5 dB and at least −15 dB, respectively, in a bandwidth of 2 GHz. We also investigate the effect of reflections at the superconductor-normal conductor contact by means of low temperature laser scanning microscopy. Our measurements show that our hybrid rings are well suited for on-chip applications in circuit quantum electrodynamics experiments.
Superconducting coplanar waveguide (CPW) can be widely used as two-dimensional (2D) resonator, transmission line or feedline, providing an important component for superconducting quantum circuit which is a promising candidate for quantum information processing. Due to the discontinuities and asymmetries in the ground planes, CPW usually exhibits the spurious resonance, which is a common source of decoherence in circuit quantum electrodynamics experiments. To mitigate the spurious resonance, we fabricated superconducting aluminum air-bridges on Nb CPW. The fabricated air-bridges are approximately 3 μm high and up to 120 μm long. Compared with other methods, the fabrication procedures of our air-bridges are simpler, and the air-bridge can withstand strong ultrasound.
Superconducting quantum electrical circuit based on Co-planar waveguide technology is considered one of the most promising candidates for practical realization of a\nquantum computer in future. For purpose of designing a multi-qubit quantum processor which is the heart of a quantum computer, it is required to couple qubits to a large number of superconducting transmission lines and resonators. Placing all these transmission lines and\nresonators in a practical design unavoidably creates bending and discontinuities in the ground\nplanes which have higher order parasitic modes that can be excited. These undesired parasitic\nmodes are detrimental for quantum measurement, for example during readout of a qubit.\nUnwanted modes can easily interfere with a resonator frequency when coupling to the qubit.\nThese parasitic modes can propagate throughout CPW structure and make unequal ground\npotential at the corresponding ground planes. The easiest way to suppress these parasitic\nmodes is to connect two unequal ground potential with superconducting air-bridges in order\nto bring them under equal potential in superconducting state. However, connecting airbridges on top of CPW adds shunt capacitor which may cause undesired reflection in the\nCPW.\nIn this thesis work, we designed superconducting air-bridges on top of\nsuperconducting CPW transmission line which is a subsystem of large quantum electrical\ncircuit. Our focus was to reduce added shunt capacitance by choosing the right height of these\nair-bridges. To keep the reflection coming from the air-bridge at minimum level, an airbridge\nheight of 8.5μm~15μm was chosen. The performance was verified by EM simulation\nand these heights were selected for device fabrication. We developed a new fabrication\nmethod for processing 8.5μm~15μm high aluminum air-bridges for quantum electrical\ncircuits. The method is compatible with the quantum circuit and easily reproducible. Finally,\ninput reflection measurement (S11) at cryogenic temperature was performed for 8.5μm high,\n300μm long aluminum air-bridges with aluminum CPW transmission line. Negligible amount\nof input reflection (S11) was observed from these measurements which is consistent with the\nresult of EM simulation.
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.
The high critical magnetic field and relatively high critical temperature of niobium nitride (NbN) make it a promising material for applications in superconducting quantum technology. However, NbN-based superconducting circuits are sensitive to decoherence sources such as two-level system (TLS) defects. Here, we numerically and experimentally investigate NbN superconducting microwave coplanar waveguide resonator arrays with a 100 nm thickness, capacitively coupled to a common coplanar waveguide on a silicon chip. We observe that the resonators' internal quality factor (QiQ_{i}Qi​) decreases fromQi∼1.07×106Q_{i}\sim 1.07\times 10^{6}Qi​∼1.07×106in a high power regime toQi∼1.36×105Q_{i}\sim 1.36\times 10^{5}Qi​∼1.36×105in the single photon regime. Data from this study is consistent with the TLS theory, which describes the TLS interactions in resonator substrates and interfaces. Moreover, we study the temperature (TTT) dependence ofQiQ_{i}Qi​and resonance frequencyfrf_{r}fr​of coplanar waveguide resonators to characterize the quasiparticle density. We observe that the increase in kinetic inductance at higher temperatures is the main reason for the frequency shift. Finally, we measurefrf_{r}fr​andQiQ_{i}Qi​in response to in-plane magnetic fields (B∣∣B_{||}B∣∣​). We verify thatQiQ_{i}Qi​stays well above10410^{4}104up toB∣∣B_{||}B∣∣​= 240 mT in the photon number< ⁣nph ⁣>< \! n_{ph} \! ><nph​>= 1.8 atTTT= 100 mK. Our results could enable the development of robust microwave superconducting circuits for circuit quantum electrodynamics at high magnetic fields required for hybrid superconducting-semiconducting quantum circuits.