(a) Schematic of a two-qubit quantum processor where the qubits are coupled via a half wavelength resonator quantum bus and two Al CPW transmission lines creating four discontinuities in the ground planes (Ref. 2 ); (b) the same circuit with Al air-bridges suppressing spurious resonant modes originating from disconnected ground planes.
Equivalent circuit of a lossless CPW transmission line terminated with a matched load including the shunt capacitance of the air-bridge .
(Color online) MICROWAVE OFFICE simulation comparing the input reflection coefficients vs frequency for a 10 mm transmission line with 1–4 air-bridges. Here, we have assumed that the sum capacitance of all air-bridges is 0.04 pF. The air-bridges are equidistantly distributed with a distance, , between neighboring ports or air-bridges.
(Color online) MICROWAVE OFFICE simulation comparing the input reflection coefficients vs frequency for a transmission line with four air-bridges but with different distances, , between them. The curve for n = 1, L = 5.0 mm is the same as in Fig. 3 , where denotes the distance to the measurement ports.
Details of the Al air-bridge fabrication process flow.
(Color online) 3D animation of 8.5 μm high SF15 support layer and SU-8 liftoff resist layer with (a) undercut <8.5 μm but greater than 2 μm, (b)undercut >10.5 μm.
Scanning electron micrograph of a (a) 300 μm long Al air-bridge with height 8.5 μm, (b) 300 μm long Al air-bridge with height 15 μm, (c) 500 μm long Al air-bridge on Al CPW, (d) 15 μm high Al air-bridge span.
Schematic of the reflection measurement microwave setup in the He cryostat. The gray boxes are microwave attenuators with the attenuation indicated in dB. The low noise amplifier (LNA) has a gain of 38 dB and the cutoff frequency of the low pass filter (LPF) is 8 GHz.
Measurement of the input reflection of a CPW with 300 μm long, 8.5 μm high Al air-bridges at a temperature of 4.2 K.
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