Bachelor defense by Marcus K. Krogsgaard

Characterization of Switches for Superconducting Qubit Measurements

Superconducting qubits require cryogenic microwave/RF wiring for control and readout, but the number of ports in a dilution refrigerator is limited. The aim of this thesis was to characterize a prototype superconducting microwave/RF switch provided by SemiQon, to determine its suitability for experiments with superconducting qubits. The switch was characterized using a Vector Network Analyser and the Quantum Machines OPX1000 Arbitrary Waveform Generator. Two critical currents were found: I1 = 3.54 mA and I2 = 4.42 mA with 95%-confidence intervals of [3.50, 3.54] mA and [4.31, 4.52] mA. The critical temperature of the switch was found to be Tc = 2.63 K with a 95%-confidence interval of [2.30, 2.63] K. Although the switch showed low transmission loss and high isolation at low frequencies, the contrast between the open and closed states decreased with increasing frequency. The data showed that a qubit in the 4 GHz to 5 GHz frequency range would experience a contrast of roughly 20 dB between the open and closed states of the switch, reducing the thermal noise level by a factor of 10 and possibly increasing the coherence time of the qubit. The transient effects of the switch were also discussed, and the fall time of the switch was found to be tf ≈ 5800 ns, while the characteristic recovery time was on the order of 200 μs. Notably, the switch showed a significantly slower recovery when exposed to continuous microwave signals during measurements.