Bachelor defense by Lukas S. B. Lund

Characterization and Optimization of a Josephson Traveling Wave Parametric Amplifier in a 5-Qubit Readout Chain

Superconducting qubit readout is fundamentally limited by the noise added by cryogenic amplifier chains, where a high-electron-mobility transistor (HEMT) at the 4\,K stage dominates the noise figure. This work characterizes the Silent Waves Argo Josephson traveling-wave parametric amplifier (JTWPA) as a near-quantum-limited amplifier for dispersive readout of a superconducting transmon qubit. The gain profile was measured via VNA transmission, showing a small-signal gain of approximately 16 dB over several GHz, with pump-induced photonic band-gap artifacts identified and excluded from analysis. A systematic sweep over pump frequency (6.10-6.35 GHz) and pump power (12.5-16.5 dBm) mapped the SNR improvement landscape and identified an optimal operating point of $f_{pump} = 6.208$ GHz, $P_{pump} = 15.17$ dBm, producing a peak $\Delta\mathrm{SNR}$ of 12.67 dB. At this operating point, IQ blob analysis of qubit 2 demonstrated an improvement in readout fidelity from $\mathcal{F}_{off} = 50.18\%$ to $\mathcal{F}_{on} = 83.47\%$, indicating that the JTWPA provided substantial improvements to readout precision.