Benchmarking the design, fabrication and shielding of superconducting qubits

Abstract

Superconducting qubits interact with their environment, which inherently limits their coherence. In this thesis, we describe measurements and present experimental results for characterising the qubits interaction with its environment. We performed flux pulse distortion measurements which allowed us to correct the flux pulse with 0.2% error of the target pulse. A two-level-system (TLS) defect spectroscopy measurement showed a TLS density of 14 ± 2 TLS/GHz, with TLS-qubit coupling between 50 kHz and 650 kHz, and TLS lifetime less than 200 ns. Repeating the TLS spectroscopy measurement every hour for twelve hours showed TLS drifting and telegraphic switching between frequencies with a timescale on the order of hours. Using a parity selective Ramsey sequence, we measured the qubit parity switching rate due to quasiparticles (QPs) as 500 ± 100 Hz, which randomly varied between 200 Hz to 800 Hz over 24 hours. We designed and measured a device where the qubit frequency could be tuned through the resonator. To this end, we measured the qubit decay rate as a function of frequency and found the single mode Purcell approximation to be negligible compared to TLS and QP losses in this device. In addition, the TLS loss was measured as F δTLS = (3±1)· 10−6 , and the quasiparticle density normalised to the density of Cooper pairs as xQP = (6±1)· 10−7 . In equilibrium, this quasiparticle density would corresponds to a qubit temperature of 147±2 mK, since the qubit is cooled down to 10 mK, we interpret this as the presence of non-equilibrium quasiparticles.