Extraction of the Bare Physical Parameters of Transmon Qubits

We discuss an algorithm to accurately extract the physical parameters E_c, E_J, and g of a transmon qubit coupled to a cavity given experimentally determined spectroscopic data. In our technique, we use an optimization-based procedure tominimize [BP1] a cost function in conjunction with the python library scQubits.³ The cost function is defined with respect to four spectroscopic transition frequencies and weighted by the measured uncertainties. As a check, the extracted physical parameters are used to accurately predict the dressed dispersive cavity frequencies in a wide range of qubits as well as the measured charge dispersion of qubits with a small E_J/E_c.

³Peter Groszkowski and Jens Koch, scqubits: a Python package for superconducting qubits, Quantum 5, 583 (2021).