Modeling and Characterization of a Tunable Coupler-Flux Qubit Ultrastrongly Coupled to an Open Transmission Line

Abstract

Studies in relativistic quantum information have shown that the vacuum state of a quantum field contains pre-existing correlations. Vacuum correlations can be extracted locally by coupling spatially separated probes to a quantum field even without direct light-matter interaction. Despite extensive theoretical work, there has been no experimental demonstration of entanglement harvesting to date. A superconducting circuit design has been proposed as a platform for investigating entanglement harvesting. The device consists of a flux qubit coupled to a one-dimensional transmission line through a tunable coupler. The tunability of the coupler provides control over the qubit-field interaction, allowing the coupling strength to be tuned over a range from the weak regime to the ultrastrong coupling regime at nanosecond time scales. These features make the design a promising platform for investigating entanglement harvesting from vacuum correlations under controlled experimental conditions. However, as the system approaches the ultrastrong-coupling regime, commonly used approximations, such as the rotating-wave approximation (RWA) in the spin-boson model and the two-level approximation for the qubit, may no longer provide an accurate description of the system. In this thesis, we investigate a superconducting qubit design as a potential platform for future studies of entanglement harvesting.

We first develop a theoretical model of a tunable coupler–flux qubit galvanically coupled to a transmission line and derive the Hamiltonian of the coupled system. The model is then used to calculate the transverse and longitudinal coupling strengths between the qubit and the transmission-line modes and to investigate their dependence on the coupler flux. We then characterize a proposed three-loop superconducting qubit design through numerical simulations and comparison with experimental spectroscopy data. The system is calibrated to establish the relation between the applied voltages and external fluxes. We then investigate how the junction parameters and junction asymmetry affect the agreement between the simulations and experimental data. Finally, the coupler flux is varied to study its effect on the simulated response. The comparison shows that variations in the junction parameters and junction asymmetry can improve the agreement with the experimental data, while variations in the coupler flux do not lead to a systematic improvement under the assumptions considered.