Quantum Tunneling of Josephson Vortices in High-Impedance Long Junctions

In the last decades, superconducting devices have emerged as a promising platform for quantum technologies, including quantum sensing and quantum computing. Their key elements are Josephson junctions, which allow for coherent supercurrent tunneling between two weakly linked superconductors. If such a junction is extended in one direction to a long junction, the superconducting phase difference can vary in space and time and may allow for quantized phase windings that drive supercurrent vortices. The physics of such Josephson vortices spans from nonlinear soliton dynamics with relativistic effects to technical applications like microwave generation and amplification. In all these cases the vortices behave as non-quantum particles. This restriction to classical behavior originates in the long junction's limited geometrical properties, in particular its electrode inductance.
The advent of superconducting high-kinetic inductance circuits relaxes these constraints and enables an increased junction impedance, which goes along with the vortices' quantumness. In this work it is demonstrated that the junction impedance can be enhanced beyond the geometric limitations, which facilitates various applications. ... mehrA hybrid system is studied particularly, that consists of a high-impedance long junction embedded in a microwave resonator. This galvanic coupling enables the dispersive readout scheme to determine the quantum states of different vortex configurations. In the vortices' quantum regime, coherent tunneling of single vortices in a two-level system is observed, forming a Josephson vortex quantum bit. Quantum Rabi oscillations with energy relaxation and dephasing times in the microsecond range are measured, making the system promising for future quantum technologies.