Practical Guide to Quantum Phase Transitions in Quantum-Dot-Based Tunable Josephson Junctions

Quantum dots attached to BCS superconducting leads exhibit a 0 - pi impurity quantum phase transition, which can be experimentally controlled either by the gate voltage or by the superconducting phase difference. For the pertinent superconducting single-impurity Anderson model, we present two simple analytical formulae describing the position of the phase boundary in parameter space for the weakly correlated and Kondo regime, respectively. Furthermore, we show that the two-level approximation provides an excellent description of the low-temperature physics of superconducting quantum dots near the phase transition. We discuss reliability and mutual agreement of available finite-temperature numerical methods (numerical renormalization group and quantum Monte Carlo) and suggest an alternative approach for efficient determination of the quantum phase boundary from measured finite-temperature data. Our results enable fast and efficient, yet reliable characterization and design of such nanoscopic tunable Josephson-junction devices.