Absence of a Dissipative Quantum Phase Transition in Josephson Junctions*

In 1962 Josephson predicted that an electric current can flow with no applied voltage through a thin insulating layer separating two superconductors. Since then, this "Josephson junction" has taken the central role in quantum electronic devices (squids magnetometers, parametric amplifiers, superconducting qubits,…), among which, notably, the Josephson voltage standard which helped rebasing the International System of Units on quantum effects.
Although the understanding of all the above Josephson devices is exquisitely accurate, here we show that, surprisingly, the interaction of a Josephson junction with a high-resistance electromagnetic environment was long misconceived and inconsistent. Specifically, we show experimentally that, contrary to what is commonly believed, a Josephson junction connected to a resistor does not become insulating above some value of the resistance due to a dissipative quantum phase transition.
We propose a new theoretical analysis which accounts for our observation and resolves previous inconsistencies in the theory. This reveals a subtle difference between this system and others, previously thought equivalent, in which a phase transition was confirmed.
[1] Murani et al., Phys. Rev. X 10, 021003 (2020).