  Although quantum mechanics applies to many macroscopic superconducting
devices, one basic prediction remained controversial for decades. Namely, a
Josephson junction connected to a resistor must undergo a dissipation-induced
quantum phase transition from superconductor to insulator once the resistor's
value exceeds $h/4e^2 \approx 6.5~\textrm{k}\Omega$ ($h$ is Planck's constant,
$e$ is the electron charge). Here we finally demonstrate this transition by
observing the resistor's internal dynamics. Implementing our resistor as a long
transmission line section, we find that a junction scatters electromagnetic
excitations in the line as either inductance (superconductor) or capacitance
(insulator), depending solely on the line's wave impedance. At the phase
boundary, the junction itself acts as ideal resistance: in addition to elastic
scattering, incident photons can spontaneously down-convert with a
frequency-independent probability, which provides a novel marker of
quantum-critical behavior.
