The a.c. Josephson effect without superconductivity

Superconductivity derives its most salient features from the coherence of the associated macroscopic wave function. The related physical phenomena have now moved from exotic subjects to fundamental building blocks for quantum circuits such as qubits or single photonic modes. Here we predict that the a.c. Josephson effect—which transforms a d.c. voltage V(b) into an oscillating signal cos (2eV(b)t/ħ)—has a mesoscopic counterpart in normal conductors. We show that when a d.c. voltage V(b) is applied to an electronic interferometer, there exists a universal transient regime where the current oscillates at frequency eV(b)/h. This effect is not limited by a superconducting gap and could, in principle, be used to produce tunable a.c. signals in the elusive 0.1–10-THz ‘terahertz gap’.