Coherent Radiation from High-Order Quantum Transitions in Small-Area Superconducting Contacts

We report a new type of oscillation arising from the coherent motion of large numbers of flux quanta across small-area superconducting contacts. It produces a unique spectrum at the frequency ${\omega }_D$ when $2\pi n{V}_0=N{\Phi }_0{\omega }_D$, where $V_0$ is the dc voltage across the contact, $n$ is an integer, ${\Phi }_0=\frac{h}{2e}$, and $N$ is the number of flux quanta crossing per cycle. The power dependence varies as $N^2$ with values of $N$ from 1 to 200 observed.