Collapse of the Josephson Emission in a Carbon Nanotube Junction

We have measured a surprising collapse of the high frequency emission of a superconducting junction made with a carbon nanotube.

We probe the high frequency emission of a carbon nanotube based Josephson junction and compare it to
its dc Josephson current. The ac emission is probed by coupling the carbon nanotube to an on-chip detector
(a superconductor-insulator-superconductor junction), via a coplanar waveguide resonator. The measure-
ment of the photoassisted current of the detector gives direct access to the signal emitted by the carbon
nanotube. We focus on the gate regions that exhibit Kondo features in the normal state and demonstrate that
when the dc supercurrent is enhanced by the Kondo effect, the ac Josephson effect is strongly reduced. This
result is compared to numerical renormalization group theory and is attributed to a transition between the
singlet ground state and the doublet excited state which is enabled only when the junction is driven out-of-
equilibrium by a voltage bias.

Schematic picture of a nanotube Josephson junction in the Kondo regime, emitting a photon

This work has been published in Physical Review Letters.