Resonator induced quantum phase transitions in a hybrid Josephson junction

TL;DR

This paper explores how a vibrational mode in a hybrid Josephson junction with a carbon nanotube quantum dot can induce quantum phase transitions, affecting the Josephson current and enabling sensitive displacement detection.

Contribution

It demonstrates the control of quantum phase transitions via electron-vibration coupling in a superconducting hybrid device, introducing a triple point in the Josephson current.

Findings

Vibration coupling can lift triplet blockade in the quantum dot.

Quantum phase transitions are observable through critical current measurements.

The setup is promising for ultra-sensitive displacement detection.

Abstract

We investigate the Josephson current through a suspended carbon nanotube double quantum dot which, at sufficiently low temperatures, is characterized by the ground state of the electronic subsystem. Depending on parameters like a magnetic field or the inter-dot coupling, the ground state can either be a current-carrying singlet or doublet, or a blockaded triplet state. Since the electron-vibration interaction has been demonstrated to be electrostatically tuneable, we study in particular its effect on the current-phase relation. We show that the coupling to the vibration mode can lift the current-suppressing triplet blockade by inducing a quantum phase transition to a ground state of a different total spin. Our key finding is the development of a triple point in the Josephson current parameterized by the resonator coupling and the Josephson phase. The quantum phase transitions around the triple point are directly accessible through the critical current and resilient to moderately finite temperatures. The proposed setup makes the mechanical degree of freedom part of a superconducting hybrid device which is interesting for ultra-sensitive displacement detectors.