Quantum tunneling of a single semifluxon in a 0-pi Josephson junction

TL;DR

This paper investigates quantum tunneling in a symmetric 0-pi Josephson junction, analyzing conditions for macroscopic quantum coherence and discussing challenges for qubit implementation.

Contribution

It models the junction as a particle in a double well potential and estimates parameters for observing quantum coherence, highlighting practical limitations.

Findings

Quantum tunneling can occur in small 0-pi junctions.

Macroscopic quantum coherence requires extremely low temperatures.

Practical use as a qubit is limited due to readout and sensitivity issues.

Abstract

We consider a symmetric 0-pi Josephson junction of length $L$, which classically can be in one of two degenerate ground states up or down, corresponding to supercurrents circulating clockwise or counterclockwise around the 0-pi boundary. When the length $L$ of the junction becomes smaller than the Josephson penetration depth $\lambda_J$, the system can switch from one state to the other due to thermal fluctuations or quantum tunneling. We map this problem to the dynamics of a single particle in a periodic double well potential and estimate parameters for which macroscopic quantum coherence may be observed. We conclude that this system is not very promising to build a qubit because (a) it requires very low temperatures to reach the quantum regime, (b) its tiny flux is hard to read out and (c) it is very sensitive to the asymmetries between the 0 and pi parts of the junction.