Coherent superpositions of single semifluxon states in a 0- Josephson junction

We consider a symmetric 0- Josephson junction of length L, which classically can be in one of two degenerate ground states or , corresponding to supercurrents circulating clockwise or counterclockwise around the 0- boundary. When the length L of the junction becomes smaller than the Josephson penetration depth _J, the system can switch from one state to the other due to thermal fluctuations or quantum tunneling to the neighboring well. 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 experimentally. 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 parts of the junction. ©2010 The American Physical Society