QUROPE Quantum Information Processing and Communication in Europe

We study quantum phase-slip (QPS) processes in a superconducting ring containing N Josephson junctions and threaded by an external static magnetic flux ΦB. In such a system, a QPS consists of a quantum tunneling event connecting two distinct classical states of the phases with different persistent currents [ Matveev et al. Phys. Rev. Lett. 89 096802 (2002)]. When the Josephson coupling energy EJ of the junctions is larger than the charging energy EC=e2/2C, where C is the junction capacitance, the quantum amplitude for the QPS process is exponentially small in the ratio EJ/EC. At given magnetic flux, each QPS can be described as the tunneling of the phase difference of a single junction of almost 2π, accompanied by a small harmonic displacement of the phase difference of the other N−1 junctions. As a consequence, the total QPS amplitude νring is a global property of the ring. Here, we study the dependence of νring on the ring size N, taking into account the effect of a finite capacitance C0 to ground, which leads to the appearance of low-frequency dispersive modes. Josephson and charging effects compete and lead to a nonmonotonic dependence of the ring's critical current on N. For N→∞, the system converges either towards a superconducting or an insulating state, depending on the ratio between the charging energy E0=e2/2C0 and the Josephson coupling energy EJ.