Measurement of the effect of quantum phase slips in a Josephson junction chain

The interplay between superconductivity and Coulomb interactions has been studied for more than 20 years now(1-13). In low-dimensional systems, superconductivity degrades in the presence of Coulomb repulsion: interactions tend to suppress fluctuations of charge, thereby increasing fluctuations of phase. This can lead to the occurrence of a superconducting-insulator transition, as has been observed in thin superconducting films(5,6), wires(7) and also in Josephson junction arrays(4,9,11-13). The last of these are very attractive systems, as they enable a relatively easy control of the relevant energies involved in the competition between superconductivity and Coulomb interactions. Josephson junction chains have been successfully used to create particular electromagnetic environments for the reduction of charge fluctuations(14-16). Recently, they have attracted interest as they could provide the basis for the realization of a new type of topologically protected qubit(17,18) or for the implementation of a new current standard(19). Here we present quantitative measurements of quantum phase slips in the ground state of a Josephson junction chain. We tune in situ the strength of quantum phase fluctuations and obtain an excellent agreement with the tight-binding model initially proposed by Matveev and colleagues(8).