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

Abstract

Coulomb interactions can cause a rapid change in the phase of the wavefunction along a very narrow superconducting system. Such a phase slip is now measured in a chain of Josephson junctions. The interplay between superconductivity and Coulomb interactions has been studied for more than 20 years now1,2,3,4,5,6,7,8,9,10,11,12,13. 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 films5,6, wires7 and also in Josephson junction arrays4,9,11,12,13. 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 fluctuations14,15,16. Recently, they have attracted interest as they could provide the basis for the realization of a new type of topologically protected qubit17,18 or for the implementation of a new current standard19. 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 colleagues8.