Coherent quantum phase slip

The magnetic-flux analogue to coherent Josephson tunnelling of electric charge has been observed in a strongly disordered superconducting nanowire. Coherent quantum phase slip (CQPS) has not, until now, been observed experimentally. It is a phenomenon exactly dual to the Josephson effect, but whereas the latter is a coherent transfer of charges between superconducting contacts, CQPS is a coherent transfer of vortices or fluxes across a superconducting wire. This paper reports direct observation of CQPS in a strongly disordered indium oxide superconducting wire inserted in a loop; the effect manifests through the superposition of quantum states with different fluxes. The CQPS may — like the Josephson effect before it — lead to innovative applications in superconducting electronics and quantum metrology. A hundred years after the discovery of superconductivity, one fundamental prediction of the theory, coherent quantum phase slip (CQPS), has not been observed. CQPS is a phenomenon exactly dual1 to the Josephson effect; whereas the latter is a coherent transfer of charges between superconducting leads2,3, the former is a coherent transfer of vortices or fluxes across a superconducting wire. In contrast to previously reported observations4,5,6,7,8 of incoherent phase slip, CQPS has been only a subject of theoretical study9,10,11,12. Its experimental demonstration is made difficult by quasiparticle dissipation due to gapless excitations in nanowires or in vortex cores. This difficulty might be overcome by using certain strongly disordered superconductors near the superconductor–insulator transition. Here we report direct observation of CQPS in a narrow segment of a superconducting loop made of strongly disordered indium oxide; the effect is made manifest through the superposition of quantum states with different numbers of flux quanta13. As with the Josephson effect, our observation should lead to new applications in superconducting electronics and quantum metrology1,10,11.