F or a superconductor, charge and phase are dual quantum variables. A phase-slip event in a superconducting nanowire changes the phase difference over the wire by 2π; it is the dual process to Cooper-pair tunnelling in a Josephson junction. Phase slip by thermal activation at high temperatures is well understood 1 . Phase slip by quantum tunnelling at low temperatures is considered plausible 2,3 , but experiments on the resistance of nanowires 4,5 are inconclusive on this point. Büchler et al. 6 conclude that successive quantum phase slip (QPS) events can be coherent. Here, we demonstrate that, if it exists, coherent QPS is the exact dual to Josephson tunnelling. A narrow nanowire should act as a QPS junction that shows kinetic capacitance, a plasma resonance and current plateaus of interest for nanoelectronic applications. We suggest feasible experiments to unequivocally confirm the existence for coherent QPS.Phase slip in a thin superconducting wire occurs on the scale of the superconducting coherence length. Phase slip by thermal activation 1 is observed as a resistive tail below the critical temperature. In wires with diameter below 10 nm and very high resistance, the energy barrier is small enough that phase slip by quantum tunnelling can be expected 2,3 . Wires of Mo-Ge deposited on suspended carbon nanotubes have been studied [3][4][5] , and the results are in reasonable agreement with microscopic calculations of phase-slip rates 7,8 . All experiments consisted of passing a small d.c. current through the sample and measuring the voltage. As each phase-slip event in the presence of a current I releases an energy IΦ 0 , where Φ 0 = h/2e is the flux quantum, with h the Planck constant and e the electron charge, such measurements are dissipative. Unambiguous experimental evidence of QPS is still absent. It has been concluded 5 that results can be described by thermally activated phase slip for wires with larger cross-section, and as mesoscopic diffusive normal-metal conductance for the weaker wires. The theoretical analysis is complicated by the fact that the behaviour of the bosonic superfluid may be overshadowed by the fermionic effects of localization and interaction. The superconducting energy gap in the wire may be suppressed and quasiparticles may be generated. However, we are not aware of any reason that would forbid QPS to be a physical reality. We thus assume that coherent QPS may take place, that it is characterized by a transition amplitude E S /2 and that no quasiparticles are present (E S < Δ, Δ being the superconducting energy gap). On the The diamond-shaped symbol in the QPS qubit circuit represents the quantum phase-slip process. The capacitive energy is E = E C (n − n g ) 2 and the inductivebasis of Wentzel-Kramers-Brillouin (WKB)-type estimates 3 and microscopic calculations 7 , we assume that the transition amplitude for QPS in practical superconducting nanowires of 1 μm length can be as high as E S /h = 100 GHz. Wires in which significant QPS occurs have large kinetic inductance L and smal...