Josephson junction chains have already been successfully used to create particular electromagnetic environments for the reduction of charge fluctuations. Recently, they have attracted interest as they could provide the basis for the realization of a new type of topologically protected qubit  or for the implementation of a new current standard .
We present measurements that show clearly the effect of quantum phase-slips on the ground state of a Josephson junction chain. We can tune in situ the strength of the phase-slips and obtain for the first time an excellent quantitative agreement with theory . These phase slips are the result of fluctuations induced by the finite charging energy of each junction in the chain. Our measurements demonstrate that a Josephson junction chain under phase bias constraint behaves in a collective way, very similar to a single macroscopic quantum object .
We also show evidence of coherent phase-slip interference, the so called Aharonov-Casher effect. This phenomenon is the dual of the well known Aharonov-Bohm interference. As we sweep the charge capacitively induced on an island in the middle of the chain, the strength of the phase slips oscillates with a periodicity of 2e.
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