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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 dual (1) to the Josephson effect; whereas the latter is a coherent transfer of charges between superconducting leads (2,3), the former is a coherent transfer of vortices or fluxes across a superconducting wire. In contrast to previously reported observations (4-8) of incoherent phase slip, CQPS has been only a subject of theoretical study (9-12). 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 quanta (13). As with the Josephson effect, our observation should lead to new applications in superconducting electronics and quantum metrology (1,10,11)., Phase slips across superconducting wires (14) lead to non-zero resistance (2,15); they may also lead to qubit dephasing, as result of charge noise (16) in a chain of Josephson junctions [...]

Low temperature scanning electron microscopic technique is employed for the spatially resolved investigation of single-electron tunneling (SET) circuits. Signals from various parts of the circuits are imaged with the help of circuit voltage, recording as a function of the beam coordinates. Charge trapping near SET-transistor induces memory effect. The efficiency of this technique is demonstrated with a system having 11 electrons.