  Zero resistance, diamagnetism (Meissner effect) and the energy gap in the
excitation spectrum are the fundamental attributes of superconductivity. In
nanoparticles with characteristic size 10 nm superconductivity is suppressed by
the quantization of the electron energy spectrum. In narrow quasi-1D
superconducting channels thermal fluctuations enable zero resistance only at
temperatures noticeably below the critical temperature Tc, while quantum
fluctuations suppress the dissipationless electric current in ultra-narrow
superconducting nanowires even at temperatures close to T=0. Here we
demonstrate that the same essentially nanoscale phenomenon, quantum
fluctuations, is responsible for quenching diamagnetism in superconducting
nanorings dramatically affecting the magnitude, the period and the shape of the
circulating persistent currents (PCs). The effect is of fundamental importance
for our understanding of the behavior of a quantum coherent system at
nanoscales; and should be taken in consideration in various nanoscience
applications.
