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Experimentally, the coherent tunneling of Cooper pairs and the related properties of quantum mechanical superpositions of charge states have been demonstrated in soectacar experiments by Y.Nakamura and others.
They observed the quantum coherent oscillations of a Josephson charge qubit prepared in a superposition of eigenstates in the time-domain.
As the result, a small superconducting grain ( called a Cooper pair ‘box’ ) attached to a superconducting reservoir by two Josephson junctions could be found out.
