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The Josephson charge qubit ( ‘box’ ) is prepared in a ground state far from a degeneracy point with a DC gate.
In this regime, the ground state is close to a charge state |0〉. Then, the gate voltage is changed to a different value with a pulse gate for a short time which is less than one nanosecond.
If the Josephson charge qubit is switched to the degeneracy point, the initial state ( a pure charge state ) is at an equal-amplitude superposition of a ground state |0〉and an excited state |1〉.
These two eigenstates have different energies, accordingly, they have different phase factors in time.
