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Y.Nakamura and others estimated the de-coherence time to be about 2 ns in their experiment with superconducting charge qubits ( about 1999 ).
In the case, de-coherence seems to be originated in a probe junction directly coupled to their circuit and 1/f-noises caused by motions of back-ground charges.
Under the absence of those external disturbances, a main de-phasing mechanism was considered so as to spontaneously induce photon emissions for such an electromagnetic environment.
So that, it was made clear that de-coherence times of 1 μs-order are possible for charge qubits.
