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Two states of charge qubit differ by one Cooper pair charge on a superconducting island.
Though ignored all other charge states have much higher energy, a certain voltage range near a degeneracy point of only two states ( n = 0 and n = 1 ) works. Because, in this case, the superconducting charge box behaves as a two-state quantum system. The Hamiltonian for spin-1/2 notation is written as follows.
H ＝ −1/2・Bz・σ’z −1/2・Bχ・σ’χ
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