Jeongho Bang, Seung-Woo Lee, Hyunseok Jeong, Jinhyoung Lee
We attempt to employ measures of average fidelity and fidelity deviation in characterizing a quantum operation when realizing the universal NOT gate for a single qubit. Based on these measures, we evaluate quantum operations with respect to optimality and universality. We show that one-qubit operations have a sharp trade-off relation, i.e. a linear relation between average fidelity and fidelity deviation. Two-qubit operations for the same task show a looser trade-off relation between the two quantities. We show that the genuine universality comes to hold for operations of more than 2 qubits, while those of even more qubits are nevertheless beneficial to compensating imperfection of control. In addition, we take into account operational noises which contaminate quantum operation in realistic circumstances. We show that the operation recovers from the contamination by a feedback procedure of differential evolution. It is discussed that our feedback scheme is also applicable to finding an optimal and universal operation of NOT.
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http://arxiv.org/abs/1208.2111
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