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We propose a nonlinear-interferometric scheme for generating two fermion states correlated in particle number or in wave phase. This scheme can be used for surpassing either the standard quantum limit of the number-partition noise or that of the interferometric phase sensitivity, up to a factor of the order of (total particle number) ^1/3. A possible experimental scheme in a mesoscopic system is discussed.
Kitagawa et al. (Mon,) studied this question.