Physical aspect of the improvement of quantum-noise characteristics caused by unitary transformation with a nonlinear optical medium

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Abstract
The standard quantum limit (SQL) as an error probability can be broken by means of a unitary transformation such as a Kerr medium installed before the conventional homodyne scheme. Such an improvement in the error probability cannot be realized by the conventional unitary transformer like squeezers for fixed coherent state inputs. The physical origin of this effect is nonlinear quantum interference for the components of the output state evolved from the input coherent state. We clarify the conditions for overcoming the SQL by unitary transformation and show several examples of the interaction Hamiltonian which generate unitary processes to break the SQL.