Enhancement of steady-state squeezing in the resonance fluorescence of a coherently driven four-level atom of Λ configuration via quantum interference

We investigate steady-state squeezing in the phase quadratures of the resonance fluorescence emitted from a coherently driven four-level atom of [Formula Presented] configuration. Quantum fluctuation spectra of the optimum-phase quadratures are calculated. We show that if quantum interference between spontaneous emission channels from closed excited states to lower levels of the atom takes place, squeezing in the optimum-phase quadrature can occur in a very wider region of the parameter space than that predicted by previous studies. We also show that the quantum interference can greatly change the atomic coherences and then lead to this interesting result.