Quantum interference in two-photon excitation with squeezed and coherent fields

Two-photon excitation of a three-level atom in a ladder configuration (1--> 2-->3) by simultaneous illumination with fields in squeezed vacuum and cohe rent states results in quantum interference for the excitation process. The particular configuration considered here is one for which the signal and i dler output fields of a subthreshold nondegenerate optical parametric oscil lator are in resonance with the two-stepwise dipole atomic transitions (1-- >2,2-->3), while a "reference oscillator'' field is in two-photon resonance with the quadrupole transition (1-->3). In an extension of the work of Fic ek and Drummond [Phys. Rev. A 43, 6247 (1991)], a theoretical formulation b ased on the full quantum master equation for the problem is presented. The combined effects of quantum interference and the nonclassical character of the squeezed state are investigated, and offer the potential for a new dete ction strategy for quantum fluctuations of the electromagnetic field with u ltrahigh frequencies (10's-100's THz). Based on the theory developed, we an alyze quantum interference in excitation in several special cases relevant to experimental realizations, including the effects of a small focusing ang le of the squeezing onto the atoms, and unusual population inversions. Spec ial emphasis is given to identifying intrinsically quantum optical field ef fects versus classical field effects. Procedures that could distinguish bet ween the two (i.e., classical and nonclassical) are suggested. [S1050-2947( 98)06109-5].