N. Ph. Georgiades, E. S. Polzik, and H. J. Kimble,
"Quantum Interference in Two-Photon Excitation with Squeezed and Coherent Fields,"
Phys. Rev. A 59, 676 (1999).
Two-photon excitation of a three-level atom in a ladder configuration (1 --> 2 --> 3) by simultaneous illumination with
fields in squeezed vacuum and coherent states results in quantum interference for the excitation process. The particular
configuration considered here is one for which the signal and idler output fields of a subthreshold nondegenerate optical
parametric oscillator 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 Ficek and Drummond [Phys. Rev. A 43, 6247 (1991)], a theoretical formulation based 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 detection strategy for quantum
fluctuations of the electromagnetic field with ultrahigh frequencies (10's-100's THz). Based on the theory developed, we
analyze quantum interference in excitation in several special cases relevant to experimental realizations, including the
effects of a small focusing angle of the squeezing onto the atoms, and unusual population inversions. Special emphasis is
given to identifying intrinsically quantum optical field effects versus classical field effects. Procedures that could
distinguish between the two (i.e., classical and nonclassical) are suggested.
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