| Non-classical properties and quantum interference (QI) in two-photon excitation of a three level atom {dollar}(vert 1rangle, vert 2rangle, vert3rangle){dollar} in a ladder configuration, illuminated by multiple fields in non-classical (squeezed) and/or classical (coherent) states, is studied. Fundamentally new effects associated with quantum correlations in the squeezed fields and QI due to multiple excitation pathways have been observed. Theoretical studies and extrapolations of these findings have revealed possible applications which are far beyond any current capabilities, including ultrafast nonlinear mixing, ultrafast homodyne detection and frequency metrology. The atom used throughout the experiments was Cesium, which was magneto-optically trapped in a vapor cell to produce a Doppler-free sample. For the first part of the work the {dollar}vert1rangletovert2rangletovert3rangle{dollar} transition (corresponding to the {dollar}6Ssb{lcub}1/2{rcub}F=4to6Psb{lcub}3/2{rcub}Fspprime=5to 6Dsb{lcub}5/2{rcub}Fsp{lcub}primeprime{rcub}=6{dollar} transition) was excited by using the quantum-correlated signal {dollar}({lcub}cal E{rcub}sb{lcub}s{rcub}){dollar} and idler {dollar}({lcub}cal E{rcub}sb{lcub}i{rcub}){dollar} output fields of a subthreshold non-degenerate optical parametric oscillator, which was tuned so that the signal and idler fields were resonant with the {dollar}vert 1rangletovert 2rangle{dollar} and {dollar}vert2rangletovert3rangle{dollar} transitions, respectively. In contrast to excitation with classical fields for which the excitation rate as a function of intensity has always an exponent greater than or equal to two, excitation with squeezed-fields has been theoretically predicted to have an exponent that approaches unity for small enough intensities. This was verified experimentally by probing the exponent down to a slope of 1.3, demonstrating for the first time a purely non-classical effect associated with the interaction of squeezed fields and atoms. In the second part excitation of the two-photon transition by three phase coherent fields {dollar}{lcub}cal E{rcub}sb1,{lcub}cal E{rcub}sb2{dollar} and {dollar}{lcub}cal E{rcub}sb0,{dollar} resonant with the dipole {dollar}vert1rangletovert2rangle{dollar} and {dollar}vert2rangletovert3rangle{dollar} and quadrupole {dollar}vert1rangletovert3rangle{dollar} transitions, respectively, is studied. QI in the excited state population is observed due to two alternative excitation pathways. This is equivalent to nonlinear mixing of the three excitation fields by the atom. Realizing that in the experiment the three fields are spaced in frequency over a range of 25 THz, and extending this scheme to other energy triplets and atoms, leads to the discovery that ranges up to 100's of THz can be bridged in a single mixing step. Motivated by these results, a master equation model has been developed for the system and its properties have been extensively studied. |
