Nonclassical excitation and quantum interference in a three level atom

[摘要] Non-classical properties and quantum interference (QI) in two-photon excitation ofa three level atom (|1〉), |2〉, |3〉) in a ladder configuration, illuminated by multiplefields in non-classical (squeezed) and/or classical (coherent) states, is studied. Fundamentallynew effects associated with quantum correlations in the squeezed fieldsand QI due to multiple excitation pathways have been observed. Theoretical studiesand extrapolations of these findings have revealed possible applications which arefar beyond any current capabilities, including ultrafast nonlinear mixing, ultrafasthomodyne detection and frequency metrology. The atom used throughout the experimentswas Cesium, which was magneto-optically trapped in a vapor cell to producea Doppler-free sample. For the first part of the work the |1〉 → |2〉 → |3〉 transition(corresponding to the 6S_1/2F = 4 → 6P_3/2F' = 5 → 6D_5/2F" = 6 transition) wasexcited by using the quantum-correlated signal (Ɛ_s) and idler (Ɛ_i) output fields of asubthreshold non-degenerate optical parametric oscillator, which was tuned so thatthe signal and idler fields were resonant with the |1〉 → |2〉 and |2〉 → |3〉 transitions,respectively. In contrast to excitation with classical fields for which the excitationrate 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 exponentthat approaches unity for small enough intensities. This was verified experimentallyby probing the exponent down to a slope of 1.3, demonstrating for the first time apurely 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 coherentfields Ɛ₁ , Ɛ₂ and Ɛ₀, resonant with the dipole |1〉 → |2〉 and |2〉 → |3〉 and quadrupole|1〉 → |3〉 transitions, respectively, is studied. QI in the excited state population isobserved due to two alternative excitation pathways. This is equivalent to nonlinearmixing of the three excitation fields by the atom. Realizing that in the experimentthe three fields are spaced in frequency over a range of 25 THz, and extending thisscheme to other energy triplets and atoms, leads to the discovery that ranges up to100'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 havebeen extensively studied.

[发布机构] University:California Institute of Technology;Department:Physics, Mathematics and Astronomy

[关键词] Physics