Nonclassical excitation and quantum interference in a three level atom

摘要

Non-classical properties and quantum interference (QI) in two-photon excitation ofuda three level atom (|1〉), |2〉, |3〉) in a ladder configuration, illuminated by multipleudfields in non-classical (squeezed) and/or classical (coherent) states, is studied. Fundamentallyudnew effects associated with quantum correlations in the squeezed fieldsudand QI due to multiple excitation pathways have been observed. Theoretical studiesudand extrapolations of these findings have revealed possible applications which areudfar beyond any current capabilities, including ultrafast nonlinear mixing, ultrafastudhomodyne detection and frequency metrology. The atom used throughout the experimentsudwas Cesium, which was magneto-optically trapped in a vapor cell to produceuda Doppler-free sample. For the first part of the work the |1〉 → |2〉 → |3〉 transitionud(corresponding to the 6SF = 4 → 6PF' = 5 → 6DF" = 6 transition) wasudexcited by using the quantum-correlated signal (Ɛ) and idler (Ɛ) output fields of audsubthreshold non-degenerate optical parametric oscillator, which was tuned so thatudthe signal and idler fields were resonant with the |1〉 → |2〉 and |2〉 → |3〉 transitions,udrespectively. In contrast to excitation with classical fields for which the excitationudrate as a function of intensity has always an exponent greater than or equal to two,udexcitation with squeezed-fields has been theoretically predicted to have an exponentudthat approaches unity for small enough intensities. This was verified experimentallyudby probing the exponent down to a slope of 1.3, demonstrating for the first time audpurely non-classical effect associated with the interaction of squeezed fields and atoms.udIn the second part excitation of the two-photon transition by three phase coherentudfields Ɛ , Ɛ and Ɛ, resonant with the dipole |1〉 → |2〉 and |2〉 → |3〉 and quadrupoleud|1〉 → |3〉 transitions, respectively, is studied. QI in the excited state population isudobserved due to two alternative excitation pathways. This is equivalent to nonlinearudmixing of the three excitation fields by the atom. Realizing that in the experimentudthe three fields are spaced in frequency over a range of 25 THz, and extending thisudscheme to other energy triplets and atoms, leads to the discovery that ranges up toud100's of THz can be bridged in a single mixing step. Motivated by these results,uda master equation model has been developed for the system and its properties haveudbeen extensively studied.