Coherent control experiments quantum interference

Here we describe the development of the coherent-control toolbox with gas-phase iodine molecules [37 1, 48]. The gas-phase molecules are isolated from each other, so that they have long coherence lifetime, serving as an ideal platform to observe and control quantum coherence. First, we describe our experiments to observe and control the temporal evolution of the WP interference. Second, the eigenstate picture of the WP interference is presented. Finally, we demonstrate the application of WPI to ultrafast molecular computing. 

The possibility to use laser radiations to achieve the so-called "coherent control" of molecular dissociation or of atomic photoionization has been predicted since the advent of laser sources in the early sixties. It was expected that, thanks to the coherence and monochromaticity properties of the laser light, one could selectively choose a dissociation channel and the spatial orientation of ejection of the fragments (either ions or electrons or even neutrals) in an elementary chemical process. However, earlier attempts, based on simple photoabsorption processes, have been unsuccessful and it is only recentiy that experiments have been shown to enable one to achieve such a goal in some selected systems. Amongst the various scenarios which have been explored, one of the most promising is based on the realization of quantum interferences in so-called "two-colour" photodissociation or... 

Here we extend the simple three-level EIT system to mote complicated and versatile configurations in a multi-level atomic system coupled by multiple laser fields. We show that with multiple excitation paths provided by different laser fields, phase-dependent quantum interference is induced either constractive or destractive interfereiKe can be realized by varying the relative phases among the laser fields. Two specific examples are discussed. One is a three-level system coupled by bichromatic coupling and probe fields, in which the phase dependent interference between the resonant two-photon Raman transitions can be initiated and controlled. Another is a four-level system coupled by two coupling fields and two probe fields, in which a double-EIT confignration is created by the phase-dependent interference between three-photon and one-photon excitation processes. We analyze the coherently coupled multi-level atomic system and discuss the control parameters for the onset of constructive or destructive quantum interference. We describe two experiments performed with cold Rb atoms that can be approximately treated as the coherently coupled three-level and four-level atomic systems respectively. The experimental results show the phase-dependent quantum coherence and interference in the multi-level Rb atomic system, and agree with the theoretical calculations based on the coherently coupled three-level or four-level model system. 

Phase-dependent coherence and interference can be induced in a multi-level atomic system coupled by multiple laser fields. Two simple examples are presented here, a three-level A-type system coupled by four laser fields and a four-level double A-type system coupled also by four laser fields. The four laser fields induce the coherent nonlinear optical processes and open multiple transitions channels. The quantum interference among the multiple channels depends on the relative phase difference of the laser fields. Simple experiments show that constructive or destructive interference associated with multiple two-photon Raman channels in the two coherently coupled systems can be controlled by the relative phase of the laser fields. Rich spectral features exhibiting multiple transparency windows and absorption peaks are observed. The multicolor EIT-type system may be useful for a variety of application in coherent nonlinear optics and quantum optics such as manipulation of group velocities of multicolor, multiple light pulses, for optical switching at ultra-low light intensities, for precision spectroscopic measurements, and for phase control of the quantum state manipulation and quantum memory. 

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