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Physical Review A

Detecting quantum superpositions of classically distinguishable states of a molecule

Ian A. Walmsley, Michael G. Raymer

Published Jul 1, 1995• 29 citations
Publisher

Abstract

We present a method for the unambiguous detection of a classically distinguishable coherent superposition state in the vibrational coordinate of a diatomic molecule. The configuration of the state may be observed via the time-dependent spectrum of spontaneous emission from the molecule. The signature of the states in the emission spectra is discussed, as are the practical possibilities for experiments on a realistic molecular system.

DOI
10.1103/physreva.52.681

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•Oct 30, 2006•0 citations
We present a method for the unambiguous detection of a classically distinguishable coherent superposition state in the vibrational coordinate of a diatomic molecule. The configuration of the state may be observed via the time-dependent spectrum of spontaneous emission from the molecule. The signature of the states in the emission spectra is discussed, as are the practical possibilities for experiments on a realistic molecular system.
It is well known that the application of a pulse to a molecular electronic transition leaves the molecules vibrational modes in different electronic states in well-localized states, or wave packets[1]. One method of detecting the wave packets is via the time-dependent spectra of spontaneous emission from the excited molecule[2]. The emission consists in general of two parts, an incoherent part that depends only on the wave packet in the excited state, and a coherent part, that depends on the wave packets in both excited and ground electronic states (of a two-state molecule). This can be seen from the following simple calculation.
A pair of coherent femtosecond pulse excitations applied to a molecule with strong electron-phonon coupling creates a coherent superposition of a low momentum and a high momentum wavepacket in the vibrational states of both the excited state and the ground state of the coherent transition. As the excited state is accelerated further, interference between the high momentum ground state contribution and the low momentum excited state contribution causes a photon echo. This photon echo is a direct consequence of quantum interference between separate vibrational trajectories and can therefore provide experimental evidence of the non-classical properties of molecular vibrations.
Emission tomography provides a method for the characterization of quantum states of those degrees of freedom of material quantum systems that are coupled to electronic states of the system. In such systems, the time-dependent fluorescence spectrum contains complete information about the non-electronic degree of freedom. Phase-space or, in certain cases, density matrix representations of the state can be reconstructed from the spectrum using standard methods of computer-assisted tomography or linear algebra. The principles and implementation of this procedure are described in detail and applied to the specific example of the reconstruction of the quantum state of the vibrational mode of a diatomic molecule excited into a non-stationary wavepacket state.
This work is devoted to the study of the quantum coherent effects in diatomic molecular systems by using high resolution laser spectroscopy. In particular, we have studied\nthe rubidium diatomic molecular gaseous medium's absorption spectrum with high\nresolution single mode laser spectroscopy. The derived electronic and rotational vibrational constants were used in the backward Raman amplification experiment of\nRb diatomic molecule. Both experimental results and theoretical calculation confirms that there is strong backward directionally dependent radiation. This effect can\nfurther be utilized in remote detection of chemical material.\nIn the saturated spectroscopy experiment of the cesium diatomic molecule, long-lived ground state coherence was observed. The coherence would decay at a rate less\nthan the natural life time of the excited states, which indicates great possibility for\nperforming the quantum optics experiments previously performed in atomic systems\nonly.\nElectromagnetically induced transparency has been observed in many atomic\nsystems for many years, while it has been seldom realized in molecular systems.\nIn our experiment of electromagnetically induced transparency in cesium diatomic\nmolecules, we utilized  energy levels, and observed subnatural linewidth. This is the first time to realize a  type EIT in a molecular ensemble. This experiment will\nlead to many other experiments of quantum effects in a molecular system, such like\nmagnetic optical rotation, light storage in ensemble of molecules.\nMagnetically induced chirality in an atomic ensemble is also investigated in my\nresearch.