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arXiv:1109.0588 · Physical Review A

Vacuum-induced coherence in ultracold photoassociative rovibrational excitations

Sumanta Das, Arpita Rakshit, Bimalendu Deb

Published Jan 3, 2012• 10 citations
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Abstract

We show that coherence between two excited rovibrational states belonging to the same molecular electronic configuration arises quite naturally due to their interaction with electromagnetic vacuum. For initial preparation of a molecule in the desired rovibrational states, we propose to employ the method of ultracold photoassociation. Spontaneous decay of the excited molecule then gives rise to vacuum-induced coherence between the excited ro-vibrational states. We demonstrate theoretically an interesting interplay of effects due to vacuum-induced coherence and photoassociation. We apply our theory to photoassociation of bosonic ytterbium (${}^{174}$Yb) atoms, which appear to be a promising system for exploring such interplay. The effects discussed here can be important for controlling decoherence and dissipation in molecular systems.

DOI
10.1103/physreva.85.011401

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We show that coherence between two excited rovibrational states belonging to the same molecular electronic configuration arises quite naturally due to their interaction with electromagnetic vacuum. For initial preparation of a molecule in the desired rovibrational states, we propose to employ the method of ultracold photoassociation. Spontaneous decay of the excited molecule then gives rise to vacuum-induced coherence between the excited ro-vibrational states. We demonstrate theoretically an interesting interplay of effects due to vacuum-induced coherence and photoassociation. We apply our theory to photoassociation of bosonic ytterbium (174{}^{174}174Yb) atoms, which appear to be a promising system for exploring such interplay. The effects discussed here can be important for controlling decoherence and dissipation in molecular systems.
Photoassociation of ultracold atoms with pulsed laser light is shown to lead to alignment of the product molecules along the excitation laser polarization axis. We theoretically investigate pulsed two-photon photoassociation of 87Rb^{87}\mathrm{Rb}87Rb atoms into a specific weakly bound level of the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ metastable electronic state and find both stationary and time-dependent field-free alignment. Although a transform-limited pulse yields significant alignment, a frequency-chirped pulse dramatically enhances the ultracold molecular formation rate at the cost of a slight decrease in the alignment. Employing multiple pulses synchronized with the vibrational and rotational periods leads to coherent enhancement of both population and alignment of the target state. Inclusion of the rotational degree of freedom in the model gives rise to processes with multiple quantum paths, which lead to quantum effects such as interference and coherence revivals.
The formation of diatomic molecules with rotational and vibrational coherence is demonstrated experimentally in free-to-bound two-photon femtosecond photoassociation of hot atoms. In a thermal gas at a temperature of 1000 K, pairs of magnesium atoms, colliding in their electronic ground state, are excited into coherent superpositions of bound rovibrational levels in an electronically excited state. The rovibrational coherence is probed by a time-delayed third photon, resulting in quantum beats in the UV fluorescence. A comprehensive theoretical model based on ab initio calculations rationalizes the generation of coherence by Franck-Condon filtering of collision energies and partial waves, quantifying it in terms of an increase in quantum purity of the thermal ensemble. Our results open the way to coherent control of a binary reaction.
The formation of the NaH molecule from unaligned ultracold atoms via photoassociation (PA) is theoretically investigated by numerically solving the time-dependent Schrödinger equation including the vibrational and rotational degrees of freedom. The PA processes via one- and two-photon excitations are described. The calculated results show that the associated molecule can be prepared to a desired rovibrational state of the ground electronic state for NaH. Compared with the one-photon association probability, the two-photon association probability is obviously enhanced.
We theoretically explore photoassociation by adiabatic passage of two colliding cold 85Rb^{85}\mathrm{Rb}85Rb atoms in an atomic trap to form an ultracold Rb2{\mathrm{Rb}}_{2}Rb2​ molecule. We consider the incoherent thermal nature of the scattering process in a trap and show that coherent manipulations of the atomic ensemble, such as adiabatic passage, are feasible if performed within the coherence time window dictated by the temperature, which is relatively long for cold atoms. We show that a sequence of $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ pulses of moderate intensities, each lasting $\ensuremath{\sim}750\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, can photoassociate a large fraction of the atomic ensemble at temperature of $100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and density of 1011atoms∕cm3{10}^{11}\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{cm}}^{3}1011atoms∕cm3. Use of multiple pulse sequences makes it possible to populate the ground vibrational state. Employing spontaneous decay from a selected excited state, one can accumulate the molecules in a narrow distribution of vibrational states in the ground electronic potential. Alternatively, by removing the created molecules from the beam path between pulse sets, one can create a low-density ensemble of molecules in their ground rovibrational state.
We report the production of ultracold heteronuclear Cs∗Yb{\mathrm{Cs}}^{*}\mathrm{Yb}Cs∗Yb molecules through one-photon photoassociation applied to an ultracold atomic mixture of Cs and Yb confined in an optical dipole trap. We use trap-loss spectroscopy to detect molecular states below the Cs(2P1/2)+Yb(1S0)\mathrm{Cs}\phantom{\rule{0.16em}{0ex}}(^{2}P_{1/2})+\mathrm{Yb}(^{1}S_{0})Cs(2P1/2​)+Yb(1S0​) asymptote. For 133Cs174Yb^{133}\mathrm{Cs}\phantom{\rule{0.16em}{0ex}}^{174}\mathrm{Yb}133Cs174Yb, we observe 13 rovibrational states with binding energies up to $\ensuremath{\sim}500\phantom{\rule{0.28em}{0ex}}\mathrm{GHz}$. For each rovibrational state we observe two resonances associated with the Cs hyperfine structure and show that the hyperfine splitting in the diatomic molecule decreases for more deeply bound states. In addition, we produce ultracold fermionic 133Cs173Yb^{133}\mathrm{Cs}\phantom{\rule{0.16em}{0ex}}^{173}\mathrm{Yb}133Cs173Yb and bosonic 133Cs172Yb^{133}\mathrm{Cs}\phantom{\rule{0.16em}{0ex}}^{172}\mathrm{Yb}133Cs172Yb and 133Cs170Yb^{133}\mathrm{Cs}\phantom{\rule{0.16em}{0ex}}^{170}\mathrm{Yb}133Cs170Yb molecules. From mass scaling, we determine the number of vibrational levels supported by the 2(1/2) excited-state potential to be 154 or 155.