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arXiv:0806.0821 · Physical Review A

Formation of deeply bound molecules via chainwise adiabatic passage

Elena Kuznetsova, Philippe Pellegrini, Robin Côté, Mikhail D. Lukin, Susanne F. Yelin

Published Aug 5, 2008• 28 citations
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Abstract

We suggest and analyze a technique for efficient and robust creation of dense ultracold molecular ensembles in their ground rovibrational state. In our approach a molecule is brought to the ground state through a series of intermediate vibrational states via a multistate chainwise stimulated Raman adiabatic passage technique. We study the influence of the intermediate states decay on the transfer process and suggest an approach that minimizes the population of these states, resulting in a maximal transfer efficiency. As an example, we analyze the formation of $^{87}\mathrm{Rb}_{2}$ starting from an initial Feshbach molecular state and taking into account major decay mechanisms due to inelastic atom-molecule and molecule-molecule collisions. Numerical analysis suggests a transfer efficiency $>90%$, even in the presence of strong collisional relaxation as are present in a high density atomic gas.

DOI
10.1103/physreva.78.021402

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arXiv:0806.2284•Science•Jul 10, 2008•385 citations
We suggest and analyze a technique for efficient and robust creation of dense ultracold molecular ensembles in their ground rovibrational state. In our approach a molecule is brought to the ground state through a series of intermediate vibrational states via a multistate chainwise stimulated Raman adiabatic passage technique. We study the influence of the intermediate states decay on the transfer process and suggest an approach that minimizes the population of these states, resulting in a maximal transfer efficiency. As an example, we analyze the formation of 87Rb2^{87}\mathrm{Rb}_{2}87Rb2​ starting from an initial Feshbach molecular state and taking into account major decay mechanisms due to inelastic atom-molecule and molecule-molecule collisions. Numerical analysis suggests a transfer efficiency >90>90%>90, even in the presence of strong collisional relaxation as are present in a high density atomic gas.
Abstract Stimulated Raman adiabatic passage allows robust transfer between two ends of a three-state quantum system and has been employed to transfer weakly bound Feshbach molecules into their deeply bound rovibrational ground state. However, the efficient transfer remains to be explored. Here we propose a possible alternative route, based on a recently developed non-Hermitian shortcut to adiabaticity method. It is able to realize single-step transfer efficiencies up to 100% even in the presence of a decaying excited level, surpassing all the previous methods. We also prove that our scheme is robust against the external field parameter fluctuations and is expected to be applicable for abundant molecular species.
Using the technique of stimulated Raman adiabatic passage (STIRAP) we have coherently transferred ultracold (87)Rb(2) Feshbach molecules into a more deeply bound vibrational quantum level. Our measurements indicate a high transfer efficiency of up to 87%. Because the molecules are held in an optical lattice with not more than a single molecule per lattice site, inelastic collisions between the molecules are suppressed and we observe long molecular lifetimes of about 1 s. Using STIRAP we have created quantum superpositions of the two molecular states and tested their coherence interferometrically. These results represent an important step towards Bose-Einstein condensation of molecules in the vibrational ground state.
Chainwise stimulated Raman adiabatic passage (C-STIRAP) in a M-type molecular system is a good alternative in creating ultracold deeply bound molecules when the typical STIRAP in a $\mathrm{\ensuremath{\Lambda}}$-type system does not work due to weak Frank-Condon factors between states. However, its creation efficiency under smooth evolution is generally low. During the process, the population in the intermediate states may decay out quickly and the strong laser pulses may induce multiphoton processes. In this paper, we find that the shortcut to adiabaticity (STA) fits very well in improving the performance of the C-STIRAP. Currently, related discussions on the so-called chainwise stimulated Raman shortcut-to-adiabatic passage (C-STIRSAP) are rare. Here, we investigate this topic under adiabatic elimination. Given a relation among the four incident pulses, it is quite interesting that the M-type system can be generalized into an effective $\mathrm{\ensuremath{\Lambda}}$-type structure with the simplest resonant coupling. Consequently, all possible methods of STA for the three-state system can be borrowed. We take the counterdiabatic driving and ``chosen path'' methods as instances to demonstrate our treatment of the molecular system. Although the chosen path method does not work well in a real three-state system if there is strong decay in the excited state, our C-STIRSAP protocol under both methods can create ultracold deeply bound molecules with high efficiency in the M-type system. The evolution time is shortened without strong laser pulses and the robustness of STA is well preserved. Finally, the detection of ultracold deeply bound molecules is discussed.
One possibility for the creation of ultracold, high phase space density quantum gases of molecules in the rovibronic ground state relies on first associating weakly-bound molecules from quantum-degenerate atomic gases on a Feshbach resonance and then transferring the molecules via several steps of coherent two-photon stimulated Raman adiabatic passage (STIRAP) into the rovibronic ground state. Here, in ultracold samples of Cs2 Feshbach molecules produced out of ultracold samples of Cs atoms, we observe several optical transitions to deeply-bound rovibrational levels of the excited 0(u)+ molecular potentials with high resolution. At least one of these transitions, although rather weak, allows efficient STIRAP transfer into the deeply-bound vibrational level (see text for symbols)v = 73 > of the singlet X1 sigma(g)+ ground state potential, as recently demonstrated (J. G. Danzl, E. Haller, M. Gustavsson, M. J. Mark, R. Hart, N. Bouloufa, O. Dulieu, H. Ritsch, and H.-C. Nägerl, Science, 2008, 321, 1062). From this level, the rovibrational ground state (see text for symbols)v = 0, J = 0 > can be reached with one more transfer step. In total, our results show that coherent ground state transfer for Cs2 is possible using a maximum of two successive two-photon STIRAP processes or one single four-photon STIRAP process.
In this paper, we carry out the non-Hermitian stimulated Raman shortcut-to-adiabatic passage (NH-STIRSAP) to create ultracold deeply-bound molecules. The open three-level molecular system is supposed to have irreversible decays out of all levels. Under the assumption of large single-photon detuning and/or strong loss rate of the excited level, the NH three-level system can be theoretically reduced to an effective two-level system. By continuously using the counter-diabatic driving of the shortcut-to-adiabaticity, an auxiliary field is produced to couple the Feshbach level with the ground level. In the numerical calculations, we firstly consider the case of decaying only from the excited level, which is the main loss of the system. Under the NH-STIRSAP, the calculations reveal that complete population transfer from the Feshbach to the ground level can occur even if the adiabatic condition is not fulfilled. Therefore, the creation efficiency of ultracold deeply-bound molecules is greatly improved. The results hold equally for counter-intuitive and intuitive time sequence of the Stokes and pump pulse. If the decays from the Feshbach and ground level are also included, the NH-STIRSAP can still restore most of that efficiency, which performs much better than typical stimulated Raman adiabatic passage. Finally, we point out that the robustness and fast transfer speed of the NH-STIRSAP facilitate the creation and detection of ultracold deeply-bound molecules.
Molecular cooling techniques face the hurdle of dissipating translational as well as internal energy in the presence of a rich electronic, vibrational, and rotational energy spectrum. In our experiment, we create a translationally ultracold, dense quantum gas of molecules bound by more than 1000 wave numbers in the electronic ground state. Specifically, we stimulate with 80% efficiency, a two-photon transfer of molecules associated on a Feshbach resonance from a Bose-Einstein condensate of cesium atoms. In the process, the initial loose, long-range electrostatic bond of the Feshbach molecule is coherently transformed into a tight chemical bond. We demonstrate coherence of the transfer in a Ramsey-type experiment and show that the molecular sample is not heated during the transfer. Our results show that the preparation of a quantum gas of molecules in specific rovibrational states is possible and that the creation of a Bose-Einstein condensate of molecules in their rovibronic ground state is within reach.