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Frontiers in Optics

Quantum Interference in an Atom Molecule System

Rainer Dumke, Jonathan D. Weinstein, M. Johanning, K. M. Jones, Paul D. Lett

Published Jan 1, 2005• 0 citations
Publisher

Abstract

By driving photoassociation transitions with continuous-wave lasers, we stateselectively form excited-state molecules from ultra cold atoms. Using a second laser to drive transitions from the excited molecular state to another long-lived molecular state, we are able to observe an interference dip narrower than the natural linewidth of the excited molecular state. The linewidth of the dip is strongly effected by the temperature of the atoms. To reach smaller linewidths we have set up an all optical BEC.

DOI
10.1364/ls.2005.lwd1

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By driving photoassociation transitions with continuous-wave lasers, we stateselectively form excited-state molecules from ultra cold atoms. Using a second laser to drive transitions from the excited molecular state to another long-lived molecular state, we are able to observe an interference dip narrower than the natural linewidth of the excited molecular state. The linewidth of the dip is strongly effected by the temperature of the atoms. To reach smaller linewidths we have set up an all optical BEC.
By driving photoassociation transitions, we form electronically excited molecules (Na2∗)({\mathrm{Na}}_{2}^{*})(Na2∗​) from ultracold $(50--300\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K})$ Na atoms. Using a second laser to drive transitions from the excited state to a level in the molecular ground state, we are able to split the photoassociation line and observe features with a width smaller than the natural linewidth of the excited molecular state. The quantum interference which gives rise to this effect is analogous to that which leads to electromagnetically induced transparency in three-level atomic $\ensuremath{\Lambda}$ systems, but here one of the ground states is a pair of free atoms while the other is a bound molecule. The linewidth is limited primarily by the finite temperature of the atoms.
New development in cold collisions has revived the interest in the process of photoassociation, because the free-bound transitions are very selective at extremely low temperatures. Some molecular systems where photoassociation has been observed in the past may now be studied in much greater detail than ever before. The photoassociation from the weakly bound Cs2 molecules at elevated and ultra cold temperatures will form peculiar molecules in their excited states in the intermediate internuclear distances.
In contrast to atoms, laser cooling of molecules is very difficult because of the lack of a closed two-level scheme for population recycling. Molecular photoassociation of cold atoms has permitted the first observation of the formation of cold ground state Cs 2 molecules. In this scheme, two free cold atoms absorb one photon, to form an electronically excited molecule in a well-defined ro-vibrational state, which yields translationally cold molecules, after spontaneous decay, in their ground state or in their lowest triplet state.
Laser cooling and magneto-optical trapping of neutral atoms has been a driving force in physics over the last few decades by providing an efficient method to produce cold and dense samples of cold atoms. Polar molecules which can have strong, long-range dipolar interactions and possess more rich internal structure promise a wide range of new physics and chemistry studies if they can be controlled efficiently. Until recently, the only way to achieve sub-milliKelvin level temperatures in molecules was to associate two precooled atomic species, which while very effective, has a firm bound on the chemical diversity accessible. While laser cooling of molecules relies on fortuitous molecular properties, it will significantly increase the choices of species available for molecular cooling.In this thesis I present the improved optical deceleration of the molecule yttrium monoxide (YO) and the loading of molecules into a 3-D magneto-optical trap (MOT). By making an improved choice of repumping scheme from our groups’ previous work, our photon scattering rate has been increased. By also improving our buffer gas cell source, we produce a much larger flux (2000 times higher) of slowed molecules at 5 m/s to reach the trapping region. I have designed and implemented 3-D MOT field coils operating at 5 MHz, and demonstrated a successful operation of a 3-D MOT. This is the first 3-D laser trapping of an oxide and the first of a molecule with an electronic state intermediate to the main cycling transition states.I have also performed a study of the transition from the ground state to the intermediate electronic state, which has a narrower linewidth of 150 kHz, in contrast to the main cycling transition’s linewidth of 5 MHz. This transition is well suited to second-stage narrow line cooling after loading molecules with the main MOT transition, which will allow direct laser cooling of a molecule to the 10 uK regime.
Chemical reactions typically proceed via stochastic encounters between reactants. Going beyond this paradigm, we combined exactly two atoms in a single, controlled reaction. The experimental apparatus traps two individual laser-cooled atoms [one sodium (Na) and one cesium (Cs)] in separate optical tweezers and then merges them into one optical dipole trap. Subsequently, photoassociation forms an excited-state NaCs molecule. The discovery of previously unseen resonances near the molecular dissociation threshold and measurement of collision rates are enabled by the tightly trapped ultracold sample of atoms. As laser-cooling and trapping capabilities are extended to more elements, the technique will enable the study of more diverse, and eventually more complex, molecules in an isolated environment, as well as synthesis of designer molecules for qubits.