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Ultracold Atoms and Molecules

Broadly speaking, we are interested in

  • Formation of ultracold polar molecules by associating ultracold atoms
  • Quantum optics and quantum control
  • Quantum state controlled elementary chemical reactions and collisions in the ultracold regime

Research on ultracold polar molecules has gained momentum in recent years. Precise control and manipulation of ultracold molecules, which have a plethora of electronic, vibrational and rotational degrees of freedom, can open exciting new research directions. In particular, ultracold polar diatomic molecules, with strong anisotropic dipole-dipole interaction, are interesting systems for studying quantum state controlled chemical reactions, novel quantum phases and quantum information. It is also proposed that ultracold polar molecules arranged in optical lattices are useful systems for studies on quantum entanglement and quantum computing.

One such class of diatomic molecules are the hetero-nuclear bi-alkali molecules e.g. LiNa, LiRb, NaCs, KRb, RbCs etc. These molecules possess a substantial electric dipole moment in their vibronic ground state. In the next few years we plan to create ultracold polar molecules by associating ultracold atoms. Major construction and development projects are underway to set up an apparatus for laser cooling of atoms in a dual-species magneto-optical trap, creation of molecules and state-selective detection of molecules. In the long term, the system will be used for precise studies on quantum-state dependent chemistry, coherent control, quantum simulation etc.

Some of my prior works are given below

Ultracold polar molecules

There are two broad approaches to cool molecules: direct cooling of ground state molecules (using buffer gas cooling, Stark deceleration, laser cooling etc.) and associating ultracold atoms to create ultracold molecules. The latter approach has been extensively used to create heteronuclear bi-alkali molecules, partly because ultracold alkali atoms can be routinely produced in the lab using the well-established techniques of laser cooling. In particular, two approaches of associating alkali atoms stand out. The first is photoassociation (PA), and the second is magneto-association (MA) followed by Stimulated Raman Adiabatic Passage (STIRAP). The second approach, although elegant, is technically more challenging and works only for species with favorable Feshbach resonances. The first approach is simpler and more general, but the population of molecules is distributed over many rovibronic levels.

My doctoral research at Purdue University focussed on creating ultracold polar LiRb molecules with the aim of studying the dipole-dipole interaction. In our typical PA experiments at Purdue University, LiRb molecules are created by shining a PA laser on ultracold Li and Rb atoms that are trapped in a dual species magneto-optical trap (MOT). A picture of the apparatus is shown above. The molecules thus created leave the trap and molecule formation reduces the number of trapped atoms. Therefore one way to know that molecules are formed by PA is to monitor the number of trapped atoms and look for reduction in the number of atoms as the PA laser frequency is scanned - such a "trap loss PA spectrum" near the Li (2s) + Rb (5p) asymptote is shown below. We have used the PA approach (PA followed by spontaneous emission) to create ultracold LiRb molecules in their electronic ground state. Such ground state molecules can be detected by resonance enhanced multi photon ionization (REMPI) of the molecules and counting the resulting ions - a PA spectrum using REMPI detection is also shown below. By choosing the PA level appropriately we have been able to produce deeply bound LiRb molecules continuously. We also demonstrated two-photon photoassociation spectroscopy of LiRb molecules and used the spectroscopic information to refine molecular potentials and to predict alternative ways to produce ultracold LiRb molecules.

Simultaneous trapping of ultracold atoms and ions

During my post-doctoral career, I worked on cooling and trapping of atoms and ions in order to study cold collisions and interactions at low temperatures. These hybrid systems enable controlled studies on charge exchange reactions at low temperatures and provide better understanding of quantum state dependent chemical reactions. Hybrid traps also offer the possibility of cooling trapped ions (typically at hundreds of Kelvin) by collisions with ultracold neutral atoms (at sub milli-Kelvin temperatures), much like buffer gas cooling but with subtle differences. Much remains to be explored in these hybrid systems, for example the possibility of reaching the s-wave (quantum) limit, ion-atom photoassociation and molecule formation.

One of our interests has been to develop new methods for cooling of ions. In a recent experiment we showed, for the first time, that trapped ions of low mass can be cooled by elastic collisions with heavier neutral atoms. Our experiment provided a resolution to a long-standing controversy in theoretical literature. The primary reason that allowed such cooling is the localized and the precisely centered nature of the ultracold neutral atoms (as opposed to a cold buffer gas that is uniformly distributed). At the centre of the ion trap, the ion’s micromotion is negligible while the ion’s secular speed is the greatest – thus a collision with an ultracold atom, that is essentially at rest, always results in reduction in the ion’s secular motion and hence in cooling of the ion.

We have also recently been able to establish a new ion cooling method based on resonant charge exchange between ions and ultracold atoms. The efficiency of cooling by resonant charge exchange was experimentally estimated and found to be higher than cooling by elastic collisions. The result provides the experimental basis for future studies on charge transport and hopping in the ultracold atom-ion systems. We have also tried to cool Rb2+ molecular ions by using ultracold Rb atoms but here the experiment threw a surprise – we found that Rb2+ ions are dissociated by the light used to cool the Rb atoms.

Cavity based quantum optics

Strong interactions between neutral atoms and light can be achieved by trapping light in a high Q cavity. This enables several aspects of strongly coupled light-matter interaction to be studied. We place ultracold atoms within a Fabry Perot cavity and study the collective strong coupling between the atoms and the cavity modes. The signature of such coupling is vacuum Rabi splitting (VRS) in the transmission spectrum of a weak probe beam.

The collective strong coupling was established by the observation of vacuum Rabi splitting (VRS) in the transmission spectrum of an on-axis weak probe beam. A closer look at VRS measurements done with different probe light intensities showed that VRS decreases with increasing probe intensity. The measurements also revealed an asymmetry in the line shape of the vacuum-Rabi peaks – a signature of optical bistability. When the probe laser is locked to the atomic transition and its power is scanned, the cavity transmission shows bistable behaviour and the cavity input-output curve shows hysteresis. The shape of the hysteresis could be controlled by another off-axis control laser, with a few μW power (Pc), tuned near a different atomic transition (see figure). This results from the probe and control beams forming a Λ-type system. We also demonstrated that the cavity transmission can be switched on and off in micro-second timescales using micro-Watt control powers.

We have also used VRS as a non-destructive probe to detect and measure the presence of trapped ions. We demonstrated that the vacuum-Rabi splitting (VRS), arising from collective strong coupling of ultracold Rb atoms and a cavity, changes in the presence of trapped Rb+ ions (see figure below). The Rb+ ions are optically dark and the Rb atoms are prepared in a dark magneto-optical trap. The VRS was measured on an optically open transition of the initially dark Rb atoms. The measurement itself is fast, nondestructive, and has sufficient fidelity to permit the measurement of the atomic-state-selective ion-atom collision rate. This demonstration illustrates a method based on atom-cavity coupling to measure two-particle interactions generically and nondestructively.