Ultracold Atoms and Molecules Laboratory - Past work: Simultaneous trapping of ultracold atoms and ions

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Ultracold Atoms and Molecules Laboratory

Past work: Simultaneous trapping of ultracold atoms and ions

The interaction between an atom and an ion falls off as 1/R⁴ (for large internuclear separation R) which is different from atom-atom 1/R⁶ and dipole-dipole 1/R³ interactions. Hybrids systems with simultaneously trapped atoms and ions thus offer a new regime to be explored. For example, charge exchange reactions at low temperatures can be investigated leading to better understanding of quantum state dependent chemical reactions and cross sections. Hybrid traps also offer the possibility of cooling trapped ions (typically at thousands 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.

Collisional cooling of light ions by cotrapped heavy atoms:

Among the different methods to cool trapped ions, cooling by elastic collisions with a cold buffer gas is arguably the most generic. Indeed, buffer gas cooling of trapped ions has been extensively used when the mass of the ion (m_ion) is higher than the mass of the buffer gas atoms (m_atom). Surprisingly however, the counterpart, that of cooling of trapped ions when m_ion < m_atom, has never been demonstrated experimentally. The most likely reason dates back to a seminal work by Major and Dehmelt [Phys. Rev. 170, 91 (1968)] where it was predicted that an ion trapped in a Paul trap can be cooled by a uniform buffer gas if and only if m_ion > m_atom. For the broader community, this became a rule of thumb. However, recent advances in laser cooling and trapping enable a different class of experiments where the atomic ensemble is well localized within a trap. In such experiments with simultaneously trapped ions and atoms, the original analysis of Major and Dehmelt needs to be revisited.

We demonstrated, for the first time, cooling of low-mass ions by co-trapped heavier atoms. We showed that ³⁹K⁺ ions trapped in a Paul trap are cooled by ultracold ⁸⁵Rb atoms trapped in a magneto-optical trap (MOT), provided the MOT is centered with the Paul trap. A similar cooling of ⁸⁵Rb⁺ ions by ultracold ¹³³Cs atoms was also demonstrated. In both cases, the cooling of ions increases the duration for which ions remain trapped in the Paul trap (see Figure below). We argue that cooling of ions by atoms may be possible for any atom-ion mass ratio. The primary reason for this 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. Our result raises hope that cooling of H₂⁺, H₃⁺ and HD⁺ with ultracold ⁶Li may be possible.

Cooling of trapped ions by resonant charge exchange:

The two most widely used ion cooling methods are laser cooling and sympathetic cooling by elastic collisions. Recent experiments with interacting trapped ion-atom mixtures have extensively studied ion cooling or heating through elastic ion-atom collisions, an example is our recent work described above. However, for homonuclear systems such as Rb-Rb⁺ or Na-Na⁺, the sympathetic ion cooling could be due to (i) elastic collisions between the fast ion and an ultracold atom, resulting in a slow ion after collision, or (ii) resonant charge exchange between a fast ion and an ultracold atom, resulting in an ion essentially at rest, or a combination of both. It is difficult to distinguish the contribution of (i) and (ii) individually in an experiment and therefore the role of resonant charge exchange in the ion cooling process has eluded direct experimental verification.

We demonstrated a method of cooling ions that is based on resonant charge exchange between the trapped ion and the ultracold parent atom. The efficiency of cooling by resonant charge exchange is 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.

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