Molecules are intrinsic coherent quantum system that have long been recognized as promising building blocks for quantum simulations, ideal natural laboratory to search for new physics, and a playground to answer fundamental questions in chemical reactions. Achieving full quantum control of molecules in bulk gases have been a forefront goal of the field that generally take either the approach of cooling molecules directly or coherent assembling ultracold molecules from ultracold atoms. Only in recent years, researchers at the CUA recognized that gaining single particle control of cold molecules [1] could be done and could potentially revolutionize the field not only in terms of achieving total control but also in terms of bypassing technical hurdles to harness them for future quantum applications.

This year, a breakthrough had been made in the Ni group at CUA that allows single particle control of ultracold molecule in optical tweezers [2]. In their demonstration, exactly two atoms are combined in a single, controlled reaction to from one molecule, which also extended chemical reactions to a new paradigm beyond stochastic encounters between reactants.

To achieve this level of control to build individual molecules, they employed laser cooling, optical tweezer trapping, and photoassociation. Their experiment began by using optical tweezers to grab single atoms from an ensemble of laser-cooled atoms. To manipulate two different atoms independently, two different color laser beams were used for the optical tweezers. Atoms were electro-optically steered until the tweezers overlap. Subsequently, one tweezer was turned off, allowing two atoms to be held by the same tweezer. The atoms confined by the tweezer potential can collide but do not stick together to form a molecule without the removal or supply of the exact amount of energy needed for bonding. To forge the bond of a NaCs molecule, a pulse of light with a carefully chosen photon energy was supplied. This novel yet simple to comprehend experiment was a cover story of Science [2] and captured much public imagination. Future work is underway to obtain full control of molecular internal degrees of freedom in an array of tweezers with configurable geometry [3].

[1] L. R. Liu, J. T. Zhang, Y. Yu, N. R. Hutzler, Y. Liu, T. Rosenband, K.-K. Ni.   Ultracold Molecular Assembly. arXiv:1701.03121(2017)

[2] L. R. Liu, J. D. Hood, Y. Yu, J. T. Zhang, N. R. Hutzler, T. Rosenband, K.-K. Ni. Building one molecule from a reservoir of two atoms. Science 360, 900 (2018)

[3] Endres, M., Bernien, H., Keesling, A., Levine, H., Anschuetz, E.R., Krajenbrink, A., Senko, C., Vuletic, V., Greiner, M. and Lukin, M.D., 2016. Atom-by-atom assembly of defect-free one-dimensional cold atom arrays. Science 354, 1024 (2016)

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