Ultracold Molecular Assembler

Molecule Tweezers

The Team: Jessie Zhang, Kenneth Wang, Lewis Picard, Dr. Will Cairncross, Dr. Yichao Yu

For the latest news, see the Ni group Publications page.

Ultracold polar molecules posess rich internal structure and long-rang dipole-dipole interactions that lend themself useful for a variety of applications in quantum simulation and quantum information studies. A key ingredient in many of these proposed applications is the fine control necessary over the quantum degrees of freedom. both internal and external, of the individual constituent molecules. In this project, we are working toward assembling a configurable molecular array atom-by-atom. Our approach draws on the bulk gas approach of forming ultracold molecules initially demonstrated in Prof. Ni's PhD thesis work at JILA, single molecule manipulation and detection schemes demonstrated in ions, as well as optical tweezer techniques developed more recently in other groups, and relies heavily on high fidelity internal and external quantum-state control of atoms and molecules. Such an approach would allow single molecules to be formed and manipulated without ensemble averaging, and for interactions between individual molecules to be flexibly engineered.

In particular, we start from single sodium (Na) and single cesium (Cs) atoms trapped in optical tweezers to form single NaCs molecules. NaCs molecules are chemically stable and have one of the largest dipole moments (4.6 Debye) of the bi-alkalis when polarized in the lab frame. The steps necessary for the creation of a single molecule include (1) trapping of single Na and Cs atoms, (2) Raman sideband cooling to their individual motional ground states, (3) adiabatic merging of the optical tweezers, (4) coherent formation of a molecule. Our efforts towards such an ultracold molecular assembler are chronicled below.


  • (2016) A single sodium atom and a single cesium atom side-by-side. :) :) Getting single sodium atoms to cooperate takes some tricks.
  • (2017) Proof-of-principle demostration of "building a molecule from two atoms". Single molecules built here are in the electronic excited state by photoassociation of single pairs of atoms in an optical tweezer. This was a cover story in Science (May 2018).
PA Spectrum

Next, we are working hard to get the single atom motion under control...

  • (2017) Raman motional sideband cooling of a Na atom in an optical tweezer...reaching 3D ground-state population of ~94%!!. Cooling of Na atoms is much more challenging due to their light mass and high initial temperatures. Data shows cooling in the most challenging direction utilizing higher order sidebands.
Na RSC axial direction
Raman Transfer
  • (2018) In the mean time, we characterized the interactions of two atoms in a tweezer and search for NaCs Feshbach resonances with just two atoms.
Interaction of Na-Cs

Working towards coherent molecule formation...

NaCs Feshbach Resonance

And we have a second way of making weakly bound molecules coherently - an all-optical appraoch without relying on Feshbach resonances. This approach can be generalized to allow a more diverse set of species to be assembled into molecules atom-by-atom!

Optical Transfer to weakly bound state
  • (2020) We completed the final step of the "ultracold molecular assembler" by transferring a weakly-bound NaCs Feshbach molecule to its rotational, vibrational, and electronic ground state. With one molecular qubit fully controlled, we are ready to scale up to an array of many molecules and to unleash the full potential of molecular qubits.