Optical formation of ground state ultracold tetratomic molecules

Project Information

UltracoldTetramers

Grant agreement ID: 101203827

DOI

10.3030/101203827

Project terminated on 14 December 2025

EC signature date 31 March 2025

Start date 7 April 2025

End date 6 April 2027

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 179 006,16

Coordinated by

UNIWERSYTET WARSZAWSKI
Poland

Periodic Reporting for period 1 - UltracoldTetramers (Optical formation of ground state ultracold tetratomic molecules)

Reporting period: 2025-04-07 to 2027-04-06

Summary of the context and overall objectives of the project

It is now routine in many laboratories to produce ultracold diatomic molecular gases at temperatures below a microkelvin. Following the success of creating ultracold diatoms over the last two decades, there is also growing interest in forming ultracold polyatomic molecules containing three or more atoms. Owing to their rich internal structure, they offer unique opportunities for studies of cold chemistry, precision measurements, the realization of exotic quantum phases, and quantum information processing. Their increased complexity, however, presents challenges in using conventional cooling methods. Nevertheless, in a recent breakthrough experiment [X.-Y. Chen et al., Nature 626, 283 (2024)], ultracold tetratomic (NaK)2 molecules have been created by associating two fermionic NaK molecules in the presence of microwave fields. These are called field-linked molecules because an external field is required for their formation. This has stimulated immense interest in the field of ultracold research, as their method is near-universal and can be applied to a wide range of ultracold polar molecules.

The observed ultracold tetratomic molecules (“tetramers”) are formed in extremely weakly bound states at a very long range, with distances between the two constituent diatoms of about 100 nm. They have limited lifetimes due to loss induced by the external field. On the other hand, the realization of long-lived samples of ultracold tetramer molecular gas, produced near the global minimum of their ground-state interaction potential energy surface, will allow us to explore the aforementioned new physics. In my Marie Skłodowska-Curie Action, my objectives were to develop new theoretical methods for transferring weakly bound FL tetramers to their absolute ground state using optical fields.

Objective 1: Developing methodology for mitigating loss of ultracold molecules in ground and excited electronic states using external static electric and microwave fields.

Objective 2: Developing a new methodology for stimulated adiabatic transfer of tetramer molecules from the excited state to the ground rovibrational electronic state.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We set out two objectives in the MSCA project. The project was to be carried out over a 24-month period. However, the researcher obtained a permanent academic position elsewhere, which meant the project lasted about 8 months.

The two objectives described above was distributed over two work packages (WP), with objective 1 being WP 1 and objective being WP 2.

WP 1 had two deliverables (D), which were:
D1: Exploring new methodologies for controlling and mitigating loss of colliding ultracold diatomic molecules in an excited electronic state with the aid of an external static electric field.
D2: Developing new models with the same motivation as above, but with external microwave fields.

We worked on WP1 within the 8-month period and completed most of the work package as follows.

We proposed a coherent optical population transfer of weakly bound field-linked (FL) tetratomic molecules (tetramers) to deeper FL-bound states using stimulated Raman adiabatic passage. We considered static-electric-field-shielded polar alkali-metal diatomic molecules and their corresponding FL tetramers in their ground X1Σ++X1Σ+ electronic state. We showed that the excited metastable X1Σ++b3Π electronic manifold supports FL tetramers over a broader range of electric fields, with collisional shielding extending to zero field. We calculated the Franck-Condon factors between the ground and excited FL tetramers and showed that they are highly tunable with the electric field. We also predicted photoassociation of ground-state shielded molecules to the excited FL states in free-bound optical transitions. We proposed proof-of-principle experiments to implement stimulated Raman adiabatic passage and photoassociation using FL tetramers, paving the way for the formation of deeply bound ultracold polyatomic molecules.

The above work was published in Phys. Rev. Lett. 136, 013401 (2026), which was D1.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We proposed the first steps toward creating stable, deeply bound polyatomic molecules at ultracold temperatures. This is challenging because, as the number of atoms increases, such molecules are prone to collisional heating, which threatens experimental control and cooling. Laser cooling enables atoms to be cooled to ultralow temperatures, and weakly bonded cold diatomic molecules can be created from these atoms using an external magnetic field. Recent advances in ultracold research have shown that weakly bonded tetratomic molecules can be formed from pairs of ultracold diatomic molecules using external electric or microwave fields, thereby advancing the
assembly process. These tetratomic molecules are exotic, with bond lengths on the order of tens of nanometers, unlike typical polyatomic molecules found in everyday life. For diatomic molecules, a pair of lasers can transfer molecules from weakly bound states to their strongly bound ground state. In our project, we extended this approach to tetratomic molecules and propose transferring their weakly bound states to deeper bound states with unprecedented control. Stable ultracold polyatomic molecules, if realized, will provide unique opportunities for studying cold chemistry, precision measurements, exotic quantum phases, and quantum information processing.

Schematic showing the proposed conversion of weakly-bound tetramers to deeper bound tetramers.