Theses Doctoral

Reaching the Bose-Einstein Condensation of Dipolar Molecules: a Journey from Ultracold Atoms to Molecular Quantum Control

Bigagli, Niccolò

Achieving the quantum control of ever more complex systems has been a driving force of atomic, molecular, and optical physics. This goal has materialized in the harnessing of systems with increasingly rich structures and interactions: the more sophisticated the system, the more faceted and fascinating its application to fields as varied as quantum simulation, quantum information, many body physics, metrology, and quantum chemistry. One of the current frontiers of quantum control is ultracold dipolar molecules. They present rich internal structures and long-range, anisotropic dipole-dipole interactions which promise to revolutionize AMO physics, for example by realizing realistic Hamiltonians in quantum simulation, by providing a new platform for quantum information, and by achieving a novel kind of quantum liquid.

Despite its promises, the full quantum control of dipolar molecules has been over a decade in the making. The difficulties in either directly laser cooling molecules or in collisionally stabilizing their bulk samples have been major roadblocks that have hampered the development of this quantum system. The realization of a Bose-Einstein condensate of dipolar molecules has been a particularly elusive milestone. In this thesis, I report on the first observation of this quantum state of matter.

The work that brought us to this achievement parallels the historical evolution of AMO physics in the last thirty years. To reach a BEC of molecules, we initially constructed a dual species experiment capable of realizing the simultaneous Bose-Einstein condensation of atomic sodium (Na) and cesium (Cs). Individual BECs of sodium and cesium were first reported in 1995 and 2003 respectively, while our experiment was the first instance of their concurrent condensation. The study of the Na-Cs interatomic scattering properties in an homogeneous magnetic field showed us the path to the Feshbach association of loosely-bound sodium-cesium (NaCs) molecules, a technique first demonstrated in 2006 for heteronuclear molecules but never attempted on our species. Following the Feschbach association, we determined a novel pathway to the molecular electronic, vibrational and rotational ground state using STIRAP.

From this point, we found ourselves at the forefront of the field: bulk samples of bosonic molecules such as NaCs had neither been stabilized against collisional losses nor evaporatively cooled. At first, we successfully applied a single-frequency microwave shielding approach to decrease in-bulk losses by a factor of 200 and reach lifetimes on the order of 2 s, allowing us to measure high elastic scattering rates and characterize their dipolar anisotropy. Moreover, we demonstrated the first evaporative cooling of a bosonic molecular gas by increasing its phase-spacedensity by a factor of 20 and reaching a temperature of 36(5) nK. Since this proved insufficient to achieve Bose-Einstein condensation due to unexpected three-body losses, we introduced an enhanced microwave shielding technique, double microwave shielding. This further decreased loss rates enabling efficient evaporative cooling of our sample to a long-lived Bose-Einstein condensate of dipolar molecules. This new double microwave shielding technique also allows the tunability of the strength of dipole-dipole interaction, establishing ultracold bosonic dipolar molecules as a new quantum liquid for the exploration of many body physics.

In addition to the experimental work on dipolar NaCs, we have theoretically explored the field of direct molecular laser cooling. Our aim was twofold: we aimed to expand the category of molecules that can be laser cooled and to simplify the identification of laser cycling schemes. For the former goal, we lifted the widespread assumption that only molecules with diagonal Franck-Condon factors could be laser cooled. For the latter, we decided to employ publicly available repositories of molecular transitions. A second consequence of the use of these databases is that they contain data on molecules of interest to other scientific fields, further establishing direct laser cooling as a technique that could be of interest beyond AMO physics. Our work was successful in that we identified laser cycling schemes for C₂ and OH+. To simplify the determination of laser cycling schemes, we developed a graph-based algorithm form their identification starting from spectroscopic data.


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More About This Work

Academic Units
Thesis Advisors
Will, Sebastian
Ph.D., Columbia University
Published Here
July 3, 2024