Can directly laser-cooled molecules reach quantum degeneracy?
In plain words
So far every ultracold molecular quantum gas was made by gluing together ultracold atoms. Molecules cooled directly with lasers, which include species useful for precision measurements, have not yet been cooled far enough to behave as a single quantum wave.
Precise statement
Directly laser-cooled molecules (CaF, SrF, YO, CaOH and similar) in optical traps or tweezer-loaded traps. Reach phase-space density $n \lambda_{\mathrm{dB}}^3 \ge 1$ ($\lambda_{\mathrm{dB}}$ the thermal de Broglie wavelength) by evaporative, sympathetic or collisional cooling, which requires a large ratio of elastic to inelastic collisions, for example through microwave or optical shielding. Answer: observation of a Bose-Einstein condensate or degenerate Fermi gas of a laser-cooled species, or a measured collisional obstruction.
What would settle it
Bimodal momentum distributions (bosons) or Fermi-surface signatures (fermions) in a gas of directly laser-cooled molecules.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Blue-detuned magneto-optical traps raised the densities of laser-cooled YO and CaF (Burau et al., PRL 2023; Li et al., PRL 2024), but reported phase-space densities remain far below 1, and a 2025 perspective still describes condensation as a goal (Zeng et al., Quantum Review Letters 2025).
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