🔬 atomic physics

Simultaneous sub-Doppler laser cooling and optical trapping of bosonic $^{39}$ K- $^{133}$ Cs and $^{41}$ K- $^{133}$ Cs mixtures

This paper reports the first simultaneous sub-Doppler laser cooling and optical trapping of $^{41}$ K- $^{133}$ Cs and $^{39}$ K- $^{133}$ Cs mixtures, achieving temperatures of $\sim10$ $\mu$ K and utilizing the $^{39}$ K-Cs samples to discover nine previously unobserved heteronuclear Feshbach resonances while characterizing the faster nonexponential loss dynamics that currently limit systematic spectroscopy in the $^{41}$ K-Cs mixture.

Original authors: Mateusz Bocheński, Jakub Dobosz, Koray Dinçer, Paweł Arciszewski, Mariusz Semczuk

Published 2026-09-09

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CC BY 4.0

Original authors: Mateusz Bocheński, Jakub Dobosz, Koray Dinçer, Paweł Arciszewski, Mariusz Semczuk

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quiet, controlled world of atomic physics, scientists strive to slow down the frantic motion of atoms until they are nearly still. When atoms are cooled to temperatures just a fraction of a degree above absolute zero, they stop behaving like a chaotic gas and begin to act like a single, coherent wave of matter. This state, known as an ultracold mixture, allows researchers to study how different types of atoms interact with one another in ways that are impossible to observe at room temperature. By using carefully tuned lasers to drain the energy from these atoms, physicists can trap them in invisible bowls of light. Within these traps, the atoms can be coaxed to form new, exotic molecules or to reveal hidden forces that govern their behavior. The ultimate goal for many in this field is to create a stable, dense collection of different atomic species that can be manipulated with extreme precision, serving as a foundation for building new quantum technologies or simulating complex materials.

A team of researchers at the University of Warsaw has now successfully brought two specific types of atomic mixtures to this ultra-cold state, achieving a milestone that had remained out of reach for one of the combinations. They focused on mixing potassium and cesium, two different chemical elements that are both alkali metals. While scientists had previously managed to cool and trap a mixture of the most common form of potassium with cesium, they had never before succeeded in doing the same with a heavier, less common version of potassium. The researchers managed to cool both mixtures simultaneously in a single vacuum chamber, using a specialized technique involving "gray molasses"—a method where lasers are tuned to slow atoms down without heating them up. They then captured the chilled atoms in a trap made of focused laser light, creating a dense cloud containing millions of atoms of each type, all cooled to a temperature of about 10 microkelvin. This achievement marks the first time a laser-cooled and optically trapped mixture of the heavier potassium isotope with cesium has been created.

The researchers used these new samples to explore how the atoms interact, a process that reveals the invisible forces between them. By applying a magnetic field and watching how many atoms were lost from the trap, they mapped out the specific conditions under which the atoms would stick together or scatter apart. For the mixture containing the common potassium isotope, they employed a clever detection method that separated the atoms based on their internal magnetic orientation, allowing them to see three different interaction pathways at once. This approach revealed fourteen distinct features where the atoms lost energy, nine of which had never been seen in an experiment before. These new observations included interactions that occur in specific, complex ways that were only predicted by theory, providing a much clearer picture of how these two elements behave together.

However, the story was different for the mixture containing the heavier potassium isotope. While the team successfully created the cold cloud, they found that the atoms disappeared from the trap much faster than in the other mixture. The loss of atoms did not follow a simple, steady pattern; instead, it happened rapidly at first and then slowed down, suggesting that the atoms were colliding with each other in a way that caused them to vanish from the trap. This rapid decay currently prevents the researchers from performing the same detailed interaction studies on this mixture that they did on the other. Despite this limitation, the mere creation of this cold, trapped mixture is a significant step forward. It proves that the experimental setup can handle this specific combination of atoms, opening the door for future work to understand why they are so unstable and, eventually, to use them to build the first ultracold molecules made from this previously unexplored pair of isotopes.

Technical Summary: Simultaneous Sub-Doppler Laser Cooling and Optical Trapping of Bosonic 39K-133Cs and 41K-133Cs Mixtures

Problem Statement
Ultracold mixtures of distinct atomic species are essential for accessing interaction regimes and molecular states unavailable in single-species systems, particularly for the production of polar ground-state heteronuclear molecules via magnetoassociation. While potassium–cesium (K-Cs) systems are attractive due to the availability of both bosonic and fermionic isotopes and their strong permanent electric dipole moments, experimental progress has been concentrated almost exclusively on the 39K-Cs mixture. To date, a laser-cooled and optically trapped 41K-Cs mixture had not been realized. Furthermore, efficient preparation of these mixtures requires overcoming challenges in simultaneous sub-Doppler cooling and high-efficiency transfer into optical dipole traps (ODT) without compromising atom number or temperature.

Methodology
The authors performed experiments in a single-chamber apparatus where potassium and cesium atoms were loaded directly from background vapor using atomic dispensers. The experimental sequence involved:

Cooling Schemes: Potassium isotopes were cooled using D1-line gray molasses (specifically $\Lambda$ -enhanced gray molasses), while cesium cooling utilized the entire sequence on the D2 line.
Transfer Optimization: To address low transfer efficiencies from gray molasses to the ODT, the authors introduced a "Transfer-Optimized Gray Optical Molasses" (TO-GMC) stage. This stage optimized beam intensities and duration specifically for loading into a crossed 1064 nm optical dipole trap, distinct from the preceding cooling stage.
Spin Polarization and Trapping: Atoms were spin-polarized and confined in the ODT. For the 39K-Cs mixture, a parallel spin-resolved detection method was implemented using Stern–Gerlach separation to distinguish the three $F=1$ potassium spin components ( $|a\rangle, |b\rangle, |c\rangle$ ) after a single interaction cycle.
Spectroscopy and Decay Analysis: Feshbach spectroscopy was conducted by ramping magnetic fields and measuring atom loss. The stability of the 41K-Cs mixture was quantitatively compared to 39K-Cs by measuring decay dynamics under comparable temperature and density conditions.

Key Contributions and Results

First Realization of 41K-Cs: The authors report the first realization of a laser-cooled and optically trapped 41K-Cs mixture. Both 39K-Cs and 41K-Cs mixtures were cooled to temperatures of $\sim 10$ µK, achieving performance comparable to single-species operations.
High Transfer Efficiency: The TO-GMC stage significantly improved loading efficiency. In single-species operation, the authors obtained approximately $2.0 \times 10^6$ 41K atoms and $3.6 \times 10^6$ 39K atoms in the ODT. For mixtures, they achieved $N_{39K} = 3.1 \times 10^6$ , $N_{Cs} = 3.8 \times 10^6$ and $N_{41K} = 1.1 \times 10^6$ , $N_{Cs} = 4.3 \times 10^6$ atoms, respectively.
Multichannel Feshbach Spectroscopy (39K-Cs): Using the spin-resolved Stern–Gerlach detection method, the authors simultaneously measured loss spectra for three potassium entrance channels. They observed 14 heteronuclear Feshbach loss features. Five matched previously reported resonances, while nine were observed experimentally for the first time, including p-wave features and resonances in additional spin channels.
Decay Dynamics Comparison: The 41K-Cs mixture exhibited substantially faster decay in the ODT compared to 39K-Cs. The decay was non-exponential, indicating a strong density-dependent loss contribution (likely heteronuclear few-body processes). While the 39K-Cs mixture showed weaker additional loss, the 41K-Cs mixture's rapid decay currently hinders systematic Feshbach spectroscopy for that isotopologue.

Significance and Claims
The paper claims that the demonstrated preparation of ultracold 41K-Cs mixtures provides a necessary starting point for Feshbach-resonance and photoassociation spectroscopy of this previously unexplored isotopologue. The work extends the experimentally accessible K-Cs systems beyond 39K-Cs. The authors state that the additional spectroscopic data for 39K-Cs provides further constraints on interaction potentials. Ultimately, these results establish a route toward systematic interaction spectroscopy and the production of ultracold ground-state KCs molecules for multiple isotopologues. The authors note that the shorter lifetime of the 41K-Cs mixture currently limits systematic resonance searches for that specific mixture, a limitation they quantify but do not resolve in this work. The cooling approach is also noted as potentially applicable to the fermionic 40K-Cs mixture, though natural abundance limitations prevented its exploration in this study.

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