Although Einstein’s theory of general relativity has been remarkably successful, it remains a classical framework that excludes quantum effects. A comprehensive understanding of gravity will require tests that can elucidate the relationship of gravity with the quantum world.

Microgravity creates unique opportunities for experiments involving quantum mixtures of ultracold gases, including those that investigate the most fundamental cornerstone of general relativity: the Einstein equivalence principle. Now, researchers from Germany report on a high-flux generation of Bose-Einstein condensate (BEC) mixtures of two different elements in an atom chip under microgravity conditions (Nat. Commun., doi: 10.1038/s41467-026-75968-9).

“Comparing the free fall of two different atomic species is the key to executing next-generation, high-precision tests of Einstein’s equivalence principle in space,” said study author André Wenzlawski, Institute of Physics at Johannes Gutenberg University Mainz, where a team developed a unique optical system used to control the atoms.

From one to two species

The study furthers a longstanding, multi-institutional effort from the German Space Agency at the Germany Aerospace Center (DLR) to transition quantum technologies from highly controlled laboratory environments into space. Previously, the team succeeded in generating the first single-species BECs in space with atoms of ⁸⁷Rb as part of the MAIUS-1 sounding rocket mission in 2017. Future missions will focus on two-species BEC generation and their mixture dynamics by adding ⁴¹K, as well as dual-species atom interferometry.

“By demonstrating that we can perfectly co-locate and control expanding rubidium and potassium mixtures in microgravity, we lay the groundwork for satellite-based tests of the weak equivalence principle,” said Wenzlawski. “This could help physicists search for dark matter or test new gravity models beyond the Standard Model.”

The starting point for most cold-atom or BEC experiments is a magneto-optical trap, which uses a combination of magnetic fields and near-resonant laser light to slow down atoms to microkelvin temperatures. For two species of atoms, the lasers must be able to provide frequencies with a specific offset to the D2 transitions of ⁴¹K (767.7 nm) and ⁸⁷Rb (780.2 nm) simultaneously. However, fitting twice as many lasers, as well as additional optical and electronic components, into a sounding rocket payload is an enormous challenge.

Highest atomic flux

Wenzlawski and his colleagues developed a miniaturized laser system by leveraging a glass–ceramic material with a near-zero coefficient of thermal expansion, named Zerodur, as the substrate for their optical benches. To manage the doubled number of laser paths within the same payload volume, they transitioned to a highly compact and modular architecture of seven specialized Zerodur benches comprising stabilization, overlapping and switching modules.

Lastly, the researchers studied the release of the atoms on the ground and in microgravity, provided by the Einstein-Elevator at Leibniz-University Hannover. The system achieved the highest atomic flux of BEC mixture generation reported to date, outperforming existing mobile systems by an order of magnitude.

“The next scientific step is to study dual-species atom interferometry in microgravity. To achieve this, we will perform extensive campaigns using the Einstein-Elevator in Hannover,” Wenzlawski said. “This platform allows us to systematically investigate the dynamics and interactions of the overlapping mixtures without the distorting effects of gravity.”