A Free-Fall Tower in Hannover Made Two Different Bose-Einstein Condensates at Once, Ten Times Faster Than Any Portable Rig

An international team has produced mixtures of two different ultracold atomic gases at an unprecedented particle flux, using an apparatus dropped repeatedly inside a free-fall facility in Germany. The results, published in Nature Communications, clear a technical path toward testing one of the foundations of general relativity from orbit.

The experiments used the MAIUS-B apparatus in the Einstein Elevator at Leibniz University Hannover, a facility that produces brief periods of microgravity on demand. Inside it, the team generated Bose-Einstein condensates — the so-called fifth state of matter, in which atoms cooled to near absolute zero collapse into a single collective quantum state that makes quantum behavior visible at a macroscopic scale — out of two different atomic species at once, rubidium and potassium.

Doing it with one species is hard but established; a BEC was first created in space during the MAIUS-1 mission in 2017. Doing it with two simultaneously is a different problem. Each species needs its own laser cooling and trapping scheme, which normally means roughly twice the optical hardware — a serious constraint for an instrument that has to survive a rocket launch and fit in a payload bay.

The solution came from a group at Johannes Gutenberg University Mainz led by professor Patrick Windpassinger and Dr. André Wenzlawski, working with Humboldt-Universität zu Berlin and the Ferdinand-Braun-Institut. They built a miniaturized laser system that carries twice as many lasers plus the extra optical and electronic components while occupying almost the same volume and mass as the single-species version. "Our task was to develop the optical interfaces between the laser modules and the vacuum system that are essential for cooling and manipulating the atoms," Wenzlawski explained.

The central component is a set of optical benches, developed jointly by Mainz and the University of Hamburg, that form the interface between the laser modules and the vacuum system. They are built from Zerodur, a glass-ceramic with an exceptionally low coefficient of thermal expansion. "This stability is crucial for maintaining precise control of the atoms under the extreme mechanical loads of a rocket launch and varying temperature conditions," Wenzlawski said. Long-term operation in both the Einstein Elevator and conventional laboratory settings validated the design. The system reached the highest atomic flux reported for a dual-species BEC mixture, outperforming existing mobile systems by an order of magnitude.

The point of all this engineering is a specific measurement. Einstein's equivalence principle holds that all objects fall at the same rate regardless of composition — the reason a hammer and a feather hit the lunar surface together in a vacuum. Atom interferometry can test that claim to extraordinary precision by dropping two different atomic species side by side and comparing their accelerations, and a condensate makes the measurement far cleaner than a warm gas. Microgravity extends the free-fall time, which is what sets the sensitivity.

The technology developed in Mainz is intended as the foundation for BECCAL, a German-American atom laboratory planned for the International Space Station. Instruments of that class would let physicists check whether rubidium and potassium fall at exactly the same rate to a precision unreachable on the ground — and any deviation, however small, would be a crack in the theory that has held for over a century. The publication is Baptist Piest et al., "Apparatus for quantum-mixture research in microgravity," Nature Communications (2026).