PSI Scientists Generate 2.18-km/s Superthermal Muonium Beam for 1% Gravity Tests

Q: Why did natural helium fail to produce this groundbreaking muonium beam, and what does this reveal about quantum interactions?

Natural helium failed because its small fraction of helium-3 impurities disrupted the superfluid-helium environment that the experiment depends on. The PSI team reported that earlier runs with natural-abundance helium did not produce muonium emission into vacuum, whereas isotopically purified helium did. In this setup, muonium must form inside the superfluid, move quickly and cleanly to the surface, and then be expelled by its positive chemical potential. Even trace isotopic impurities can alter that delicate transport and surface-ejection process. That matters because superfluid helium is not just a cold liquid here; it is a quantum medium. At about 0.2 K, the helium behaves collectively, and the muonium beam quality depends on extremely clean quantum propagation with minimal scattering or trapping. Helium-3 atoms, being a different isotope with different quantum statistics from helium-4, act as impurities in that medium and can change how muonium interacts with the liquid, how fast it diffuses, and whether it survives long enough to escape before the muon decays. The result reveals a broader point about quantum interactions: in ultracold quantum systems, tiny changes in isotopic composition can have outsized effects on particle behavior. The failure of natural helium shows that the beam was not produced by simple classical cooling alone, but by a finely balanced quantum interaction between muonium and an exceptionally pure superfluid helium environment.

2 articles · Updated · Nature.com · Sep 14

A superfluid-helium source at PSI produced a high-brightness muonium beam with a best-fit longitudinal speed of 2,180 m/s and an 8.2% vacuum conversion efficiency from stopped muons.
Time-resolved decay data across four detector layers showed muonium diffusing to the helium surface in about 2 microseconds, and simulations strongly favored a superthermal beam over a thermal source.
The beam’s narrow spread—80 to 150 m/s, with roughly 12 mrad minimum divergence—keeps yields comparable to the best room-temperature aerogel sources while delivering a much tighter velocity distribution.
At projected PSI rates of about 1×10^5 muonium atoms per second, the source could support interferometry measuring muonium’s gravitational acceleration to about 1% in roughly 100 days.
The same compact, slow beam could boost addressable atoms for 1S-2S laser spectroscopy by nearly 10^3 times, opening sub-kilohertz measurements of the muon mass and bound-state QED.

Sources

Nature.com4d ago

PSI Scientists Generate High-Brightness Superthermal Muonium Beam from Superfluid Helium for Gravity and Spectroscopy Experiments

ScienceDaily4h ago

Scientists are about to test Einstein's gravity with exotic matter | ScienceDaily

Why did natural helium fail to produce this groundbreaking muonium beam, and what does this reveal about quantum interactions?

How does a tiny drop of superfluid helium unlock the secrets of gravity for second-generation particles like never before?

Measuring Gravity on Muonium: The 1% Precision Test of the Equivalence Principle with a Superthermal Beam from Superfluid Helium

Overview

In September 2026, researchers at ETH Zurich and PSI achieved a major breakthrough by creating a high-intensity, superthermal muonium beam using superfluid helium cooled to 0.2 Kelvin. By firing positive muons into this quantum fluid, neutral muonium atoms formed and were ejected upward at a uniform speed, thanks to a unique chemical potential. This cold, collimated beam allows the weak gravitational force on second-generation leptonic matter to be measured precisely with an atom interferometer. Planned upgrades will boost muonium yield, enabling not only gravity tests but also high-precision spectroscopy and searches for new fundamental forces.

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