Lab Detects Single Gas Atoms Kicking a Levitated Nanoparticle, One Collision at a Time

Atoms and molecules are bombarding everything around us all the time. Physicists routinely measure the large-scale result, but the effect of a single collision has been far harder to pin down. Now researchers have done it. In a study published in Physical Review Letters, a team describes detecting the momentum transferred by individual gas particles striking a nanoparticle levitated in laser light.

On a large scale, billions of constant collisions create what people experience as pressure. At high altitude the particles are spread out and pressure is lower than at sea level. The same principle makes pressure rise with depth in liquids. In 1827 Robert Brown noticed pollen grains jiggling in water under a microscope, and Einstein later showed that this random motion comes from constant collisions with fast-moving molecules in the surrounding fluid, now called Brownian motion. Usually the individual impacts are too small and too frequent to tell apart from background noise.

Researchers later realized that laser-trapped nanoparticles offer a way to measure collisions without interference from a supporting surface. In the new experiment, the team isolated a silica sphere about 100 nanometers across in a laser trap inside an ultrahigh-vacuum chamber. They then introduced three heavy gases, krypton, xenon and sulfur hexafluoride, at controlled pressures and tracked the sphere's motion through scattered laser light. They calibrated the sensor against expected values and compared collision measurements with background recordings and theoretical predictions.

The kicks from individual krypton atoms, xenon atoms and sulfur hexafluoride molecules were detectable even though each gas particle is roughly 10 million times lighter than the silica sphere. The measured distribution of momentum transfers agreed well with predictions from gas theory. Pressure estimates built from counting collisions also matched a conventional vacuum gauge within uncertainties, a proof of principle for a new kind of pressure sensor.

The result also addressed an open question. Earlier experiments reported differing amounts of laser heating in levitated silica particles, leaving doubt about their surface temperatures. In this experiment the patterns were consistent with a surface near room temperature, though the researchers could only set upper limits rather than a precise value.

The authors say the work matters for quantum experiments. "Detection of individual gas collisions with a macroscopic mechanical system is directly relevant to ongoing experimental programs studying such systems in the quantum regime," they write. Levitation removes many sources of decoherence, but "uncontrolled information loss due to scattering from background gas molecules remains a fundamental challenge." Theorists have also proposed using single collisions to measure extremely low pressures, and better detection of this motion could even help researchers measure the mass of a neutrino.

More work is needed. Noise obscured some smaller kicks, which limited detection of lighter gases and kept the sensor from resolving every collision. The team expects that better motion measurements could reveal weaker impacts, lighter gas species and lower pressures.