Levitated Nanoparticle Detects Collisions With Single Atoms
A silica nanoparticle levitated by a laser in a vacuum chamber can register collisions with individual gas atoms and molecules, physicists at Yale University and Lawrence Berkeley National Laboratory report in Physical Review Letters. The proof-of-principle experiment suggests the sphere could serve as a tiny pressure sensor, and it grew out of the team's work on new neutrino and dark-matter detectors.
Why background collisions matter
Physicists have proposed that hypothetical particles such as dark matter might be detected when they occasionally scatter from nanometre-sized objects held in optical traps, which use a focused laser beam to hold a particle in place. Alternatively, if a nano-object contains a radioactive substance, the recoil of the object could reveal the momentum of the decay products. Either approach requires a clear picture of the unavoidable background: collisions between the particle and residual gas in the vacuum chamber.
"Usually when an atom collides with an atom it just disappears because the impact is so large that you won't be able to keep it in the trap," explains Yu-Han Tseng of Yale. He adds that this is the dominant reason experiments with atom tweezers have a limited lifetime, since gas keeps colliding with the trapped atoms.
How the experiment worked
Tseng and colleagues in the group of David Moore at Yale trapped a silica nanosphere of radius 50 nm with a focused laser beam inside a vacuum chamber. At the focus of the trap the particle undergoes simple harmonic motion, swinging back and forth like a mass on a spring. The researchers then injected small amounts of different gases, whose atoms or molecules struck the much larger sphere at random.
Without the trap, such impacts would make the sphere wander at random, a familiar effect called Brownian motion, and tracking it would be impossible. In the trap, each collision instead perturbed the sphere's regular oscillation without knocking it loose. The team monitored the motion along the direction of the trapping laser through the phase of backscattered laser light, and from the perturbations detected the impulse of individual collisions.
They tested three gases, krypton, xenon and sulphur hexafluoride. Starting from the highest vacuum they could reach, they gradually raised the pressure to about 1.5 nbar, far below atmospheric pressure of roughly 1 bar. For each gas they took data at seven pressures, recording the rate and size of individual impulses. The rate of impulses rose with pressure and was broadly consistent with what the kinetic theory of gases predicts.
Possible uses
"I think what's quite unique is that, even with a macroscopic object, we can do a very precise measurement," says Tseng. The researchers believe resolving single collisions with a macroscopic object could be useful in a variety of experiments, and potentially as a future pressure standard. Yale colleague Clarke Hardy says the team would be delighted if someone turned it into a workable pressure sensor that could be integrated into another system. The group's own aim lies elsewhere: Hardy says they plan to dope the nanospheres with radioactive isotopes whose decays would let them reconstruct the momentum of emitted neutrinos.
A parallel result from Zurich
Maxime Perdriat of ETH Zurich, lead author of a paper published earlier this year in Physical Review A, and colleagues have also detected single-atom impulses on a nanosphere in an optical tweezer. They compared impulses from a supersonic beam of xenon atoms with thermal impulses from gas flowing into the chamber. Perdriat, who was not involved in the Yale work, called it "an amazing result" because it distinguishes the regime where single atoms can be resolved from the one where there are too many atoms to resolve, which matters for dark-matter and sterile-neutrino searches.
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