Trapped ion force sensing in three papers

Short answer: A trapped atomic ion behaves like a very light, very low-loss mechanical oscillator, so a tiny electric force pushes it by a measurable amount. Three papers report using ion motion for force sensing, with sensitivities in the zeptonewton and yoctonewton range per root hertz. These are laboratory measurements, and one of the three is an arXiv preprint.

Why can an ion sense forces?

The 2017 Nature Communications paper explains the principle. Most force sensors estimate force from the displacement of a mechanical oscillator, and the best sensitivity is typically reached when the force alternates at the oscillator's resonance, because the response is boosted by the quality factor. The authors say trapped atomic ions can be approximated as harmonic mechanical oscillators that are highly sensitive to electrical forces, and that the extremely high quality factor of ion traps lets even very small resonant forces produce relatively large displacements. The displacement is usually read out through the Doppler shift of an optical transition.

What did the Penning-trap experiment report?

A publication mirrored on the exa.ai library page (the venue was not confirmed on that page, so treat it as unconfirmed) describes force detection with crystals of 60 beryllium-9 ions in a Penning trap. The authors state that normal motional modes are excited by externally applied electric fields and detected through phase-coherent Doppler velocimetry, which can discriminate ion motion with amplitudes on the scale of nanometres. They report detecting forces as small as 174 yoctonewtons, with a sensitivity of 390 plus or minus 150 yoctonewtons per root hertz, and say this is more than three orders of magnitude better than existing reports. They add that their single-ion-limit results validate proposals that trapped ions could approach 1 yoctonewton per root hertz. The abstract also expresses an expectation that the demonstration will motivate a new class of ion-based sensors. That is an expectation, and no sensor product is described.

What did the single-ion experiment report?

The 2017 Nature Communications paper uses a single trapped strontium-88 ion as a force sensor, electrically driven at a frequency much lower than the trap resonance. Low-frequency forces below resonance are harder to measure, the authors note, because the oscillation amplitudes are much smaller. They measure the small motion through the periodic Doppler shift of an atomic optical clock transition, enhanced with the quantum lock-in technique, and report a force detection sensitivity as low as 2.8 x 10^-20 newtons per root hertz.

What does the preprint add?

An arXiv preprint from 2017, which may not have been peer reviewed, studies one and two laser-cooled calcium-40 ions in a trap. The authors drive the axial motion with either a harmonic dipolar field or white noise. The dipolar field gives a coherent oscillation, while the noise gives an enhanced Brownian motion through an additional contribution to the heating rate. They evaluate and compare the sensitivity of an isolated ion and a two-ion crystal. They say understanding this dynamics matters for a planned Penning-trap mass-spectroscopy technique based on optical detection, aiming to improve precision and sensitivity. The abstract in the text we read gives no sensitivity figures.

How to read this

The three experiments use different ions, traps and readout methods, so their sensitivities should not be ranked against each other without reading the papers. All numbers are the authors' own. This post is based on abstracts and visible introductions and full texts were not checked. It makes no claim about products or field use.

FAQ

What is the smallest force reported here?

174 yoctonewtons, in the Penning-trap experiment described on the mirrored page.

What is a yoctonewton?

10^-24 newtons, as defined in that abstract.

Why use a clock transition?

In the 2017 paper, the clock transition's Doppler shift reveals the ion's tiny motion, helped by the quantum lock-in technique.

Is this a preprint?

One of the three papers is an arXiv preprint. The mirrored paper's venue was not confirmed.