By Geoff Brumfiel
By pushing a cluster of just 60 ions with a tiny electric field, researchers have measured the most minuscule force ever.
The result, measuring mere yoctonewtons (10^-24 newtons), beats previous record lows by several orders of magnitude. The group behind the measurements, based at the National Institute of Standards and Technology in Boulder, Colorado, hopes that the technique can eventually lead to new tools for measuring the minuscule features of materials' surfaces.
Tiny force measurements are crucial for imaging atomic surfaces and detecting nuclear spins, but they are difficult to make because of the tiny dimensions involved.
To date, researchers have successfully measured around an attonewton (10^-18 N) of force by giving small pushes to microscopic paddles or wires and then watching them vibrate. These systems work well, but are limited by factors such as their relatively large size.
The new technique eschews the paddle-type systems in favor of just 60 beryllium-9 ions. The group flattened the ions into a tiny "pancake" and suspended this in mid-air using magnetic fields. They then fired a laser at the ions, lead author Michael Biercuk, now at the University of Sydney in Australia, writes in a paper on the physics preprint server arXiv.org.
By carefully tuning the laser, they extracted energy from the atomic pancake until it reached a temperature of just 0.5 millikelvins.
The team then nudged their pancake with a small electric field. The nudge shook the ions and caused a discernible change in the reflected laser light. On the basis of the size of the change, the team estimates that it has measured a force as small as 174 yoctonewtons--about a thousand times smaller than previous measurements.
Tiny force shunts tiny mass
"What makes it work is that the system is so light," says Chris Monroe, a physicist at the University of Maryland in College Park who was not involved in the research.
Newton's second law of motion states that force is equal to the product of mass and acceleration, so a tiny mass is sensitive to a tiny force. Weighing in at around 0.1 yoctokilograms, 60 beryllium-9 ions make one of the lightest force probes possible.
There is nothing particularly new about the technique, Monroe adds. Clusters of ultracold atoms are already the focus of many studies in their own right. The team's insight was that the ultracold ions would make for supersensitive force detectors. In their paper, the researchers say that even more sensitive detections might be possible with fewer ions.
Monroe says that he agrees in principle, but notes that as the number of ions shrinks, so will the laser signal crucial to the measurement. The team behind the work says that a single ion could detect an even smaller force. True enough, says Monroe, assuming the ion itself can be accurately measured.
Ultimately, the team hopes that beryllium ions could be used as tiny force detectors in all sorts of measurement. "In principle, you could try to use this for fundamental force measurements," says Konrad Lehnert, a researcher at JILA in Boulder, who held the previous measurement record for work using a vibrating wire. In particular, it might be possible to test gravity and quantum effects at ultra-short scales.
But Monroe cautions that the technique should not be oversold. The ions must be kept isolated in a vacuum chamber in order to work, he notes, making actual applications tricky.
"It's not going to be used to find oil tomorrow or anything," he says. But he adds that it may well be possible to develop the atomic pancakes into something more practical.