The atomic force microscope is a powerful tool in physics, able to image individual atoms by relying on a tiny probe dragged across a surface. Responding to the repulsive forces from the atoms it encounters, the probe gauges the topographical contours of the surface like a needle tracing the inside of a record groove.

A paper published online recently in Nature Physics proposes what might be the ultimate scale-down of such scanning-probe approaches—using a single atom, suspended in space, as a probe tip. (Scientific American is part of the Nature Publishing Group.)

The researchers, from the National Institute of Standards and Technology (NIST) in Boulder, Colo., and the University of Erlangen–Nuremberg and the Max Planck Institute for the Science of Light in Erlangen, Germany, devised a so-called stylus trap that freezes a magnesium ion, or charged atom, in place with lasers and electromagnetic fields, where it can be used as an ultrasensitive atomic-scale probe.

That probe would be several orders of magnitude more sensitive to electric and magnetic forces than an atomic force microscope, according to the study's authors. Physicists could use such a fine-scale force sensor to identify sources of spurious "noise" in extremely delicate systems, says physicist Christian Ospelkaus of NIST, who did not participate in the research. The manipulation of atoms in experimental setups, including potential approaches to quantum computing, suffers from disruptive rogue electric fields, he says. Identifying which surfaces and assembly processes best curb that noise would be beneficial.

Physicists have harnessed trapped ions by other means, but "more traditional ion traps typically trap the ion in the middle of a set of rods or wafers," limiting their access from outside the trap, Ospelkaus says. Even relatively open surface-electrode traps, which suspend ions dozens of microns above a surface, are only accessible from above the surface plane, he notes.

The stylus trap leaves the atom exposed by levitating it over a central, pillarlike electrode. By elevating the electrode above the rest of the assembly, the level of access can be increased, although that comes at the cost of weakening the strength of the trap, says lead study author Robert Maiwald, a graduate student in the Institute of Optics, Information and Photonics at the University of Erlangen–Nuremberg.

Another implementation of the exposed trap involves placing a parabolic mirror around the ion to focus light onto it, which Maiwald compares to reversing the process by which a flashlight's reflector directs its bulb's glow outward. That is the application that most excites Boris Blinov, a physicist at the University of Washington in Seattle. Using the mirror to collect light on an ion would allow better coupling of matter to light for quantum information processing—photons (particles of light) are good carriers of information, but ions provide a more stable medium for data storage.

In recent experiments, poor photon collection has been a hindrance in establishing entanglement between ions—that is, the coordinated behavior of the ions at a distance—which could form the foundation of quantum computation networks.

"Many groups, including mine, work on...ion–photon entanglement experiments," Blinov says, "and the major limitation is the small collection efficiency of photons." NIST's Ospelkaus is also intrigued by the stylus trap's potential usefulness in quantum information systems. "I can see this being implemented without major roadblocks in the near future," he says. "I think this would be a really big step forward."