14 July 2009

Geometry brings new sensor possibilities

Miniature devices for trapping ions (electrically charged atoms) are common components in atomic clocks and quantum computing research. But now ion trap geometry could bring a new generation of applications because the device could be a stylus for sensing very small forces or as an interface for efficient transfer of individual light particles for quantum communications.

The “stylus trap” can hold a single ion (electrically charged atom) above any of the three sets of concentric cylinders on the centerline. Source: NIST

The “stylus trap” uses fairly standard techniques to cool ions with laser light and trap them with electromagnetic fields, said researchers at the National Institute of Standards and Technology (NIST) and Germany’s University of Erlangen-Nuremberg. But whereas in conventional ion traps, trapping electrodes surround the ions, in the stylus trap a single ion ends up above the tip of a set of steel electrodes, forming a point-like probe. The open trap geometry allows access to the trapped ion, and the electrodes can maneuver close to surfaces. The researchers theoretically modeled and then built several different versions of the trap and characterized them using single magnesium ions.

The new trap, if used to measure forces with the ion as a stylus probe tip, is about 1 million times more sensitive than an atomic force microscope using a cantilever as a sensor because the ion is lighter in mass and reacts more strongly to small forces. In addition, ions offer combined sensitivity to electric and magnetic fields or other force fields, producing a more versatile sensor than, for example, neutral atoms or quantum dots.

By either scanning the ion trap near a surface or moving a sample near the trap, a user could map out the near-surface electric and magnetic fields. The ion is extremely sensitive to electric fields oscillating at between approximately 100 kilohertz and 10 megahertz.

The new trap also might go in the focus of a parabolic (cone-shaped) mirror so light beams could focus directly on the ion. Under the right conditions, single photons, particles of light, could transfer between an optical fiber and the single ion with close to 95% efficiency. Efficient atom-fiber interfaces are crucial in long-distance quantum key cryptography, the best method known for protecting the privacy of a communications channel. In quantum computing research, fluorescent light emitted by ions could gather with similar efficiency as a read-out signal. The new trap also could compare heating rates of different electrode surfaces, a rapid approach to investigating a long-standing problem in the design of ion-trap quantum computers.

For related information, go to www.isa.org/sensors.