Miniature devices for trapping ions are common components in atomic clocks and in quantum-computing research. Now, a novel ion-trap geometry demonstrated at the National Institute of Standards and Technology (NIST; Boulder, CO) could usher in a new generation of applications because the device holds promise as a stylus for sensing very small forces, or as an interface for efficient transfer of individual photons for quantum communications.
The "stylus trap," built by physicists from NIST and Germany's University of Erlangen-Nuremberg, uses fairly standard techniques to cool ions with laser light and trap them with electromagnetic fields. But whereas in conventional ion traps the ions are surrounded by the trapping electrodes, in the stylus trap a single ion is captured above the tip of a set of steel electrodes, forming a point-like probe. The open trap geometry allows unprecedented access to the trapped ion (of up to 96% of the 4π solid angle); in addition, the electrodes can be maneuvered close to surfaces. The researchers theoretically modeled and then built several different versions of the trap and characterized them using single magnesium ions.
Far more sensitive than an AFM
The new trap, if used to measure forces with the ion as a stylus-probe tip, is about six orders of magnitude more sensitive (down to an estimated force of 1 x 10-24 N Hz-1/2) than an atomic-force microscope (AFM) using a cantilever as a sensor, because the ion is much lighter in mass and reacts more strongly to small forces. In addition, ions offer combined sensitivity to both 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 kHz and 10 MHz.
The new trap also could be placed in the focus of a parabolic mirror so that light beams could be focused directly on the ion. Under the right conditions, single photons could be transferred 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 be collected with similar efficiency as a readout signal. The new trap also could be used to compare heating rates of different electrode surfaces--a rapid approach to investigating a longstanding problem in the design of ion-trap quantum computers.
Research on the stylus trap was supported by the Intelligence Advanced Research Projects Activity (College Park, MD).
REFERENCE
1. R. Maiwald et al., "Stylus ion trap for enhanced access and sensing," Nature Physics, published online June 28, 2009; doi:10.1038/nphys1311.
