Physicists² at the National Institute of Standards and Technology (NIST) have used a small crystal of ions（离子） (electrically charged atoms) to detect forces at the scale of yoctonewtons. Measurements of slight forces—one yoctonewton is equivalent to（等于，相当于） the weight of a single copper³ atom on Earth—can be useful in force microscopy（显微镜检查） , nanoscale science, and tests of fundamental physics theories. A newton is already a small unit: roughly the force of Earth's gravity on a small apple. A yoctonewton is one septillionth（第10的） of a newton (yocto means 23 zeros after the decimal place小数位, or 0.000000000000000000000001).

Measurements of vanishingly（难以觉察地） small forces typically are made with tiny mechanical oscillators（振荡器） , which vibrate like guitar strings⁴. The new NIST sensor⁵, described in Nature Nanotechnology, is even more exotic（异国的，外来的） —a flat crystal of about 60 beryllium ions trapped inside a vacuum chamber⁶ by electromagnetic fields（电磁场） and cooled to 500 millionths of a degree above absolute zero with an ultraviolet laser. The apparatus⁷（装置，设备） was developed over the past 15 years for experiments related to ion plasmas（等离子体） and quantum computing⁸. In this case, it was used to measure yoctonewton-scale forces from an applied⁹ electric field. In particular, the experiment showed that it was possible to measure about 390 yoctonewtons in just one second of measurement time, a rapid speed that indicates the technique's high sensitivity. Sensitivity is an asset（资产，优点） for practical applications.

The previous force measurement record with this level of sensitivity was achieved by another NIST physicist¹ who measured forces 1,000 times larger, or 500 zeptonewtons (0.0000000000000000005 newtons) in one second of measurement time using a mechanical oscillator. Previous NIST research indicated that a single trapped ion could sense forces at yoctonewton scales but did not make calibrated¹⁰ measurements.

The ion sensor described in Nature Nanotechnology works by examining how an applied force affects ion motion, based on changes in laser light reflected off the ions. A small oscillating electric field applied to the crystal causes the ions to rock back and forth¹¹; as the ions rock, the intensity¹² of the reflected laser light wobbles（摇晃，摇摆） in sync with the ion motion. A change in the amount of reflected laser light due to the force is detectable¹³, providing a measure of the ions' induced motion using a principle similar to the one at work in a police officer's radar¹⁴ gun. The technique is highly sensitive because of the low mass of the ions, strong response of charged particles to external electric fields, and ability to detect nanometer-scale changes in ion motion.