Physicists at the National Institute of Standards and Technology
(NIST) have used a small crystal of ions (electrically charged atoms) to
detect forces at the scale of yoctonewtons. Measurements of slight
forces—one yoctonewton is equivalent to the weight of a single copper
atom on Earth—can be useful in force microscopy, nanoscale science, and
tests of fundamental physics theories.
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The NIST force sensor is a crystal of ions (charged atoms) trapped Credit: Bollinger/NIST |
A newton is already a small unit: roughly the force of Earth's
gravity on a small apple. A yoctonewton is one septillionth of a newton
(yocto means 23 zeros after the decimal place, or
0.000000000000000000000001).
Measurements of vanishingly small forces typically are made with tiny
mechanical oscillators, which vibrate like guitar strings. The new NIST
sensor, described in Nature Nanotechnology,* is even more
exotic—a flat crystal of about 60 beryllium ions trapped inside a vacuum
chamber by electromagnetic fields and cooled to 500 millionths of a
degree above absolute zero with an ultraviolet laser. The apparatus was
developed over the past 15 years for experiments related to ion plasmas
and quantum computing. In this case, it was used to measure
yoctonewton-scale forces from an applied 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 for
practical applications.
The previous force measurement record with this level of sensitivity
was achieved by another NIST physicist 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 calibrated 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 forth; as the
ions rock, the intensity of the reflected laser light wobbles in sync
with the ion motion. A change in the amount of reflected laser light due
to the force is detectable, providing a measure of the ions' induced
motion using a principle similar to the one at work in a police
officer's radar 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.
The research was funded in part by the Defense Advanced Research
Projects Agency. The first author, M.J. Biercuk, did the work as a
post-doctoral researcher at NIST and is now at the University Sydney in
Australia. Co-author H. Uys did the work as a NIST guest researcher and
has since returned to the Council for Scientific and Industrial
Research, Pretoria, South Africa.
* M.J. Biercuk, H. Uys, J.P. Britton, A.P. VanDevender and J. J.
Bollinger. Ultrasensitive force and displacement detection using trapped
ions. Nature Nanotechnology. Posted online Aug. 22, 2010.
** J.D. Teufel, T. Donner, M.A. Castellanos-Beltran, J.W. Harlow and
K.W. Lehnert. Nanomechanical motion measured with an imprecision below
that at the standard quantum limit. Nature Nanotechnology 4, 820–823.
*** See TechBeat article "NIST Develops Novel Ion Trap for Sensing Force and Light," NIST Tech Beat, June 30, 2009, at www.nist.gov/public_affairs/techbeat/tb2009_0630.htm#trap.
Media Contact: Laura Ost, laura.ost@nist.gov, 303-497-4880
