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A bowl of trapped ions have been used to measure the smallest forces ever recorded, report scientists.

The discovery opens the door to solving unanswered questions in materials science areas such as computing and nanotechnology.

Reporting in the pre-publication blog ArXiv, Professor Michael Biercuk of the University of Sydney and colleagues from the National Institute of Standards and Technology in Colorado have measured forces as small as 174 yoctonewtons, beating the previous best measure by three orders of magnitude.

Yocto is the smallest prefix recognised by the International System of Units (SI), which was first developed in the 18th century.
Shrinking units

The initial list of prefixes ranged from mega (one million) down to milli (one thousandth).

In 1960, additional prefixes were introduced, such as giga (billion), tera (trillion), nano (billionth) and pico (trillionth). The prefix yocto (septillionth or 10 to the power of minus 24) was added in 1991.

The new record, may force the listing of a new prefix for an octillionth, or 10 to the power of minus 27.

The device developed by Biercuk and colleagues consists of a few dozen beryllium ions in a device called a Penning trap. Any movement caused by an applied force are measured by a laser.

Dr Peter Fisk, general manager of the Physical Metrology Branch at the National Measurement Institute in Sydney, says the trap uses very cold ions held by an electro-magnetic field.

"They need to be cold so motion due to thermal effect (such as room temperature) is completely removed leaving them extremely still.

"In this state the ions, being charged, are highly susceptible to the effects of stray magnetic and electric fields responding to very small forces which cause them to vibrate."
Bowl of marbles

Fisk says it's a lot like marbles in a bowl.

"They'll normally sit in the bottom of the bowl until you apply a force causing them to roll back and forth.

"In the same way, the atoms will normally line up in the centre of the trap, getting displaced from the centre by an applied force.

"It's this motion which is detected by bouncing a laser off the ions and measuring any doppler shift they cause. The reflected laser light will be a slightly higher frequency as the ion moves towards you compared to when it's moving away.

"It turns out, this system can measure forces from these stray fields that are only a few dozen yoctonewtons in strength."

The most sensitive force detectors until now have been resonating spring boards that are limited to attonewtons, or 10 to the power of minus 18 newtons.

Biercuk and colleagues achieved their 174 yoctonewtons measurement using a trap holding about 60 beryllium ions.

The researchers hope to eventually measure a single yoctonewton using an individual beryllium ion.

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