Physicists say they've found a "cool" way to make atomic-force microscope so sensitive they can detect forces as small as the weight of an individual virus -- literally, by cooling it to more than 400 degrees Fahrenheit below zero.

Scientists at the Research School of Physics and Engineering of the Australian National University in Canberra say the technique uses laser beams to cool microscope's nanowire probe to those frigid temperatures.

Atomic-force microscopes are considered the most desirable tools for measuring tiny forces operating between molecules and studying nanoscale structures.

The work by moving the nanowire probe over the surface to be measured, but the probe -- 500 times thinner than a human hair -- are susceptible to vibrations that can interfere with accurate measurements.

"At room temperature the probe vibrates, just because it is warm, and this can make your measurements noisy," says Ben Buchler, one of the authors of a study present in Nature Communications.

A laser can cool the probe to overcome those vibrations, the researchers in the school's Quantum Optics Group found.

"The level of sensitivity achieved after cooling is accurate enough for us to sense the weight of a large virus that is 100 billion times lighter than a mosquito," Buchler says.

One drawback to the technique is that the microscope's probe can't be utilized when the laser is on, because the laser's energy overwhelms the probe's sensitive measurements.

The solution, the researchers found, is to turn the laser off and make the microscopic measurements in the few milliseconds before the nano-probe returns to room temperature.

Repeating those measurements through a number of heating and subsequent cooling cycles can return an accurate final value, they said.

"If you imagine that during one time period you just observe the motion of the wires, that gives you information that in the next time period you use to [predict] something," says researcher Ping Koy Lam. "By observing the period before, you can predict if there was anything present to cause the wire to behave in a certain way, for example."

That effect can then be subtracted from subsequent measurements to provide an increasingly accurate result, he said.

Atomic-force microscopes have resolution capabilities a thousand times greater than optical microscopes can manage.

There were invented in the 1980s be researchers at IBM Research-Zurich, and brought its developers the Nobel Prize for Physics in 1986.

Atomic-force microscopes became commercially available at the end of that decade.

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