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Journal Club: New ability to measure the recoil from single photons could help address fundamental physics mysteries

The recoil of individual photons has been measured directly for the first time in a laser trap. Credit: Physical Review Letters/American Physical Society

The recoil of individual photons has been measured directly for the first time in a laser trap. Credit: Physical Review Letters/American Physical Society

Scientists have for the first time directly measured the recoil that single photons exert on levitating nanoparticles in a vacuum. The work that could help lead to more sensitive measurements of ultra-weak forces, according to research detailed online June 13 in Physical Review Letters.

The ability to measure ultra-weak forces, such as the gravitational pull between electrons, could help answer major questions in physics. For example, some theoreticians question Newton’s inverse-square law of gravitation, hypothesizing that additional factors may come into play at very short distances between masses. “We may need very sensitive detectors to pick up such forces,” says study senior author Lukas Novotny, a physicist and optical engineer at the Swiss Federal Institute of Technology in Zürich.

Observing single photon recoil was a multi-step process. First, the researchers levitated a silica sphere, in the range of 100 to 140 nanometers in diameter, in a vacuum using an “optical tweezers” setup, which traps and controls particles through the forces exerted by light on the sphere. Researchers could then probe the position or motion of the nanoparticle sphere by measuring the properties of a laser beam that interacted with the nanoparticle.

The scientists used near-infrared laser beams to cool the nanoparticle to a temperature of roughly a half-thousandth of a degree above absolute zero. They monitored their lasers to measure the motions of the nanoparticle, then varied laser strength to reduce its jittering and thus temperature.

When the researchers stopped cooling the nanoparticle, they estimated that photons from the lasers recoiled from the particle at rates of about 20,000 to 29,000 times per second, findings that match theoretical predictions based on the sizes of the particle.

The key to this advance was the use of ultra-high vacuum, says Novotny. This significantly reduced jostling of the nanoparticle from any surrounding gas molecules, helping researchers look at only the kicks that photons imparted on the particle.

“It’s certainly a significant result—when it came to individual atoms, kicks of photons were known to cause particles to jitter around, but this is the first time that this has really been demonstrated with optically levitated particles of larger sizes,” says physicist Andrew Geraci at the University of Nevada, Reno, who did not take part in this research.

In the future, such a setup could help answer an enduring mystery: the point at which the classical physics that describes the world at large ends and the quantum physics that describes the universe at its smallest level begins, Novotny suggests. “The playground of quantum mechanics is usually examined from its smaller end, that of atoms and ions — you scale up these systems until classical physics takes over,” Novotny says. “Here we can go from the top down, starting with a relatively massive body made of many atoms that behaves classically and seeing when quantum mechanics appears.”

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