“Quantum squeezing” a nanoscale particle for the first time

Researchers Mitsuyoshi Kamba, Naoki Hara, and Kiyotaka Aikawa of the University of Tokyo have successfully demonstrated quantum squeezing of the motion of a nanoscale particle, a motion whose uncertainty is smaller than that of quantum mechanical fluctuations. As enhancing the measurement precision of sensors is vital in many modern technologies, the achievement paves the way not only for basic research in fundamental physics but also for applications such as accurate autonomous driving and navigation without a GPS signal. The findings were published in the journal Science.

The physical world at the macroscale, from dust particles to planets, is governed by the laws of classical mechanics discovered by Newton in the 17th century. The physical world at the microscale, atoms and below, is governed by the laws of quantum mechanics, which lead to phenomena generally not observed at the macroscale. One of such phenomena is “uncertainty” in the quantum world: the precision of measurement is inherently limited by quantum mechanical fluctuations. For example, zero-point fluctuation is the quantum mechanical fluctuation of the position and velocity of a trapped particle even when it is at its lowest possible energy state. Quantum squeezing is the generation of a quantum mechanical state whose uncertainty is less than the zero-point fluctuation. Precision measurement of an object with a quantum mechanical limit is vital not only for understanding the natural world accurately but also for designing next-generation technologies that may be affected by quantum phenomena.

“Although quantum mechanics has been successful with microscopic particles, such as photons and atoms, it has not been explored to what extent quantum mechanics is correct at macroscopic scales,” says Aikawa, the principal investigator. “One reason for this is that it has been challenging to prepare an appropriate experimental condition to explore quantum mechanics for large, that is, nanoscale, objects.”

The researchers set out to find a particle that could be used as a platform to investigate quantum phenomena at the nanoscale. They used a nanoscale particle made of glass levitated in a vacuum and cooled it to near the lowest possible energy level to reduce its uncertainty. After making sure its trapping potential was modulated optimally, the researchers released the particle and let it fly for a short time, measuring the velocity just before the release. By repeating this procedure, they obtained the velocity distribution of the particle in this potential.

“When the time before the release is optimal,” explains Aikawa, “the velocity distribution is narrower than the velocity uncertainty of the lowest energy level, which is a signature of quantum squeezing.”

The researchers could finally demonstrate quantum squeezing after a years-long process, as the many technical issues they had faced added fluctuations to the particle. The levitation itself also posed fundamental problems. However, these challenges had not stopped them, and they do not plan on stopping now, either.

“When we found a condition that could be reliably reproduced,” says Aikawa, “we were surprised how sensitive the levitated nanoscale particle was to the fluctuations of its environment. This levitated small particle isolated in a vacuum environment will be an ideal system to explore the transition between quantum mechanics and classical mechanics and to develop new kinds of quantum devices in the future.”

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