Super-cool nanoparticle bridges quantum and classical worlds

In our macroscopic world, we aren’t usually concerned about the strange consequences of quantum mechanics – we assume they only affect the smallest of particles such as protons and electrons. However, recent laser cooling techniques are enabling scientists to trap ever larger systems in their lowest quantum states, where at least one macroscopic property, such as mechanical motion or electrical conductivity, can only be described using quantum mechanics rather than classical physics. Researchers hope that such ‘macroscopic quantum states’ can be used as high-fidelity sensors.

Now a team, led by Lukas Novotny at ETH Zürich in Switzerland and including René Reimann at the Technology Innovation Institute in Abu Dhabi, UAE, has reported a new achievement in cooling a relatively large nanoparticle to such low temperatures that there is no thermal motion to mask its quantum nature. Their work, published in Nature Physics, describes how they cooled a silica nanoparticle, 100 million times the mass of a water molecule, so that not one but two of the nanoparticle’s mechanical modes of motion were in their quantum ground states.

The experimental setup used by Novotny and co-workers involves trapping the nanoparticle between powerful lasers in a high vacuum chamber. The nanoparticle cools down by scattering light, which is picked up by photodetectors to measure the particle’s quantum state.

“A nanoparticle levitated in a strongly focused light beam, known as an optical tweezer, has peculiar characteristics: it is a naturally three-dimensional system,” explains Francesco Marin, an expert in quantum optomechanics from the University of Florence, Italy, who was not involved in the Novotny study. “Since 2020, the oscillatory motion of silica nanospheres - tiny grains of sand - has been frozen down to the quantum ground state in only one direction at a time: along the tweezer axis exploiting electrical feedback, and along an axis in the transverse plane using a technique imported from atomic physics. In this work, for the first time, the authors succeeded in ground-state cooling the full two-dimensional planar motion.” 

Achieving such control over the planar motion implies that Novotny and co-workers also had control over the nanoparticle’s orbital angular momentum. This could allow them to realise a highly sensitive theoretical device called a quantum gyroscope, which could detect effects such as tiny changes in the Earth’s rotation.

“This result paves the way for the complete three-dimensional localization of a levitated nanoparticle at the quantum limit,” says Marin, “or even more intriguing experiments investigating the evolution of the wavefunction of a macroscopic system, and the transition from quantum to classical worlds.”

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