Physicists caused the levitation of the glass nanosphere by moving it into the realm of quantum mechanics

Quantum mechanics deals with the behavior of the universe on ultra-small scales: atoms and subatomic particles act in ways that classical physics cannot explain. To explore this contradiction between the quantum and the classical, scientists try to make all larger objects behave in a quantum-like way.

In this particular study, we are talking about a tiny glass nanosphere 100 nanometers in diameter – about a thousand times smaller than the thickness of a human hair. In our view, this is very, very small, but in terms of quantum physics, it is actually quite huge and consists of 10 million atoms.

To translate such a nanosphere into the realm of quantum mechanics is a huge achievement, but that’s exactly what physicists have now achieved.

Using carefully calibrated laser beams, the nanosphere was suspended in the lowest quantum mechanical state, an extremely limited motion in which quantum behavior can begin.

“This is the first time that such a method has been used to control the quantum state of a macroscopic object in free space,” says Lukas Novotny, professor of photonics at ETH Zurich in Switzerland.

To achieve a quantum state, motion and energy must be minimized. Novotny and his colleagues used a vacuum container cooled to -269 degrees Celsius (-452 degrees Fahrenheit) and then used a feedback system to make further adjustments.

Using interference patterns created by two laser beams, the researchers calculated the exact position of the nanosphere in the chamber and then, using the electric field created by the two electrodes, made the fine-tuning needed to bring the object’s motion down to zero.

This is not much different from slowing down the swing on the playground, when it is pushed and pulled until it comes to a state of rest. Once this lowest quantum mechanical state is reached, further experiments can proceed.

“In order to clearly see quantum effects, the nanosphere must be slowed down… down to its moving ground state,” says electrical engineer Felix Tebbenyohannes of ETH Zurich.

“This means that we freeze the energy of sphere motion to a minimum that is close to quantum mechanical zero motion.”

Although similar results have been achieved before, they used a so-called optical resonator to balance objects with light.

The approach used here better protects the nanosphere from extraneous influences and allows the object to be observed in isolation after the laser is turned off – although much further research will be needed to realize this.

The researchers hope their results may be useful in studying how quantum mechanics makes elementary particles behave like waves. It’s possible that ultra-sensitive installations like this nanosphere could also help develop the next generation of sensors beyond anything we have today.

The ability to levitate such a large sphere in a cryogenic environment represents a significant leap to the macroscopic scale, where the line between classical and quantum can be explored.

“Together with the fact that the potential of the optical trap is well controlled, our experimental platform offers a way to explore quantum mechanics at macroscopic scales,” the researchers conclude in their paper.

The study was published in the journal Nature.

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