Physicists have handled levitating nanoparticles enough as small glass spheres have been suspended in a vacuum and brought close enough to interact with one another. The milestone has opened new routes for probing the difficult borderland between our familiar world and the counterintuitive quantum mechanics that governs atomic-scale things.
Glass nanoparticle suspended in an optical cavity
How Physicists Levitate Nanoparticles
The initial step toward juggling numerous levitated particles was made in the most recent study published in Nature by Deli, Aspelmeyer, and their partners. They split a laser beam in half by reflecting it off a liquid crystal panel within a vacuum chamber. They then utilized an ultrasonic nebulizer, like those used to treat asthma, to inject 200 nanometer-wide glass spheres into the chamber until one was trapped in the focal point of each of the two laser beams.
Because of how quickly the electric fields of the laser oscillate, the optical levitation technique causes electric charges to appear equally quickly at each nanosphere's opposing ends, much like the poles of a bar magnet. This polarization produces a force that pulls the particles in the direction of the light's focal point, which in this case is where the light is most intense.
According to co-author Benjamin Stickler, a theoretical physicist at the University of Duisburg-Essen in Duisburg, Germany, as the polarization rapidly flips back and forth, it behaves like the electric current within an antenna that creates electromagnetic waves.
The researchers were able to move the two focal points closer together by modifying the liquid-crystal panels. Since there are accelerated charges, it generates radiation. The particles started to detect one another's signals at a few micrometers apart, and the researchers were able to get them to vibrate together like weights connected by springs.
The team's ability to turn off the force that one particle applied to the other without also turning off the opposing force from the second particle was made possible by tuning the laser. As a result, artificial physics laws were created that appeared to defy Isaac Newton's third law, according to which there must be an equal and opposite reaction to every action.
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Quantum Physics Research Progress
Stickler says the next step will be to cool both particles to their quantum ground state using laser light. At that point, it may be possible to induce quantum entanglement in the particles, which means that some of their measurable properties (in this case, their positions) are more strongly correlated than would be permitted by the laws of classical, non-quantum physics.
He added that another advantage of the levitation technique is that it should work just as well for trapping more than two particles. Peter Zoller, a theoretical physicist at the University of Innsbruck in Austria, concurs that it is instantly scalable to a much higher number.
When applied to individual atoms or ions, levitation and laser cooling have been described as like a secret sauce in quantum computing. The same could be said for nanoparticles.
Quantum behavior, which is typically only visible at subatomic sizes, is characterized by entanglement. According to Wikipedia, when a set of particles are created, interact, or share spatial proximity in a way that prevents the quantum states of any one of them from being independently described, regardless of how far apart they are from one another, this physical phenomenon known as quantum entanglement takes place.
Since the beginning of time, physicists have argued over whether macroscopic things are subject to a unique set of rules or whether quantum phenomena are simply too difficult to perceive at those scales. This subject is being investigated by several experimental projects that show quantum behavior at ever-larger sizes. The first time this had been done for macroscopic items was when two teams separately entangled pairs of micrometer-scale drums last year.
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