“Levitating” nanoparticles could push the limits of quantum entanglement

Glass particles suspended in laser beams can be made to interact (artist’s view).Credit: Equinox Graphics Ltd.

Physicists suspended tiny glass spheres in a vacuum and made them interact with each other at close range. “Levitating” nanoparticles have now been manipulated with enough precision to open up new avenues for probing the enigmatic twilight zone between the everyday world and the counterintuitive quantum physics that governs objects at the atomic scale.

“This is certainly an important step that opens up new opportunities,” says Romain Quidant, a physicist who conducts similar experiments at the Swiss Federal Institute of Technology (ETH) in Zurich. The results were published on August 25 in Science1. Levitating particles could one day serve as a platform for quantum computing or pave the way for extremely sensitive measuring devices.

laser levitation

Over the past decade, physicists have mastered various techniques for manipulating objects the size of virus particles – a few hundred nanometers in diameter – in a vacuum, including using the slight pressure exerted by laser light.

In 2020, Uroš Delić of the University of Vienna and his collaborators stunned the physics community by slowing down the centers of mass of particles to what physicists call the quantum ground state, as if the particles were as cold as possible.2. Reaching the ground state is the first step towards accessing and manipulating quantum behavior, which is normally only achieved at subatomic scales, and requires objects to be cooled to near absolute zero. Although their centers of mass were in the ground state, the particles continued to be otherwise hot, thermally vibrating and spinning.

Physicist Lia Li recalls the community’s excitement when University of Vienna physicist Markus Aspelmeyer, the lead author of this paper, reported on the quantum ground state at a conference and then published a preprint on the arXiv server. “People were frantic, says Li, who is managing director of engineering firm Zero Point Motion in Bristol, UK. A handful of labs rushed to replicate the results – and some succeeded.

Some physicists, including Giorgio Gratta of Stanford University in California, are working with slightly larger particles – a micrometer in diameter or larger – that have sufficient mass to exert an appreciable gravitational pull. “The main idea is to look for new interactions on the microscopic scale, or departures from Newtonian gravity,” he says.

Two by two

In the last paper, Delić, Aspelmeyer and their collaborators took the first step towards juggling multiple levitating particles. They bounced a laser off a liquid crystal panel inside a vacuum chamber, which split the beam in two. Then they injected 200-nanometer-wide glass spheres into the chamber using an ultrasonic nebulizer, similar to devices used to treat asthma, until a nanosphere was captured at the point. focal length of each of the two laser beams.

This “optical levitation” technique works because the rapid oscillations of the laser’s electric fields induce electrical charges to appear just as quickly at opposite ends of each nanosphere, like the poles of a magnetic bar. This polarization creates a force that pushes the particles toward the regions where the light is most intense — in this case, toward the focal point of the laser beam.

As the polarization rapidly oscillates back and forth, it acts like the electric current inside an antenna that emits electromagnetic waves, explains co-author Benjamin Stickler, a theoretical physicist at the University of Duisburg-Essen at Duisburg, Germany. “Since you have accelerated charges, it emits radiation.” By adjusting the liquid crystal panels, the researchers were able to bring the two focal points closer together. At distances of a few micrometers, the particles began to sense each other’s waves and the researchers were able to make them vibrate in unison, like masses connected by a series of springs.

The laser setting also allowed the team to turn off the force exerted by one particle on the other, without turning off the opposing force of the second particle. This produced “artificial” laws of physics that seemed to violate Isaac Newton’s third law that for every action there is an equal and opposite reaction.

quantum leap

Stickler says the next task will be to use laser light to cool the two particles to their quantum ground state. At this point, it might become possible to put the particles into a state of quantum entanglement, which means that some of their measurable properties – in this case, their positions – are more strongly correlated than the laws of nonsense would allow. – classic entanglement. quantum physics.

Entanglement is a feature of quantum behavior, which is usually observed only at subatomic scales. Physicists have long debated whether macroscopic objects are governed by their own set of laws or whether quantum effects are simply too difficult to observe at these scales. A number of experimental efforts are probing this question by demonstrating quantum behavior at increasingly larger scales. Last year, two teams independently put pairs of micrometer-scale drums into an entangled state – the first time this had been done for macroscopic objects.

But the researchers say such “restrained” objects pose limitations: They are physically connected to a device, making it difficult to keep delicate quantum states from being disrupted. With this in mind, Peter Zoller, a theoretical physicist at the University of Innsbruck in Austria, and others first considered using levitating nanoparticles for quantum experiments in 2010.35. “You might even think of a nanoparticle as a little computer that you can control with laser light and move around,” says Zoller.

Another benefit of the levitation technique is that it should work just as well for trapping more than two particles, Stickler adds. Zoller agrees. “It’s immediately scalable to a much larger number,” he says.

When applied to individual atoms or ions, laser levitation and cooling has been “like a secret sauce in quantum computing,” says Zoller. The same could happen with nanoparticles.

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