Experiments with optical tweezers race to test the laws of quantum mechanics

One might think that the optical tweezer – a focused laser beam that can trap small particles – is old hat by now. After all, the tweezer was invented by Arthur Ashkin in 1970. And he received the Nobel Prize for it this year – presumably after its main implications had been realized during the last half-century.

Amazingly, this is far from true. The optical tweezer is revealing new capabilities while helping scientists understand quantum mechanics, the theory that explains nature in terms of subatomic particles.

This theory has led to some weird and counterintuitive conclusions. One of them is that quantum mechanics allows for a single object to exist in two different states of reality at the same time. For example, quantum physics allows a body to be at two different locations in space simultaneously – or both dead and alive, as in the famous thought experiment of Schrödinger’s cat.

The two states of Schrodinger’s cat: dead (on the left) and alive (on the right). Quantum physics says the cat can exist in both states simultaneously. Rhoeo / Shutterstock.com
The technical name for this phenomenon is superposition. Superpositions have been observed for tiny objects like single atoms. But clearly, we never see a superposition in our everyday lives. For example, we do not see a cup of coffee in two locations at the same time.

To explain this observation, theoretical physicists have suggested that for large objects – even for nanoparticles containing about a billion atoms –superpositions collapse quickly to one or the other of the two possibilities, due to a breakdown of standard quantum mechanics. For larger objects the rate of collapse is faster. For Schrodinger’s cat, this collapse – to “alive” or “dead” – would be practically instantaneous, explaining why we never see the superposition of a cat being in two states at once.

Until recently, these “collapse theories,” which would require modifications of textbook quantum mechanics, could not be tested, as it is difficult to prepare a large object in a superposition. This is because larger objects interact more with their surroundings than atoms or subatomic particles – which leads to leaks in heat that destroys quantum states.

As physicists, we are interested in collapse theories because we would like to understand quantum physics better, and specifically because there are theoretical indications that the collapse could be due to gravitational effects. A connection between quantum physics and gravity would be exciting to find, since all of physics rests on these two theories, and their unified description – the so-called Theory of Everything – is one of the grand goals of modern science.

Enter the optical tweezer

Optical tweezers exploit the fact that light can exert pressure on matter. Although the radiation pressure from even an intense laser beam is quite small, Ashkin was the first person to show that it was large enough to support a nanoparticle, countering gravity, effectively levitating it.

In 2010 a group of researchers realized that such a nanoparticle held by an optic tweezer was well-isolated from its environment, since it was not in contact with any material support. Following these ideas, several groups suggested ways to create and observe superpositions of a nanoparticle at two distinct spatial locations.