Electrons as "quantum triggers"

What happens when you bombard nanoparticles with individual electrons? A research team from the University of Siegen and TU Wien has found a surprising answer to this question - and thus opened up a potential way to investigate quantum phenomena in much larger objects than previously possible. The results have now been published in the renowned journal Physical Review Letters.

"We want to understand why the world we see has a classical effect - even though the fundamental laws of quantum mechanics should actually also apply to large objects," explains Philipp Haslinger from TU Wien. Quantum mechanics describes how particles behave at the smallest level - for example electrons in atoms. They follow rules that contradict our "normal" everyday experience: for example, particles can be in different places at the same time or be "entangled" with each other, even if they are far apart. In theory, these laws should also apply to larger objects. In practice, however, this is difficult to observe: The larger an object is, the more easily its quantum state is disturbed by influences from its surroundings.

In order to detect quantum phenomena in larger objects, the team uses a trick from electron microscopy. Electrons are used in highly sensitive microscopes to make tiny structures visible. The researchers are now proposing to use electrons not only for imaging, but also to put nanoparticles into a quantum state in a targeted manner: When a single electron hits a nanoparticle, it is scattered by the atomic lattice structure of this particle, causing the particle to recoil minimally - an effect that is normally overlooked. The recoil causes a quantum mechanical entanglement between the electron and the particle. The nanoparticle is thus put into a superposition state, as if it were in different places at the same time: a state that has so far only been known from the quantum world.

"You bend the nanoparticle on its own internal structure, so to speak. It doesn't get much smaller than that," says Stefan Nimmrichter, junior professor at the University of Siegen and first author of the study. Due to the tiny structure of its own atomic crystal lattice, quantum effects can be generated in an extremely short time - in hundreds of millionths of a second. This means that disruptive environmental influences have virtually no chance of destroying the effect. A single nanoparticle could therefore be used several times, enabling completely new types of experiments.

"Our concept opens up new possibilities for high-precision experiments with heavy particles. This will allow us to investigate fundamental questions - perhaps even whether gravity itself has quantum mechanical properties," says author Dennis Rätzel.

The planned experiments will take place in specially adapted electron microscopes. These devices can already make individual atoms visible today. In the future, they could do even more: they could be used to directly produce and measure macroscopic quantum states. "We are entering new scientific territory - with a technology that already exists," says Isobel Bicket from TU Wien.