Researchers at the Universities of York, Munich and Cardiff have discovered that all quantum particles exhibit a unique property known as ‘backflow,’ which gives them the ability to move in the opposite direction to the way in which they are being pushed. This is the first time such movement has been observed in a particle where external forces are acting on it. Previously, it had only been seen in ‘free’ quantum particles, where no force is acting on them.

In the study, the team used a combination of analytical and numerical methods to obtain precise estimates about the strength of this phenomenon. They found that the phenomenon applies to all quantum particles, not just free ones. Although the results demonstrate that backflow is always there, researchers say it is a rather small effect, which may explain why it has remained elusive in particles that are not ‘free’.

“This new theoretical analysis into quantum mechanical particles shows that this ‘backflow’ effect is ubiquitous in quantum physics,” says Dr Henning Bostelmann, a researcher in York’s Department of Mathematics. “We have shown that backflow can always occur, even if a force is acting on the quantum particle while it travels. The backflow effect is the result of wave-particle duality and the probabilistic nature of quantum mechanics, and it is already well understood in an idealised case of force-free motion.”

Researchers Discover That Quantum Particles Can Move Opposite To Applied Force [Credit: CC0 Public Domain]

“Forces can of course make a particle go backwards – that is, they can reflect it, and this naturally leads to increased backflow,” explains Dr Gandalf Lechner, a researcher at Cardiff’s School of Mathematics. “But we could show that even in a completely reflection-free medium, backflow occurs. In the presence of reflection, on the other hand, we found that backflow remains a small effect, and estimated its magnitude.”

“The backflow effect in quantum mechanics has been known for quite a while, but it has always been discussed in regards to ‘free’ quantum particles, i.e., no external forces are acting on the particle.” says Dr Daniela Cadamuro, a researcher at the Technical University of Munich. “As ‘free’ quantum particles are an idealised, perhaps unrealistic situation, we have shown that backflow still occurs when external forces are present. This means that external forces don’t destroy the backflow effect, which is an exciting new discovery.”

“These new findings allow us to find out the optimal configuration of a quantum particle that exhibits the maximal amount of backflow, which is important for future experimental verification.”

Researchers say their discovery paves the way for further research into quantum mechanics, and could be applied to future experiments in quantum technology fields such as computer encryption.

The study, entitled “Quantum Backflow And Scattering” has been published in the journal Physical Review A.

Source: University of York