Thermodynamic processes cannot be reversed in a quantum system. Nathan Nelson/Flickr

Time moves forward even when it is a quantum state, physicists have confirmed. The concept of irreversibility comes up in thermodynamics a lot – once the thermodynamics of something has been changed, it cannot be restored to its exact original state. But in a quantum system the same physics laws do not apply – at the microscopic level, it's been assumed that time can go in reverse.

Scientists from the Federal University of ABC, Brazil were looking to test this idea and their experiment showed thermodynamic processes cannot be reversed in a quantum system, reports. They published their findings in the journal Physical Review Letters.

In normal life, we only observe time going forwards. But the idea of irreversibility is complicated when it comes to quantum physics because of laws such as the Schrödinger equation, which say things can be reversible.

"Irreversibility is one of the most intriguing concepts in physics," they wrote. "While microscopic physical laws are perfectly reversible, macroscopic average behaviour has a preferred direction of time."

To find out what happens at a quantum level, the scientists did an experiment where they applied a magnetic field pulse so an atoms' nuclear spins flipped. The team said that if the procedure was reversible, they should return to their starting point – their original state. However, they didn't. Instead, they were driven out of an equilibrium. Scientists say this shows the quantum thermodynamic process is not reversible.

But this finding throws up more questions than it answers, the team said. "Our experiment shows the irreversible nature of quantum dynamics but does not pinpoint, experimentally, what causes it at the microscopic level, what determines the onset of the arrow of time," study co-author Mauro Paternostro told the website. "Addressing it would clarify the ultimate reason for its emergence."

"Any progress towards the management of finite-time thermodynamic processes at the quantum level is a step forward towards the realisation of a fully-fledged thermo-machine that can exploit the laws of quantum mechanics to overcome the performance limitations of classical devices. This work shows the implications for reversibility (or lack thereof) of non-equilibrium quantum dynamics. Once we characterise it, we can harness it at the technological level."