Time can flow backwards in quantum systems

Time can flow backwards in quantum systems

Technology News |
The boundary between time moving forward and backward – and even doing both at the same time – may be blurred in quantum mechanical systems, according to a team of physicists drawn from Bristol, Vienna and the Balearic Islands.
By Peter Clarke

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A study published in Nature Communications Physics has shown how quantum systems can evolve simultaneously along two opposite time arrows – both forward and backward in time, n manner similar to the superposition of states. This necessitates a rethinking of how the flow of time is understood and represented in contexts where quantum mechanics play a crucial role.

It has long been understood that time flows in one direction, that of increasing entropy which is equivalent to more disorder. And the second law of thermodynamics states that entropy can never decrease.

“If a phenomenon produces a large amount of entropy, observing its time-reversal is so improbable as to become essentially impossible,” said Giulia Rubino from the University of Bristol’s Quantum Engineering Technology Labs (QET labs) and lead-author of the publication. “However, when the entropy produced is small enough, there is a non-negligible probability of seeing the time-reversal of a phenomenon occur naturally.”

Rubino used the idea of a tube of toothpaste to illustrate the underlying concept.

“If we were shown our toothpaste moving from the toothbrush back into its tube, we would be in no doubt it was a rewinding recording of our day. However, if we squeezed the tube gently so only a small part of the toothpaste came out, it would not be so unlikely to observe it re-entering the tube, sucked in by the tube’s decompression.”

The authors of the study, under the lead of Professor Caslav Brukner of the University of Vienna and the Institute for Quantum Optics and Quantum Information in Vienna, applied this idea to the quantum realm.

According to the principle of quantum superposition if two states of a quantum system are both possible, then that system can also be in both states at the same time.

Next: Putting the toothpaste back in the tube


“Extending this principle to time’s arrows, it results that with quantum systems evolving in one or other temporal direction – the toothpaste coming out of or going back into the tube – can also find themselves evolving simultaneously along both temporal directions,” said Rubino. “Although this idea seems rather nonsensical when applied to our day-to-day experience, at its most fundamental level, the laws of the universe are based on quantum-mechanical principles. This begs the question of why we never encounter these superpositions of time flows in nature.”

As part of the work the team quantified the entropy produced by a system evolving in quantum superposition of processes with opposite time arrows. It was found that when small amounts of entropy are involved then the consequences of the system having evolved along the forward and backward temporal directions can both be observed.

Aside from the fundamental feature that time itself might not be well-defined, the work also has practical implications in quantum thermodynamics. Placing a quantum system in a superposition of alternative time’s arrows could offer advantages in the performance of thermal machines and refrigerators.

Rubino said: “Although time is often treated as a continuously increasing parameter, our study shows the laws governing its flow in quantum mechanical contexts are much more complex. This may suggest that we need to rethink the way we represent this quantity in all those contexts where quantum laws play a crucial role.”

Paper: ‘Quantum superposition of thermodynamic evolutions with opposing time’s arrows’ by Rubino et al. in Nature Communications Physics

Related links and articles:

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