Twente photonics experiment resolves longstanding quantum paradox

05 Jul 2023

Theoretical solution to quantum-thermodynamics dichotomy reinforced with optical chip demonstration.

Convention has it that quantum mechanics and thermodynamics cannot be true simultaneously – so say scientists at the University of Twente (“UT”), The Netherlands.

But now, in a new journal paper, UT researchers use photons in an optical chip to demonstrate how both theories can be true at the same time. The results are described in Nature Communications.

In quantum mechanics, time can be reversed and information is always preserved. That is, one can always “look back” at the previous state of particles, as UT puts it. For a long time it was not known how this could be true at the same time as established thermodynamics laws. In the latter, time has a direction and information can also be lost.

There was already a theoretical solution to this quantum puzzle and even an experiment with atoms, but now the UT researchers have also demonstrated it with photons. “Photons have the advantage that it is quite easy to reverse time with them,” said Dr. Jelmer Renema, who is assistant professor in the Adaptive Quantum Optics research group at UT.

Entanglement of subsystems

In the experiment, the researchers used an optical chip with channels through which the photons can pass. At first, they could determine exactly how many photons there were in each channel, but after that, the photons shuffled positions.

“When we looked at the individual channels, they obeyed the laws of thermodynamics and built up disorder. Based on measurements on one channel, we didn't know how many photons were still in that channel, but the overall system was consistent with quantum mechanics,” said Dr. Renema. The various channels – also known as subsystems – were entangled. The missing information in one subsystem “disappears” into the other subsystem.

Dr. Renema is also one of the featured scientists at the University of Twente. He did the research with a team, including the research group of Prof. Dr. Jens Eisert of the Freie Universität Berlin, who played an important role in demonstrating the reversibility of the experiment.

Paper abstract

The abstract of the Nature Communications paper reads: “One of the core questions of quantum physics is how to reconcile the unitary evolution of quantum states, which is information-preserving and time-reversible, with evolution following the second law of thermodynamics, which, in general, is neither.

“The resolution to this paradox is to recognize that global unitary evolution of a multi-partite quantum state causes the state of local subsystems to evolve towards maximum-entropy states.

“In this work, we experimentally demonstrate this effect in linear quantum optics by simultaneously showing the convergence of local quantum states to a generalized Gibbs ensemble constituting a maximum-entropy state under precisely controlled conditions, while introducing an efficient certification method to demonstrate that the state retains global purity.”

“Our quantum states are manipulated by a programmable integrated quantum photonic processor, which simulates arbitrary non-interacting Hamiltonians, demonstrating the universality of this phenomenon. Our results show the potential of photonic devices for quantum simulations involving non-Gaussian states.”