Photon Experiment Shows a Qubit Farther From Equilibrium Can Settle Faster

A system that starts farther from equilibrium should take longer to settle than one that starts closer. In the Mpemba effect, it does not. Physicists at Southeast University in China now report the first optical demonstration of a genuine quantum version of that paradox in a single qubit, published in Physical Review Letters under the title "Genuine Quantum Strong Mpemba Effect by Eliminating Coherence." The work was led by Xiang Zhan with senior author Peng Xue.

The classic Mpemba effect is the old observation that hot water can sometimes freeze faster than cold. The "strong" version is more specific: relaxation speeds up dramatically when the starting state has no overlap with the slowest-decaying mode of the system.

The experiment tests a 2024 theory from researchers at Trinity College Dublin and Ludwig-Maximilians University Munich. Using a model of a quantum system weakly coupled to an environment at fixed temperature, known as the Davies map, they argued that the slowest decay modes are purely coherent. A qubit that begins without coherence between its energy levels can skip those slow processes.

To test it, the Southeast team encoded a qubit in the polarization of a single photon, with the photon's paths acting as ancillas. They split the Davies map into a purely dissipative step and a rotation built from standard linear optics. Temperature and evolution time were set by wave-plate angles, so no real time elapsed in the setup; the photon simulated the relaxation.

They prepared two starting states: an energy eigenstate with no coherence, and a superposition with coherence. Using quantum state tomography, they tracked each state's nonequilibrium free energy, a measure of distance from equilibrium. The eigenstate started with higher free energy, so it was farther from equilibrium, yet it reached equilibrium faster. To check accuracy, they confirmed that applying the time-t channel twice matched the time-2t channel.

"Removing coherence accelerates relaxation," Xue said in summarizing the result. Coherence also carries free energy beyond the classical populations, which helps explain why it slows things down.

The team plans to extend the work to high-dimensional systems, many-body open systems and mixed initial states. They also point to possible applications in quantum thermal machines and faster cooling, where steering a system through its relaxation path could save time. For now, the result is a clean one-qubit demonstration that preparing the right starting state can beat the clock.

The Mpemba effect is named for Erasto Mpemba, a Tanzanian student who noticed in the 1960s that hot ice-cream mix froze before cold mix. Whether the classical version is real has been debated for decades, which is why a clean quantum test, with every variable controlled, matters to physicists. The new result is not a claim about water; it is a demonstration that the underlying mathematics of shortcuts to equilibrium works in a quantum system.