This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Typically, after a physical system is disturbed, it can gradually return toward a state of thermal equilibrium by exchanging heat with its surroundings. Thermal equilibrium is a state in which there is no net transfer of heat between the system and its surroundings.
One may assume that the further a disturbed system is from equilibrium, the longer it will take for it to settle. Under some conditions, however, the opposite occurs, and systems that start farther from equilibrium approach it faster than those that start closer. This counterintuitive phenomenon is known as the Mpemba effect.
In some cases, the Mpemba effect occurs because different components of a system's initial disturbance fade at different rates, and if a system's starting state does not contain the slowest-decaying components, it can approach equilibrium faster. This is the main idea behind the so-called strong Mpemba effect. Researchers at Southeast University recently ran an optical experiment that demonstrated this effect in a single qubit, a unit of quantum information that can be in a superposition of two states.
Their observations, outlined in a paper published in Physical Review Letters, offer new insight into how the strong Mpemba effect could shape a system's journey back to equilibrium. "Among the various mechanisms proposed for the Mpemba effect, the 'strong' Mpemba effect offers the conceptually most transparent one: relaxation is exponentially accelerated when the initial state has no overlap with the slowest decay mode," Peng Xue, senior author of the paper, told Phys.org. "However, an obvious problem immediately arises.
This mechanism is commonly believed to require a unique slowest decay mode." A theoretical model introduced by researchers at Trinity College Dublin and Ludwig-Maximilians University Munich in 2024 suggests that in the dynamics described by the Davies map, several modes can share the slowest decay rate, and all these modes involve the loss of quantum coherence. The Davies map is a mathematical rule describing how a quantum system changes over time when interacting weakly with its environment at a fixed temperature. In more technical terms, the theory suggests that for the so-called Davies map, the slowest degenerate modes are purely coherent.
This means that if a qubit starts without coherence between its energy levels, it could skip these slow processes. "Based on this theory, removing coherence from the initial state could restore the effect," said Xue. "Inspired by this, we aimed to demonstrate the mechanism experimentally in the minimal quantum system—a single qubit—and to identify the precise role of quantum coherence." In their experiment, Xue and their colleagues simulated a qubit approaching a state of thermal equilibrium using a single photon (i.e., a particle of light).
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