A single photon, guided through an optical setup at Southeast University in Nanjing, has helped show how a quantum system can sometimes reach equilibrium faster when it starts farther away from it. The experiment demonstrated the strong Mpemba effect in a single qubit, the smallest unit of quantum information.
Usually, a disturbed system gradually settles as it exchanges heat with its surroundings. It is natural to expect that a system starting farther from equilibrium will take longer to settle. The Mpemba effect turns that expectation around: under some conditions, a system that begins farther away gets there sooner. In the strong version, the speed-up happens when the starting state avoids the slowest processes that would otherwise hold it back.
The Southeast University team focused on quantum coherence: the links between different possible energy states in a quantum system. A theoretical model published in 2024 by researchers at Trinity College Dublin and Ludwig-Maximilians University Munich suggested that, in a particular description of quantum relaxation called the Davies map, several slowest-decaying modes can share the same rate. These modes involve the loss of coherence. A qubit that begins without coherence between its energy levels may therefore avoid those slow modes and move toward equilibrium more quickly.
To test the idea, Peng Xue and colleagues used one photon to simulate a qubit. They encoded the qubit in the photon’s polarization, which describes how its light oscillates. The different paths available to the photon acted as ancillas—extra parts of the setup used to model the qubit’s interaction with its surroundings.
The researchers represented the Davies map as a dissipative step followed by a simple rotation, using standard optical components. They set the simulated temperature and evolution time by changing the angles of wave plates. That meant they could study the relaxation by rotating the plates, without rebuilding the optical setup each time.
The team compared two initial states. One was an energy eigenstate, with a definite energy and no coherence between energy levels. The other was a superposition, in which the qubit occupied a combination of two energy states and did have coherence. The first could bypass the slowest part of the relaxation; the second remained subject to it. The researchers tracked both states with quantum state tomography, a method for measuring a quantum state, and followed their nonequilibrium free energies. They also checked that their optical simulator followed the expected Davies-map evolution over time.
The findings, reported in Physical Review Letters, offer an experimental way to examine how coherence shapes a quantum system’s return to equilibrium. They also test an important part of the proposed mechanism in the simplest quantum setting: one qubit. The result does not mean that every system with less coherence will settle faster. Rather, it shows how a system’s starting state can determine whether particular slow processes affect its path. Future work can use this kind of controlled optical platform to explore that relationship in greater detail.