Optical experiment demonstrates the strong Mpemba effect in a single qubit

Optical Demonstration of the Strong Quantum Mpemba Effect

Researchers at Southeast University have provided the first experimental evidence that a single qubit can relax to thermal equilibrium faster when it starts farther from equilibrium—a phenomenon known as the strong Mpemba effect. Their work, published in Physical Review Letters (2026), uses linear‑optical techniques to simulate the qubit’s interaction with a thermal bath via the Davies map.

From Theory to Photonic Simulation

The strong Mpemba effect predicts an exponential acceleration of relaxation when the initial state lacks overlap with the system’s slowest decay mode. A 2024 theoretical study by teams at Trinity College Dublin and LMU Munich showed that, for the Davies map, the slowest modes are purely coherent. Removing coherence from the qubit’s initial state should therefore bypass these slow processes.

To test this, the Southeast University team encoded a qubit in the polarization of a single photon. Ancillary optical paths acted as the environment, and wave‑plate angles directly set the effective temperature and evolution time. By rotating these plates, the researchers could scan the simulated relaxation without reconfiguring the circuit.

Two Contrasting Initial States

• Energy‑eigenstate (no coherence): The photon’s polarization was aligned with an energy eigenstate, giving the qubit a definite energy but no superposition. This state started farther from equilibrium in terms of nonequilibrium free energy.
• Coherent superposition: The polarization was set to a superposition of the two energy levels, retaining quantum coherence.

Quantum state tomography tracked the evolution of each state’s density matrix and nonequilibrium free energy. The eigenstate relaxed noticeably faster, confirming that eliminating coherence accelerates thermalization—a clear signature of the strong Mpemba effect.

Verifying the Simulated Dynamics

The authors validated their optical simulator by exploiting the semigroup property of the Davies map: applying the simulated channel for time t twice produced the same result as a single application for time 2t. This check ensured that the linear‑optics implementation faithfully reproduced the intended open‑system dynamics.

Implications and Future Directions

Beyond confirming a subtle thermodynamic paradox, the experiment opens pathways for:

• Designing quantum thermal machines that exploit accelerated cooling.
• Developing photonic platforms for broader studies of open‑system quantum thermodynamics.
• Extending the approach to higher‑dimensional systems, many‑body lattices, and mixed initial states.

The authors plan to scale the technique to more complex configurations, potentially informing new strategies for quantum state preparation and control.

--- The article is based on the Phys.org news release and the cited Physical Review Letters paper (Xiang Zhan et al., 2026). All interpretations are drawn from the provided text.