Accelerating qubit reset through the Mpemba effect

Passive qubit reset is a key primitive for quantum information processing, whereby qubits are initialized by allowing them to relax to their ground state through natural dissipation, without the need for active control or feedback. However, passive reset occurs on timescales that are much longer than those of gate operations and measurements, making it a significant bottleneck for algorithmic execution. Here, we show that this limitation can be overcome by exploiting the Mpemba effect, originally indicating the faster cooling of hot systems compared to cooler ones. Focusing on the regime where coherence times exceed energy relaxation times ($T_2 > T_1$), we propose a simple protocol based on a single entangling two-qubit gate that converts local single-qubit coherences into fast-decaying global two-qubit coherences. This removes their overlap with the slowest decaying Liouvillian mode and enables a substantially faster relaxation to the ground state. For realistic parameters, we find that our protocol can reduce reset times by up to $50%$ compared to standard passive reset. We analyze the robustness of the protocol under non-Markovian noise, imperfect coherent control and finite temperature, finding that the accelerated reset persists across a broad range of realistic error sources. Finally, we present an experimental implementation of our protocol on an IQM superconducting quantum processor. Our results demonstrate how Mpemba-like accelerated relaxation can be harnessed as a practical tool for fast and accurate qubit initialization.

💡 Research Summary

The paper addresses a fundamental bottleneck in quantum computing: the time required to reset qubits to a known ground state before each algorithmic run. While passive reset—simply waiting for natural relaxation—is simple and hardware‑friendly, its duration is set by the energy relaxation time T₁ and can be dramatically longer than gate times. In modern superconducting devices, coherence times T₂ often exceed T₁, meaning that residual off‑diagonal elements (coherences) persist far longer than populations, potentially contaminating subsequent computations.

The authors propose exploiting the quantum Mpemba effect—where a system initially farther from equilibrium can relax faster—to accelerate passive reset. They focus on the regime T₂ > T₁ and show that, under a Markovian Davies map, the Liouvillian generator separates into a block governing populations (real eigenvalues) and a block governing coherences (complex eigenvalues). When the slowest eigenmode λ₂ is complex, it belongs to the coherence block and dictates the overall equilibration rate. By eliminating the overlap of the initial state with this slow mode, the system relaxes at the rate of the next eigenvalue λ₃, which can be substantially larger.

The key technical insight is that a single entangling two‑qubit gate, applied between the target qubit (q₁) and an incoherent ancilla (q₂), can convert local coherences of q₁ into global two‑qubit coherences. Global coherences are exposed to dissipation on both qubits and therefore decay roughly twice as fast as local ones. The authors derive the condition for complete removal of q₁’s local coherence: the controlled unitary must satisfy κ = Tr