Hot Water Freezes Faster? A Quantum Answer ✛ ✎ ✦

Original: "Tuning the Quantum Mpemba Effect in Isolated System by Initial State Engineering"
arXiv:2505.02040v2 · 2025-05-04 · arXiv non-exclusive · independent analysis · 1 min
Physicists have found a way to control the speed at which quantum systems reach equilibrium.

Hot water sometimes freezes faster than cold — this paradox, called the Mpemba effect, also shows up in the quantum world, where the laws discovered by John von Neumann reign. Only instead of water, we have tiny magnets (spins), whose ordered state dissolves into chaos.

The reason lies in how the environment is prepared. Like sugar in tea: sprinkled in a thin layer, it melts instantly; dropped in a lump, it takes time. The same goes for quantum particles: if the environment is 'scattered' across many states, the main system loses its superposition — the ability to be in many states at once — faster. Scientists confirmed this on a chain of 15 spins linked by quantum entanglement, tracking the disappearance of disequilibrium through measurements. The result: the wider the spread of the environment, the more rapidly entropy — the measure of disorder introduced by Boltzmann — grows.

This discovery is not just a curiosity. It shows how to control quantum decoherence — the process that destroys fragile states and hinders the operation of quantum computers, dreamed up by Feynman. Now engineers will be able to speed up or slow down the loss of properties, preserving quantum information. And the most unexpected conclusion: sometimes, to make a system 'cool down' faster, you need to 'heat it up'.

The classical Mpemba effect was noticed by Aristotle, and in 1963 it was rediscovered by Tanzanian schoolboy Erasto Mpemba while making ice cream.

Controlling the speed of thermal equilibrium echoes the idea of 'slowing down entropy' from Greg Egan's novel 'Permutation City'.

S = k_B \ln \Omega
Entropy is proportional to the logarithm of the number of accessible microstates; a dense bath spectrum means rapid entropy growth and, as a result, acceleration of relaxation.
\Delta S_A = \mathrm{tr}\left[ \rho ( \log\rho - \log\rho_Q ) \right]
Quantum-information measure of deviation from equilibrium; ρ is the subsystem's density matrix, ρ_Q is its fully charge-symmetrized version. The faster this quantity decreases, the sooner the system thermalizes.
Scientists
Daniel GottesmanDavid DiVincenzoGilles BrassardIgnacio CiracJohn PreskillPeter Zoller
Concepts
quantum mpemba effect quantum freezing phenomena thermal equilibrium ground state squeezed state single spin manipulation bell states qubit lifetime feedback error suppression geometric phase control mixed state Berry phase quantum effect thermal state single qubit operation vacuum two level system medicine qubit noise reduction