 Quantum phase transition

A quantum phase transition is a change of the ground state as a nonthermal control parameter is varied at zero temperature. Pressure, interaction strength or an applied magnetic field can tune the competing contributions to the Hamiltonian. The transition is driven by changes in the quantum ground state, whereas a thermal phase transition is driven by thermal occupation and entropy.

The Transverse-field Ising model illustrates the competition between aligned-spin order and a field favoring a different spin direction. A continuous transition has a quantum critical point, with a closing excitation gap and growing correlation scales; a quantum phase transition can also be first order. Experiments operate at finite temperature, where the nearby quantum-critical regime can reveal the influence of the zero-temperature transition without placing the sample exactly at zero.

Quantum phase transition refers to a fundamental change in the state of matter that occurs at absolute zero temperature (0 K) due to quantum mechanical effects rather than thermal fluctuations, which are more common in classical phase transitions. Unlike classical phase transitions, which occur as a system is heated or cooled and are often driven by changes in temperature and pressure (like the melting of ice to water), quantum phase transitions are induced by changes in external parameters such as magnetic fields, pressure, or chemical composition.