
In physics, a quantum phase transition (QPT) is a phase transition between different quantum phases (phases of matter at zero temperature). Contrary to classical phase transitions, quantum phase transitions can only be accessed by varying a physical parameter  such as magnetic field or pressure  at absolute zero temperature. The transition describes an abrupt change in the ground state of a manybody system due to its quantum fluctuations. Such quantum phase transitions can be firstorder phase transition or continuous. To understand quantum phase transitions, it is useful to contrast them to classical phase transitions (CPT) (also called thermal phase transitions). A CPT describes a discontinuity in the thermodynamic properties of a system. It signals a reorganization of the particles; A typical example is the freezing transition of water describing the transition between liquid and ice. The classical phase transitions are driven by a competition between the energy of a system and the entropy of its thermal fluctuations. A classical system does not have entropy at zero temperature and therefore no phase transition can occur. In contrast, even at zero temperature a quantummechanical system has quantum fluctuations and therefore can still support phase transitions. As a physical parameter is varied, quantum fluctuations can drive a phase transition into a different phase of matter. A canonical quantum phase transition is the wellstudied superconductor/insulator transition in disordered thin films which separates two quantum phases having different symmetries. Quantum magnets provide another example of QPT. References S. Sachdev. Quantum Phase Transitions. Cambridge University Press (2000) Quantum phase transitions, Subir Sachdev Retrieved from "http://en.wikipedia.org/" 
