In physics, phase transitions are typical everyday occurrences, for example, melting: when a solid is heated, its atoms travel more freely, and the solid abruptly turns into a liquid. In contrast, quantum phase transitions cannot be seen in daily lives and pose a number of questions for science.
Currently, Professor Sabine Andergassen and Dominik Maile at the University of Tübingen, working with Professor Wolfgang Belzig and Dr. Gianluca Rastelli at the University of Konstanz have applied theoretical calculations to examine what mechanisms and effects define the physical behavior in the proximity of a quantum phase transition. They discovered odd behavior associated with Heisenberg's uncertainty principle—and which could influence data processing in a proposed quantum computer. Their research findings can be found in Physical Review B.
At the absolute zero, there is no motion because of temperature, merely quantum fluctuations. Phase transitions in a quantum system may be triggered by variations in magnetic field or pressure. These have an equivalent effect matching that of heat on atoms in condensed matter.
"This dynamic is influenced by the interaction of the quantum system with its environment. It is generally described as dissipation," Sabine Andergassen explains. Dissipation means that the potential of the system is restricted by energy loss or by the particles’ inability to move. Dissipation quenches some quantum fluctuations, favoring an ordered state - the "solid" - instead of a disordered state - the "liquid," she says.
"As Heisenberg’s uncertainty principle dictates, this leads to an increase of the fluctuations of the momentum," Wolfgang Belzig adds.
"When the system interacts with two environments trying to quench the fluctuations of two conjugate variables simultaneously, Heisenberg’s principle prohibits this. You could say the system becomes frustrated because it fundamentally cannot satisfy both demands at the same time," says Dominik Maile. Gianluca Rastelli adds: "At a quantum phase transition un unexpected behavior may emerge." In the comparison with melting in classical physics, the quantum system would be torn between the solid and the liquid states.
"This competition between different dissipative mechanisms leads to a peculiar behavior at the transition," Gianluca summarizes. This signifies a real quantum signature and opens the door to study novel aspects of dissipative quantum phase transitions using engineered environments. This recently discovered characteristic could have an impact in the field of information processing, for instance by a quantum computer, the researchers state.