This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Most of us recognize basic phases of matter and their properties: solids maintain their shape, liquids flow freely with constant volume, and gases expand to fill their container. Quantum phases, on the other hand, are much harder to categorize, overturning traditional notions of what a phase even is and forcing physicists to rethink their definitions.
Even so, physicists have made substantial advances in classifying quantum phases for isolated systems. Yet categorizing those for open, nonequilibrium systems—those that freely interact with their environment—has presented numerous technical challenges, hindering our understanding of fundamentally new quantum behaviors and their implementation in quantum-computing applications. Now, in a new paper published in Physical Review X, Illinois physicists at the Anthony J.
Leggett Institute for Condensed Matter Theory have developed a framework for classifying nonequilibrium quantum phases of matter, generalizing principles governing closed systems. Their work overcomes the shortcomings of current classification schemes, enabling scientists to distinguish between a wider range of quantum phenomena than ever before. Ordinary phases of matter are generally classified by their symmetries.
Liquids, for instance, look roughly the same from any direction and have a high degree of symmetry, whereas crystalline solids possess symmetries only along well-defined axes, a difference showing that they're distinct phases. This classification paradigm, pioneered by Lev Landau in the 1930s, has extraordinary explanatory power, describing everything from solids, liquids and gases to magnets and even superconductors. Since the 1980s, however, physicists have realized that many phases can't be explained by symmetry alone.
They also require topology, a branch of mathematics that studies fundamental, global properties of shapes while ignoring their local, small-scale details. To give an oft-cited example, although seemingly different, a coffee cup can be mathematically "massaged," or continuously deformed, into a doughnut like clay, showing that they're globally identical and belong to the same class of shapes. Each class is characterized by so-called topological invariants, special numbers such as the number of holes a shape possesses, that don't change under continuous deformations.
On the other hand, making a new, distinct topological shape requires a discontinuous change such as tearing—a banned operation in topology. Like the topological shapes they correspond to, topological phases are distinguished by their invariants too, maintaining their order when locally perturbed by external interactions. Physically, they arise when temperatures are brought so low that quantum fluctuations, once masked by thermal jostling, emerge to produce system-wide quantum entanglement.
Extract — continue reading at the source.
