Topological order

In physics, topological order is a kind of order in zero-temperature phase of matter (also known as quantum matter). Macroscopically, topological order is defined/described by robust ground state degeneracy and quantized non-Abelian geometric phases of degenerate ground states. Microscopically, topological order corresponds to patterns of long-range quantum entanglement. States with different topological orders (or different patterns of long range entanglements) cannot change into each other without a phase transition.

Topologically ordered states have some interesting properties, such as (1) topological degeneracy and fractional statistics/non-abelian statistics that can be used to realize topological quantum computer; (2) perfect conducting edge states that may have important device applications; (3) emergent gauge field and Fermi statistics that suggest a quantum information origin of elementary particles (it from qubit); (4) topological entanglement entropy that reveals the entanglement origin of topological order, etc. Topological order is important in the study of several physical systems such as spin liquids, the quantum Hall effect, along with potential applications to fault-tolerant quantum computation.

Topological insulators and topological superconductors (beyond 1D) do not have topological order as defined above, their entanglements being only short-ranged.

Although all matter is formed by atoms, matter can have different properties and appear in different forms, such as solid, liquid, superfluid, magnet, etc. These various forms of matter are often called states of matter or phases. According to condensed matter physics and the principle of emergence, the different properties of materials originate from the different ways in which the atoms are organized in the materials. Those different organizations of the atoms (or other particles) are formally called the orders in the materials.

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