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Although all matter is formed by atoms, matter can have very different properties and appear in very different forms, such as solid, liquid, superfluid, magnet, etc. 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.
Atoms can organize in many ways which lead to many different orders and many different types of materials. With so many different orders, we need a general understanding of the orders. Landau symmetry-breaking theory provides such a general understanding. It points out that different orders really correspond to different symmetries in the organizations of the constituent atoms. As a material changes from one order to another order (i.e., as the material undergoes a phase transition), what happens is that the symmetry of the organization of the atoms changes.
For example, atoms have a random distribution in a liquid, so a liquid remains the same as we displace it by an arbitrary distance. We say that a liquid has a continuous translation symmetry. After a phase transition, a liquid can turn into a crystal. In a crystal, atoms organize into a regular array (a lattice). A lattice remains unchanged only when we displace it by a particular distance, so a crystal has only discrete translation symmetry. The phase transition between a liquid and a crystal is a transition that reduces the continuous translation symmetry of the liquid to the discrete symmetry of the crystal. Such change in symmetry is called symmetry breaking. The essence of the difference between liquids and crystals is therefore that the organizations of atoms have different symmetries in the two phases.
Landau symmetry-breaking theory is a very successful theory. For a long time, physicists believed that Landau symmetry-breaking theory describes all possible orders in materials, and all possible (continuous) phase transitions.
The discovery and characterization of topological order
However, in last twenty years, it has become more and more apparent that Landau symmetry-breaking theory may not describe all possible orders. In 1987, physicists introduced chiral spin state in an attempt to explain high temperature superconductivity [Kalmeyer and Laughlin, 1987; Wen etal, 1989]. At first people still wanted to use Landau symmetry-breaking theory to describe the chiral spin state. They identified the chiral spin state as a state that breaks the time reversal and parity symmetries, but not the spin rotation symmetry. However, it was quickly realized that there are many different chiral spin states that have exactly the same symmetry, so symmetry alone was not enough to characterize different chiral spin states. This means that the chiral spin states contain a new kind of order that is beyond symmetry description.[Wen, 1989] This new kind of order was named topological order.[Wen, 1990] (The name "topological order" is motivated by the low energy effective theory of the chiral spin states, which is a topological quantum field theory.[Witten, 1989]) New quantum numbers, such as ground state degeneracy [Wen, 1989] and the non-Abelian Berry's phase of degenerate ground states [Wen, 1990], were introduced to characterize the different topological orders in chiral spin states. Recently, it was shown that topological orders can also be characterized by topological entropy.[Kitaev and Preskill, 2006; Levin and Wen, 2006]
But experiments soon indicated that chiral spin states do not describe high-temperature superconductors, and the theory of topological order became a theory with no experimental realization. However, the similarity between chiral spin states and quantum Hall states allows one to use the theory of topological order to describe different quantum Hall states.[Wen and Niu, 1990] Just like chiral spin states, different quantum Hall states all have the same symmetry and are beyond the Landau symmetry-breaking description. One finds that the different orders in different quantum Hall states can indeed be described by topological orders, so the topological order does have experimental realizations.
Mechanism of topological order
A large class of topological orders is realized through a mechanism called string-net condensation.[Levin and Wen, 2003] This class of topological orders is described and classified by tensor category theory. One finds that string-net condensation can generate infinitely many different types of topological orders, which may indicate that there are many different new types of materials remaining to be discovered.
The collective motions of condensed strings give rise to excitations above the string-net condensed states. Those excitations turn out to be gauge bosons. The ends of strings are defects which correspond to another type of excitations. Those excitations are the gauge charges and can carry Fermi or fractional statistics.
The condensations of other extended objects such as membranes,[Hamma etal, 2005] brane-nets,[Bombin, M.A. Martin-Delgado, 2006] and fractals [Chamon, 2005] also lead to topologically ordered phases.
The materials described by Landau symmetry-breaking theory have had a substantial impact on technology. For example, Ferromagnetic materials that break spin rotation symmetry can be used as the media of digital information storage. A hard drive made of ferromagnetic materials can store gigabytes of information. Liquid crystals that break the rotational symmetry of molecules find wide application in display technology; nowadays one can hardly find a household without a liquid crystal display somewhere in it. Crystals that break translation symmetry lead to well defined electronic bands which in turn allow us to make semiconducting devices such as transistors. Topologically ordered states are a new class of materials that are even richer than symmetry breaking states. This may suggest an exciting potential for applications.
One theorized application would be to use topologically ordered states as media for quantum computing. A topologically ordered state is a state with complicated non-local quantum entanglement. The non-locality means that the quantum entanglement in a topologically ordered state is distributed among many different particles. As a result, the pattern of quantum entanglements cannot be destroyed by local perturbations. This significantly reduces the effect of decoherence. This suggests that if we use different quantum entanglements in a topologically ordered state to encode quantum information, the information may last much longer.[Dennis etal, 2002] The quantum information encoded by the topological quantum entanglements can also be manipulated by dragging the topological defects around each other. This process may provide a physical apparatus for performing quantum computations.[Freedman etal, 2003] Therefore, topologically ordered states may provide natural media for both quantum memory and quantum computation. Such realizations of quantum memory and quantum computation may potentially be made fault tolerant.[Kitaev, 2003]
Why is topological order important? Landau symmetry-breaking theory is a cornerstone of condensed matter physics. It is used to define the territory of condensed matter research. The existence of topological order appears to indicate that nature is much richer than Landau symmetry-breaking theory has so far indicated. The exciting time of condensed matter physics is still ahead of us. Some suggest that topological order (or more precisely, string-net condensation) has a potential to provide a unified origin for photons, electrons and other elementary particles in our universe.
Fractional quantum Hall states:
Chiral spin states:
Early characterization of FQH states:
Characterization of topological order:
Mechanism of topological order
Emergence of elementary particles
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Topological_order". A list of authors is available in Wikipedia.|