Introduction to special issue

Quantum Wonderland

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Science  29 Feb 2008:
Vol. 319, Issue 5867, pp. 1201
DOI: 10.1126/science.319.5867.1201
CREDIT: I. A. WALMSLEY

Like Alice and her wonderland, physicists also have access to two worlds: the classical and the quantum. Although both worlds are inhabited by the same two species, bosons and fermions, their behavior in either world can be remarkably different. The macroscopic or classical world is filled with the familiar and modeled with classical laws. Lowering the temperature sufficiently to enter the quantum world reveals that these species can interact in cooperative ways, giving rise to exotic phases of matter—quantum matter—not seen in the classical world. Here, things get interesting (and weird); solids, liquids, and electrons can flow without dissipation; exotic phases can emerge; fluctuations can be critical; and objects can be entangled and be in multiple places at once.

Many experimentalists and theorists have been exploring this quantum regime for some time now, studying how individual particles and ensembles of particles behave, in attempts to unravel the underlying physics producing these exotic properties and phases. Some others are heading straight to applications. The six Perspectives in this special section provide a taste of some of the topics that occupy the world of quantum matter.

With atoms trapped in a lattice of optical microtraps, Bloch (p. 1202) discusses how the ability to manipulate the magnitude and sign of the interaction between the atoms can provide a model system in which to explore the formation of the exotic phases seen in quantum gases, liquids, solids, and electronic and magnetic systems. Leggett (p. 1203) sets out the theoretical basics of such quantum systems, explaining how their behavior depends on which family of statistics (Bose-Einstein or Fermi-Dirac) the atoms belong to. Choosing the example of quantum criticality in fermionic systems, Zaanen (p. 1205) points out that the fermions and their statistical family are troublemakers. Trying to explain the complexity emerging from what are simple constituents, he tells us that our present mathematical toolbox is incapable of describing how these exotic electronic phases emerge and that new mathematical tools need to be developed. Another recent example of an observation in need of an explanation is the supersolid effect found in helium-4, where a solid crystal seems to move like a superfluid. Chan (p. 1207) presents the latest on this new phase and argues that imperfections in the crystal appear to be necessary for the effect to be seen. Communication is a vital technology in the classical world, and Walmsley (p. 1211) describes how developments made in the quantum world are carrying over to applications through the use of quantum optics in areas such as secure communication and cryptography. Lloyd (p. 1209) expands on the topic of communication and information, describing how quantum information can be considered as matter, as concrete as any of the matter we are familiar with in our classical world, and how theoretical ideas in quantum error correction will lend themselves to the realization of an operational quantum computer.

Outside the special section, Adrian Cho's story in News Focus (p. 1180) describes research in Fermi condensates, gases composed of fermionic atoms, which may help researchers model materials as diverse as high-temperature superconductors and the interiors of neutron stars.

So, armed with an Alice-like curiosity, let's take a short walk in this quantum landscape.

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