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Quantum physics

Engineering decoherence

Nature volume 403, pages 256–257 (2000)Cite this article

The superposition principle lies at the very heart of quantum mechanics. As Paul Dirac stated in the first chapter of his classic textbook1: “…any two or more states may be superposed to give a new state”. This principle sounds rather innocent but is the source of all the paradoxes of quantum mechanics, ranging from the Einstein–Podolsky–Rosen situation to Schrödinger's cat. Superpositions can exist only in quantum systems that are free from external influences. But real quantum systems cannot stay isolated because a measurement requires interaction with an apparatus. If classical apparatus is used, which typically consists of a large number of atoms, then these atoms act as a reservoir. The interaction of the quantum system with the reservoir destroys the quantum-mechanical superposition. This decay from a superposition to a statistical mixture of states is called decoherence. On page 269 of this issue, Myatt et al.2 from the National Institute of Standards and Technology in Boulder, Colorado, report a pioneering experiment that engineers decoherence.

Myatt et al. induce decoherence by coupling a single ion in a Paul trap to a reservoir that is under the experimenter's control. Paul traps store individual charged particles such as ions and have been very successful in demonstrating quantum gates, the building blocks of a quantum computer. This is because laser cooling reduces the motional energy of the ion to the point where quantum effects become important. Myatt et al. engineer three different reservoirs, which allows them to confirm that the rate of decoherence scales exponentially with the size of the superposition, demonstrating why we never see macroscopic Schrödinger cats. This work is the culmination of several impressive experiments carried out by Dave Wineland's group at Boulder.

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Figure 1: How the quantum world slides into the classical world.

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  1. the Abteilung für Quantenphysik, Universität Ulm, Ulm, D-89069, Germany

    Wolfgang P. Schleich

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  1. Wolfgang P. Schleich
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Correspondence to Wolfgang P. Schleich.

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Schleich, W. Engineering decoherence. Nature 403, 256–257 (2000). https://doi.org/10.1038/35002223

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