Science's breakthrough of 2010: A visible quantum device

At what point does a mechanical system—governed by classical dynamics—become a quantum system, or vice-versa? In 1920, Neils Bohr argued that quantum systems became describable with classical mechanics when the correspondence (or classical) limit had been reached. He described this as occurring "when the quantum numbers describing the system are large." The exact interpretation of "large" has been left to the reader as a 90-year-old take-home exercise.

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