Quantum mechanics<span> (QM -- also known as </span>quantum physics<span>, or </span>quantum<span> theory) is a branch of </span>physics<span> which deals with physical phenomena at nanoscopic scales where the action is on the order of the Planck constant. It departs from classical </span>mechanics<span> primarily at the </span>quantum<span>realm of atomic and subatomic length scales.</span>
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