In 1927, during the formative years of quantum mechanics, Friedrich Hund posed a paradox: Why is a chiral molecule found in either its left-handed or right-handed isomeric forms and not in a superposition of the two? After all, both isomers are equally likely and the molecular Hamiltonian is parity-invariant.

At first glance, the answer seems clear. If the tunneling time between the two isomers is long, their superposition is unlikely to arise. That answer might hold for a sugar, protein, or other large chiral molecule whose tunneling time may exceed the age of the universe, but it fails for ammonia and other small molecules. Nor can it explain why the habitual states of a molecule, large or small, are its left-handed and right-handed isomers and not its parity eigenstates.

Now, Klaus Hornberger of Ludwig-Maximilians University in Munich and his postdoc Johannes Trost have resolved Hund’s venerable paradox. 1 The...

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