Quantum mechanics is a powerful theory that explains much of the world at the level of atoms and molecules. Its predictive and explanatory powers are only matched by relativity and evolution. There are, however, gaps. These gaps are not the sort that you can see in everyday experiments—for instance, if you shine the right color of light on an atom, an electron might be excited from one state to another, and quantum mechanics predicts this perfectly. But there are some questions it doesn't handle so easily. How long does it take the electron to enter an excited state? How does the the probability of finding an electron in a region of space around an atomic nucleus change as the transition takes place?

To measure events like this, you need to probe the atom with light pulses that are shorter in duration than the time it takes the electron to make the transition. That means pulses of light that are less than a single femtosecond in duration, made by a process called high harmonic generation (HHG). Now, in a follow-up to earlier research, a group from Korea has shown how to do HHG in a tiny tube, opening up all sorts of experimental possibilities.

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