Smile, hydrogen atom, you're on quantum camera

How on earth do you make an image of such an object?

 Measuring the position of a single electron "collapses" the wave function, forcing it to pick a particular position, but that alone is not representative of its normal, quantum presence in the atom. "Wave functions are difficult to measure. They're exquisite quantum objects that change their appearance upon observation," says Aneta Stodolna of the FOM Institute AMOLF in Amsterdam, the Netherlands. Her team decided to make a picture using a technique dreamed up 30 years ago that can be thought of as a quantum microscope. Rather than taking an image of a single atom, they sampled a bunch of atoms. This removes the quantum nature of each individual atom's electron, forcing it to choose a particular location from those it is allowed to reside in. Do it with enough atoms and the number choosing each spot will reflect the quantum probabilities laid out by the wave function. Stodolna's team made a beam of atomic hydrogen and zapped it with two separate lasers that excited the atoms' electrons by precise amounts. An applied electric field then pushed the excited electrons away from their respective nuclei, towards a detector about half a metre away. The electrons emitted waves that produced an interference pattern on the detector (see "An atom undressed"). Crucially, the pattern was a projection of the spacings of the energy levels in the hydrogen atom, as laid out in the wave function, with bright rings where electrons were present and dark lanes where they were not (Physical Review Letters, doi.org/mmz). "You can think about our experiment as a tool that allows you to look inside the atom and see what's going on,"

  Stodolna says.