Talk about taking a tough shot. Physicists have, for the first time, been able to image the quantum workings of electrons in hydrogen atoms, an advance that could open the door to a deeper understanding of the quantum world.
Snapping a picture of the inside of an atom—the electrons, the protons, the neutrons—is no easy task. Quantum mechanics makes it virtually impossible to pin down these subatomic particles. Instead of having the ability to describe where a particle is, quantum theory provides a description of its whereabouts called a wave function. Wave functions work like sound waves, except that whereas the mathematical description of a sound wave defines the motion of molecules in air at a particular place, a wave function describes the probability of finding the particle.
Physicists can theoretically predict what a wave function is like, but measuring a wave function is very hard because they are exquisitely fragile. In another bit of quantum weirdness, most attempts to directly observe wave functions actually destroy them in a process called collapse. So to experimentally measure the properties of a wave function requires researchers to reconstruct it from many separate destructive measurements on identically prepared atoms or molecules.
Physicists at AMOLF, a lab of the Netherlands' Foundation for Fundamental Research on Matter (FOM), in Amsterdam demonstrated a new nondestructive approach in a paper published this week in Physical Review Letters. Building on a 1981 proposal by three Russian theorists and more recent work that brought that proposal into the realm of possibility, the team first fired two lasers at hydrogen atoms inside a chamber, kicking off electrons at speeds and directions that depended on their underlying wave functions. A strong electric field inside the chamber guided the electrons to positions on a planar detector that depended on their initial velocities rather than on their initial positions. So the distribution of electrons striking the detector matched the wave function the electrons had at the moment they left their hydrogen nuclei behind. The apparatus displays the electron distribution on a phosphorescent screen as light and dark rings, which the team photographed using a high-resolution digital camera.
"We are really happy with our results," says team leader Aneta Stodolna, noting that although quantum mechanics is part of daily life for physicists, it is rarely understood in such a visceral way. She says that there may be practical applications in the future—a commentary accompanying the paper suggests that the method could aid in the development of technologies such as molecular wires, atom-thick conductors that could help shrink electronic devices—but that their result concerns "extremely fundamental" physics that might be just as valuable for developing quantum intuition in the next generation of physicists.
"It's an interesting experiment, mostly because it's investigating hydrogen," an element that is both a textbook example in undergraduate physics classes and also makes up three-quarters of the universe, says Jeff Lundeen, a physicist at the University of Ottawa in Canada who's performed related experiments on photon wave functions. Stodolna's team "basically developed a new technique" for observing wave functions, Lundeen says, though it's not yet clear whether it applies to more complicated atoms that physicists understand less well than hydrogen. "If it ends up being fairly universal … then it would be a very useful tool" for studying those atoms in the lab, improving physicists' understanding of the atomic physics underlying chemical reactions and nanotechnology.