In a hall with a thousand spectators, applause blends into a general noise, but if someone claps out of rhythm, a ripple runs through the crowd. The quantum world works similarly: atoms in a trap can behave as a single whole, or they can scatter into independent individuals. Seeing this synchrony down to the last atom had long been elusive — microscopes showed where particles were, but not how their oscillations were coordinated. A new tool looks deeper.
Physicists switch off the optical lattice that holds atoms in place. The atoms fly apart like a crowd after a concert. After 29 microseconds, the lattice is switched back on, and they watch how the atoms settle back into place. If they were moving coherently before the switch-off, they will not return randomly but in a specific pattern. This pattern reveals quantum coherence — the ability of particles to act in concert.
The method was tested on erbium atoms. In the center of the cloud, where atoms are densely packed, coherence disappeared when interactions were strengthened. At the edges, where atoms have more room, it survived. Two different states coexisted in a single cloud, and the new microscope saw them separately. Previously, this was impossible: measurements averaged over the entire cloud, and details were lost.
The Talbot effect used here was described by Henry Fox Talbot in 1836 for light passing through a grating. Now it helps to examine quantum gases with single-atom precision.