A laser strikes a strontium atom, and two electrons fly out at once. Previously, it was thought that in such double ionization each electron acts on its own: one is knocked out first, the second later, randomly. The new experiment with cold atoms showed the opposite: the electrons leave the atom in a coordinated manner, like a pair of dancers holding hands.
Using spectroscopy, the scientists peered into the process. They irradiated strontium atoms with laser pulses and caught the ejected electrons with detectors that recorded their energy and emission angle. The resulting images revealed clear stripes — a sign that before ejection, both electrons were in a special excited state of the atom where their motions are tightly coupled.
The key point is resonance. The laser is tuned so that its energy exactly matches the energy of the transition to this doubly excited state. Then the atom first absorbs the light and transitions to a bound state, and only then, under the same pulse, both electrons break free and fly away. This is called non-sequential double ionization, as opposed to sequential, where electrons are knocked out one by one.
The signature in the data, stripes on the energy map, indicates that the correlation embedded in the excited state is not erased during ionization. The electrons carry it away with them, like two shards of a broken vase that still fit together. Such behavior breaks the old picture where a strong laser field supposedly destroys all internal atomic correlations.
This result is important not only for atomic physics. Similar correlations between two particles occur in molecules, solids, and even atomic nuclei. Understanding how correlated particles behave under an external impact helps to more accurately describe chemical reactions and material properties.
Strontium is the element that gives fireworks their bright red color. In this experiment, its atoms were cooled to nearly absolute zero so they would freeze and not interfere with observations.
The idea that particles retain a connection after separation echoes quantum entanglement from science fiction, although here the connection is classical, just synchronized motion.