Some classical physical systems can explore many configurations over time until their long-term behavior no longer reveals their starting conditions. The sampling of available configurations according to the appropriate statistical distribution is a property called ergodicity.

Quantum systems, which are governed by the laws of quantum mechanics, could instead retain some traces of their beginnings. In a paper published in Physical Review X, researchers at Harvard University and other institutions introduced the idea of quantum birthmarks, statistical traces of initial states and early development that persist in quantum systems that do not exchange information with their surroundings.

“The paper grew out of a simple tension in quantum chaos: classically, an ergodic system is expected to forget its initial conditions, but quantum evolution can retain traces of its past through coherence effects,” Joonas Keski-Rahkonen, senior author of the paper, told Phys.org. “We wanted to know whether this memory is a special feature of phenomena like quantum scars, where individual eigenstates break the expectation of classical-like ergodicity, or something far more general, as it turned out to be.”