Quantum Birthmarks Endure: Initial-State Memories Survive Chaotic Evolution

Physicists show that chaotic quantum systems retain a memory of their starting conditions: the time-averaged probability to return to the initial state is enhanced by at least a factor of 2 over unrelated states (the quantum birthmark), and this nonergodic signature persists in the infinite-time limit absent decoherence. The effect stems from quantum interference among energy eigenstates and can be amplified by classical periodic orbits (scars), linking quantum chaos to ergodicity-breaking and offering a new angle on quantum thermalization. The stadium billiard serves as a testbed, with potential experimental tests in quantum simulators; the work deepens the bridge between quantum and classical descriptions of chaos.