  Energy transfer through quantum coherences plays an essential role in diverse
natural phenomena and technological applications, such as human vision,
light-harvesting complexes, quantum heat engines, and quantum information and
computing. The understanding of the long-lived coherence involved in these
phenomena requires a detailed modeling of the system-bath interactions beyond
the so-called secular and/or Markovian approximations. Despite continuous
theoretical progress on understanding nonsecular dynamics in the last decades,
convincing experimental observations are still lacking. By using the
laser-kicked molecular rotor as a model system, we here experimentally unveil
the nonsecular dynamics in the rotational relaxation of molecules due to
thermal collisions. Specifically, the rotational coherence in gas-phase
molecules is systematically probed and characterized by the recently discovered
rotational alignment echoes featuring a decoherence and dissipation process
which can only be explained by the nonsecular quantum master equations for
modeling molecular collisions.
