In this interesting work, the authors investigate the responses of various physical systems to external chirped fields. The key insight is that if a system can tell the difference between an up-chirp and a down-chirp, it is sensitive to the phase of the field and, for instance, can be used as a sensitive probe. As often happens with quantum mechanical systems and driving fields, it all comes down to symmetry arguments. What the authors found is that harmonic oscillators, two-level atoms, and electron-positron pairs in a spatially homogeneous field will absorb an equal amount of energy and therefore cannot be chirped coherently. On the other hand, less symmetric systems such as anharmonic oscillators, three-level atoms, and electron-positron pairs in a spatially inhomogeneous field will display a different final state depending on the nature of the chirp. As the authors point out, we are at the beginning of our understanding of such types of dynamical systems.
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