Mechanism and Lineshapes of Sub-Fourier Resonances

Subtle internal interference effects allow quantum-chaotic systems to display "sub-Fourier" resonances, i.e. to distinguish two neighboring driving frequencies in a time shorter than the inverse of the difference of the two frequencies. We report experiments on the atomic version of the kicked rotor showing the unusual properties of the sub-Fourier resonances, and develop a theoretical approach (based on the Floquet theorem) explaining these properties, and correctly predicting the widths and lineshapes.