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Pierre Agostini's experiment illuminated an argon jet with an annular $800\mathrm{nm}$ beam. A pinhole removed the annulus; a tungsten-coated mirror relayed harmonics and weak central reference into another argon jet. A magnetic-bottle electron spectrometer used time of flight and microchannel plate detectors. Glass plates varied delay. Reference-on sidebands contrasted with reference-off peaks. Atomic-phase-corrected reconstruction yielded $250$-attosecond bursts.^[ref]

The two-path attosecond phase reconstruction explains the control's purpose: adding the reference opens an interference channel. Its fitted oscillation tests phase relationships under the weak-field and atomic-response model. It does not independently reveal every target phase or an arbitrary nonperiodic optical pulse.

RABBITT measures adjacent harmonic spectral phases through two-photon photoionization. A weak delayed infrared reference opens two interfering paths to a shared photoelectron energy; the two-path attosecond phase reconstruction spells out the inference.