Mutually coherent high-order harmonic pulses for XUV Ramsey spectroscopy

We describe the series of experiments that led to the development of the Ramsey technique for achieving high-resolution atomic spectroscopy in the extreme ultraviolet with high-order harmonics. We start with a demonstration of phase coherence in the harmonic generation process, then, we illustrate the appearance of spectral interference fringes in the case of collinear, time-delayed harmonic pulses, and we finally present the first demonstration of a Ramsey technique that is based on pairs of time-delayed and phase-coherent harmonic pulses and that is used for the study of the excited states of krypton.