Tunnelling is one of the key features of quantum mechanics. A related debate, ongoing since the inception of quantum theory, is about the value, meaning and interpretation of 'tunnelling time' 1-5 . Simply put, the question is whether a tunnelling quantum particle spends a finite and measurable time under a potential barrier. Until recently the debate was purely theoretical, with the process considered to be instantaneous for all practical purposes. This changed with the development of ultrafast lasers and attosecond metrology 6 , which gave physicists experimental access to the attosecond (1 as = 10 -18 s) domain. It is at this time scale

Extreme ultraviolet and X-ray free-electron lasers (FELs) produce short-wavelength pulses with high intensity, ultrashort duration, well-defined polarization and transverse coherence, and have been utilized for many experiments previously possible only at long wavelengths: multiphoton ionization, pumping an atomic laser and four-wave mixing spectroscopy. However one important optical technique, coherent control, has not yet been demonstrated, because self-amplified spontaneous emission FELs have limited longitudinal coherence. Single-colour pulses from the FERMI seeded FEL are longitudinally coherent, and two-colour emission is predicted to be coherent. Here, we demonstrate the phase correlation of two colours, and manipulate it to control an experiment. Light of wavelengths 63.0 and 31.5nm ionized neon, and we controlled the asymmetry of the photoelectron angular distribution by adjusting the phase, with a temporal resolution of 3as. This opens the door to new short-wavelength coherent control experiments with ultrahigh time resolution and chemical sensitivity

We investigate two-pathway interferences between nonresonant one-photon and resonant twophoton ionization of atomic hydrogen. In particular, we analyze in detail the photoionization mediated by the fundamental frequency and the second harmonic of a femtosecond VUV pulse when the fundamental is tuned near an intermediate atomic state. Following our recent study [Phys. Rev. A 91, 063418 (2015)] of such effects with linearly polarized light, we analyze a similar situation with circularly polarized radiation. As a consequence of the richer structure in circularly polarized light, characterized by its right-handed or left-handed helicity, we present and discuss various important features associated with the photoelectron angular distribution.

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