This Article contains an error in the final two sentences of the section entitled “53-attosecond pulses retrieved by PROOF method”. These sentences should read: ‘Knowing the relative intensity change between the ungated and gated HHG, as well as the tin filter transmission, the photon flux for the 53 as pulse is estimated to be ~ 5 × 105 photons per laser shot. The photon flux above carbon K-edge (284 eV) is >1 × 104 photons per laser shot’.