摘要： Probing vectorial properties of light-matter interactions requires control over the polarization state of light. The generation of extreme-ultraviolet (XUV) attosecond pulses opened new perspectives in measurements of chiral phenomena. Recently, new methods for polarization control in the XUV range, which are based on manipulation of the high harmonic generation (HHG) process were demonstrated [1-4]. However, the limited polarization control in this regime prevents the development of advanced measurement schemes for weak vectorial signals, which require polarization modulation. In our work [5], we establish an XUV lock-in detection scheme, allowing the isolation and amplification of extremely weak chiral signals, by achieving a dynamical control over the polarization state of the XUV light. We demonstrate a time-domain approach to control and modulate the polarization state. This scheme enables us to characterize the polarization state via an in-situ measurement (see Figure 1a,d,e). Our approach, resembling a birefringent crystal for the visible range, is based on the collinear superposition of two independent, phase-locked, orthogonally polarized XUV sources and the control of their relative delay with sub-cycle accuracy (see Figure 1b). We achieve lock-in detection of XUV magnetic circular dichroism (XMCD) in cobalt, transferring weak amplitude variations into a phase modulation (see Figure 1c), by controlling the relative angle, θ , between the two linearly polarized sources. This approach holds the potential of significantly extending the scope of vectorial measurements to the attosecond and nanometer frontiers.