Fig. 1. Schematic diagram of terahertz-streaking photoioniza-tion experiments. (a) The probe pulses consist of a ultraviolet pulse and a terahertz pulse. The ultraviolet pulse is locked at the zero point of terahertz vector potential. The superposition state of rubidium atoms is excited by an infrared pulse. (b) Schematic diagram in energy representation. First, the electrons in the superposition state of rubidium atoms are ionized by a UV femtosecond pulse to form two characteristic spectral lines in the photoelectron energy spectrum. Second, the continuous electron with different final momenta is driven by the terahertz pulse. The spectral lines are broadened and the interference occurs.

Fig. 2. The evolution of photoelectron momentum spectra as a function of time delay: (a) Photoelectron momentum spectra without a terahertz field; (b) photoelectron momentum spectra with a terahertz field.

Fig. 3. Time-domain resolution of quantum coherence and the reconstruction of density matrix: (a) Coherent dynamics of the quantum system. The solid line is the theoretical prediction of the coherence term . The dotted line is the cross section along the blue line in Fig. 2 (b) to extract the phase information. (b) Yellow line is the photoelectron momentum distribution driven by THz electric field at . The red line represents the population contribution; and the green line represents the coherence contribution.