If another linearly polarized attosecond pulse (the probe pulse) centered at 100 eV doubly ionizes the helium atom in the mixed state as illustrated in the right graph of Figure 9.1, the ionization probability depends on the time delay between the pump and the probe pulses. The probability is higher when the two electrons are located near each other, as depicted in Figure 9.2. Experimentally, one can measure the doubly ionized helium ion (He2þ) signal as function of time delay between the probe pulse and the pump pulse. The He2þ yield as a function of the time delay would inform us how the electron separation changes with time. Hu and Collins’s simulation shows that the separation changes periodically with a 2 fs cycle, which can be observed with the attosecond pulses. The cycle is much larger than the 200 as corresponding to the energy difference between the ground state and the first excited state (1s2p) because other excited states are involved.