## 4 Summary and Outlook  
In this article, we have focused on several fundamental issues of angular momentum (ring current) of \(\pi\) electrons in a low-symmetry aromatic ring molecule. One of the issues is on characteristics of the angular momentum generated by ultrashort polarized laser pulse(s). The generated angular momentum, called coherent angular momentum, is time-dependent, i.e., its direction changes in time, the duration of which is inversely proportional to the energy difference between quasi-degenerate, two electronic eigenstates. The phase in the oscillatory behavior of angular momentum, which is directly related to the relative quantum phase of the superposed quasi-degenerate states, can be controlled by the ellipticity and orientation of the incident laser. This is contrasted with the angular momentum generated in a highly symmetric aromatic molecule, which is time-independent, i.e., unidirectional. That is, stationary angular momentum is generated by an excitation of a degenerate excited state by a circularly polarized laser. Another issue is the nonadiabatic coupling between laser-driven \(\pi\)-electron ring currents and molecular vibration. The coherent angular momentum is gradually attenuated by the nonadiabatic coupling. On the other hand, the amplitude of induced molecular vibration is prominently dependent on the direction of linear polarization vectors but insensitive to the helicity of circular polarization. The characteristic feature in vibrational amplitudes is ascribed to the WP interference effects in nonadiabatic transition. This offers a new possibility of attosecond/several-femtosecond polarized laser pulses as a promising tool to manipulate molecular vibration (and chemical reactions) through the WP interference in nonadiabatic transition. Recently, there has been new progress on the issue of coherent angular momentum. [PERSON] et al. have performed optimal control simulations of coherent \(\pi\)-electron rotations in (\(P\))-2,2\({}^{\prime}\)-biphenol by taking into account two types of control targets: one is generation of the maximum angular momentum of the \(x\) component, which corresponds to CC or AA rotation of \(\pi\) electrons shown in Figure 12, and the other is the maintaining of the generated unidirectional angular momentum during a setting time duration [98]. The optimal control pulse for each target was designed. The analysis of the simulation results shows that the effective maintaining of unidirectional angular momentum can be realized by applying the \(2\pi\) pulse to one of the two excited states forming a coherent electronic state. The \(2\pi\) pulse prevents the reverse rotation of \(\pi\) electrons by dumping the excited state population to the ground state and subsequently by pumping the population back to the excited state. In addition, [PERSON] et al. have found an effectively stationary angular momentum in a low-symmetry molecule [99]. This is realized by applying two linearly polarized lasers with the same frequency and with an appropriate relative phase to quasi-degenerate two excited states. This is due to creation of a dressed state with equal populations for the two excited eigenstates. [PERSON] et al. have proposed another laser-control scenario of generation of unidirectional angular momentum in a low-symmetry aromatic ring molecule [100]. This is intuitively explained in terms of dynamic Stark shifts of two relevant excited states, which are induced by using two linearly polarized stationary lasers: Each laser is set to selectively interact with one of the two excited states. The lower and higher excited states are shifted up and down with the same rate, respectively, and the two excited states become degenerate at their midpoint. The control scenario with the ground state as the initial condition was applied to toluene. The derived time-dependent angular momentum consists of a train of unidirectional angular momentum.