Quantum Localization in Laser-Driven Molecular Rotation

The periodically-kicked rotor is a paradigmatic system in nonlinear dynamics studies. The classical kicked rotor exhibits a truly chaotic motion with an unlimited diffusive growth of the angular momentum. In the quantum regime, the chaotic dynamics is either suppressed by a mechanism similar to Anderson localization in disordered solids, or rotational excitation is enhanced due to the so-called quantum resonance. 

Although these fundamental quantum phenomena have been theoretically studied for several decades already, until recent times there has not been a single experiment demonstrating Anderson localization in a real rotating quantum system. Quantum chaotic dynamics was primarily studied by using ultra-cold atoms in pulsed optical lattices. Recently we revisited the problem of the periodically-kicked quantum rotor, and predicted that several well-known quantum localization phenomena in solid-state systems – Anderson localization, Bloch oscillations, and Tamm-Shockley surface states – may manifest themselves in the rotational dynamics of the laser-kicked molecules.  We showed that current femtosecond technology used for laser alignment of molecules offers tools for exploring these effects, and we defined conditions for their experimental observation. 

In this talk, I will give an overview of the physical mechanisms behind these new molecular rotational phenomena, and will present the results of recent femtosecond experiments in which the rotational Bloch oscillations and the dynamical Anderson localization were observed along the lines of our proposal.  These results introduce molecular gases at ambient conditions as a new platform for studying localization phenomena in quantum transport. The observed rotational effects are important for many applications, ranging from selective excitation in the mixtures of molecular species (i.e. molecular isotopes or nuclear spin isomers) to controlling propagation of powerful laser pulses in the atmosphere.