Quantum Control of Laser-driven Chiral Molecular Motors

Abstract

The design and control of functional molecular machines and devices is one of the fascinating and challenging research targets in molecular science (Feringa et al., 2000; Kinbara & Aida, 2005; Kay et al., 2007). They were originally inspired from biological machines such as ATP synthases (Boyer, 1993; Abrahams et al., 1994) and myosin and kinesin (Julicher et al., 1997). They now include various kinds of artificial molecular machines such as transmitters, shuttles, nanocars and logic gates (Balzani et al., 2008), which can be driven by external forces at the molecular level. Some of them are not simply sizeddown versions of macroscopic machines and are controlled at the quantum level (Roncaglia & Tsironis, 1998). Lasers are energy sources over a wide range of wave lengths from mid-infrared to ultraviolet, which make it possible to drive various sizes of molecular machines without any direct contact. Lasers are expected to play an important role as a source of external forces for controlling molecular machines because lasers have various controlling-parameters such as central frequencies, pulse shapes, photon polarizations and time differences between two pulses (Assion et al., 1998; Gouliemakis et al., 2004). Based on coherent control theory (Kosloff et al. 1989; Shi & Rabitz, 1990; Shapiro & Brumer, 2000), laser pulses can be designed to produce the maximum desired target with minimum laser energy (Assion et al., 1998; Rice & Zhao, 2000; Gordon & Fujimura, 2002; Bandrauk et al., 2002). Molecular machines can be controlled through coherent interactions between lasers and molecules at a quantum level (Hoki et al., 2003). The procedures are sometimes called “quantum ignition” for driving molecular motors (Fujimura et al., 2004). The time evolution is obtained by solving the time-dependent Schrodinger equation or the Liouville equation (Sugawara & Fujimura, 1994; Ohtsuki et al., 1999; Hoki et al., 2001). Application of coherent control theory enables extraction of key factors for driving molecular motors with a unidirectional motion, though we have to wait for further experimental progress to carry out coherent control experiments on artificial molecular machines. In this chapter, we present fundamental principles for unidirectional motions of chiral molecular motors driven by linearly polarized laser pulses having no photon helicity.