Experimental Demonstration of Coherent Control in Quantum Chaotic Systems

Abstract

We experimentally demonstrate coherent control of a quantum system, whose dynamics is chaotic in the classical limit. Interaction of diatomic molecules with a periodic sequence of ultrashort laser pulses leads to the dynamical localization of the molecular angular momentum, a characteristic feature of the chaotic quantum kicked rotor. By changing the phases of the rotational states in the initially prepared coherent wave packet, we control the rotational distribution of the final localized state and its total energy. We demonstrate the anticipated sensitivity of control to the exact parameters of the kicking field, as well as its disappearance in the classical regime of excitation.PACS numbers: 05.45. Mt, 42.50.Hz Control of molecular dynamics with external fields is a long-standing goal of physics and chemistry research. Great progress has been made by exploiting the coherent nature of light-matter interaction. At the heart of coherent control is the interference of quantum pathways leading to the desired target state from a well-defined initial state [1]. In this context, an exponential sensitivity to the initial conditions, characteristic for classically chaotic systems, poses an important question about the controllability in the quantum limit (for a comprehensive review of this topic, see [2]). As the underlying classical rovibrational dynamics of the majority of large polyatomic molecules is often chaotic, the answer to this question has far reaching implications for the ultimate prospects of using coherence to control chemical reactions.Success in steering the outcome of chemical reactions by means of feedback-based adaptive algorithms [3], using the methods of optimal control theory [4], proved that such control is feasible. Theoretical works on quantum controllability in the presence of chaos, both in general [5] and with regard to specific molecular systems [6,7], pointed at the importance of coherent evolution. To investigate the roles of coherence, chaoticity and quantumness further, Gong and Brumer considered a paradigm system for studying quantum effects on classically chaotic dynamics -the quantum kicked rotor (QKR) [2,6,8]. The latter is known to exhibit dynamical localization (closely related to Anderson localization in disordered solids [9,10]), in which quantum interferences suppress the classically chaotic diffusion after the "quantum break time" [11,12]. Gong and Brumer demonstrated that the energy of the localized state can be controlled by modifying the initial wave packet. They showed that quantum coherences, as opposed to the classical structures in the rotor's phase space [13], are indeed responsible for the achieved control over the chaotic dynamics of the QKR.In this report, we present an experimental proof of the Gong-Brumer control scheme. Following a theoretical proposal of Averbukh and co-workers [14, 15], we investigate the dynamics of true quantum rotors by exposing diatomic molecules to a periodic sequence of ultra-short laser pulses. A number of representative QKR effects h...

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