Quantum control experiment reveals solvation-induced decoherence

Abstract

Coherent control holds the promise of becoming a powerful spectroscopic tool for the study of complex molecular systems. Achieving control requires coherence in the quantum system under study. In the condensed phase, coherence is typically lost rapidly because of fluctuating interactions between the solvated molecule and its surrounding environment. We investigate the degree of attainable control on a dye molecule when the fluctuations of its environment are systematically varied. A single successful learning curve for optimizing stimulated emission from the dye in solution is reapplied for a range of solvents with varying viscosity, revealing a striking trend that is correlated directly with the dephasing time. Our results provide clear evidence that the environment limits the leverage of control on the molecular system. This insight can be used to enhance the yield of control experiments greatly.coherent control ͉ quantum-control spectroscopy ͉ solvation dynamics C oherent control has been recognized as a potential spectroscopic tool for the study of complex molecular systems (1). With this technique, a molecular system is selectively guided through a potential energy landscape toward a target quantum state by shaped light fields, often employing a closed-loop learning algorithm (2-9). The central idea is that the pulse shapes corresponding to the highest yield contain spectroscopic information concerning the quantum system, yet often the complexity of these pulses hinders their interpretation. Further, achieving control requires coherence in the quantum system under study. In the condensed phase, coherence is typically lost rapidly because of fluctuating interactions between the solvated molecule and its surrounding environment. In simple molecular systems in the gas phase, coherent control has been used to guide the system into a subspace with enhanced decoherence times (10). For solvated molecules, this approach is infeasible because of the complexity added by the environment. Nonetheless, it has been found surprisingly easy to come up with control fields for a wide range of systems (11), from simple atomic systems to large biomolecules. These control fields exert their influence even when the system is interacting with a fluctuating environment such as a protein or a solvent (8). However, in only a few isolated cases has the goal of deriving fundamental properties of the system and the physical mechanism of control been achieved (3, 4, 7). The complexity of the pulse shape hinders the usefulness of coherent control as a research tool, and more progress needs to be made in understanding the basic mechanisms of control and how these apply to a variety of molecular systems. An important objective is to find ''rules of thumb'' for coherent control, where a given shaped pulse produces a predictable and universal response. A promising route to the realization of this goal is to study a set of systems in which some parameter is systematically varied (12). In this article, we show how the amount of attainable cont...

Training Schrödinger’s cat: quantum optimal control

“…The phenomena were attributed to the influence of the environment. A subsequent study showed that such controllability is solvent dependent [211]. A theoretical demonstration that phaseonly control is possible in weak field for an open quantum system soon followed [212].…”

The scaling of weak field phase-only control in Markovian dynamics

“…A subsequent study by van der Walle et al showed that such controllability is solvent dependent. 8 A careful examination of the assumptions can resolve the discrepancy between theory and experiment, considering that the experiments were carried out for an open quantum system. It has been suggested that the coupling to the environment changes the conditions under which the statement of impossibility holds.…”

Searching for an optimal control in the presence of saddles on the quantum-mechanical observable landscape

“…OCEs have been successfully performed for a wide range of goals, including the control of molecular vibrational [13][14][15][16][17][18][19][20] and electronic states [21][22][23][24][25][26][27][28][29], the generation and coherent manipulation of X-rays [30][31][32][33][34], the control of decoherence processes [35,36], the selective cleavage and formation of chemical bonds [37][38][39][40][41][42][43], the manipulation of energy flow in macromolecular complexes [44][45][46][47], and the control of photoisomerization reactions [48][49][50][51][52]. Optimal control theory (OCT) [7,9,[53][54][55][56] has provided insights into the coherent control of a variety of quantum phenomena, such as electron transfer …”