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Physical Review Letters

Selective Enhancement of Spectroscopic Features by Quantum Optimal Control

Daniel Keefer, Shaul Mukamel

Published Apr 19, 2021• 14 citations
Publisher

Abstract

Tailored light can be used to steer atomic motions into selected quantum pathways. In optimal control theory (OCT), the target is usually expressed in terms of the molecular wave function, a quantity that is not directly observable in experiment. We present simulations using OCT that optimize the spectroscopic signal itself. By shaping the optical pump, the x-ray stimulated Raman signal, which occurs solely during the passage through conical intersections, is temporally controlled and amplified by up to 2 orders of magnitude. This enhancement can be crucial in order to bring small coherence-based signatures above the detectable threshold. Our approach is applicable to any signal that depends on the expectation value of a positive definite operator.

DOI
10.1103/physrevlett.126.163202

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Tailored light can be used to steer atomic motions into selected quantum pathways. In optimal control theory (OCT), the target is usually expressed in terms of the molecular wave function, a quantity that is not directly observable in experiment. We present simulations using OCT that optimize the spectroscopic signal itself. By shaping the optical pump, the x-ray stimulated Raman signal, which occurs solely during the passage through conical intersections, is temporally controlled and amplified by up to 2 orders of magnitude. This enhancement can be crucial in order to bring small coherence-based signatures above the detectable threshold. Our approach is applicable to any signal that depends on the expectation value of a positive definite operator.
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We demonstrate that Optimal Control Theory (OCT) with a state-dependent constraint which depends on the state of the system at each instant can reproduce the famous counterintuitive mechanism of Stimulated Raman adiabatic passage (STIRAP). We examine this behavior in a Λ-type three-level system and we show that could be applied for sequentially coupled many-level systems. We study the robustness of the two methods with respect to pulse fluctuations and the decays. We show that new OCT formulation appears to be more robust than STIRAP when a perturbation is introduced in the pulses. Such method is of great use for systems involving coherence loss such as molecular systems with dissociation or ionization limits. It also may find potential applications in the control of chemical reactions, quantum optics, and quantum information processing.
We theoretically demonstrate that selective resonant excitation can be achieved in a dense collection of V-type three-level atoms by optimizing the pulse delay and peak intensity ratio of an applied phase-tailored ultrashort pulse pair. Near-dipole-dipole interaction plays an important role in the quantum control of selective excitations since it brings about an intrinsic frequency shift in the atomic resonance, which builds up various excitation pathways. As a consequence, we can control the quantum interference between various pathways by shaping the excitation pulse pair to steer the atomic excitation selectively toward a desired quantum state.
We propose and theoretically analyze a new approach for generating and shaping 1-fs pulses. It combines the ideas of strong-field molecular optics and optimal control to manipulate light generation in a pump-probe Raman regime. Flexible phase control over the generated spectrum of about 3 eV width is achieved by controlling the input pulses and maximizing the coherence of medium excitation by adiabatically aligning molecules in the medium with a specially shaped pump pulse. The generated pulse is optimized for an output window, precompensating for its dispersion to all orders.
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