Quantum control of molecular rotation

Abstract

The angular momentum of molecules, or, equivalently, their rotation in three-dimensional space, is ideally suited for quantum control. Molecular angular momentum is naturally quantized, time evolution is governed by a well-known Hamiltonian with only a few accurately known parameters, and transitions between rotational levels can be driven by external fields from various parts of the electromagnetic spectrum. Control over the rotational motion can be exerted in one-, two-and many-body scenarios, thereby allowing to probe Anderson localization, target stereoselectivity of bimolecular reactions, or encode quantum information, to name just a few examples. The corresponding approaches to quantum control are pursued within separate, and typically disjoint, subfields of physics, including ultrafast science, cold collisions, ultracold gases, quantum information science, and condensed matter physics. It is the purpose of this review to present the various control phenomena, which all rely on the same underlying physics, within a unified framework. To this end, we recall the Hamiltonian for free rotations, assuming the rigid rotor approximation to be valid, and summarize the different ways for a rotor to interact with external electromagnetic fields. These interactions can be exploited for control -from achieving alignment, orientation, or laser cooling in a one-body framework, steering bimolecular collisions, or realizing a quantum computer or quantum simulator in the many-body setting. IntroductionMolecules, unlike atoms, are extended objects that possess a number of different types of internal motion. In particular, the geometric arrangement of their constituent atoms endows molecules with the basic capability to rotate in three-dimensional space. Rotations can couple to vibrations of the atomic nuclei as well as to the orbital and spin angular momentum of the electrons. The resulting complexity of the energy level structure [1,2,3,4] may be daunting. It offers, on the other hand, a variety of knobs for control and thus is at the core of numerous applications, from the classic example of the ammonia maser [5] all the way to recent measurements of the electron's electric dipole moment in a cryogenic molecular beam of thorium monoxide [6].A key advantage of internal degrees of freedom such as rotation is that they occupy the low-energy part of the energy spectrum. Quantization of the rotational motion has been an early hallmark of quantum mechanics due to its connection to selection rules that govern all light-matter interactions [7]. Nowadays, rotational states and rotational molecular dynamics feature prominently in all active areas of AMO physics research as well as in neighbouring fields such as physical chemistry and quantum information science. Control over the rotational motion is crucial in one-body, two-body and many-body scenarios. For example, rotational state-selective excitation could pave the way towards separating left-and right handed enantiomers of chiral molecules [8,9]. Still within the one-body scenar...

Field-free molecular orientation by delay- and polarization-optimized two fs pulses

“…In recent decades, laser-based techniques for fixing molecules in space have become an important tool for a range of experiments. This is because intrinsic structural information of molecules is averaged out unless the molecular axis is fixed in the laboratory frame 1 . The most widely-adopted technique is a transient alignment achieved by irradiating a fs laser pulse, so-called nonadiabatic (impulsive) alignment technique 2 – 6 .…”

Encoding a qubit in a molecule

“…V.D]. Nevertheless, significant steps toward the realization of molecular codes may be feasible during the NISQ era [31] as the technology for trapping and controlling molecules [33][34][35][36][37][38] continues to advance.…”

Spatiotemporal rotational dynamics of laser-driven molecules

“…Using nonresonant laser pulses, molecular axes can be aligned or oriented along specific directions in space. The control of molecular rotation by laser pulses has been a very active field, [1][2][3][4][5] where the focus has been on aligning/orienting otherwise isotropic molecular gases. Here alignment is defined as constraining a molecular axis along the polarization axis of the field, and orientation further requires breaking head-versustail symmetry, if such symmetry exists in the molecules.…”

“…The topic of molecular alignment and orientation has been reviewed several times and by different authors: Stapelfeldt and Seideman 1 covered the historical development of molecular alignment; Ohshima and Hasegawa 2 focused on state-resolved probing of impulsively excited alignment and UDR; and Fleischer et al 3 reviewed selective rotational excitation by a pair of pulses, including isotope identification and unidirectional control, together with surface-scattering rotational dynamics. A more recent review by Koch et al 5 provides a more general perspective of quantum control. To minimize overlap with these previous reviews, here we focus on the most recent progress on laser-induced molecular alignment and orientation, including optically induced molecular UDR, all-optical field-free partial and 3-D orientation of asymmetric-top molecules, and rotational alignment echoes.…”