arXiv subjects

Monika Leibscher

Publications and source records attributed to Monika Leibscher.

11 recordsLinked to original sources

arXiv2026

A linear rotor trapped and coupled to the vibrational modes of an ion crystal

When molecular ions are trapped together with atomic ions in a Paul trap, their dipole moment couples the molecular rotation to the joint vibrational motion of the particles in the trap. To leverage this coupling for quantum control, a rotational transition in the molecule should be resonant with one of the ion crystal vibrational modes. Focusing on the example of singly charged thorium fluoride molecular ions, cotrapped with two ytterbium ions, we determine the conditions for resonant dipole-phonon coupling, fully accounting for the molecular hyperfine structure. We identify several choices for resonant coupling and discuss its detection using sideband-resolved laser spectroscopy and measurements of decoherence.

quant-ph↗

arXiv2026

Chiral rotational dynamics in the molecular frame: Breaking symmetry with angular momentum

Achiral molecules can be prepared in superposition states that are chiral. Here, we propose angular momentum orientation in the molecular frame to achieve the required symmetry breaking, exerting rotational control without the need for laboratory frame orientation. We derive the conditions for chiral rotational dynamics from the requirement to simultaneously break the continuous spatial rotational symmetry and the molecular point group symmetry. This can be achieved by three microwave pulses as well as two non-resonant optical pulses in combination with a THz pulse or three THz pulses, all with mutually orthogonal polarization directions, and the ensuing dynamics can be probed by photoelectron circular dichroism. Our results open the way for distinguishing structural from dynamical enantioselectivity and investigating time-odd chiroptical phenomena in randomly oriented molecules.

physics.chem-ph↗

arXiv2026

Sympathetic rotational cooling of large trapped molecular ions

We suggest a protocol for the sympathetic cooling of a molecular asymmetric top rotor co-trapped with laser-cooled atomic ions, based on resonant coupling between the molecular ion's electric dipole moment and a common normal mode of the trapped particles. By combining sympathetic sideband laser cooling with coherent microwave excitation, we demonstrate the efficient depopulation of arbitrary rotational subspaces and the ability to cool an incoherent distribution of rotational states into a single, well-defined quantum state. This capability opens the door to exploiting the rotational Hilbert space for applications in quantum information processing and high-precision spectroscopy.

quant-ph↗

arXiv2026

Enhancing the controllability of quantum systems via a static field

We provide a sufficient condition for the controllability of a bilinear closed quantum system steered by a static field and a time-varying field, based on the notion of weakly conically connected spectrum. More precisely, we show that if a controlled Hamiltonian with two inputs has a weakly conically connected spectrum, then, freezing one of the two inputs at almost every constant value, the obtained single-input system is controllable. The result is illustrated with two examples, enantio-selective excitation in a chiral molecule and the driven Jaynes-Cummings Hamiltonian.

math.OC↗

arXiv2023

Quantum control of ro-vibrational dynamics and application to light-induced molecular chirality

Achiral molecules can be made temporarily chiral by excitation with electric fields, in the sense that an average over molecular orientations displays a net chiral signal [Tikhonov et al., Sci. Adv. 8, eade0311 (2022)]. Here, we go beyond the assumption of molecular orientations to remain fixed during the excitation process. Treating both rotations and vibrations quantum mechanically, we identify conditions for the creation of chiral vibrational wavepackets -- with net chiral signals -- in ensembles of achiral molecules which are initially randomly oriented. Based on the analysis of symmetry and controllability, we derive excitation schemes for the creation of chiral wavepackets using a combination of (a) microwave and IR pulses and (b) a static field and a sequence of IR pulses. These protocols leverage quantum rotational dynamics for pump-probe spectroscopy of chiral vibrational dynamics, extending the latter to regions of the electromagnetic spectrum other than the UV.

quant-ph↗

arXiv2022

Graph test of controllability in qubit arrays: A systematic way to determine the minimum number of external controls

The ability to implement any desired quantum logic gate on a quantum processing unit is equivalent to evolution-operator controllability of the qubits. Conversely, controllability analysis can be used to minimize the resources, i.e., the number of external controls and qubit-qubit couplings, required for universal quantum computing. Standard controllability analysis, consisting in the construction of the dynamical Lie algebra, is, however, impractical already for a comparatively small number of qubits. Here, we show how to leverage an alternative approach, based on a graph representation of the Hamiltonian, to determine controllability of arrays of coupled qubits. We provide a complete computational framework and exemplify it for arrays of five qubits, inspired by the ibmq_quito architecture. We find that the number of controls can be reduced from five to one for complex qubit-qubit couplings and to two for standard qubit-qubit couplings.

quant-ph↗

arXiv2022

Rational pulse design for enantiomer-selective microwave three-wave mixing

Microwave three-wave mixing allows for enantiomer-selective excitation of randomly oriented chiral molecules into rotational states with different energy. The random orientation of molecules is reflected in the degeneracy of the rotational spectrum with respect to the orientational quantum number $M$ and reduces, if not accounted for, enantiomer-selectivity. Here, we show how to design pulse sequences with maximal enantiomer-selectivity from an analysis of the $M$-dependence of the Rabi frequencies associated with rotational transitions induced by resonant microwave drives. We compare different excitations schemes for rotational transitions and show that maximal enantiomer-selectivity at a given rotational temperature is achieved for synchronized three-wave mixing with circularly polarized fields.

quant-ph↗

arXiv2021

Lie algebra for rotational subsystems of a driven asymmetric top

We present an analytical approach to construct the Lie algebra of finite-dimensional subsystems of the driven asymmetric top rotor. Each rotational level is degenerate due to the isotropy of space, and the degeneracy increases with rotational excitation. For a given rotational excitation, we determine the nested commutators between drift and drive Hamiltonians using a graph representation. We then generate the Lie algebra for subsystems with arbitrary rotational excitation using an inductive argument.

quant-ph↗

arXiv2020

Complete Controllability Despite Degeneracy: Quantum Control of Enantiomer-Specific State Transfer in Chiral Molecules

We prove complete controllability for rotational states of an asymmetric top molecule belonging to degenerate values of the orientational quantum number M. Based on this insight, we construct a pulse sequence that energetically separates population initially distributed over degenerate M-states, as a precursor for orientational purification. Introducing the concept of enantio-selective controllability, we determine the conditions for complete enantiomer-specific population transfer in chiral molecules and construct pulse sequences realizing this transfer for population initially distributed over degenerate M-states. This degeneracy presently limits enantiomer-selectivity for any initial state except the rotational ground state. Our work thus shows how to overcome an important obstacle towards separating, with electric fields only, left-handed from right-handed molecules in a racemic mixture.

quant-ph↗

arXiv2019

Principles of enantio-selective excitation in three-wave mixing spectroscopy of chiral molecules

Three-wave mixing spectroscopy of chiral molecules, which exist in left-handed and right-handed conformations, allows for enantio-selective population transfer despite random orientation of the molecules. This is based on constructive interference of the three-photon pathways for one enantiomer and destructive one for the other. We prove here that three mutually orthogonal polarization directions are required to this end. Two different dynamical regimes exist to realize enantio-selective population transfer, and we show that they correspond to different phase conditions in the three-wave mixing. We find the excitation scheme used in current rotational three-wave mixing experiments of chiral molecules with $C_1$ symmetry to be close to optimal and discuss prospects for ro-vibrational three-wave mixing experiments of axially chiral molecules. Our comprehensive study allows us to clarify earlier misconceptions in the literature.

quant-ph↗

arXiv2017

A systematic four-dimensional approach to strong field control of molecular torsions

We introduce a four-dimensional quantum model for describing the torsional control of $\rm G_{16}$-type molecules in the electronic ground state, based on the symmetry-adapted variational method. We define conditions for which lower-dimensional models, commonly used to simulate the strong-field control of molecular torsions, are reliable approximations to a four-dimensional treatment. In particular, we study the role of different types of rotational-torsional couplings---the field-free coupling and the field-induced coupling---and show that the conclusions recently drawn on the role of rotational-torsional couplings in the process of torsional alignment are not correct. Furthermore, we demonstrate how important an adequate description of the molecular polarizability is for reliably predicting the torsional alignment.

physics.chem-ph↗