15 research outputs found
Nuclear spin selective alignment of ethylene and analogues
We investigate the alignment of ethylene and of some of its analogues via
short, non-resonant laser pulses and show that it depends crucially on the
nuclear spin of the molecules. We calculate the time-dependent alignment
factors of the four nuclear spin isomers of ethylene and analyze them by
comparison with the symmetric top molecule allene. Moreover, we explore how
the nuclear spin selective alignment depends on the asymmetry of the molecules
and on the intensity of the laser pulse. As an application, we discuss how
nuclear spin selective alignment could be applied in order to separate
different isotopomers of ethylene
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.Comment: 18 pages, 7 figures, 3 tables, 3 algorithm
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.Comment: 16 pages, 8 figure
Full quantum control of enantiomer-selective state transfer in chiral molecules despite degeneracy
The driven quantum asymmetric top is an important paradigm in molecular physics with applications ranging from quantum information to chiral-sensitive spectroscopy. A key prerequisite for these applications is the ability to completely control the rotational dynamics. The inherent degeneracy of quantum rotors poses a challenge for quantum control since selecting a particular rotational state cannot be achieved by spectral selection alone. Here, we prove complete controllability for rotational states of an asymmetric top belonging to degenerate values of the orientational quantum number M. Based on this insight, we construct a pulse sequence that energetically separates population in degenerate M-states. Introducing the concept of enantio-selective controllability, we determine the conditions for complete enantiomer-specific population transfer in chiral molecules and construct pulse sequences for the example of propanediol and carvone molecules for population initially distributed over degenerate M-states. Our work shows how to leverage controllability analysis for the solution of practical quantum control problems
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
Effects of Molecular Symmetry on Quantum Reaction Dynamics: Novel Aspects of Photoinduced Nonadiabatic Dynamics
Nonadiabatic coupling terms (NACTs)
between different electronic
states lead to fast radiationless decay in photoexcited molecules.
Using molecular symmetry, i.e., symmetry with respect to permutation
of identical nuclei and inversion of the molecule in space, the irreducible
representations of the NACTs can be determined with a combination
of molecular symmetry arguments and quantization rules. Here, we extend
these symmetry rules for electronic states and coupling elements and
demonstrate the importance of molecular symmetry for nonadiabatic
nuclear dynamics. As an example, we consider the NACTs related to
the torsion around the CN bond in C<sub>5</sub>H<sub>4</sub>NH. We
present the results of quantum dynamical simulations of the photoinduced
large amplitude torsion on three coupled electronic states and show
how the interference between wavepackets leads to radiationless decay,
which depends on the symmetry of the NACTs. Moreover, we show that
the nuclear spin of the system determines the symmetry of the initial
nuclear wave function and thus influences the torsional dynamics.
This may open new possibilities for nuclear spin selective laser control
of nuclear dynamics
Lie algebra for rotational subsystems of a driven asymmetric top
Gefördert im Rahmen eines Open-Access-Transformationsvertrags mit dem Verla
Lie algebra for rotational subsystems of a driven asymmetric top
10 pages, 7 figuresInternational audienceWe 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