179 research outputs found
Mode-selective vibrational excitation induced by nonequilibrium transport processes in single-molecule junctions
In a nanoscale molecular junction at finite bias voltage,the intra-molecular
distribution of vibrational energy can strongly deviate from the thermal
equilibrium distribution and specific vibrational modes can be selectively
excited in a controllable way,regardless of the corresponding mode frequency.
This is demonstrated for generic models of asymmetric molecular junctions with
localized electronic states, employing a master equation as well as a
nonequilibrium Green's function approach. It is shown that the applied bias
voltage controls the excitation of specific vibrational modes coupled to these
states, by tuning their electronic population,which influences the efficiency
of vibrational cooling processes due to energy exchange with the leads.Comment: 12 pages, 4 figures, and Support Informatio
Advances in decoherence control
I address the current status of dynamical decoupling techniques in terms of
required control resources and feasibility. Based on recent advances in both
improving the theoretical design and assessing the control performance for
specific noise models, I argue that significant progress may still be possible
on the road of implementing decoupling under realistic constraints.Comment: 14 pages, 3 encapsulated eps figures. To appear in Journal of Modern
Optics, Special Proceedings Volume of the XXXIV Winter Colloquium on the
Physics of Quantum Electronics, Snowbird, Jan 200
Quantum Control of the Hyperfine Spin of a Cs Atom Ensemble
We demonstrate quantum control of a large spin-angular momentum associated
with the F=3 hyperfine ground state of 133Cs. A combination of time dependent
magnetic fields and a static tensor light shift is used to implement
near-optimal controls and map a fiducial state to a broad range of target
states, with yields in the range 0.8-0.9. Squeezed states are produced also by
an adiabatic scheme that is more robust against errors. Universal control
facilitates the encoding and manipulation of qubits and qudits in atomic ground
states, and may lead to improvement of some precision measurements.Comment: 4 pages, 4 figures (color
Hydrodynamic View of Wave-Packet Interference: Quantum Caves
Wave-packet interference is investigated within the complex quantum
Hamilton-Jacobi formalism using a hydrodynamic description. Quantum
interference leads to the formation of the topological structure of quantum
caves in space-time Argand plots. These caves consist of the vortical and
stagnation tubes originating from the isosurfaces of the amplitude of the wave
function and its first derivative. Complex quantum trajectories display
counterclockwise helical wrapping around the stagnation tubes and hyperbolic
deflection near the vortical tubes. The string of alternating stagnation and
vortical tubes is sufficient to generate divergent trajectories. Moreover, the
average wrapping time for trajectories and the rotational rate of the nodal
line in the complex plane can be used to define the lifetime for interference
features.Comment: 4 pages, 3 figures (major revisions with respect to the previous
version have been carried out
Cavity cooling of internal molecular motion
We predict that it is possible to cool rotational, vibrational, and translational degrees of freedom of molecules by coupling a molecular dipole transition to an optical cavity. The dynamics is numerically simulated for a realistic set of experimental parameters using OH molecules. The results show that the translational motion is cooled to a few μK and the internal state is prepared in one of the two ground states of the two decoupled rotational ladders in a few seconds. Shorter cooling times are expected for molecules with larger polarizability
Dynamic interference of photoelectrons produced by high-frequency laser pulses
The ionization of an atom by a high-frequency intense laser pulse, where the
energy of a single-photon is sufficient to ionize the system, is investigated
from first principles. It is shown that as a consequence of an AC Stark effect
in the continuum, the energy of the photoelectron follows the envelope of the
laser pulse. This is demonstrated to result in strong dynamic interference of
the photoelectrons of the same kinetic energy emitted at different times.
Numerically exact computations on the hydrogen atom demonstrate that the
dynamic interference spectacularly modifies the photoionization process and is
prominently manifested in the photoelectron spectrum by the appearance of a
distinct multi-peak pattern. The general theory is shown to be well
approximated by explicit analytical expressions which allow for a transparent
understanding of the discovered phenomena and for making predictions on the
dependence of the measured spectrum on the properties of the pulse.Comment: 5 figure
Optimal Control of Quantum Dynamics : A New Theoretical Approach
A New theoretical formalism for the optimal quantum control has been
presented. The approach stems from the consideration of describing the
time-dependent quantum system in terms of the real physical observables, viz.,
the probability density rho(x,t) and the quantum current j(x,t) which is well
documented in the Bohm's hydrodynamical formulation of quantum mechanics. The
approach has been applied for manipulating the vibrational motion of HBr in its
ground electronic state under an external electric field.Comment: 4 figure
Quantum process tomography of molecular dimers from two-dimensional electronic spectroscopy I: General theory and application to homodimers
Is it possible to infer the time evolving quantum state of a
multichromophoric system from a sequence of two-dimensional electronic spectra
(2D-ES) as a function of waiting time? Here we provide a positive answer for a
tractable model system: a coupled dimer. After exhaustively enumerating the
Liouville pathways associated to each peak in the 2D-ES, we argue that by
judiciously combining the information from a series of experiments varying the
polarization and frequency components of the pulses, detailed information at
the amplitude level about the input and output quantum states at the waiting
time can be obtained. This possibility yields a quantum process tomography
(QPT) of the single-exciton manifold, which completely characterizes the open
quantum system dynamics through the reconstruction of the process matrix. This
is the first of a series of two articles. In this manuscript, we specialize our
results to the case of a homodimer, where we prove that signals stemming from
coherence to population transfer and viceversa vanish upon isotropic averaging,
and therefore, only a partial QPT is possible in this case. However, this fact
simplifies the spectra, and it follows that only two polarization controlled
experiments (and no pulse-shaping requirements) suffice to yield the elements
of the process matrix which survive under isotropic averaging. The angle
between the two site transition dipole moments is self-consistently obtained
from the 2D-ES. Model calculations are presented, as well as an error analysis
in terms of the angle between the dipoles and peak overlap. In the second
article accompanying this study, we numerically exemplify the theory for
heterodimers and carry out a detailed error analysis for such case. This
investigation provides an important benchmark for more complex proposals of
quantum process tomography (QPT) via multidimensional spectroscopic
experiments
Coherent Optimal Control of Multiphoton Molecular Excitation
We give a framework for molecular multiphoton excitation process induced by
an optimally designed electric field. The molecule is initially prepared in a
coherent superposition state of two of its eigenfunctions. The relative phase
of the two superposed eigenfunctions has been shown to control the optimally
designed electric field which triggers the multiphoton excitation in the
molecule. This brings forth flexibility in desiging the optimal field in the
laboratory by suitably tuning the molecular phase and hence by choosing the
most favorable interfering routes that the system follows to reach the target.
We follow the quantum fluid dynamical formulation for desiging the electric
field with application to HBr molecule.Comment: 5 figure
Quantum Driven Dissipative Parametric Oscillator in a Blackbody Radiation Field
We consider the general open system problem of a charged quantum oscillator
confined in a harmonic trap, whose frequency can be arbitrarily modulated in
time, that interacts with both an incoherent quantized (blackbody) radiation
field and with an arbitrary coherent laser field. We assume that the oscillator
is initially in thermodynamic equilibrium with its environment, a
non-factorized initial density matrix of the system and the environment, and
that at the modulation of the frequency, the coupling to the incoherent
and the coherent radiation are switched on. The subsequent dynamics, induced by
the presence of the blackbody radiation and the laser field, is studied in the
framework of the influence functional approach. This approach allows
incorporating, in \emph{analytic closed formulae}, the non-Markovian character
of the oscillator-environment interaction at any temperature as well the
non-Markovian character of the blackbody radiation and its zero-point
fluctuations. Expressions for the time evolution of the covariance matrix
elements of the quantum fluctuations and the reduced density-operator are
obtained.Comment: 11 pages. It matches the published versio
- …