102 research outputs found
Ab initio calculation of H + He charge transfer cross sections for plasma physics
The charge transfer in low energy (0.25 to 150 eV/amu) H() + He
collisions is investigated using a quasi-molecular approach for the as
well as the first two singlet states. The diabatic potential energy
curves of the HeH molecular ion are obtained from the adiabatic potential
energy curves and the non-adiabatic radial coupling matrix elements using a
two-by-two diabatization method, and a time-dependent wave-packet approach is
used to calculate the state-to-state cross sections. We find a strong
dependence of the charge transfer cross section in the principal and orbital
quantum numbers and of the initial or final state. We estimate the
effect of the non-adiabatic rotational couplings, which is found to be
important even at energies below 1 eV/amu. However, the effect is small on the
total cross sections at energies below 10 eV/amu. We observe that to calculate
charge transfer cross sections in a manifold, it is only necessary to
include states with , and we discuss the limitations of our
approach as the number of states increases.Comment: 14 pages, 10 figure
Simulation of the elementary evolution operator with the motional states of an ion in an anharmonic trap
Following a recent proposal of L. Wang and D. Babikov, J. Chem. Phys. 137,
064301 (2012), we theoretically illustrate the possibility of using the
motional states of a ion trapped in a slightly anharmonic potential to
simulate the single-particle time-dependent Schr\"odinger equation. The
simulated wave packet is discretized on a spatial grid and the grid points are
mapped on the ion motional states which define the qubit network. The
localization probability at each grid point is obtained from the population in
the corresponding motional state. The quantum gate is the elementary evolution
operator corresponding to the time-dependent Schr\"odinger equation of the
simulated system. The corresponding matrix can be estimated by any numerical
algorithm. The radio-frequency field able to drive this unitary transformation
among the qubit states of the ion is obtained by multi-target optimal control
theory. The ion is assumed to be cooled in the ground motional state and the
preliminary step consists in initializing the qubits with the amplitudes of the
initial simulated wave packet. The time evolution of the localization
probability at the grids points is then obtained by successive applications of
the gate and reading out the motional state population. The gate field is
always identical for a given simulated potential, only the field preparing the
initial wave packet has to be optimized for different simulations. We check the
stability of the simulation against decoherence due to fluctuating electric
fields in the trap electrodes by applying dissipative Lindblad dynamics.Comment: 31 pages, 8 figures. Revised version. New title, new figure and new
reference
Towards Laser Control of Open Quantum Systems: Memory Effects
Laser control of Open Quantum Systems (OQS) is a challenging issue as
compared to its counterpart in isolated small size molecules, basically due to
very large numbers of degrees of freedom to be accounted for. Such a control
aims at appropriately optimizing decoherence processes of a central two-level
system (a given vibrational mode, for instance) towards its environmental bath
(including, for instance, all other normal modes). A variety of applications
could potentially be envisioned, either to preserve the central system from
decaying (long duration molecular alignment or orientation, qubit decoherence
protection) or, to speed up the information flow towards the bath (efficient
charge or proton transfers in long chain organic compounds). Achieving such
controls require some quantitative measures of decoherence in relation with
memory effects in the bath response, actually given by the degree of
non-Markovianity. Characteristic decoherence rates of a Spin-Boson model are
calculated using a Nakajima-Zwanzig type master equation with converged HEOM
expansion for the memory kernel. It is shown that, by adequately tuning the
two-level transition frequency through a controlled Stark shift produced by an
external laser field, non-Markovianity can be enhanced in a continuous way
leading to a first attempt towards the control of OQS
The role of the multiple excitation manifold in a driven quantum simulator of an antenna complex
Biomolecular light-harvesting antennas operate as nanoscale devices in a
regime where the coherent interactions of individual light, matter and
vibrational quanta are non-perturbatively strong. The complex behaviour arising
from this could, if fully understood, be exploited for myriad energy
applications. However, non-perturbative dynamics are computationally
challenging to simulate, and experiments on biomaterials explore very limited
regions of the non-perturbative parameter space. So-called `quantum simulators'
of light-harvesting models could provide a solution to this problem, and here
we employ the hierarchical equations of motion technique to investigate recent
superconducting experiments of Poto{\v{c}}nik (Nat. Com.
9, 904 (2018)) used to explore excitonic energy capture. By explicitly
including the role of optical driving fields, non-perturbative dephasing noise
and the full multi-excitation Hilbert space of a three-qubit quantum circuit,
we predict the measureable impact of these factors on transfer efficiency. By
analysis of the eigenspectrum of the network, we uncover a structure of energy
levels that allows the network to exploit optical `dark' states and excited
state absorption for energy transfer. We also confirm that time-resolvable
coherent oscillations could be experimentally observed, even under strong,
non-additive action of the driving and optical fields
Observation of resonance trapping in an open microwave cavity
The coupling of a quantum mechanical system to open decay channels has been
theoretically studied in numerous works, mainly in the context of nuclear
physics but also in atomic, molecular and mesoscopic physics. Theory predicts
that with increasing coupling strength to the channels the resonance widths of
all states should first increase but finally decrease again for most of the
states. In this letter, the first direct experimental verification of this
effect, known as resonance trapping, is presented. In the experiment a
microwave Sinai cavity with an attached waveguide with variable slit width was
used.Comment: to be published in Phys. Rev. Let
Not gate in a cis-trans photoisomerization model
We numerically study the implementation of a NOT gate by laser pulses in a
model molecular system presenting two electronic surfaces coupled by non
adiabatic interactions. The two states of the bit are the fundamental states of
the cis-trans isomers of the molecule. The gate is classical in the sense that
it involves a one-qubit flip so that the encoding of the outputs is based on
population analysis which does not take the phases into account. This gate can
also be viewed as a double photo-switch process with the property that the same
electric field controls the two isomerizations. As an example, we consider
one-dimensional cuts in a model of the retinal in rhodopsin already proposed in
the literature. The laser pulses are computed by the Multi Target Optimal
Control Theory with chirped pulses as trial fields. Very high fidelities are
obtained. We also examine the stability of the control when the system is
coupled to a bath of oscillators modelled by an Ohmic spectral density. The
bath correlation time scale being smaller than the pulse duration the dynamics
is carried out in the Markovian approximation.Comment: 29 pages, 7 figure
Resonance trapping and saturation of decay widths
Resonance trapping appears in open many-particle quantum systems at high
level density when the coupling to the continuum of decay channels reaches a
critical strength. Here a reorganization of the system takes place and a
separation of different time scales appears. We investigate it under the
influence of additional weakly coupled channels as well as by taking into
account the real part of the coupling term between system and continuum. We
observe a saturation of the mean width of the trapped states. Also the decay
rates saturate as a function of the coupling strength. The mechanism of the
saturation is studied in detail. In any case, the critical region of
reorganization is enlarged. When the transmission coefficients for the
different channels are different, the width distribution is broadened as
compared to a chi_K^2 distribution where K is the number of channels. Resonance
trapping takes place before the broad state overlaps regions beyond the
extension of the spectrum of the closed system.Comment: 18 pages, 8 figures, accepted by Phys. Rev.
Collectivity, Phase Transitions and Exceptional Points in Open Quantum Systems
Phase transitions in open quantum systems, which are associated with the
formation of collective states of a large width and of trapped states with
rather small widths, are related to exceptional points of the Hamiltonian.
Exceptional points are the singularities of the spectrum and eigenfunctions,
when they are considered as functions of a coupling parameter. In the present
paper this parameter is the coupling strength to the continuum. It is shown
that the positions of the exceptional points (their accumulation point in the
thermodynamical limit) depend on the particular type and energy dependence of
the coupling to the continuum in the same way as the transition point of the
corresponding phase transition.Comment: 22 pages, 4 figure
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