1,199 research outputs found
Instantaneous coherent destruction of tunneling and fast quantum state preparation for strongly pulsed spin qubits in diamond
Qubits driven by resonant strong pulses are studied and a parameter regime is
explored in which the dynamics can be solved in closed form. Instantaneous
coherent destruction of tunneling can be seen for longer pulses, whereas
shorter pulses allow a fast preparation of the qubit state. Results are
compared with recent experiments of pulsed nitrogen-vacancy center spin qubits
in diamond.Comment: 9 pages, 4 figures. Published in the special issue of Chemical
Physics in honor of Peter Hangg
Quantum optical effective-medium theory for layered metamaterials
The quantum optics of metamaterials starts with the question whether the same
effective-medium theories apply as in classical optics. In general the answer
is negative. For active plasmonics but also for some passive metamaterials, we
show that an additional effective-medium parameter is indispensable besides the
effective index, namely the effective noise-photon distribution. Only with the
extra parameter can one predict how well the quantumness of states of light is
preserved in the metamaterial. The fact that the effective index alone is not
always sufficient and that one additional effective parameter suffices in the
quantum optics of metamaterials is both of fundamental and practical interest.
Here from a Lagrangian description of the quantum electrodynamics of media with
both linear gain and loss, we compute the effective noise-photon distribution
for quantum light propagation in arbitrary directions in layered metamaterials,
thereby detailing and generalizing our recent work [ E. Amooghorban et al.,
Phys. Rev. Lett. , 153602 (2013)]. The effective index with its
direction and polarization dependence is the same as in classical
effective-medium theories. As our main result we derive both for passive and
for active media how the value of the effective noise-photon distribution too
depends on the polarization and propagation directions of the light.
Interestingly, for TE-polarized light incident on passive metamaterials, the
noise-photon distribution reduces to a thermal distribution, but for
TM-polarized light it does not. We illustrate the robustness of our quantum
optical effective-medium theory by accurate predictions both for power spectra
and for balanced homodyne detection of output quantum states of the
metamaterial.Comment: 8 figure
Foerster resonance energy transfer rate and local density of optical states are uncorrelated in any dielectric nanophotonic medium
Motivated by the ongoing debate about nanophotonic control of Foerster
resonance energy transfer (FRET), notably by the local density of optical
states (LDOS), we study an analytic model system wherein a pair of ideal dipole
emitters - donor and acceptor - exhibit energy transfer in the vicinity of an
ideal mirror. The FRET rate is controlled by the mirror up to distances
comparable to the donor-acceptor distance, that is, the few-nanometer range.
For vanishing distance, we find a complete inhibition or a four-fold
enhancement, depending on dipole orientation. For mirror distances on the
wavelength scale, where the well-known `Drexhage' modification of the
spontaneous-emission rate occurs, the FRET rate is constant. Hence there is no
correlation between the Foerster (or total) energy transfer rate and the LDOS.
At any distance to the mirror, the total energy transfer between a
closely-spaced donor and acceptor is dominated by Foerster transfer, i.e., by
the static dipole-dipole interaction that yields the characteristic
inverse-sixth-power donor-acceptor distance dependence in homogeneous media.
Generalizing to arbitrary inhomogeneous media with weak dispersion and weak
absorption in the frequency overlap range of donor and acceptor, we derive two
main theoretical results. Firstly, the spatially dependent Foerster energy
transfer rate does not depend on frequency, hence not on the LDOS. Secondly the
FRET rate is expressed as a frequency integral of the imaginary part of the
Green function. This leads to an approximate FRET rate in terms of the LDOS
integrated over a huge bandwidth from zero frequency to about 10 times the
donor emission frequency, corresponding to the vacuum-ultraviolet. Even then,
the broadband LDOS hardly contributes to the energy transfer rates. We discuss
practical consequences including quantum information processing.Comment: 17 pages, 9 figure
Entanglement creation in circuit QED via Landau-Zener sweeps
A qubit may undergo Landau-Zener transitions due to its coupling to one or
several quantum harmonic oscillators. We show that for a qubit coupled to one
oscillator, Landau-Zener transitions can be used for single-photon generation
and for the controllable creation of qubit-oscillator entanglement, with
state-of-the-art circuit QED as a promising realization. Moreover, for a qubit
coupled to two cavities, we show that Landau-Zener sweeps of the qubit are well
suited for the robust creation of entangled cavity states, in particular
symmetric Bell states, with the qubit acting as the entanglement mediator. At
the heart of our proposals lies the calculation of the exact Landau-Zener
transition probability for the qubit, by summing all orders of the
corresponding series in time-dependent perturbation theory. This transition
probability emerges to be independent of the oscillator frequencies, both
inside and outside the regime where a rotating-wave approximation is valid.Comment: 12 pages, 7 figure
Projected-Dipole Model for Quantum Plasmonics
Quantum effects of plasmonic phenomena have been explored through ab-initio
studies, but only for exceedingly small metallic nanostructures, leaving most
experimentally relevant structures too large to handle. We propose instead an
effective description with the computationally appealing features of classical
electrodynamics, while quantum properties are described accurately through an
infinitely thin layer of dipoles oriented normally to the metal surface. The
nonlocal polarizability of the dipole layer is mapped from the free-electron
distribution near the metal surface as obtained with 1D quantum calculations,
such as time-dependent density-functional theory (TDDFT), and is determined
once and for all. The model can be applied to any system size that is tractable
within classical electrodynamics, while capturing quantum plasmonic aspects of
nonlocal response and a finite work function with TDDFT-level accuracy.
Applying the theory to dimers we find quantum-corrections to the hybridization
even in mesoscopic dimers as long as the gap is sub-nanometric itself.Comment: Supplemental Material is available upon request to author
How nonlocal damping reduces plasmon-enhanced fluorescence in ultranarrow gaps
The nonclassical modification of plasmon-assisted fluorescence enhancement is
theoretically explored by placing two-level dipole emitters at the narrow gaps
encountered in canonical plasmonic architectures, namely dimers and trimers of
different metallic nanoparticles. Through detailed simulations, in comparison
with appropriate analytical modelling, it is shown that within classical
electrodynamics, and for the reduced separations explored here, fluorescence
enhancement factors of the order of can be achieved, with a divergent
behaviour as the particle touching regime is approached. This remarkable
prediction is mainly governed by the dramatic increase in excitation rate
triggered by the corresponding field enhancement inside the gaps. Nevertheless,
once nonclassical corrections are included, the amplification factors decrease
by up to two orders of magnitude and a saturation regime for narrower gaps is
reached. These nonclassical limitations are demonstrated by simulations based
on the generalised nonlocal optical response theory, which accounts in an
efficient way not only for nonlocal screening, but also for the enhanced Landau
damping near the metal surface. A simple strategy to introduce nonlocal
corrections to the analytic solutions is also proposed. It is therefore shown
that the nonlocal optical response of the metal imposes more realistic, finite
upper bounds to the enhancement feasible with ultrasmall plasmonic cavities,
thus providing a theoretical description closer to state of the art
experiments
Robustness of the Rabi splitting under nonlocal corrections in plexcitonics
We explore theoretically how nonlocal corrections in the description of the
metal affect the strong coupling between excitons and plasmons in typical
examples where nonlocal effects are anticipated to be strong, namely small
metallic nanoparticles, thin metallic nanoshells or dimers with narrow
separations, either coated with or encapsulating an excitonic layer. Through
detailed simulations based on the generalised nonlocal optical response theory,
which simultaneously accounts both for modal shifts due to screening and for
surface-enhanced Landau damping, we show that, contrary to expectations, the
influence of nonlocality is rather limited, as in most occasions the width of
the Rabi splitting remains largely unaffected and the two hybrid modes are well
distinguishable. We discuss how this behaviour can be understood in view of the
popular coupled-harmonic-oscillator model, while we also provide analytic
solutions based on Mie theory to describe the hybrid modes in the case of
matryoshka-like single nanoparticles. Our analysis provides an answer to a so
far open question, that of the influence of nonlocality on strong coupling, and
is expected to facilitate the design and study of plexcitonic architectures
with ultrafine geometrical details
Spontaneous-emission rates in finite photonic crystals of plane scatterers
The concept of a plane scatterer that was developed earlier for scalar waves
is generalized so that polarization of light is included. Starting from a
Lippmann-Schwinger formalism for vector waves, we show that the Green function
has to be regularized before T-matrices can be defined in a consistent way.
After the regularization, optical modes and Green functions are determined
exactly for finite structures built up of an arbitrary number of parallel
planes, at arbitrary positions, and where each plane can have different optical
properties. The model is applied to the special case of finite crystals
consisting of regularly spaced identical planes, where analytical methods can
be taken further and only light numerical tasks remain. The formalism is used
to calculate position- and orientation-dependent spontaneous-emission rates
inside and near the finite photonic crystals. The results show that emission
rates and reflection properties can differ strongly for scalar and for vector
waves. The finite size of the crystal influences the emission rates. For
parallel dipoles close to a plane, emission into guided modes gives rise to a
peak in the frequency-dependent emission rate.Comment: 18 pages, 6 figures, to be published in Phys. Rev.
Incomplete pure dephasing of N-qubit entangled W states
We consider qubits in a linear arrangement coupled to a bosonic field which
acts as a quantum heat bath and causes decoherence. By taking the spatial
separation of the qubits explicitly into account, the reduced qubit dynamics
acquires an additional non-Markovian element. We investigate the time evolution
of an entangled many-qubit W state, which for vanishing qubit separation
remains robust under pure dephasing. For finite separation, by contrast, the
dynamics is no longer decoherence-free. On the other hand, spatial noise
correlations may prevent a complete dephasing. While a standard Bloch-Redfield
master equation fails to describe this behavior even qualitatively, we propose
instead a widely applicable causal master equation. Here we employ it to
identify and characterize decoherence-poor subspaces. Consequences for quantum
error correction are discussed.Comment: 14 pages, 6 figures, revised version, to appear in Phys. Rev.
Stochastic dynamic simulation of fruit abortion: a case study of sweet pepper
Abortion of reproductive organs diminishes yields in many crops. In indeterminate greenhouse crops, alternating periods of fruit abortion and fruit set exist, resulting in fluctuations in fruit yield. Factors affecting the level of abortion are e.g., the supply and demand for assimilates (source and sink strength, respectively), temperature and cultivar. However, simulation of fruit abortion is still a weak part of crop simulation models. Variation in fruit abortion exists between plants, which results in differences in the timing and the number of set fruits. Therefore, simulating fruit abortion with variation could give more realistic simulation results. The probability of a fruit to abort should be related to factors like source strength and sink strength. The more favourable the circumstances are for fruit abortion, e.g., low source strength or high sink strength, the more likely it is that the fruit aborts. Survival analysis estimates parameters quantifying the influence of explanatory variables on the abortion rate. Time-varying explanatory variables can be used in the analysis. In a case study, we used survival analysis to analyse a data set with observations on flowering, fruit abortion and fruit harvest for sweet pepper. Source and sink strength were used as explanatory variables. The resulting equation determining the probability of abortion per day was implemented in a simple simulation model to simulate fruit set. The model output, as an average of 100 plants, showed similar timing in the fluctuations in fruit set as the observations, although the amplitude of the fluctuations was in some cases underestimated. The percentage fruit set was simulated correctl
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