164 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
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
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
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.
Two-fluid hydrodynamic model for semiconductors
The hydrodynamic Drude model (HDM) has been successful in describing the
optical properties of metallic nanostructures, but for semiconductors where
several different kinds of charge carriers are present, an extended theory is
required. We present a two-fluid hydrodynamic model for semiconductors
containing electrons and holes (from thermal or external excitation) or light
and heavy holes (in -doped materials). The two-fluid model predicts the
existence of two longitudinal modes, an acoustic and an optical, whereas only
an optical mode is present in the HDM. By extending nonlocal Mie theory to two
plasmas, we are able to simulate the optical properties of two-fluid
nanospheres and predict that the acoustic mode gives rise to peaks in the
extinction spectra that are absent in the HDM.Comment: Accepted in PRB. 17 pages, 9 figures, 1 tabl
Qubit coherence decay down to threshold: influence of substrate dimensions
Keeping single-qubit quantum coherence above some threshold value not far
below unity is a prerequisite for fault-tolerant quantum error correction
(QEC). We study the initial dephasing of solid-state qubits in the
independent-boson model, which describes well recent experiments on quantum dot
(QD) excitons both in bulk and in substrates of reduced geometry such as
nanotubes. Using explicit expressions for the exact coherence dynamics, a
minimal QEC rate is identified in terms of the error threshold, temperature,
and qubit-environment coupling strength. This allows us to systematically study
the benefit of a current trend towards substrates with reduced dimensions.Comment: 4 pages, 4 figure
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