2,073 research outputs found
The Optical Excitation of Zigzag Carbon Nanotubes with Photons Guided in Nanofibers
We consider the excitation of electrons in semiconducting carbon nanotubes by
photons from the evanescent field created by a subwavelength-diameter optical
fiber. The strongly changing evanescent field of such nanofibers requires
dropping the dipole approximation. We show that this leads to novel effects,
especially a high dependence of the photon absorption on the relative
orientation and geometry of the nanotube-nanofiber setup in the optical and
near infrared domain. In particular, we calculate photon absorption
probabilities for a straight nanotube and nanofiber depending on their relative
angle. Nanotubes orthogonal to the fiber are found to perform much better than
parallel nanotubes when they are short. As the nanotube gets longer the
absorption of parallel nanotubes is found to exceed the orthogonal nanotubes
and approach 100% for extremely long nanotubes. In addition, we show that if
the nanotube is wrapped around the fiber in an appropriate way the absorption
is enhanced. We find that optical and near infrared photons could be converted
to excitations with efficiencies that may exceed 90%. This may provide
opportunities for future photodetectors and we discuss possible setups.Comment: 14 pages, 14 figure
Long Distance Entanglement Generation in 2D Networks
We consider 2D networks composed of nodes initially linked by two-qubit mixed
states. In these networks we develop a global error correction scheme that can
generate distance-independent entanglement from arbitrary network geometries
using rank two states. By using this method and combining it with the concept
of percolation we also show that the generation of long distance entanglement
is possible with rank three states. Entanglement percolation and global error
correction have different advantages depending on the given situation. To
reveal the trade-off between them we consider their application on networks
containing pure states. In doing so we find a range of pure-state schemes, each
of which has applications in particular circumstances: For instance, we can
identify a protocol for creating perfect entanglement between two distant
nodes. However, this protocol can not generate a singlet between any two nodes.
On the other hand, we can also construct schemes for creating entanglement
between any nodes, but the corresponding entanglement fidelity is lower.Comment: 10 pages, 9 figures, 1 tabl
Singlet Generation in Mixed State Quantum Networks
We study the generation of singlets in quantum networks with nodes initially
sharing a finite number of partially entangled bipartite mixed states. We prove
that singlets between arbitrary nodes in such networks can be created if and
only if the initial states connecting the nodes have a particular form. We then
generalize the method of entanglement percolation, previously developed for
pure states, to mixed states of this form. As part of this, we find and compare
different distillation protocols necessary to convert groups of mixed states
shared between neighboring nodes of the network into singlets. In addition, we
discuss protocols that only rely on local rules for the efficient connection of
two remote nodes in the network via entanglement swapping. Further improvements
of the success probability of singlet generation are developed by using
particular forms of `quantum preprocessing' on the network. This includes
generalized forms of entanglement swapping and we show how such strategies can
be embedded in regular and hierarchical quantum networks.Comment: 17 pages, 21 figure
Fast initialization of a high-fidelity quantum register using optical superlattices
We propose a method for the fast generation of a quantum register of
addressable qubits consisting of ultracold atoms stored in an optical lattice.
Starting with a half filled lattice we remove every second lattice barrier by
adiabatically switching on a superlattice potential which leads to a long
wavelength lattice in the Mott insulator state with unit filling. The larger
periodicity of the resulting lattice could make individual addressing of the
atoms via an external laser feasible. We develop a Bose-Hubbard-like model for
describing the dynamics of cold atoms in a lattice when doubling the lattice
periodicity via the addition of a superlattice potential. The dynamics of the
transition from a half filled to a commensurately filled lattice is analyzed
numerically with the help of the Time Evolving Block Decimation algorithm and
analytically using the Kibble-Zurek theory. We show that the time scale for the
whole process, i.e. creating the half filled lattice and subsequent doubling of
the lattice periodicity, is significantly faster than adiabatic direct quantum
freezing of a superfluid into a Mott insulator for large lattice periods. Our
method therefore provides a high fidelity quantum register of addressable
qubits on a fast time scale.Comment: 22 pages, 9 figures, IOP style. Revised version to appear in NJ
Dissipative quantum light field engineering
We put forward a dissipative preparation scheme for strongly correlated
photon states. Our approach is based on a two-photon loss mechanism that is
realised via a single four-level atom inside a bimodal optical cavity. Each
elementary two-photon emission event removes one photon out of each of the two
modes. The dark states of this loss mechanism are given by NOON states and
arbitrary superpositions thereof. We find that the steady state of the two
cavity modes exhibits entanglement and for certain parameters, a mixture of two
coherent entangled states is produced. We discuss how the quantum correlations
in the cavity modes and the output fields can be measured.Comment: 11 pages, 5 figure
Broadband study of blazar 1ES 1959+650 during flaring state in 2016
Aim : The nearby TeV blazar 1ES 1959+650 (z=0.047) was reported to be in
flaring state during June - July 2016 by Fermi-LAT, FACT, MAGIC and VERITAS
collaborations. We studied the spectral energy distributions (SEDs) in
different states of the flare during MJD 57530 - 57589 using simultaneous
multiwaveband data to understand the possible broadband emission scenario
during the flare. Methods : The UV/optical and X-ray data from UVOT and XRT
respectively on board Swift and high energy -ray data from Fermi-LAT
are used to generate multiwaveband lightcurves as well as to obtain high flux
states and quiescent state SEDs. The correlation and lag between different
energy bands is quantified using discrete correlation function. The synchrotron
self Compton (SSC) model was used to reproduce the observed SEDs during flaring
and quiescent states of the source. Results : A decent correlation is seen
between X-ray and high energy -ray fluxes. The spectral hardening with
increase in the flux is seen in X-ray band. The powerlaw index vs flux plot in
-ray band indicates the different emission regions for 0.1 - 3 GeV and
3-300 GeV energy photons. Two zone SSC model satisfactorily fits the observed
broadband SEDs. The inner zone is mainly responsible for producing synchrotron
peak and high energy -ray part of the SED in all states. The second
zone is mainly required to produce less variable optical/UV and low energy
-ray emission. Conclusions : Conventional single zone SSC model does
not satisfactorily explain broadband emission during observation period
considered. There is an indication of two emission zones in the jet which are
responsible for producing broadband emission from optical to high energy
-rays.Comment: 11 pages, 12 figures, Accepted in A&
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