975 research outputs found
Noise assisted Ramsey interferometry
I analyze a metrological strategy for improving the precision of frequency
estimation via Ramsey interferometry with strings of atoms in the presence of
correlated dephasing. This strategy does not employ entangled states, but
rather a product state which evolves into a stationary state under the
influence of correlated dephasing. It is shown that by using this state an
improvement in precision compared to standard Ramsey interferometry can be
gained. This improvement is not an improvement in scaling, i.e. the estimation
precision has the same scaling with the number of atoms as the standard quantum
limit, but an improvement proportional to the free evolution time in the Ramsey
interferometer. Since a stationary state is used, this evolution time can be
substantially larger than in standard Ramsey interferometry which is limited by
the coherence time of the atoms.Comment: 8+1 pages; 5 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
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
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
Quantum phase estimation with lossy interferometers
We give a detailed discussion of optimal quantum states for optical two-mode
interferometry in the presence of photon losses. We derive analytical formulae
for the precision of phase estimation obtainable using quantum states of light
with a definite photon number and prove that maximization of the precision is a
convex optimization problem. The corresponding optimal precision, i.e. the
lowest possible uncertainty, is shown to beat the standard quantum limit thus
outperforming classical interferometry. Furthermore, we discuss more general
inputs: states with indefinite photon number and states with photons
distributed between distinguishable time bins. We prove that neither of these
is helpful in improving phase estimation precision.Comment: 12 pages, 5 figure
Entangled states of trapped ions allow measuring the magnetic field gradient of a single atomic spin
Using trapped ions in an entangled state we propose detecting a magnetic
dipole of a single atom at distance of a few m. This requires a
measurement of the magnetic field gradient at a level of about 10
Tesla/m. We discuss applications e.g. in determining a wide variation of
ionic magnetic moments, for investigating the magnetic substructure of ions
with a level structure not accessible for optical cooling and detection,and for
studying exotic or rare ions, and molecular ions. The scheme may also be used
for measureing spin imbalances of neutral atoms or atomic ensembles trapped by
optical dipole forces. As the proposed method relies on techniques well
established in ion trap quantum information processing it is within reach of
current technology.Comment: 4 pages, 2 fi
Entangling strings of neutral atoms in 1D atomic pipeline structures
We study a string of neutral atoms with nearest neighbor interaction in a 1D
beam splitter configuration, where the longitudinal motion is controlled by a
moving optical lattice potential. The dynamics of the atoms crossing the beam
splitter maps to a 1D spin model with controllable time dependent parameters,
which allows the creation of maximally entangled states of atoms by crossing a
quantum phase transition. Furthermore, we show that this system realizes
protected quantum memory, and we discuss the implementation of one- and
two-qubit gates in this setup.Comment: 4 pages, REVTEX, revised version: improvements in introduction and
figure
Entanglement Percolation with Bipartite Mixed States
We develop a concept of entanglement percolation for long-distance singlet
generation in quantum networks with neighboring nodes connected by partially
entangled bipartite mixed states. We give a necessary and sufficient condition
on the class of mixed network states for the generation of singlets. States
beyond this class are insufficient for entanglement percolation. We find that
neighboring nodes are required to be connected by multiple partially entangled
states and devise a rich variety of distillation protocols for the conversion
of these states into singlets. These distillation protocols are suitable for a
variety of network geometries and have a sufficiently high success probability
even for significantly impure states. In addition to this, we discuss possible
further improvements achievable by using quantum strategies including
generalized forms of entanglement swapping.Comment: 6+ pages, 5 figures; Published versio
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