1,114 research outputs found

### Probing BEC phase fluctuations with atomic quantum dots

We consider the dephasing of two internal states |0> and |1> of a trapped
impurity atom, a so-called atomic quantum dot (AQD), where only state |1>
couples to a Bose-Einstein condensate (BEC). A direct relation between the
dephasing of the internal states of the AQD and the temporal phase fluctuations
of the BEC is established. Based on this relation we suggest a scheme to probe
BEC phase fluctuations nondestructively via dephasing measurements of the AQD.
In particular, the scheme allows to trace the dependence of the phase
fluctuations on the trapping geometry of the BEC.Comment: 11 pages, 3 figure

### Out-of-equilibrium Correlated Systems : Bipartite Entanglement as a Probe of Thermalization

Thermalization play a central role in out-of-equilibrium physics of ultracold
atoms or electronic transport phenomena. On the other hand, entanglement
concepts have proven to be extremely useful to investigate quantum phases of
matter. Here, it is argued that **bipartite** entanglement measures provide key
information on out-of-equilibrium states and might therefore offer stringent
thermalization criteria. This is illustrated by considering a global quench in
an (extended) XXZ spin-1/2 chain across its (zero-temperature) quantum critical
point. A non-local **bipartition** of the chain **preserving translation
symmetry** is proposed. The time-evolution after the quench of the **reduced**
density matrix of the half-system is computed and its associated
(time-dependent) entanglement spectrum is analyzed. Generically, the
corresponding entanglement entropy quickly reaches a "plateau" after a short
transient regime. However, in the case of the integrable XXZ chain, the
low-energy entanglement spectrum still reveals strong time-fluctuations. In
addition, its infinite-time average shows strong deviations from the spectrum
of a Boltzmann thermal density matrix. In contrast, when the integrability of
the model is broken (by small next-nearest neighbor couplings), the
entanglement spectra of the time-average and thermal density matrices become
remarkably similar.Comment: extended version: 15 pages, 9 figure

### High field fractional quantum Hall effect in optical lattices

We consider interacting bosonic atoms in an optical lattice subject to a
large simulated magnetic field. We develop a model similar to a bilayer
fractional quantum Hall system valid near simple rational numbers of magnetic
flux quanta per lattice cell. Then we calculate its ground state, magnetic
lengths, fractional fillings, and find unexpected sign changes in the Hall
current. Finally we study methods for detecting these novel features via shot
noise and Hall current measurements.Comment: 4 pages, 4 figures, accepted by PR

### Multipartite entanglement detection in bosons

We propose a simple quantum network to detect multipartite entangled states
of bosons, and show how to implement this network for neutral atoms stored in
an optical lattice. We investigate the special properties of cluster states,
multipartite entangled states and superpositions of distinct macroscopic
quantum states that can be identified by the network.Comment: 4 pages, 2 figure

### Signatures of the superfluid to Mott-insulator transition in the excitation spectrum of ultracold atoms

We present a detailed analysis of the dynamical response of ultra-cold
bosonic atoms in a one-dimensional optical lattice subjected to a periodic
modulation of the lattice depth. Following the experimental realization by
Stoferle et al [Phys. Rev. Lett. 92, 130403 (2004)] we study the excitation
spectrum of the system as revealed by the response of the total energy as a
function of the modulation frequency Omega. By using the Time Evolving Block
Decimation algorithm, we are able to simulate one-dimensional systems
comparable in size to those in the experiment, with harmonic trapping and
across many lattice depths ranging from the Mott-insulator to the superfluid
regime. Our results produce many of the features seen in the experiment, namely
a broad response in the superfluid regime, and narrow discrete resonances in
the Mott-insulator regime. We identify several signatures of the
superfluid-Mott insulator transition that are manifested in the spectrum as it
evolves from one limit to the other.Comment: 18 pages and 12 figures; Some improved results and additional
references. To appear in a special issue of New J. Phy

### 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

### 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

### Creation of effective magnetic fields in optical lattices: The Hofstadter butterfly for cold neutral atoms

We investigate the dynamics of neutral atoms in a 2D optical lattice which
traps two distinct internal states of the atoms in different columns. Two Raman
lasers are used to coherently transfer atoms from one internal state to the
other, thereby causing hopping between the different columns. By adjusting the
laser parameters appropriately we can induce a non vanishing phase of particles
moving along a closed path on the lattice. This phase is proportional to the
enclosed area and we thus simulate a magnetic flux through the lattice. This
setup is described by a Hamiltonian identical to the one for electrons on a
lattice subject to a magnetic field and thus allows us to study this equivalent
situation under very well defined controllable conditions. We consider the
limiting case of huge magnetic fields -- which is not experimentally accessible
for electrons in metals -- where a fractal band structure, the Hofstadter
butterfly, characterizes the system.Comment: 6 pages, RevTe

### Creation of resilient entangled states and a resource for measurement-based quantum computation with optical superlattices

We investigate how to create entangled states of ultracold atoms trapped in
optical lattices by dynamically manipulating the shape of the lattice
potential. We consider an additional potential (the superlattice) that allows
both the splitting of each site into a double well potential, and the control
of the height of potential barrier between sites. We use superlattice
manipulations to perform entangling operations between neighbouring qubits
encoded on the Zeeman levels of the atoms without having to perform transfers
between the different vibrational states of the atoms. We show how to use
superlattices to engineer many-body entangled states resilient to collective
dephasing noise. Also, we present a method to realize a 2D resource for
measurement-based quantum computing via Bell-pair measurements. We analyze
measurement networks that allow the execution of quantum algorithms while
maintaining the resilience properties of the system throughout the computation.Comment: 23 pages, 6 figures, IOP style, published in New Journal of Physics.
Minor corrections/few typos remove

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