210 research outputs found
Probing magnetic order in ultracold lattice gases
A forthcoming challenge in ultracold lattice gases is the simulation of
quantum magnetism. That involves both the preparation of the lattice atomic gas
in the desired spin state and the probing of the state. Here we demonstrate how
a probing scheme based on atom-light interfaces gives access to the order
parameters of nontrivial quantum magnetic phases, allowing us to characterize
univocally strongly correlated magnetic systems produced in ultracold gases.
This method, which is also nondemolishing, yields spatially resolved spin
correlations and can be applied to bosons or fermions. As a proof of principle,
we apply this method to detect the complete phase diagram displayed by a chain
of (rotationally invariant) spin-1 bosons.Comment: published versio
Measuring the purity of a qubit state: entanglement estimation with fully separable measurements
Given a finite number of copies of a qubit state we compute the maximum
fidelity that can be attained using joint-measurement protocols for estimating
its purity. We prove that in the asymptotic limit,
separable-measurement protocols can be as efficient as the optimal
joint-measurement one if classical communication is used. This in turn shows
that the optimal estimation of the entanglement of a two-qubit state can also
be achieved asymptotically with fully separable measurements. The relationship
between our global Bayesian approach and the quantum Cramer-Rao bound is also
discussed.Comment: 5 pages, 1 figure, RevTeX, improved versio
Superconducting Vortex Lattices for Ultracold Atoms
We propose and analyze a nanoengineered vortex array in a thin-film type-II
superconductor as a magnetic lattice for ultracold atoms. This proposal
addresses several of the key questions in the development of atomic quantum
simulators. By trapping atoms close to the surface, tools of nanofabrication
and structuring of lattices on the scale of few tens of nanometers become
available with a corresponding benefit in energy scales and temperature
requirements. This can be combined with the possibility of magnetic single site
addressing and manipulation together with a favorable scaling of
superconducting surface-induced decoherence.Comment: Published Version. Manuscript: 5 pages, 3 figures. Supplementary
Information: 11 pages, 7 figure
Efficiency in Quantum Key Distribution Protocols with Entangled Gaussian States
Quantum key distribution (QKD) refers to specific quantum strategies which
permit the secure distribution of a secret key between two parties that wish to
communicate secretly. Quantum cryptography has proven unconditionally secure in
ideal scenarios and has been successfully implemented using quantum states with
finite (discrete) as well as infinite (continuous) degrees of freedom. Here, we
analyze the efficiency of QKD protocols that use as a resource entangled
gaussian states and gaussian operations only. In this framework, it has already
been shown that QKD is possible (M. Navascu\'es et al. Phys. Rev. Lett. 94,
010502 (2005)) but the issue of its efficiency has not been considered. We
propose a figure of merit (the efficiency ) to quantify the number of
classical correlated bits that can be used to distill a key from a sample of
entangled states. We relate the efficiency of the protocol to the
entanglement and purity of the states shared between the parties.Comment: 13 pages, 2 figures, OSID style, published versio
Hybrid Architecture for Engineering Magnonic Quantum Networks
We show theoretically that a network of superconducting loops and magnetic
particles can be used to implement magnonic crystals with tunable magnonic band
structures. In our approach, the loops mediate interactions between the
particles and allow magnetic excitations to tunnel over long distances. As a
result, different arrangements of loops and particles allow one to engineer the
band structure for the magnonic excitations. Furthermore, we show how magnons
in such crystals can serve as a quantum bus for long-distance magnetic coupling
of spin qubits. The qubits are coupled to the magnets in the network by their
local magnetic-dipole interaction and provide an integrated way to measure the
state of the magnonic quantum network.Comment: Manuscript: 4 pages, 3 figures. Supplemental Material: 9 pages, 4
figures. V2: Published version in PRA: 14 pages + 8 figures. Substantial
rearrangement of the content of the previous versio
Transport and Entanglement Generation in the Bose-Hubbard Model
We study entanglement generation via particle transport across a
one-dimensional system described by the Bose-Hubbard Hamiltonian. We analyze
how the competition between interactions and tunneling affects transport
properties and the creation of entanglement in the occupation number basis.
Alternatively, we propose to use spatially delocalized quantum bits, where a
quantum bit is defined by the presence of a particle either in a site or in the
adjacent one. Our results can serve as a guidance for future experiments to
characterize entanglement of ultracold gases in one-dimensional optical
lattices.Comment: 14 pages, 6 figure
Quantum State Transfer in Spin-1 Chains
We study the transfer of quantum information through a Heisenberg spin-1
chain prepared in its ground state. We measure the efficiency of such a quantum
channel {\em via} the fidelity of retrieving an arbitrarily prepared state and
{\em via} the transfer of quantum entanglement. The Heisenberg spin-1 chain has
a very rich quantum phase diagram. We show that the phase boundaries are
reflected in sharp variations of the transfer efficiency. In the vicinity of
the border between the dimer and the ferromagnetic phase (in the conjectured
spin-nematic region), we find strong indications for a qualitative change of
the excitation spectrum. Moreover, we identify two regions of the phase diagram
which give rise to particularly high transfer efficiency; the channel might be
non-classical even for chains of arbitrary length, in contrast to spin-1/2
chains.Comment: 4 pages, 4 figures, published versio
Large Quantum Superpositions and Interference of Massive Nanometer-Sized Objects
We propose a method to prepare and verify spatial quantum superpositions of a
nanometer-sized object separated by distances of the order of its size. This
method provides unprecedented bounds for objective collapse models of the wave
function by merging techniques and insights from cavity quantum optomechanics
and matter wave interferometry. An analysis and simulation of the experiment is
performed taking into account standard sources of decoherence. We provide an
operational parameter regime using present day and planned technology.Comment: 4 pages, 2 figures, to appear in PR
Separable Measurement Estimation of Density Matrices and its Fidelity Gap with Collective Protocols
We show that there exists a gap between the performance of separable and
collective measurements in qubit mixed-state estimation that persists in the
large sample limit. We characterize such gap in terms of the corresponding
bounds on the mean fidelity. We present an adaptive protocol that attains the
separable-measurement bound. This (optimal separable) protocol uses von Neumann
measurements and can be easily implemented with current technology.Comment: version published in PR
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