237 research outputs found
Distant Entanglement of Macroscopic Gas Samples
One of the main ingredients in most quantum information protocols is a
reliable source of two entangled systems. Such systems have been generated
experimentally several years ago for light but has only in the past few years
been demonstrated for atomic systems. None of these approaches however involve
two atomic systems situated in separate environments. This is necessary for the
creation of entanglement over arbitrary distances which is required for many
quantum information protocols such as atomic teleportation. We present an
experimental realization of such distant entanglement based on an adaptation of
the entanglement of macroscopic gas samples containing about 10^11 cesium atoms
shown previously by our group. The entanglement is generated via the
off-resonant Kerr interaction between the atomic samples and a pulse of light.
The achieved entanglement distance is 0.35m but can be scaled arbitrarily. The
feasibility of an implementation of various quantum information protocols using
macroscopic samples of atoms has therefore been greatly increased. We also
present a theoretical modeling in terms of canonical position and momentum
operators X and P describing the entanglement generation and verification in
presence of decoherence mechanisms.Comment: 20 pages book-style, 3 figure
Entanglement of bosonic modes in symmetric graphs
The ground and thermal states of a quadratic hamiltonian representing the
interaction of bosonic modes or particles are always Gaussian states. We
investigate the entanglement properties of these states for the case where the
interactions are represented by harmonic forces acting along the edges of
symmetric graphs, i.e. 1, 2, and 3 dimensional rectangular lattices, mean field
clusters and platonic solids. We determine the Entanglement of Formation (EoF)
as a function of the interaction strength, calculate the maximum EoF in each
case and compare these values with the bounds found in \cite{wolf} which are
valid for any quadratic hamiltonian.Comment: 15 pages, 8 figures, 3 tables, Latex, Accepted for publication in
Physical Review
Evolution of twin-beam in active optical media
We study the evolution of twin-beam propagating inside active media that may
be used to establish a continuous variable entangled channel between two
distant users. In particular, we analyze how entanglement is degraded during
propagation, and determine a threshold value for the interaction time, above
which the state become separable, and thus useless for entanglement based
manipulations. We explicitly calculate the fidelity for coherent state
teleportation and show that it is larger than one half for the whole range of
parameters preserving entanglemenent.Comment: several misprints correcte
Quantum teleportation between light and matter
Quantum teleportation is an important ingredient in distributed quantum
networks, and can also serve as an elementary operation in quantum computers.
Teleportation was first demonstrated as a transfer of a quantum state of light
onto another light beam; later developments used optical relays and
demonstrated entanglement swapping for continuous variables. The teleportation
of a quantum state between two single material particles (trapped ions) has now
also been achieved. Here we demonstrate teleportation between objects of a
different nature - light and matter, which respectively represent 'flying' and
'stationary' media. A quantum state encoded in a light pulse is teleported onto
a macroscopic object (an atomic ensemble containing 10^12 caesium atoms).
Deterministic teleportation is achieved for sets of coherent states with mean
photon number (n) up to a few hundred. The fidelities are 0.58+-0.02 for n=20
and 0.60+-0.02 for n=5 - higher than any classical state transfer can possibly
achieve. Besides being of fundamental interest, teleportation using a
macroscopic atomic ensemble is relevant for the practical implementation of a
quantum repeater. An important factor for the implementation of quantum
networks is the teleportation distance between transmitter and receiver; this
is 0.5 metres in the present experiment. As our experiment uses propagating
light to achieve the entanglement of light and atoms required for
teleportation, the present approach should be scalable to longer distances.Comment: 23 pages, 8 figures, incl. supplementary informatio
Extracting high fidelity quantum computer hardware from random systems
An overview of current status and prospects of the development of quantum
computer hardware based on inorganic crystals doped with rare-earth ions is
presented. Major parts of the experimental work in this area has been done in
two places, Canberra, Australia and Lund, Sweden, and the present description
follows more closely the Lund work. Techniques will be described that include
optimal filtering of the initially inhomogeneously broadened profile down to
well separated and narrow ensembles, as well as the use of advanced
pulse-shaping in order to achieve robust arbitrary single-qubit operations with
fidelities above 90%, as characterized by quantum state tomography. It is
expected that full scalability of these systems will require the ability to
determine the state of single rare-earth ions. It has been proposed that this
can be done using special readout ions doped into the crystal and an update is
given on the work to find and characterize such ions. Finally, a few aspects on
the possibilities for remote entanglement of ions in separate
rare-earth-ion-doped crystals are considered.Comment: 19 pages, 9 figures. Written for The Proceedings of the
Nobelsymposium on qubits for future quantum computers, Gothenburg, May-0
Light-Matter Quantum Interface
We propose a quantum interface which applies multiple passes of a pulse of
light through an atomic sample with phase/polarization rotations in between the
passes. Our proposal does not require nonclassical light input or measurements
on the system, and it predicts rapidly growing unconditional entanglement of
light and atoms from just coherent inputs. The proposed interface makes it
possible to achieve a number of tasks within quantum information processing
including teleportation between light and atoms, quantum memory for light and
squeezing of atomic and light variables.Comment: 4 pages, 4 figure
Experimental demonstration of quantum memory for light
The information carrier of today's communications, a weak pulse of light, is
an intrinsically quantum object. As a consequence, complete information about
the pulse cannot, even in principle, be perfectly recorded in a classical
memory. In the field of quantum information this has led to a long standing
challenge: how to achieve a high-fidelity transfer of an independently prepared
quantum state of light onto the atomic quantum state? Here we propose and
experimentally demonstrate a protocol for such quantum memory based on atomic
ensembles. We demonstrate for the first time a recording of an externally
provided quantum state of light onto the atomic quantum memory with a fidelity
up to 70%, significantly higher than that for the classical recording. Quantum
storage of light is achieved in three steps: an interaction of light with
atoms, the subsequent measurement on the transmitted light, and the feedback
onto the atoms conditioned on the measurement result. Density of recorded
states 33% higher than that for the best classical recording of light on atoms
is achieved. A quantum memory lifetime of up to 4 msec is demonstrated.Comment: 22 pages (double line spacing) incl. supplementary information, 4
figures, accepted for publication in Natur
Decay dynamics of quantum dots influenced by the local density of optical states of two-dimensional photonic crystal membranes
We have performed time-resolved spectroscopy on InAs quantum dot ensembles in
photonic crystal membranes. The influence of the photonic crystal is
investigated by varying the lattice constant systematically. We observe a
strong slow down of the quantum dots' spontaneous emission rates as the
two-dimensional bandgap is tuned through their emission frequencies. The
measured band edges are in full agreement with theoretical predictions. We
characterize the multi-exponential decay curves by their mean decay time and
find enhancement of the spontaneous emission at the bandgap edges and strong
inhibition inside the bandgap in good agreement with local density of states
calculations.Comment: 9 pages (preprint), 3 figure
Single-Photon Generation from Stored Excitation in an Atomic Ensemble
Single photons are generated from an ensemble of cold Cs atoms via the
protocol of Duan et al. [Nature \textbf{414}, 413 (2001)]. Conditioned upon an
initial detection from field 1 at 852 nm, a photon in field 2 at 894 nm is
produced in a controlled fashion from excitation stored within the atomic
ensemble. The single-quantum character of the field 2 is demonstrated by the
violation of a Cauchy-Schwarz inequality, namely , where describes detection of two events
conditioned upon an initial detection , with
for single photons.Comment: 5 pages, 4 figure
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