550 research outputs found
Quantum noise limited and entanglement-assisted magnetometry
We study experimentally the fundamental limits of sensitivity of an atomic
radio-frequency magnetometer. First we apply an optimal sequence of state
preparation, evolution, and the back-action evading measurement to achieve a
nearly projection noise limited sensitivity. We furthermore experimentally
demonstrate that Einstein-Podolsky-Rosen (EPR) entanglement of atoms generated
by a measurement enhances the sensitivity to pulsed magnetic fields. We
demonstrate this quantum limited sensing in a magnetometer utilizing a truly
macroscopic ensemble of 1.5*10^12 atoms which allows us to achieve
sub-femtoTesla/sqrt(Hz) sensitivity.Comment: To appear in Physical Review Letters, April 9 issue (provisionally
Robust entanglement generation by reservoir engineering
Following a recent proposal [C. Muschik et. al., Phys. Rev. A 83, 052312
(2011)], engineered dissipative processes have been used for the generation of
stable entanglement between two macroscopic atomic ensembles at room
temperature [H. Krauter et. al., Phys. Rev. Lett. 107, 080503 (2011)]. This
experiment included the preparation of entangled states which are continuously
available during a time interval of one hour. Here, we present additional
material, further-reaching data and an extension of the theory developed in [C.
Muschik et. al., Phys. Rev. A 83, 052312 (2011)]. In particular, we show how
the combination of the entangling dissipative mechanism with measurements can
give rise to a substantial improvement of the generated entanglement in the
presence of noise.Comment: Submitted to Journal of Physics B, special issue on "Quantum Memory
Deterministic quantum teleportation between distant atomic objects
Quantum teleportation is a key ingredient of quantum networks and a building
block for quantum computation. Teleportation between distant material objects
using light as the quantum information carrier has been a particularly exciting
goal. Here we demonstrate a new element of the quantum teleportation landscape,
the deterministic continuous variable (cv) teleportation between distant
material objects. The objects are macroscopic atomic ensembles at room
temperature. Entanglement required for teleportation is distributed by light
propagating from one ensemble to the other. Quantum states encoded in a
collective spin state of one ensemble are teleported onto another ensemble
using this entanglement and homodyne measurements on light. By implementing
process tomography, we demonstrate that the experimental fidelity of the
quantum teleportation is higher than that achievable by any classical process.
Furthermore, we demonstrate the benefits of deterministic teleportation by
teleporting a dynamically changing sequence of spin states from one distant
object onto another
Entanglement generated by dissipation and steady state entanglement of two macroscopic objects
Entanglement is a striking feature of quantum mechanics and an essential
ingredient in most applications in quantum information. Typically, coupling of
a system to an environment inhibits entanglement, particularly in macroscopic
systems. Here we report on an experiment, where dissipation continuously
generates entanglement between two macroscopic objects. This is achieved by
engineering the dissipation using laser- and magnetic fields, and leads to
robust event-ready entanglement maintained for 0.04s at room temperature. Our
system consists of two ensembles containing about 10^{12} atoms and separated
by 0.5m coupled to the environment composed of the vacuum modes of the
electromagnetic field. By combining the dissipative mechanism with a continuous
measurement, steady state entanglement is continuously generated and observed
for up to an hour.Comment: This is an update of the preprint from June 2010. It includes new
results on the creation of steady state entanglement, which has been
maintained up to one hou
Una nota sobre "la transformación correcta"
En la nota presente se intenta demostrar que hay algunos problemas conceptuales y lógicos en el artículo aludido, aunque presente aspectos incisivos y nuevos sobre la relación entre valores y precios, referidos al producto neto, a la composición orgánica del capital, a los salarios y a la relación plusvalía y ganancia
High quality anti-relaxation coating material for alkali atom vapor cells
We present an experimental investigation of alkali atom vapor cells coated
with a high quality anti-relaxation coating material based on alkenes. The
prepared cells with single compound alkene based coating showed the longest
spin relaxation times which have been measured up to now with room temperature
vapor cells. Suggestions are made that chemical binding of a cesium atom and an
alkene molecule by attack to the C=C bond plays a crucial role in such
improvement of anti-relaxation coating quality
Simulating open quantum systems: from many-body interactions to stabilizer pumping
In a recent experiment, Barreiro et al. demonstrated the fundamental building
blocks of an open-system quantum simulator with trapped ions [Nature 470, 486
(2011)]. Using up to five ions, single- and multi-qubit entangling gate
operations were combined with optical pumping in stroboscopic sequences. This
enabled the implementation of both coherent many-body dynamics as well as
dissipative processes by controlling the coupling of the system to an
artificial, suitably tailored environment. This engineering was illustrated by
the dissipative preparation of entangled two- and four-qubit states, the
simulation of coherent four-body spin interactions and the quantum
non-demolition measurement of a multi-qubit stabilizer operator. In the present
paper, we present the theoretical framework of this gate-based ("digital")
simulation approach for open-system dynamics with trapped ions. In addition, we
discuss how within this simulation approach minimal instances of spin models of
interest in the context of topological quantum computing and condensed matter
physics can be realized in state-of-the-art linear ion-trap quantum computing
architectures. We outline concrete simulation schemes for Kitaev's toric code
Hamiltonian and a recently suggested color code model. The presented simulation
protocols can be adapted to scalable and two-dimensional ion-trap
architectures, which are currently under development.Comment: 27 pages, 9 figures, submitted to NJP Focus on Topological Quantum
Computatio
Quantum memory for entangled two-mode squeezed states
A quantum memory for light is a key element for the realization of future
quantum information networks. Requirements for a good quantum memory are (i)
versatility (allowing a wide range of inputs) and (ii) true quantum coherence
(preserving quantum information). Here we demonstrate such a quantum memory for
states possessing Einstein-Podolsky-Rosen (EPR) entanglement. These
multi-photon states are two-mode squeezed by 6.0 dB with a variable orientation
of squeezing and displaced by a few vacuum units. This range encompasses
typical input alphabets for a continuous variable quantum information protocol.
The memory consists of two cells, one for each mode, filled with cesium atoms
at room temperature with a memory time of about 1msec. The preservation of
quantum coherence is rigorously proven by showing that the experimental memory
fidelity 0.52(2) significantly exceeds the benchmark of 0.45 for the best
possible classical memory for a range of displacements.Comment: main text 5 pages, supplementary information 3 page
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