146 research outputs found
Non-Gaussian distribution of collective operators in quantum spin chains
We numerically analyse the behavior of the full distribution of collective
observables in quantum spin chains. While most of previous studies of quantum
critical phenomena are limited to the first moments, here we demonstrate how
quantum fluctuations at criticality lead to highly non-Gaussian distributions
thus violating the central limit theorem. Interestingly, we show that the
distributions for different system sizes collapse after scaling on the same
curve for a wide range of transitions: first and second order quantum
transitions and transitions of the Berezinskii-Kosterlitz-Thouless type. We
propose and carefully analyse the feasibility of an experimental reconstruction
of the distribution using light-matter interfaces for atoms in optical lattices
or in optical resonators.Comment: 15 pages, 5 figures; last version close to published versio
Characterization of Bose-Hubbard Models with Quantum Non-demolition Measurements
We propose a scheme for the detection of quantum phase transitions in the 1D
Bose-Hubbard (BH) and 1D Extended Bose-Hubbard (EBH) models, using the
non-demolition measurement technique of quantum polarization spectroscopy. We
use collective measurements of the effective total angular momentum of a
particular spatial mode to characterise the Mott insulator to superfluid phase
transition in the BH model, and the transition to a density wave state in the
EBH model. We extend the application of collective measurements to the ground
states at various deformations of a super-lattice potential.Comment: 8 pages, 9 figures; published version in PRA, Editors' Suggestio
Spin squeezing of atomic ensembles via nuclear-electronic spin entanglement
Entangled many body systems have recently attracted significant attention in
various contexts. Among them, spin squeezed atoms and ions have raised interest
in the field of precision measurements, as they allow to overcome quantum noise
of uncorrelated particles. Precise quantum state engineering is also required
as a resource for quantum computation, and spin squeezing can be used to create
multi-partite entangled states. Two-mode spin squeezed systems have been used
for elementary quantum communication protocols. Until now spin squeezing has
been always achieved via generation of entanglement between different atoms of
the ensemble. In this Letter, we demonstrate for the first time ensemble spin
squeezing generated by engineering the quantum state of each individual atom.
More specifically, we entangle the nuclear and electronic spins of
Cesium atoms at room temperature. We verify entanglement and ensemble spin
squeezing by performing quantum tomography on the atomic state.Comment: 5 pages, 3 figure
Preparation of ultracold atom clouds at the shot noise level
We prepare number stabilized ultracold clouds through the real-time analysis
of non-destructive images and the application of feedback. In our experiments,
the atom number is determined by high precision Faraday imaging
with uncertainty below the shot noise level, i.e., . Based on this measurement, feedback is applied to reduce the atom
number to a user-defined target, whereupon a second imaging series probes the
number stabilized cloud. By this method, we show that the atom number in
ultracold clouds can be prepared below the shot noise level.Comment: Main text: 4 Figures, 4 pages. Supplemental Information: 4 figures, 5
page
Spin dynamics in a two dimensional quantum gas
We have investigated spin dynamics in a 2D quantum gas. Through spin-changing
collisions, two clouds with opposite spin orientations are spontaneously
created in a Bose-Einstein condensate. After ballistic expansion, both clouds
acquire ring-shaped density distributions with superimposed angular density
modulations. The density distributions depend on the applied magnetic field and
are well explained by a simple Bogoliubov model. We show that the two clouds
are anti-correlated in momentum space. The observed momentum correlations pave
the way towards the creation of an atom source with non-local
Einstein-Podolsky-Rosen entanglement.Comment: 5 pages, 4 figure
Entangled light from Bose-Einstein condensates
We propose a method to generate entangled light with a Bose-Einstein
condensate trapped in a cavity, a system realized in recent experiments. The
atoms of the condensate are trapped in a periodic potential generated by a
cavity mode. The condensate is continuously pumped by a laser and spontaneously
emits a pair of photons of different frequencies in two distinct cavity modes.
In this way, the condensate mediates entanglement between two cavity modes
which leak out and can be separated and exhibit continuous variable
entanglement. The scheme exploits the experimentally demonstrated strong,
steady and collective coupling of condensate atoms to a cavity field.Comment: 5 pages and 5 figure
Creating and observing N-partite entanglement with atoms
The Mermin inequality provides a criterion for experimentally ruling out
local-realistic descriptions of multiparticle systems. A violation of this
inequality means that the particles must be entangled, but does not, in
general, indicate whether N-partite entanglement is present. For this, a
stricter bound is required. Here we discuss this bound and use it to propose
two different schemes for demonstrating N-partite entanglement with atoms. The
first scheme involves Bose-Einstein condensates trapped in an optical lattice
and the second uses Rydberg atoms in microwave cavities.Comment: 12 pages, 4 figure
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