50 research outputs found
Entanglement and extreme spin squeezing of unpolarized states
We present criteria to detect the depth of entanglement in macroscopic
ensembles of spin-j particles using the variance and second moments of the
collective spin components. The class of states detected goes beyond
traditional spin-squeezed states by including Dicke states and other
unpolarized states. The criteria derived are easy to evaluate numerically even
for systems of very many particles and outperform past approaches, especially
in practical situations where noise is present. We also derive analytic lower
bounds based on the linearization of our criteria, which make it possible to
define spin-squeezing parameters for Dicke states. In addition, we obtain spin
squeezing parameters also from the condition derived in [A. S. Sorensen and K.
Molmer, Phys. Rev. Lett. 86, 4431 (2001)]. We also extend our results to
systems with fluctuating number of particles.Comment: 18 pages including 4 figures; v2: published versio
Detecting multiparticle entanglement of Dicke states
Recent experiments demonstrate the production of many thousands of neutral
atoms entangled in their spin degrees of freedom. We present a criterion for
estimating the amount of entanglement based on a measurement of the global
spin. It outperforms previous criteria and applies to a wide class of entangled
states, including Dicke states. Experimentally, we produce a Dicke-like state
using spin dynamics in a Bose-Einstein condensate. Our criterion proves that it
contains at least genuine 28-particle entanglement. We infer a generalized
squeezing parameter of -11.4(5) dB.Comment: 5 pages, 4 figure
Quantum engineering for optical clocks
Atomic clocks known as optical clocks are more accurate and stable than current timekeepers. Two quantum-engineering approaches could improve the performance of optical clocks even further and extend their applications
Adiabatic spin cooling using high-spin Fermi gases
Spatial entropy redistribution plays a key role in adiabatic cooling of ultra-cold lattice gases. We show that high-spin fermions with a spatially variable quadratic Zeeman coupling may allow for the creation of an inner spin-1/2 core surrounded by high-spin wings. The latter are always more entropic than the core at high temperatures and, remarkably, at all temperatures in the presence of frustration. Combining thermodynamic Bethe Ansatz with local density approximation, we study the spatial entropy distribution for the particular case of one-dimensional spin-3/2 lattice fermions in the Mott phase. Interestingly, this spatially dependent entropy opens a possible path for an adiabatic cooling technique that, in contrast to previous proposals, would specifically target the spin degree of freedom. We discuss a possible realization of this adiabatic cooling, which may allow for a highly efficient entropy decrease in the spin-1/2 core and help access antiferromagnetic order in experiments on ultracold spinor fermions.DFG/EXC/QUESTESF/EuroQUASARSCOPES/IZ73Z0-12805
Detecting metrologically useful entanglement in the vicinity of Dicke states
We present a method to verify the metrological usefulness of noisy Dicke
states of a particle ensemble with only a few collective measurements, without
the need for a direct measurement of the sensitivity. Our method determines the
usefulness of the state for the usual protocol for estimating the angle of
rotation with Dicke states, which is based on the measurement of the second
moment of a total spin component. It can also be used to detect entangled
states that are useful for quantum metrology. We test our approach on recent
experimental results.Comment: 17 pages including 4 figures; v2: Sec. IV with further numerical
examples added, Appendix added; v3: published versio
Excited-state quantum phase transitions in spinor Bose-Einstein condensates
Excited-state quantum phase transitions (ESQPTs) extend the notion of quantum
phase transitions beyond the ground state. They are characterized by closing
energy gaps amid the spectrum. Identifying order parameters for ESQPTs poses
however a major challenge. We introduce spinor Bose-Einstein condensates as a
versatile platform for studies of ESQPTs. Based on the mean-field dynamics, we
define a topological order parameter that distinguishes between excited-state
phases, and discuss how to interferometrically access the order parameter in
current experiments. Our work opens the way for the experimental
characterization of excited-state quantum phases in atomic many-body systems.Comment: 14 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