201 research outputs found
Spin nematic order in antiferromagnetic spinor condensates
Large spin systems can exhibit unconventional types of magnetic ordering
different from the ferromagnetic or N\'eel-like antiferromagnetic order
commonly found in spin 1/2 systems. Spin-nematic phases, for instance, do not
break time-reversal invariance and their magnetic order parameter is
characterized by a second rank tensor with the symmetry of an ellipsoid. Here
we show direct experimental evidence for spin-nematic ordering in a spin-1
Bose-Einstein condensate of sodium atoms with antiferromagnetic interactions.
In a mean field description this order is enforced by locking the relative
phase between spin components. We reveal this mechanism by studying the spin
noise after a spin rotation, which is shown to contain information hidden when
looking only at averages. The method should be applicable to high spin systems
in order to reveal complex magnetic phases.Comment: published versio
Classical bifurcation at the transition from Rabi to Josephson dynamics
We report on the experimental realization of an internal bosonic Josephson
junction in a Rubidium spinor Bose-Einstein condensate. The measurement of the
full time dynamics in phase space allows the characterization of the
theoretically predicted -phase modes and quantitatively confirms
analytical predictions, revealing a classical bifurcation. Our results suggest
that this system is a model system which can be tuned from classical to the
quantum regime and thus is an important step towards the experimental
investigation of entanglement generation close to critical points
Dynamic generation of spin-squeezed states in bosonic Josephson junctions
We analyze the formation of squeezed states in a condensate of ultracold
bosonic atoms confined by a double-well potential. The emphasis is set on the
dynamical formation of such states from initially coherent many-body quantum
states. Two cases are described: the squeezing formation in the evolution of
the system around the stable point, and in the short time evolution in the
vicinity of an unstable point. The latter is shown to produce highly squeezed
states on very short times. On the basis of a semiclassical approximation to
the Bose-Hubbard Hamiltonian, we are able to predict the amount of squeezing,
its scaling with and the speed of coherent spin formation with simple
analytical formulas which successfully describe the numerical Bose-Hubbard
results. This new method of producing highly squeezed spin states in systems of
ultracold atoms is compared to other standard methods in the literature.Comment: 12 pages, revised discussion + added reference
Spin fragmentation of Bose-Einstein condensates with antiferromagnetic interactions
We study spin fragmentation of an antiferromagnetic spin 1 condensate in the
presence of a quadratic Zeeman (QZ) effect breaking spin rotational symmetry.
We describe how the QZ effect turns a fragmented spin state, with large
fluctuations of the Zeemans populations, into a regular polar condensate, where
atoms all condense in the state along the field direction. We calculate
the average value and variance of the Zeeman state to illustrate clearly
the crossover from a fragmented to an unfragmented state. The typical width of
this crossover is , where is the QZ energy, the spin
temperature and the atom number. This shows that spin fluctuations are a
mesoscopic effect that will not survive in the thermodynamic limit
, but are observable for sufficiently small atom number.Comment: submitted to NJ
Einstein-Podolsky-Rosen experiment with two Bose-Einstein condensates
In 1935, Einstein, Podolsky and Rosen (EPR) conceived a Gedankenexperiment
which became a cornerstone of quantum technology and still challenges our
understanding of reality and locality today. While the experiment has been
realized with small quantum systems, a demonstration of the EPR paradox with
spatially separated, massive many-particle systems has so far remained elusive.
We observe the EPR paradox in an experiment with two spatially separated
Bose-Einstein condensates containing about 700 Rubidium atoms each. EPR
entanglement in conjunction with individual manipulation of the two condensates
on the quantum level, as demonstrated here, constitutes an important resource
for quantum metrology and information processing with many-particle systems.
Our results show that the conflict between quantum mechanics and local realism
does not disappear as the system size is increased to over a thousand massive
particles.Comment: 9 pages, 5 figure
Aluminum arsenide cleaved-edge overgrown quantum wires
We report conductance measurements in quantum wires made of aluminum
arsenide, a heavy-mass, multi-valley one-dimensional (1D) system. Zero-bias
conductance steps are observed as the electron density in the wire is lowered,
with additional steps observable upon applying a finite dc bias. We attribute
these steps to depopulation of successive 1D subbands. The quantum conductance
is substantially reduced with respect to the anticipated value for a spin- and
valley-degenerate 1D system. This reduction is consistent with
disorder-induced, intra-wire backscattering which suppresses the transmission
of 1D modes. Calculations are presented to demonstrate the role of strain in
the 1D states of this cleaved-edge structure.Comment: Submitted to Applied Physics Letter
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