42 research outputs found
Magnetically tuned spin dynamics resonance
We present the experimental observation of a magnetically tuned resonance
phenomenon resulting from spin mixing dynamics of ultracold atomic gases. In
particular we study the magnetic field dependence of spin conversion in F=2
87Rb spinor condensates in the crossover from interaction dominated to
quadratic Zeeman dominated dynamics. We discuss the observed phenomenon in the
framework of spin dynamics as well as matter wave four wave mixing. Furthermore
we show that the validity range of the single mode approximation for spin
dynamics is significantly extended in the regime of high magnetic field
Measurement of a Mixed Spin Channel Feshbach Resonance in Rubidium 87
We report on the observation of a mixed spin channel Feshbach resonance at
the low magnetic field value of (9.09 +/- 0.01) G for a mixture of |2,-1> and
|1,+1> states in 87Rb. This mixture is important for applications of
multi-component BECs of 87Rb, e.g. in spin mixture physics and for quantum
entanglement. Values for position, height and width of the resonance are
reported and compared to a recent theoretical calculation of this resonance.Comment: 4 pages, 3 figures minor changes, actualized citation
Ultracold quantum gases in triangular optical lattices
Over the last years the exciting developments in the field of ultracold atoms
confined in optical lattices have led to numerous theoretical proposals devoted
to the quantum simulation of problems e.g. known from condensed matter physics.
Many of those ideas demand for experimental environments with non-cubic lattice
geometries. In this paper we report on the implementation of a versatile
three-beam lattice allowing for the generation of triangular as well as
hexagonal optical lattices. As an important step the superfluid-Mott insulator
(SF-MI) quantum phase transition has been observed and investigated in detail
in this lattice geometry for the first time. In addition to this we study the
physics of spinor Bose-Einstein condensates (BEC) in the presence of the
triangular optical lattice potential, especially spin changing dynamics across
the SF-MI transition. Our results suggest that below the SF-MI phase
transition, a well-established mean-field model describes the observed data
when renormalizing the spin-dependent interaction. Interestingly this opens new
perspectives for a lattice driven tuning of a spin dynamics resonance occurring
through the interplay of quadratic Zeeman effect and spin-dependent
interaction. We finally discuss further lattice configurations which can be
realized with our setup.Comment: 19 pages, 7 figure
Bose-Einstein condensation at constant temperature
We present a novel experimental approach to Bose-Einstein condensation by
increasing the particle number of the system at almost constant temperature. In
particular the emergence of a new condensate is observed in multi-component F=1
spinor condensates of 87-Rb. Furthermore we develop a simple rate-equation
model for multi-component BEC thermodynamics at finite temperature which well
reproduces the measured effects.Comment: 4 pages, 3 figures, RevTe
Dynamics of F=2 Spinor Bose-Einstein Condensates
We experimentally investigate and analyze the rich dynamics in F=2 spinor
Bose-Einstein condensates of Rb87. An interplay between mean-field driven spin
dynamics and hyperfine-changing losses in addition to interactions with the
thermal component is observed. In particular we measure conversion rates in the
range of 10^-12 cm^3/s for spin changing collisions within the F=2 manifold and
spin-dependent loss rates in the range of 10^-13 cm^3/s for hyperfine-changing
collisions. From our data we observe a polar behavior in the F=2 ground state
of Rb87, while we measure the F=1 ground state to be ferromagnetic. Furthermore
we see a magnetization for condensates prepared with non-zero total spin.Comment: 4 pages, 2 figures, RevTe
Evolution of a spinor condensate: coherent dynamics, dephasing and revivals
We present measurements and a theoretical model for the interplay of spin
dependent interactions and external magnetic fields in atomic spinor
condensates. We highlight general features like quadratic Zeeman dephasing and
its influence on coherent spin mixing processes by focusing on a specific
coherent superposition state in a F=1 Rb Bose-Einstein condensate. In
particular, we observe the transition from coherent spinor oscillations to
thermal equilibration
Oscillations and interactions of dark and dark-bright solitons in Bose-Einstein condensates
Solitons are among the most distinguishing fundamental excitations in a wide
range of non-linear systems such as water in narrow channels, high speed
optical communication, molecular biology and astrophysics. Stabilized by a
balance between spreading and focusing, solitons are wavepackets, which share
some exceptional generic features like form-stability and particle-like
properties. Ultra-cold quantum gases represent very pure and well-controlled
non-linear systems, therefore offering unique possibilities to study soliton
dynamics. Here we report on the first observation of long-lived dark and
dark-bright solitons with lifetimes of up to several seconds as well as their
dynamics in highly stable optically trapped Rb Bose-Einstein
condensates. In particular, our detailed studies of dark and dark-bright
soliton oscillations reveal the particle-like nature of these collective
excitations for the first time. In addition, we discuss the collision between
these two types of solitary excitations in Bose-Einstein condensates.Comment: 9 pages, 4 figure
Dynamics and thermodynamics in spinor quantum gases
We discuss magnetism in spinor quantum gases theoretically and experimentally
with emphasis on temporal dynamics of the spinor order parameter in the
presence of an external magnetic field. In a simple coupled Gross-Pitaevskii
picture we observe a dramatic suppression of spin dynamics due to quadratic
Zeeman ''dephasing''. In view of an inhomogeneous density profile of the
trapped condensate we present evidence of spatial variations of spin dynamics.
In addition we study spinor quantum gases as a model system for thermodynamics
of Bose-Einstein condensation. As a particular example we present measurements
on condensate magnetisation due to the interaction with a thermal bath.Comment: 8 pages, 7 figure
Modified spin-wave theory with ordering vector optimization I: frustrated bosons on the spatially anisotropic triangular lattice
We investigate a system of frustrated hardcore bosons, modeled by an XY
antiferromagnet on the spatially anisotropic triangular lattice, using
Takahashi's modified spin-wave (MSW) theory. In particular we implement
ordering vector optimization on the ordered reference state of MSW theory,
which leads to significant improvement of the theory and accounts for quantum
corrections to the classically ordered state. The MSW results at zero
temperature compare favorably to exact diagonalization (ED) and projected
entangled-pair state (PEPS) calculations. The resulting zero-temperature phase
diagram includes a 1D quasi-ordered phase, a 2D Neel ordered phase, and a 2D
spiraling ordered phase. We have strong indications that the various ordered or
quasi-ordered phases are separated by spin-liquid phases with short-range
correlations, in analogy to what has been predicted for the Heisenberg model on
the same lattice. Within MSW theory we also explore the finite-temperature
phase diagram. We find that the zero-temperature long-range-ordered phases turn
into quasi-ordered phases (up to a Berezinskii-Kosterlitz-Thouless
temperature), while zero-temperature quasi-ordered phases become short-range
correlated at finite temperature. These results show that modified spin-wave
theory is very well suited for describing ordered and quasi-ordered phases of
frustrated XY spins (or, equivalently, of frustrated lattice bosons) both at
zero and finite temperatures. While MSW theory, just as other theoretical
methods, cannot describe spin-liquid phases, its breakdown provides a fast
method for singling out Hamiltonians which may feature these intriguing quantum
phases. We thus suggest a tool for guiding our search for interesting systems
whose properties are necessarily studied with a physical quantum simulator.Comment: 40 pages, 16 figure
Spin-Nematic Squeezed Vacuum in a Quantum Gas
Using squeezed states it is possible to surpass the standard quantum limit of
measurement uncertainty by reducing the measurement uncertainty of one property
at the expense of another complementary property. Squeezed states were first
demonstrated in optical fields and later with ensembles of pseudo spin-1/2
atoms using non-linear atom-light interactions. Recently, collisional
interactions in ultracold atomic gases have been used to generate a large
degree of quadrature spin squeezing in two-component Bose condensates. For
pseudo spin-1/2 systems, the complementary properties are the different
components of the total spin vector , which fully characterize the state on
an SU(2) Bloch sphere. Here, we measure squeezing in a spin-1 Bose condensate,
an SU(3) system, which requires measurement of the rank-2 nematic or quadrupole
tensor as well to fully characterize the state. Following a quench
through a nematic to ferromagnetic quantum phase transition, squeezing is
observed in the variance of the quadratures up to -8.3(-0.7 +0.6) dB
(-10.3(-0.9 +0.7) dB corrected for detection noise) below the standard quantum
limit. This spin-nematic squeezing is observed for negligible occupation of the
squeezed modes and is analogous to optical two-mode vacuum squeezing. This work
has potential applications to continuous variable quantum information and
quantum-enhanced magnetometry