226 research outputs found
Particle correlations in a fermi superfluid
We discuss correlations between particles of different momentum in a
superfluid fermi gas, accessible through noise measurements of absorption
images of the expanded gas. We include two elements missing from the simplest
treatment, based on the BCS wavefunction: the explicit use of a conserving
approximation satisfying particle number conservation, and the inclusion of the
contribution from Cooper pairs at finite momentum. We expect the latter to be a
significant issue in the strongly correlated state emerging in the BCS-BEC
crossover.Comment: Published versio
Adiabatic loading of a Bose-Einstein condensate in a 3D optical lattice
We experimentally investigate the adiabatic loading of a Bose-Einstein
condensate into an optical lattice potential. The generation of excitations
during the ramp is detected by a corresponding decrease in the visibility of
the interference pattern observed after free expansion of the cloud. We focus
on the superfluid regime, where we show that the limiting time scale is related
to the redistribution of atoms across the lattice by single-particle tunneling
Observation of two-orbital spin-exchange interactions with ultracold SU(N)-symmetric fermions
We report on the direct observation of spin-exchanging interactions in a
two-orbital SU(N)-symmetric quantum gas of ytterbium in an optical lattice. The
two orbital states are represented by two different (meta-)stable electronic
configurations of fermionic Yb-173. A strong spin-exchange between particles in
the two separate orbitals is mediated by the contact interaction between atoms,
which we characterize by clock shift spectroscopy in a 3D optical lattice. We
find the system to be SU(N)-symmetric within our measurement precision and
characterize all relevant scattering channels for atom pairs in combinations of
the ground and the excited state. Elastic scattering between the orbitals is
dominated by the antisymmetric channel, which leads to the strong spin-exchange
coupling. The exchange process is directly observed, by characterizing the
dynamic equilibration of spin imbalances between two large ensembles in the two
orbital states, as well as indirectly in atom pairs via interaction shift
spectroscopy in a 3D lattice. The realization of a stable SU(N)-symmetric
two-orbital Hubbard Hamiltonian opens the route towards experimental quantum
simulation of condensed-matter models based on orbital interactions, such as
the Kondo lattice model.Comment: Correction: In the original version of this preprint the assignment
of states with symmetric electronic wavefunction (|eg+>) and with
antisymmetric electronic wavefunction (|eg->) to the observed spectral lines
was inverted. This has been corrected in the current version. The results of
the paper remain unchanged, with the exchange coupling being inverted to a
ferromagnetic exchang
Observation of an orbital interaction-induced Feshbach resonance in 173-Yb
We report on the experimental observation of a novel inter-orbital Feshbach
resonance in ultracold 173-Yb atoms, which opens the possibility of tuning the
interactions between the 1S0 and 3P0 metastable state, both possessing
vanishing total electronic angular momentum. The resonance is observed at
experimentally accessible magnetic field strengths and occurs universally for
all hyperfine state combinations. We characterize the resonance in the bulk via
inter-orbital cross-thermalization as well as in a three-dimensional lattice
using high-resolution clock-line spectroscopy.Comment: 5 pages, 4 figure
Time-resolved Observation and Control of Superexchange Interactions with Ultracold Atoms in Optical Lattices
Quantum mechanical superexchange interactions form the basis of quantum
magnetism in strongly correlated electronic media. We report on the direct
measurement of superexchange interactions with ultracold atoms in optical
lattices. After preparing a spin-mixture of ultracold atoms in an
antiferromagnetically ordered state, we measure a coherent
superexchange-mediated spin dynamics with coupling energies from 5 Hz up to 1
kHz. By dynamically modifying the potential bias between neighboring lattice
sites, the magnitude and sign of the superexchange interaction can be
controlled, thus allowing the system to be switched between antiferromagnetic
or ferromagnetic spin interactions. We compare our findings to predictions of a
two-site Bose-Hubbard model and find very good agreement, but are also able to
identify corrections which can be explained by the inclusion of direct
nearest-neighbor interactions.Comment: 24 pages, 7 figure
Preparation and detection of d-wave superfluidity in two-dimensional optical superlattices
We propose a controlled method to create and detect d-wave superfluidity with
ultracold fermionic atoms loaded in two-dimensional optical superlattices. Our
scheme consists in preparing an array of nearest-neighbor coupled square
plaquettes or ``superplaquettes'' and using them as building blocks to
construct a d-wave superfluid state. We describe how to use the coherent
dynamical evolution in such a system to experimentally probe the pairing
mechanism. We also derive the zero temperature phase diagram of the fermions in
a checkerboard lattice (many weakly coupled plaquettes) and show that by tuning
the inter-plaquette tunneling spin-dependently or varying the filling factor
one can drive the system into a d-wave superfluid phase or a Cooper pair
density wave phase. We discuss the use of noise correlation measurements to
experimentally probe these phases.Comment: 8 pages, 6 figure
Noise Thermometry with Two Weakly Coupled Bose-Einstein Condensates
Here we report on the experimental investigation of thermally induced
fluctuations of the relative phase between two Bose-Einstein condensates which
are coupled via tunneling. The experimental control over the coupling strength
and the temperature of the thermal background allows for the quantitative
analysis of the phase fluctuations. Furthermore, we demonstrate the application
of these measurements for thermometry in a regime where standard methods fail.
With this we confirm that the heat capacity of an ideal Bose gas deviates from
that of a classical gas as predicted by the third law of thermodynamics.Comment: 4 pages, 4 figure
Hybrid 2D surface trap for quantum simulation
We demonstrate a novel optical trapping scheme for ultracold atoms. Using a
combination of evanescent wave, standing wave, and magnetic potentials we
create a deeply 2D Bose-Einstein condensate (BEC) at a few microns from a glass
surface. Using techniques such as broadband "white" light to create evanescent
and standing waves, we realize a smooth potential with a trap frequency aspect
ratio of 300:1:1 and long lifetimes. This makes the setup suitable for
many-body quantum simulations and applications such as high precision
measurements close to surfaces.Comment: 5 pages, 4 figure
Precision measurement of spin-dependent interaction strengths for spin-1 and spin-2 87Rb atoms
We report on precision measurements of spin-dependent interaction-strengths
in the 87Rb spin-1 and spin-2 hyperfine ground states. Our method is based on
the recent observation of coherence in the collisionally driven spin-dynamics
of ultracold atom pairs trapped in optical lattices. Analysis of the Rabi-type
oscillations between two spin states of an atom pair allows a direct
determination of the coupling parameters in the interaction hamiltonian. We
deduce differences in scattering lengths from our data that can directly be
compared to theoretical predictions in order to test interatomic potentials.
Our measurements agree with the predictions within 20%. The knowledge of these
coupling parameters allows one to determine the nature of the magnetic ground
state. Our data imply a ferromagnetic ground state for 87Rb in the f=1
manifold, in agreement with earlier experiments performed without the optical
lattice. For 87Rb in the f=2 manifold the data points towards an
antiferromagnetic ground state, however our error bars do not exclude a
possible cyclic phase.Comment: 11 pages, 5 figure
Probing transport and slow relaxation in the mass-imbalanced Fermi-Hubbard model
Constraints in the dynamics of quantum many-body systems can dramatically alter transport properties and relaxation time scales even in the absence of static disorder. Here, we report on the observation of such constrained dynamics arising from the distinct mobility of two species in the one-dimensional mass-imbalanced Fermi-Hubbard model, realized with ultracold ytterbium atoms in a state-dependent optical lattice. By displacing the trap potential and monitoring the dynamical response of the system, we identify suppressed transport and slow relaxation with a strong dependence on the mass imbalance and interspecies interaction strength, suggesting eventual thermalization for long times. Our observations are supported by numerical simulations and pave the way to study metastability arising from dynamical constraints in other quantum many-body systems
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