67 research outputs found
Correlations and Pair Formation in a Repulsively Interacting Fermi Gas
A degenerate Fermi gas is rapidly quenched into the regime of strong
effective repulsion near a Feshbach resonance. The spin fluctuations are
monitored using speckle imaging and, contrary to several theoretical
predictions, the samples remain in the paramagnetic phase for arbitrarily large
scattering length. Over a wide range of interaction strengths a rapid decay
into bound pairs is observed over times on the order of 10\hbar/E_F, preventing
the study of equilibrium phases of strongly repulsive fermions. Our work
suggests that a Fermi gas with strong short-range repulsive interactions does
not undergo a ferromagnetic phase transition
Observation of mHz-level cooperative Lamb shifts in an optical atomic clock
We report on the direct observation of resonant electric dipole-dipole
interactions in a cubic array of atoms in the many-excitation limit. The
interactions, mediated by single-atom couplings to the shared electromagnetic
vacuum, are shown to produce spatially-dependent cooperative Lamb shifts when
spectroscopically interrogating the mHz-wide optical clock transition in
strontium-87. We show that the ensemble-averaged shifts can be suppressed below
the level of evaluated systematic uncertainties for state-of-the-art optical
atomic clocks. Additionally, we demonstrate that excitation of the atomic
dipoles near a Bragg angle can enhance these effects by nearly an order of
magnitude compared to non-resonant geometries. Given the remarkable precision
of frequency measurements and the high accuracy of the modeled response, our
work demonstrates that such a clock is a novel platform for studies of the
quantum many-body physics of spins with long-range interactions mediated by
propagating photons
Speckle Imaging of Spin Fluctuations in a Strongly Interacting Fermi Gas
Spin fluctuations and density fluctuations are studied for a two-component
gas of strongly interacting fermions along the BEC-BCS crossover. This is done
by in-situ imaging of dispersive speckle patterns. Compressibility and magnetic
susceptibility are determined from the measured fluctuations. This new
sensitive method easily resolves a tenfold suppression of spin fluctuations
below shot noise due to pairing, and can be applied to novel magnetic phases in
optical lattices
Suppression of Density Fluctuations in a Quantum Degenerate Fermi Gas
We study density profiles of an ideal Fermi gas and observe Pauli suppression
of density fluctuations (atom shot noise) for cold clouds deep in the quantum
degenerate regime. Strong suppression is observed for probe volumes containing
more than 10,000 atoms. Measuring the level of suppression provides sensitive
thermometry at low temperatures. After this method of sensitive noise
measurements has been validated with an ideal Fermi gas, it can now be applied
to characterize phase transitions in strongly correlated many-body systems.Comment: minor edit: fixed technical problem with arxiv's processing of .eps
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Disentangling Pauli blocking of atomic decay from cooperative radiation and atomic motion in a 2D Fermi gas
The observation of Pauli blocking of atomic spontaneous decay via direct measurements of the atomic population requires the use of long-lived atomic gases where quantum statistics, atom recoil and cooperative radiative processes are all relevant. We develop a theoretical framework capable of simultaneously accounting for all these effects in a regime where prior theoretical approaches based on semi-classical non-interacting or interacting frozen atom approximations fail. We apply it to atoms in a single 2D pancake or arrays of pancakes featuring an effective A level structure (one excited and two degenerate ground states). We identify a parameter window in which a factor of two extension in the atomic lifetime clearly attributable to Pauli blocking should be experimentally observable in deeply degenerate gases with~103 atoms. Our predictions are supported by observation of a number-dependent excited state decay rate on the 1S0 - 3P1 transition in 87Sr atoms.Physic
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