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arXiv:2411.05694 · SciPost Physics

Direct production of fermionic superfluids in a cavity-enhanced optical dipole trap

Tabea Bühler, Timo Zwettler, Gaia Bolognini, Aurélien Fabre, Victor Helson, Giulia Del Pace, Jean-Philippe Brantut

Published Apr 23, 2025• 4 citations
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Abstract

We present the production of quantum degenerate, superfluid gases of ^6 6 Li through direct evaporative cooling in a cavity-enhanced optical dipole trap. The entire evaporative cooling process is performed in a trap created by the TEM _{00} 00 mode of a Fabry-Pérot cavity, simultaneously driven on several successive longitudinal modes. This leads to near-complete cancellation of the inherent lattice structure along the axial direction of the cavity, as evidenced by the observation of long-lived dipole oscillations of the atomic cloud. We demonstrate the production of molecular Bose-Einstein condensates upon adiabatic conversion of a unitary Fermi gas evaporatively cooled in this trap. The lifetime and heating in the cavity trap are similar to those in a running wave dipole trap. Our system enables the optical production of ultracold samples using a total trap-laser power below 1W 1 W , leveraging the benefits of optical resonators as dipole traps in quantum gas research while maintaining a simple resonator design and minimizing additional experimental complexity.

DOI
10.21468/scipostphys.18.4.133

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We present the production of quantum degenerate, superfluid gases of ^6 6 Li through direct evaporative cooling in a cavity-enhanced optical dipole trap. The entire evaporative cooling process is performed in a trap created by the TEM _{00} 00 mode of a Fabry-Pérot cavity, simultaneously driven on several successive longitudinal modes. This leads to near-complete cancellation of the inherent lattice structure along the axial direction of the cavity, as evidenced by the observation of long-lived dipole oscillations of the atomic cloud. We demonstrate the production of molecular Bose-Einstein condensates upon adiabatic conversion of a unitary Fermi gas evaporatively cooled in this trap. The lifetime and heating in the cavity trap are similar to those in a running wave dipole trap. Our system enables the optical production of ultracold samples using a total trap-laser power below 1W 1 W , leveraging the benefits of optical resonators as dipole traps in quantum gas research while maintaining a simple resonator design and minimizing additional experimental complexity.
We report efficient production of quantum gases of 6 Li using a sub-Doppler cooling scheme based on the D 1 transition. After loading in a standard magneto-optical trap, an atomic sample of 10 9 atoms is cooled at a temperature of 40 μ K by a bichromatic D 1 gray-molasses. More than 2 × 10 7 atoms are then transferred into a high-intensity optical dipole trap, where a two-spin state mixture is evaporatively cooled down to quantum degeneracy. We observe that D 1 cooling remains effective in the deep trapping potential, allowing an effective increase of the atomic phase-space density before starting the evaporation. In a total experimental cycle of 11 s, we produce weakly-interacting degenerate Fermi gases of 7 × 10 5 atoms at T/T F < 0.1 and molecular Bose-Einstein condensates of up 5 × 10 5 molecules. We further describe a simple and compact optical system both for high-resolution imaging and for imprinting a thin optical barrier on the atomic cloud; this represents a first step towards the study of quantum tunneling in strongly interacting superfluid Fermi gases.
We achieve a highly degenerate and strongly interacting Fermi gas in a mixture of the two lowest hyperfine states of 6 Li by direct evaporative cooling in a high power crossed optical dipole trap. The trap is loaded from a large atom number magneto-optical trap realized by a laser system of 2.5-W intracavity-frequency-doubled light output at 671 nm. With this system, we also demonstrate the production of a molecular Bose-Einstein condensate (mBEC) of 6 Li 2 , and observe the anisotropic expansion of Fermi gases in the so-called BEC-Bardeen-Cooper-Schrieffer crossover regime.
We report on the fast production and weakly destructive detection of a Fermi gas with tunable interactions in a high finesse cavity. The cavity is used both with far off-resonant light to create a deep optical dipole trap, and with near-resonant light to reach the strong light-matter coupling regime. The cavity-based dipole trap allows for an efficient capture of laser-cooled atoms, and the use of a lattice-cancellation scheme makes it possible to perform efficient intra-cavity evaporative cooling. After transfer in a crossed optical dipole trap, we produce deeply degenerate unitary Fermi gases with up to 7cdot1057 \\cdot 10^57cdot105 atoms inside the cavity, with an overall 2.852.852.85 s long sequence. The cavity is then probed with near-resonant light to perform five hundred-times repeated, dispersive measurements of the population of individual clouds, allowing for weakly destructive observations of slow atom-number variations over a single sample. This platform will make possible the real-time observation of transport and dynamics as well as the study of driven-dissipative, strongly correlated quantum matter.
Abstract We report on the fast production and weakly destructive detection of a Fermi gas with tunable interactions in a high finesse cavity. The cavity is used both with far off-resonant light to create a deep optical dipole trap, and with near-resonant light to reach the strong light–matter coupling regime. The cavity-based dipole trap allows for an efficient capture of laser-cooled atoms, and the use of a lattice-cancellation scheme makes it possible to perform efficient intra-cavity evaporative cooling. After transfer in a crossed optical dipole trap, we produce deeply degenerate unitary Fermi gases with up to 7 × 105 atoms inside the cavity, with an overall 2.85 s long sequence. The cavity is then probed with near-resonant light to perform five hundred-times repeated, dispersive measurements of the population of individual clouds, allowing for weakly destructive observations of slow atom-number variations over a single sample. This platform will make possible the real-time observation of transport and dynamics as well as the study of driven-dissipative, strongly correlated quantum matter.
We report on an efficient production scheme for a large quantum degenerate sample of fermionic lithium. The approach is based on our previous work on narrow-line $2{S}_{1/2}\ensuremath{\rightarrow}3{P}_{3/2}$ laser cooling resulting in a high phase-space density of up to $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. This allows utilizing a large-volume crossed optical dipole trap with a total power of 45W45\phantom{\rule{0.16em}{0ex}}\mathrm{W}45W, leading to high loading efficiency and $8\ifmmode\times\else\texttimes\fi{}{10}^{6}$ trapped atoms. The same optical trapping configuration is used for rapid adiabatic transport over a distance of 25cm25\phantom{\rule{0.16em}{0ex}}\mathrm{cm}25cm in 0.9s0.9\phantom{\rule{0.16em}{0ex}}\mathrm{s}0.9s, and subsequent evaporative cooling. With optimized evaporation we achieve a degenerate Fermi gas with $1.7\ifmmode\times\else\texttimes\fi{}{10}^{6}$ atoms at a temperature of 60nK60\phantom{\rule{0.28em}{0ex}}\mathrm{nK}60nK, corresponding to T/TF=0.16(2)T/{T}_{\text{F}}=0.16\left(2\right)T/TF​=0.16(2). Furthermore, the performance is demonstrated by evaporation near a broad Feshbach resonance creating a molecular Bose-Einstein condensate of $3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ lithium dimers.