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arXiv:1809.06086 · Physical Review A

Evidence for cooling in an optical lattice by amplitude modulation

Maxime Arnal, V. Brunaud, Gabriel Chatelain, Citlali Cabrera-Gutiérrez, Eric Michon, P. Cheiney, Juliette Billy, David Guéry-Odelin

Published Jul 18, 2019• 9 citations
arXivHTMLPDF

Abstract

We report on a generic cooling technique for atoms trapped in optical lattices. It consists in modulating the lattice depth with a proper frequency sweeping. This filtering technique removes the most energetic atoms and provides, with the onset of thermalization, a cooling mechanism reminiscent of evaporative cooling. However, the selection is here performed in quasimomentum space rather than in position space. Interband selection rules are used to protect the population with a zero quasimomentum, namely the Bose Einstein condensate. Direct condensation of thermal atoms in an optical lattice is also achieved with this technique. It offers an interesting complementary cooling mechanism for quantum simulations performed with quantum gases trapped in optical lattices.

DOI
10.1103/physreva.100.013416

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•Jul 17, 2015•1 citation
We report on a generic cooling technique for atoms trapped in optical lattices. It consists in modulating the lattice depth with a proper frequency sweeping. This filtering technique removes the most energetic atoms and provides, with the onset of thermalization, a cooling mechanism reminiscent of evaporative cooling. However, the selection is here performed in quasimomentum space rather than in position space. Interband selection rules are used to protect the population with a zero quasimomentum, namely the Bose Einstein condensate. Direct condensation of thermal atoms in an optical lattice is also achieved with this technique. It offers an interesting complementary cooling mechanism for quantum simulations performed with quantum gases trapped in optical lattices.
We demonstrate the possibility of energy-selective removal of cold atoms from a tight optical trap by means of parametric excitation of the trap vibrational modes. Taking advantage of the anharmonicity of the trap potential, we either selectively remove the most energetic trapped atoms or excite those at the bottom of the trap by tuning the parametric modulation frequency. This process, which had been previously identified as a possible source of heating, also appears to be a robust way for forcing evaporative cooling in anharmonic traps.
A new method of studying and cooling trapped atoms is discussed, with particular attention to atoms in far detuned, 3D optical lattices. The technique, projection cooling, uses a combination of microwave and optical fields to cycle atoms between hyperfine sublevels. A single vibrational level will remain dark to both the light and the microwaves, so atoms will accumulate there. Cooling below the photon recoil limit is possible with this technique. As a diagnostic tool it promises to yield detailed information about atoms in the lattice, including vibrational spectra and the distribution of atoms among vibrational levels, even in the limit of relatively weak binding to lattice sites. Atoms cooled in this way and then allowed to adiabatically expand in their potentials could reach the Bose-Einstein condensation point in less than a second, or at least get close enough to reach it after only a modest amount of evaporative cooling in a larger volume trap. Atoms so cooled and trapped are also of interest for precision measurements.
A key challenge for optical lattice experiments focused on simulating models of strongly correlated electronic solids has been achieving lower temperatures. Although cooling of lattice-well vibrational states has been demonstrated, the motion of atoms through the lattice, characterized by their quasimomentum, has not been cooled directly. Furthermore, quasimomentum equilibration, which is necessary for cooling, has remained unexplored. Here, we use quasimomentum-selective stimulated Raman transitions to directly measure rethermalization rates and cool quasimomentum for a bosonic lattice Hubbard gas. The measured relaxation rate is consistent with a short-range, two-particle scattering model without free parameters at high lattice depths, despite an apparent violation of the Mott-Ioffe-Regel (MIR) bound. Efficient cooling exceeding heating rates is achieved by iteratively removing high quasimomentum atoms from the lattice. Our results may have implications for models of unusual transport phenomena in materials with strong interactions, such as heavy fermion materials and transition metal oxides, and the cooling method we have developed is applicable to any species, including fermionic atoms.
We demonstrate a simple scheme to achieve fast, accelerating (runaway) evaporative cooling of optically trapped atoms by tilting the optical potential with a magnetic field gradient. Runaway evaporation is possible in this trap geometry due to the weak dependence of vibration frequencies on trap depth, which preserves atomic density during the evaporation process. Using this scheme, we show that Bose-Einstein condensation with $\ensuremath{\sim}{10}^{5}$ cesium atoms can be realized in 2−−4s2--4\phantom{\rule{0.3em}{0ex}}\mathrm{s}2−−4s of forced evaporation. The evaporation speed and energetics are consistent with the three-dimensional evaporation picture, despite the fact that atoms can only leave the trap in the direction of tilt.
This thesis describes experiments focused on investigating out-of-equilibrium phenomena in the Bose-Hubbard Model and exploring novel cooling techniques for ultracold gases in optical lattices. In the first experiment, we study quenches across the Mott-insulator-to-superfluid quantum phase transition in the 3D Bose-Hubbard Model. The quench is accomplished by continuously tuning the ratio of the Hubbard energies. We observe that the degree of excitation is proportional to the fraction of atoms that cross the phase boundary, and that the amount of excitations and energy produced during the quench have a power-law dependence on the quench rate. These phenomena suggest an excitation process analogous to the mechanism for defect generation in non-equilibrium classical phase transitions. This experiment constitutes the first observation of the Kibble-Zurek mechanism in a quantum quench. We have reported our findings in Ref. [1]. In a second experiment, published in Ref. [2], we investigate dissipation as a method for cooling a strongly interacting gas. We introduce dissipation via a bosonic reservoir to a strongly interacting bosonic gas in the Mott-insulator regime of a 3D spin-dependent optical lattice. The lattice atoms are excited to a higher energy band using laser-induced Bragg transitions. A weakly interacting superfluid comprised of atoms in a state that does not experience the lattice potential acts as a dissipative bath that interacts with the lattice atoms through collisions. We measure the resulting bath-induced decay using the atomic quasimomentum distribution, and we compare the decay rate with predictions from a weakly interacting model with no free parameters. A competing intrinsic decay mechanism arising from collisions between lattice atoms is also investigated. The presence of intrinsic decay can not be accommodated within a non-interacting framework and signals that strong interactions may play a central role in the lattice-atom dynamics. The intrinsic decay process we observe may negatively impact the success of cooling via dissipation because a fraction of intrinsic decay events can deposit a large amount of energy into the lattice gas. In a third experiment, we develop and carry out the first demonstration of cooling an atomic quasimomentum distribution. Our scheme, applied in a proof-of-principle experiment to 3D Bose-Hubbard gas in the superfluid regime, involves quasimomentum-selective Raman transitions. This experiment is motivated by the search of new cooling techniques for lattice-trapped gases. Efficient cooling exceeding heating rates is achieved by iteratively removing high quasimomentum atoms from the lattice. Quasimomentum equilibration, which is necessary for cooling, is investigated by directly measuring rethermalization rates after bringing the quasimomentum distribution of the gas out of equilibrium. The measured relaxation rate is consistent at high lattice depths with a short-range, two-particle scattering model without free parameters, despite an apparent violation of the Mott-Ioffe-Regel bound. Our results may have implications for models of unusual transport phenomena in materials with strong interactions, such as heavy fermion materials and transition metal oxides. The cooling method we have developed is applicable to any species, including fermionic atoms. Our results are available in Ref. [3]