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Physical Review A

Relaxation of atomic temperature anisotropy in a one-dimensional optical lattice enhanced by dynamic control of the aspect ratio

Meung Ho Seo, Sooyoung Park, D. Cho

Published Apr 15, 2020• 3 citations
Publisher

Abstract

We study the relaxation dynamics of temperature anisotropy between longitudinal and transverse motions of $^{87}\mathrm{Rb}$ atoms in a one-dimensional optical lattice. The temperature imbalance is established by Raman sideband cooling applied along the lattice axis. At low temperature, a large difference between the longitudinal and transverse vibration frequencies limits the relaxation. We transform the trap potential by phase modulating the lattice beam to reduce the anisotropy and thereby enhance cross-dimensional sympathetic cooling. An extension of classical theory, which incorporates the quantization of the atomic motion, explains the experimental results well. We also report that effects of trap anharmonicity and atomic vector polarizability on broadening of the Raman sideband cancel each other at a particular lattice depth, which greatly facilitates sideband cooling.

DOI
10.1103/physreva.101.043611

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We study the relaxation dynamics of temperature anisotropy between longitudinal and transverse motions of 87Rb^{87}\mathrm{Rb}87Rb atoms in a one-dimensional optical lattice. The temperature imbalance is established by Raman sideband cooling applied along the lattice axis. At low temperature, a large difference between the longitudinal and transverse vibration frequencies limits the relaxation. We transform the trap potential by phase modulating the lattice beam to reduce the anisotropy and thereby enhance cross-dimensional sympathetic cooling. An extension of classical theory, which incorporates the quantization of the atomic motion, explains the experimental results well. We also report that effects of trap anharmonicity and atomic vector polarizability on broadening of the Raman sideband cancel each other at a particular lattice depth, which greatly facilitates sideband cooling.
We report our experiment on sideband cooling with two Raman transitions in a $\mathrm{\ensuremath{\Lambda}}$ configuration that allows selective coherent population trapping (CPT) of the low-lying motional states. The cooling method is applied to 87Rb^{87}\mathrm{Rb}87Rb atoms in a circularly polarized one-dimensional optical lattice. Owing to the vector polarizability, the vibration frequency of a trapped atom depends on its Zeeman quantum number and the CPT resonance for a pair of bound states in the $\mathrm{\ensuremath{\Lambda}}$ configuration depends on their vibrational quantum numbers. We call this scheme motion-selective coherent population trapping (MSCPT) and it is a trapped-atom analog of the velocity-selective CPT developed for free He atoms. We observe a pronounced dip in temperature near a detuning for the Raman beams to satisfy the CPT resonance condition for the motional ground state. Although the lowest temperature we obtain is ten times the recoil limit owing to the heat load from motion along an axis, which is left uncooled, and the large Lamb-Dicke parameter of 2.3 in our apparatus, the experiment demonstrates that MSCPT enhances the effectiveness of Raman sideband cooling and enlarges the range of its application. Discussions of design parameters optimized for MSCPT on 87Rb^{87}\mathrm{Rb}87Rb atoms and opportunities provided by diatomic polar molecules, whose Stark shift shows a strong dependence on the rotational quantum number, are included.
Sideband laser cooling using stimulated Raman transitions is performed on trapped cesium atoms. The confinement is produced by a far–off-resonance dipole trap consisting of two crossed YAG beams which, by interference, create a one-dimensional optical lattice. In a pure intensity lattice, we measure a 1D temperature of T = 6 μK corresponding to a mean quantum vibrational number of about ⟨ n v ⟩ = 0.75. In a polarization gradient lattice, the final temperature is T = 3.6 μK corresponding to ⟨ n v ⟩ ≃ 2.4.
Sideband cooling is a key technique for improving the performance of optical atomic clocks by preparing cold atoms and single ions into the ground vibrational state. In this work, we demonstrate detailed experimental research on pulsed Raman sideband cooling in a 171Yb optical lattice clock. A sequence comprised of interleaved 578 nm cooling pulses resonant on the 1st-order red sideband and 1388 nm repumping pulses is carried out to transfer atoms into the motional ground state. We successfully decrease the axial temperature of atoms in the lattice from 6.5 μK to less than 0.8 μK in the trap depth of 24 μK, corresponding to an average axial motional quantum number 〈 nz 〉 < 0.03. Rabi oscillation spectroscopy is measured to evaluate the effect of sideband cooling on inhomogeneous excitation. The maximum excitation fraction is increased from 0.8 to 0.86, indicating an enhancement in the quantum coherence of the ensemble. Our work will contribute to improving the instability and uncertainty of Yb lattice clocks.
Sideband laser cooling using stimulated Raman transitions is performed on trapped cesium atoms. The confinement is produced by a far–off-resonance dipole trap consisting of two crossed YAG beams which, by interference, create a one-dimensional optical lattice. In a pure intensity lattice, we measure a 1D temperature of T = 6 μK corresponding to a mean quantum vibrational number of about nv = 0.75. In a polarization gradient lattice, the final temperature is T = 3.6 μK corresponding to nv 2.4.
We experimentally study the effect of a slight nonorthogonality in a two-dimensional optical lattice onto resolved-sideband Raman cooling. We find that when the trap frequencies of the two lattice directions are equal, the trap frequencies of the combined potential exhibit an avoided crossing and the corresponding eigenmodes are rotated by ${45}^{\ensuremath{\circ}}$ relative to the lattice beams. Hence, tuning the trap frequencies makes it possible to rotate the eigenmodes such that both eigenmodes have a large projection onto any desired direction in the lattice plane, in particular, onto the direction along which Raman cooling works. Using this, we achieve two-dimensional Raman ground-state cooling in a geometry where this would be impossible, if the eigenmodes were not rotated. Our experiment is performed with a single atom inside an optical resonator but this is inessential and the scheme is expected to work equally well in other situations.
We trap neutral Cs atoms in a two-dimensional optical lattice and cool them close to the zero point of motion by resolved-sideband Raman cooling. Sideband cooling occurs via transitions between the vibrational manifolds associated with a pair of magnetic sublevels, and the required Raman coupling is provided by the lattice potential itself. We obtain mean vibrational excitations ${\overline{n}}_{x}\ensuremath{\approx}{\overline{n}}_{y}<0.024$, corresponding to a population >95>95%>95 in the vibrational ground state. Atoms in the ground state of an optical lattice provide a new system in which to explore quantum state control and subrecoil laser cooling.