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arXiv:physics/0104061 · Physical Review A

Quenched narrow-line laser cooling of 40Ca to near the photon recoil limit

E. A. Curtis, Chris W. Oates, Leo W. Hollberg

Published Aug 16, 2001• 117 citations
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Abstract

We present a cooling method that should be generally applicable to atoms with narrow optical transitions. This technique uses velocity-selective pulses to drive atoms towards a zero-velocity dark state and then quenches the excited state to increase the cooling rate. We demonstrate this technique of quenched narrow-line cooling by reducing the 1D temperature of a sample of neutral ${}^{40}\mathrm{Ca}$ atoms. We cool selected velocities with the ${}^{1}{S}_{0}{(4s}^{2}{)}^{3}{P}_{1}(4s4p)$ 657-nm intercombination line and quench with the ${}^{3}{P}_{1}{(4s4p)}^{1}{S}_{0}(4s5s)$ intercombination line at 553 nm, which increases the cooling rate eightfold. Limited only by available quenching laser power, we have transferred 18% of the atoms from our initial 2-mK velocity distribution and achieved temperatures as low as 4 \ensuremath{\mu}K, corresponding to a ${v}_{\mathrm{rms}}$ of 2.8 cm/s or 2 recoils at 657 nm. This cooling technique, which is closely related to Raman cooling, can be extended to three dimensions.

DOI
10.1103/physreva.64.031403

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We present a cooling method that should be generally applicable to atoms with narrow optical transitions. This technique uses velocity-selective pulses to drive atoms towards a zero-velocity dark state and then quenches the excited state to increase the cooling rate. We demonstrate this technique of quenched narrow-line cooling by reducing the 1D temperature of a sample of neutral 40Ca{}^{40}\mathrm{Ca}40Ca atoms. We cool selected velocities with the 1S0(4s2)3P1(4s4p){}^{1}{S}_{0}{(4s}^{2}{)}^{3}{P}_{1}(4s4p)1S0​(4s2)3P1​(4s4p) 657-nm intercombination line and quench with the 3P1(4s4p)1S0(4s5s){}^{3}{P}_{1}{(4s4p)}^{1}{S}_{0}(4s5s)3P1​(4s4p)1S0​(4s5s) intercombination line at 553 nm, which increases the cooling rate eightfold. Limited only by available quenching laser power, we have transferred 18% of the atoms from our initial 2-mK velocity distribution and achieved temperatures as low as 4 \ensuremath{\mu}K, corresponding to a vrms{v}_{\mathrm{rms}}vrms​ of 2.8 cm/s or 2 recoils at 657 nm. This cooling technique, which is closely related to Raman cooling, can be extended to three dimensions.
We demonstrate three-dimensional quenched narrow-line laser cooling and trapping of 40Ca. With 5 ms of cooling time we can transfer 28% of the atoms from a magneto-optic trap based on the strong 423-nm cooling line to a trap based on the narrow 657-nm clock transition (which is quenched by an intercombination line at 552 nm), thereby reducing the atoms’ temperature from 2 mK to 10 μK. This reduction in temperature should help to reduce the overall systematic frequency uncertainty for our Ca optical frequency standard to <1 Hz. Additional pulsed, quenched, narrow-line third-stage cooling in one dimension yields subrecoil temperatures as low as 300 nK and makes possible the observation of high-contrast two-pulse Ramsey spectroscopic line shapes.
We present a new and efficient implementation of Raman cooling of trapped atoms. It uses Raman pulses with an appropriate frequency chirp to realize a velocity selective excitation through a rapid adiabatic passage. This method allows to address in a single pulse a large number of non zero atomic velocity classes and it produces a nearly unity transfer efficiency. We demonstrate this cooling method using cesium atoms in a far-detuned crossed dipole trap. Three-dimensional cooling of 1×1051 \times 10^{5}1×105 atoms down to 2μ2 μ2μK is performed in 100 ms. In this preliminary experiment the final atomic density is 1.3×10121.3\times 10^{12}1.3×1012 at/cm3^33 (within a factor of 2) and the phase-space density increase over the uncooled sample is 20. Numerical simulations indicate that temperatures below the single photon recoil temperature should be achievable with this method.
We present a new laser-cooling scheme based on velocity-selective optical pumping of atoms into a nonabsorbing coherent superposition of states. This method has allowed us to achieve transverse cooling of metastable 4He^{4}\mathrm{He}4He atoms to a temperature of 2 \ensuremath{\mu}K, lower than both the usual Doppler cooling limit (23 \ensuremath{\mu}K) and the one-photon recoil energy (4 \ensuremath{\mu}K). The corresponding de Broglie wavelength (1.4 \ensuremath{\mu}m) is larger than the atomic-transition optical wavelength.
Ultracold atoms at temperatures close to the recoil limit have been achieved by extending Doppler cooling to forbidden transitions. A cloud of (40)Ca atoms has been cooled and trapped to a temperature as low as 6 microK by operating a magnetooptical trap on the spin-forbidden intercombination transition. Quenching the long-lived excited state with an additional laser enhanced the scattering rate by a factor of 15, while a high selectivity in velocity was preserved. With this method, more than 10% of precooled atoms from a standard magnetooptical trap have been transferred to the ultracold trap. Monte Carlo simulations of the cooling process are in good agreement with the experiments.
In this paper we propose a new and simple velocity selective method, which combines the velocity selective coherent population trapping of atoms in a light field and the kinetic energy coupling between the dark state and coupled state(s) during free flight, in order to reach a very narrow momentum distribution in a short period of time. This technique combined with force-assisted velocity selective coherent population trapping can produce very efficient cooling and trapping. It can also be used to overcome the cooling limit caused by the leakage of the dark state.