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arXiv:physics/0208071 · Journal of the Optical Society of America B

Quenched narrow-line second- and third-stage laser cooling of ^40 Ca

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

Published May 1, 2003• 44 citations
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Abstract

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.

DOI
10.1364/josab.20.000977

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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 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 realize narrow-linewidth magneto-optical traps of 46^{46}46Ti, 48^{48}48Ti and 50^{50}50Ti atoms based on a 1040 nm-wavelength transition, cooling atoms to a minimum temperature in one dimension of Tz=990(20)T_z=990(20)Tz​=990(20) nK and a three-dimensional temperature of T3D=1.28(7)T_\mathrm{3D}=1.28(7)T3D​=1.28(7) μμμK. Atoms are pre-cooled in a broad-line magneto-optical trap and then transferred with about 25% efficiency to the narrow-line trap. We operate the narrow-line trap in two stages over 85 ms. First, a single cooling beam, blue-detuned from the narrow-linewidth atomic resonance, optically pumps and traps the atoms on a two-dimensional surface where the Zeeman shift from the applied spherical quadrupole magnetic field brings the light nearly to resonance. Second, four additional beams, counter-propagating in the transverse directions, cool and compress the atoms in all dimensions. The high magnetic moment of the laser cooling state makes the dynamics of the narrow-line titanium trap similar to those of other magnetic atoms. We measure the lifetime of the excited state of the transition to be τ=8.2(9)τ=8.2(9)τ=8.2(9) μμμs, indicating a transition linewidth of γ/2π=20(2)γ/2π=20(2)γ/2π=20(2) kHz, and also measure isotope shifts on the narrow-line transition. We use Stern-Gerlach separation on the ultracold Ti gas to measure the mJm_JmJ​-distribution in the narrow-line magneto-optical trap, finding over 98% of the atoms in the stretched spin state.
Research on the development of the optical frequency standard based on trapped and cold 40 Ca + with the 4s 2 S 1/2 -3d 2 D 5/2 clock transition at 729 nm is reported. A single calcium ion was trapped and laser cooled in the Paul trap and stay in trap for more than 15 days. The linewidth of a 729 nm laser was reduced to less than 10 Hz by locking to a cavity for longer than 50 hours uninterruptedly. The overall systematic uncertainty of the clock transition has been characterized to be better than 6.5×10 −16 . The absolute frequency of the clock transition was measured at 10 −15 level using an optical frequency comb referenced to a Hydrogen maser, which was calibrated to the SI second through the global positioning system (GPS). The frequency value was 411042129776393.0(1.6) Hz after the correction of the systematic shifts.
Laser cooling to micro-Kelvin temperatures requires a laser with active frequency stabilization. The linewidth of an atomic transition sets a lower bound on the Doppler cooling temperature. The 2s-2p transition in lithium-6 has a Doppler temperature of ~140 μK In contrast, the 2s-3p transition has a narrower linewidth and thus provides a lower temperature limit of ~20 μK. We present a method for stabilizing a laser to an atomic line in a vapor cell using modulation transfer spectroscopy and a home-built lock-in amplifier. Our results demonstrate successful locking of a 323 nm laser to the 2s-3p transition. The stabilized laser provides a second stage of magneto-optical trapping that results in a factor of ∼5 increase in the phase space density before evaporating to degeneracy in an optical dipole trap.
In addition to being suitable for laser cooling and trapping in a magneto-optical trap (MOT) using a relatively broad transition, the molecule YO possesses a narrow-line transition. This forbidden transition between the and states has linewidth kHz. After cooling in a MOT on the 614 nm to (orange) transition, the narrow 690 nm (red) transition can be used to further cool the sample, requiring only minimal additions to the first stage system. We estimate that the narrow line cooling stage will bring the temperature from ∼1 mK to ∼10 μ K, significantly advancing the frontier on direct cooling achievable for molecules.