Simultaneous dual-species laser cooling using an optical frequency comb

We demonstrate 1D simultaneous laser cooling of ${}^{87}\mathrm{Rb}$ and ${}^{85}\mathrm{Rb}$ atoms using an optical frequency comb. By adjusting the pulse repetition frequency and the offset frequency, the frequency comb spectrum is tuned to ensure that two distinct frequency comb modes are simultaneously red-detuned from the cooling transitions, one mode for each species. Starting from a precooled cloud of ${}^{85,87}\mathrm{Rb}$ atoms at above-Doppler temperatures, we show simultaneous cooling of both species down to the Doppler temperature using two counter-propagating ${\sigma }^{+}$ and ${\sigma }^{-}$ polarized beams from the frequency comb. The results indicate that simultaneous dual-species frequency comb cooling does not affect the cooling characteristics of individual atomic species. The results of this work imply that several atomic species could be cooled simultaneously using a single frequency comb source. This comb-based multichannel laser cooling could bring significant advances in multispecies atom interferometers for space applications and in the study of multispecies interactions.

Figure 1

Measured (a) and calculated (b) FC radiation pressure force as a function of comb detuning $\delta$ for ${}^{85}\mathrm{Rb}$ (red) and ${}^{87}\mathrm{Rb}$ (blue) atoms. $\delta$ denotes the detuning of the $n\mathrm{th}$ comb mode from the ${}^{87}\mathrm{Rb}|5S_{1/2};F=2\rangle \to |5P_{3/2};F^{\prime }=3\rangle$ transition. The calculated FC force in (b) includes contributions from three ${}^{85}\mathrm{Rb}|5S_{1/2};F=3\rangle \to |5P_{3/2};F^{\prime }=2,3,4\rangle$ and three ${}^{87}\mathrm{Rb}|5S_{1/2};F=2\rangle \to |5P_{3/2};F^{\prime }=1,2,3\rangle$ hyperfine transitions (shown separately in Ref. [26]).

Figure 2

Relative positions of comb lines within the FC spectrum with respect to the ${}^{85}\mathrm{Rb}|5S_{1/2};F=3\rangle \to |5P_{3/2};F^{\prime }=2,3,4\rangle$ and ${}^{87}\mathrm{Rb}|5S_{1/2};F=2\rangle \to |5P_{3/2};F^{\prime }=1,2,3\rangle$ hyperfine transitions for the FC parameters used in the experiment. Note that, when the $n$-th comb mode is resonant with the ${}^{87}\mathrm{Rb}|5S_{1/2};F=2\rangle \to |5P_{3/2};F^{\prime }=3\rangle$ transition, the $(n+14)$-th comb mode is 0.4 MHz blue detuned from (i.e., almost in resonance with) the ${}^{85}\mathrm{Rb}|5S_{1/2};F=3\rangle \to |5P_{3/2};F^{\prime }=4\rangle$ transition.

Figure 3

Temperatures obtained by simultaneous FC cooling of ${}^{85}\mathrm{Rb}$ (red circles) and ${}^{87}\mathrm{Rb}$ (blue circles) as a function of the comb detuning. $\delta$ denotes the detuning of the $n\mathrm{th}$ comb mode from the ${}^{87}\mathrm{Rb}|5S_{1/2};F=2\rangle \to |5P_{3/2};F^{\prime }=3\rangle$ transition. The initial cloud temperatures are $T_i=210\left(20\right)\mu \mathrm{K}$ for ${}^{85}\mathrm{Rb}$ and $T_i=260\left(10\right)\mu \mathrm{K}$ for ${}^{87}\mathrm{Rb}$, and the FC cooling time is $t_{\text{FC}}=3$ ms. For ${}^{85}\mathrm{Rb}$ the steady-state temperature, which approaches the Doppler temperature, is achieved for $\delta \approx -\mathrm{\Gamma }/2$. Due to the higher $T_i$ for ${}^{87}\mathrm{Rb}$, the steady-state is not achieved during the applied cooling time leading to slightly higher measured temperatures. In general, equal temperature dependence on the comb detuning is obtained for both isotopes. The dashed line indicates the ${}^{85}\mathrm{Rb}$ temperature calculated for the relevant experimental parameters using the relation (17) in Ref. [10], i.e., the steady-state temperature obtained for the simple model of 1D FC cooling of two-level atoms.

Figure 4

Temperatures obtained by simultaneous FC cooling of ${}^{85}\mathrm{Rb}$ (red circles) and ${}^{87}\mathrm{Rb}$ (blue circles) as a function of FC cooling time $t_{\mathrm{FC}}$. The initial cloud temperatures are $T_i=231\left(16\right)\mu \mathrm{K}$ for ${}^{85}\mathrm{Rb}$ and $T_i=262\left(8\right)\mu \mathrm{K}$ for ${}^{87}\mathrm{Rb}$, with comb detuning $\delta =-\mathrm{\Gamma }/2$. As the FC cooling time is increased, the cloud temperature decreases from the initial value $T_i$ prepared in the MOT phase and approaches the Doppler limited steady-state temperature on the time scale of a few ms. Solid lines represent the theoretical estimate for the dependence of the cloud temperature on the FC cooling time (see text for details of the model).