Sub-Doppler magneto-optical trap for calcium

We explore an efficient method for preparing large samples of ultracold calcium atoms. An optimized conventional (Doppler-limited) magneto-optical trap collects atoms from a Zeeman cooled atomic beam using a strong dipole transition within the singlet system. This transition is not completely closed thus yielding an intense flux of atoms into the metastable triplet state ${}^3{P}_2.$ A second magneto-optical trap sharing the same magnetic-field gradient is superimposed which captures and further cools the metastables using the narrow-band infrared transition ${}^3{\overrightarrow{P_2}}^3{D}_3.$ In our present experiment we were able to prepare $3\times {10}^8$ atoms at temperatures below 20 microkelvin within 250 ms. Minor technical improvements of our setup promise to yield above ${10}^{10}$ atoms at submicrokelvin temperatures within 1 s.