Doppler Cooling Beyond the Doppler Limit with Dark Resonances

High fidelity quantum computing operations with trapped ions require the ions to be cooled near the motional ground state. Cooling below the Doppler limit is typically achieved by resolved sideband cooling or electromagnetically induced transparency (EIT) cooling. Sideband cooling typically requires milliseconds to cool even a single motional mode and can become a large portion of the time budget for a quantum circuit. EIT cooling is typically faster and can simultaneously cool a broad range of motional modes. However, EIT cooling increases the required number of laser beams with added constraints on the beam angles and polarizations. Here we present results from an enhanced Doppler cooling scheme that exploits EIT features within the cooling spectrum to cool a ⁴⁰Ca⁺ ion well below the Doppler limit. We demonstrate simultaneous cooling of the axial and radial modes of a single ion to below one quantum with 1/e time constants of less than 100 µs. In contrast with traditional EIT cooling, the typical Doppler cooling beam setup can be used, provided that the linewidth of the cooling beams can be sufficiently narrowed.

This work was done in collaboration with Los Alamos National Laboratory.