Laser cooling of trapped ions via an analog of velocity-selective coherent population trapping*

We theoretically study a ground-state cooling scheme that efficiently cools the phonon modes in generic trapped ion arrays. Our scheme employs a pair of highly detuned Raman lasers that couples the electronic spins with the motion of the ions. The same Raman beams drive the spin into a dark state that is tunable by the laser parameters. At appropriate resonances, ions can escape the dark state by absorbing phonons while cooling their motion, in a way that emulates the velocity-selective coherent population trapping scheme in atomic gases. We show that the relevant heating mechanisms are suppressed in this setting allowing the ions to reach their motional ground state with high fidelity. The proposed scheme can be implemented in current trapped ion experiments such as a two-dimensional ion crystal in a Penning trap where the rotation of the ions complicates more standard cooling schemes.  For this system, we discuss the currently accessible parameter regimes in which the cooling is operational.