Double-EIT ground-state laser cooling without blue-sideband heating

– We discuss a laser cooling scheme for trapped atoms or ions which is based on double electromagnetically induced transparency (EIT) and makes use of a four-level atom in tripod configuration. The additional fourth atomic state is coupled by a strong coupling laser field to the usual three-level setup of single-EIT cooling. This effectively allows to create two EIT structures in the absorption spectrum of the system to be cooled, which may be controlled by the coupling laser field parameters to cancel both the carrierand the bluesideband excitations. In leading order of the Lamb-Dicke expansion, this suppresses all heating processes. As a consequence, the double-EIT scheme can be used to lower the cooling limit by almost two powers of the Lamb-Dicke parameter as compared to single-EIT cooling. Introduction. – Many current experiments involving the preparation or manipulation of atoms and ions require a precise coherent control of the system of interest. This does not only apply to internal degrees of freedom, but also to the external motional degrees of freedom. In the last few years, the laser cooling of trapped ions or atoms has therefore been a subject of intense research and is now a routine tool in many laboratories. Starting from the first observation of laser cooling [1], many interesting applications have been made possible by laser cooling. Examples are the direct observation of quantum jumps [2], the preparation of atoms in the motional ground state [3], and high-precision spectroscopy [4]. Apart from cooling on dipole-forbidden transitions [3,5] and cooling by stimulated Raman transitions [6], cooling facilitated by electromagnetically induced transparency (EIT) [7] is a promising recent technique to reach the mechanical ground state. EIT cooling has already been observed experimentally [9]. Once a trapped atom reaches the Lamb-Dicke regime e.g. by unresolved Doppler cooling, the interaction with a cooling laser field may excite the atom by three kinds of processes. Carrier excitations involve an excitation of the internal electronic degree of freedom of the atom without a change of the motional quantum number. Processes where simultaneously with the internal excitation the motional quantum number is increased (decreased) are known as blue (red) sideband excitations. Both excitation-deexcitation cycles involving carrierand blue-sideband excitations may lead to a heating of the trapped system. (∗) [email protected] (∗∗) [email protected]