It is shown that the possible excitation of a phonon laser instability in an ultra-cold atomic gas confined in a magneto-optical trap. Such an effect results from a negative Landau damping of the collective density perturbations in the gas, leading to the coherent emission of phonons. This laser instability can be driven by a blue-detuned laser superimposed to the usual red-detuning laser beams which usually provide the cooling mechanism. Threshold conditions, instability growth rates and saturation levels are derived. This work generalizes, on theoretical grounds, the recent results obtained with a single-ion phonon laser, to an ultra-cold atomic gas, where real phonons can be excited. Future phonon lasers could thus adequately be called phasers.
The researchers have extended the concept of the single-ion phonon laser to a large collection of atoms. They have shown that an ultra-cold atomic gas can enable collective phonon excitations. In contrast with the single-ion case, here the phonon frequency is determined by the internal oscillations of the atoms in the gas, similar to how the photon frequency in a laser is determined by internal vibrations of the optical cavity.