Ion Laser Cooling & Trapping

Ionization of neutral atoms, making ions out of atoms, is a prerequisite for ion trapping. One way is to shoot fast electrons onto atoms and take advantage of inelastic collisions. The fast electrons can kick out electrons from the atomic shell creating an atom with at least one electron less than protons, so a positively charged ion. Instead of electrons, one can also use lasers photons with an energy higher that is above the atomic ionization energy to ionize neutral atoms by inelastic collisions. Since ionization energies are in the several eV range, one typically uses lasers in the deep UV or high power pulsed lasers in the UV. In the latter case, two photons are needed for the ionization. While quite straight forward to implement using electron guns or available lasers, these methods allow one to ionize different isotopes or even different elements. This might be advantageous if many species have to be studied but is a disadvantage if a very special isotope has to be trapped. In the latter case, one uses resonantly enhanced two-photon ionization. In a first step, a photon from a laser with a wavelength that is matched to an electronic transition excites the atom. This can be done element or isotope specific using a narrow linewidth laser. Then a second photon, usually from a different laser (e.g. that laser that will be used for ion cooling) having enough photon energy, ionizes the already excited atom. Such a selective ionization process is also used in resonance ionization mass spectroscopy.

Laser cooling of ions can be performed in different ways. Most common methods are Doppler cooling and Sideband cooling. The first one is analog to Doppler cooling of atoms and uses the Doppler effect-induced imbalance in photon scattering from different laser beams in order to give photon recoil kicks in such a way that the ion looses kinetic energy. Sideband cooling makes use of the high trapping frequencies of ion traps on the order of many MHz. If the linewidth of an electronic transition within the ion is smaller than the trapping frequency, one can selectively address the ion with a narrow linewidth laser in such a way that each internal excitation comes with a reduction in kinetic energy. For this, the laser frequency is adjusted an amount of the trap frequency below the transition frequency of the ion. This way, exciting the ion takes away one quantum of vibration energy (oscillation of ion in the trap). If the confinement of the ion in the trap is very strong (Lamb-Dicke regime) the spontaneous emission does not alter the kinetic energy of the ion which is essentially a Mössbauer effect (Nobel Prize in Physics 1961). So each step of excitation/spontaneous emission can be seen as anti-Stokes Raman scattering and leads to a cooling by one quantum of ion oscillation within the trap.  The excitation stops if the atom is in the lowest vibrational state, the ground state of the ion trap.