Single-photon tweezers
Figure 2. A rubidium atom entering the high-finesse cavity of the MPQ experiment leads to an increase in the transmitted power that triggers a feedback switch that increases the power of the driving laser (dashed line). This traps the atom in a light field containing on average about one photon. The atom remains in the cavity for as long as 1.7 milliseconds. After 3 milliseconds the laser-light intensity is switched back to its original value, waiting for the next atom to be captured. The large oscillations in the transmitted light power reflect the motion of the trapped atom. In the Caltech experiment, these oscillations are more regular, indicating a more periodic motion of the atom.