An extremely clear demonstration of gain without population inversion results from doing three-level spectroscopy on atoms confined in a magneto-optic trap (MOT). Because the atoms in the MOT are extremely cold, the effects of Doppler broadening are almost completely eliminated. The atoms therefore closely approximate a stationary, driven, four-level system and can be modelled with a simple, fully analytical theoretical treatment.

In the experiment, Rb⁸⁷ atoms are first confined in a standard MOT (see MOT). The atoms are then  probed with a weak, cw laser on the D1 line at 795 nm while being driven with a strong cw drive laser (see probe) tuned to the D2 line at 780 nm. In the presence of a repump laser which puts atomic population into the probe transition levels, gain without population inversion is observed when the probe is tuned on-resonance (see figure below).

Two absorption peaks are clearly seen in the probe transmission profile above. These can be thought of as resulting from absorption from the two dressed states of the atom-drive field system, indicating that there is no population inversion between the probe excited state and either of the two dressed probe ground states. The gain that occurs in the center of the profile therefore occurs without population inversion of any kind on the probe transition and is exclusively a result of quantum interference between different probe absorption mechanisms associated with the drive field.

As an additional check on the presence or absence of a population inversion in the system, the probe and repump were tuned to a different excited state hyperfine level (F=1) for which a population inversion should occur due to the different decay rate from that level. In this case, a clear two-peaked gain profile (see gain with inversion) can be seen in contrast to the two-peaked absorption profile shown above. This measurement provides additional evidence that no population inversion is present for the data shown above.

References:

J. Kitching and L. Hollberg, "Interference-Induced Optical Gain Without Population Inversion in Cold, Trapped Atoms," Phys. Rev. A, 59, 4685-4689, 1999.

John Kitching, NIST/JILA, [email protected]