The requirement that this
The requirement that this state be achieved has been circumvented by using quantum interference in rubidium and on the D1 line of cesium by Marlan Scully. This process can be enhanced by using heterojunctions composed of gallium arsenide between layers of aluminum arsenide, and this state can be created in hydrogen by applying an inhomogeneous magnetic field to a gas undergoing the 21-cm transition. It is usually explained through a four-level system, which is characteristic of rare earth ions like neodymium, or the simpler three-level system with the top layer known as the "pump band", as seen in ruby. For 10 points, identify this situation where a majority of particles are in a more excited state, which is necessary for the operation of most lasers.
The requirement that this state be achieved has been circumvented by using quantum interference in rubidium and on the D1 line of cesium by Marlan Scully.
population inversion