Main Question or Discussion Point
Imagine a two-state system, e.g., a single nucleus that can be aligned or anti-aligned with an external magnetic field. If that nucleus is in a "mixed state" at time zero, such that it's wavefunction is 50% up and 50% down at time zero, why can that nucleus absorb a "quanta" of radiofrequency electromagnetic energy equal to the energy gap between the "down state" and the "up state". It seems to violate conservation of energy, because the system at time zero had more energy than the "down state" , and the absorbed photon had energy equal to the difference between the down state and the up state, and after absorption I assume the wavefunction is entirely in the up state? Somebody please help.