# Negative Kelvin Temperature

I remember my Physical Chemistry Professor saying that very tiny negative Kelvin temperatures have been achieved on the quantum level. Is this true?

The energy levels of some systems depends on a magnetic field. You can let the populations of the energy levels come to equilibrium at some temperature $T$ and then quickly invert the magnetic field. What was a low energy level now becomes a high one and vice versa. You have an upside down distribution of populations.