# 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?

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Welcome to chemistry.stackexchange.com @zero. This is an interesting question, but it may be more appropriate at physics.stackexchange.com. –  Ben Norris Nov 9 '12 at 11:35

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.