My assumption is that he is referring to negative absolute temperature. This describes a system where the entropy decreases as the total energy increases. The arch-example is where you reverse the magnetic field for a system of spins which have previously been cooled near absolute zero. But the most common example occurs in pumped lasers. It is in this negative temperature regime where you get actual amplification.ummm... mercury thermometers measure negative temperatures. So do thermocouples and Pt100/1000s.
what are you going for? How negative and in what unit °C/°F/K(can't be negative, but hey)?