Assuming the second law is not violated, the system of lower (negative) temperature becomes a source of energy (as opposed to sink). If this is right, that is mighty interesting.
Not really - this has been done before and the textbook example are spin systems. The newsworthy part is that the negative temperature is realized via motional degrees of freedom.
Just checked wikipedia:
"By contrast, a system with a truly negative temperature in absolute terms on the kelvin scale is hotter than any system with a positive temperature. If a negative-temperature system and a positive-temperature system come in contact, heat will flow from the negative- to the positive-temperature system."
Let's define wikipedia to be the absolute zero of information.
