FQXi's Franco Nori and colleagues at RIKEN in Japan have developed a nanodevice that act as a either a heat engine, or a mini-fridge--or, with an added quantum twist-- it can both cool and heat at the same time. Their paper was published a paper in Physical Review Letters in October.
You wouldn't usually want to combine a fridge and a heater--at least not in a macroscale device. But on the nanoscale, such a device might be useful for quickly switching between the two functions, on a quantum computing chip.
In a normal, macroscale fridge, you exchange heat between two baths of fluid: compressing one bath causes it to heat up, while expanding the other makes it cool. In the nano engine, this process is mimicked by increasing or decreasing the gap between internal energy levels in an electron. The team monitored whether the engine was functioning as a heater or a fridge by monitoring microwaves that were emitted by the device. When it functions as both simultaneously, the emitted microwaves combine to create a characteristic interference pattern.