An international team of scientists was able to create a nifty quantum experiment that at first might appear to have broken thermodynamics: They were able to create a spontaneous heat flow from a cold system to a hot system. In fact, they were able to do so without breaking any physical law. Their work highlights the complex relationship between quantum mechanics, thermodynamics, and time itself.
Heat always flows from a hot system to a cold one. If you put ice cubes in your soda, your drink won’t suddenly get hotter. When this heat flow happens, the entropy of the system increases. So by looking at the entropy of the system, it is possible to work out if we are looking at the system going “forward” in time or “backward”. The entropy increase defines a thermodynamic arrow of time, and the macroscopic world experiences this in the same direction that we do.
While this is all well and good, there is an important assumption: for heat to flow from hot to cold, the systems have to be uncorrelated. There shouldn’t be any special connections linking the two before you put them in contact. This is obvious for macroscopic systems – you don’t expect ice cubes to have a special relationship with the molecules that make up your specific soda.
However, this is not so obvious in the quantum world. It is possible to create quantum states that are correlated, where suddenly the direction of the arrow of time can be reversed. This has been suggested theoretically before, but this study shows that it's possible to prove experimentally. To do so, they set up two correlated thermal systems and witnessed the heat flow from the cold to the hot system.
So what gives? In their paper, available on the arXiv, the team says there is a trade-off between the quantum correlation and the entropy of the system. It is this trade-off that allows for the reversal of heat flow. The thermodynamic arrow of time is strongly dependent on the initial condition of a system.
There is another interesting tidbit from the study. According to the researchers, this reversal of heat flow is not limited to extremely microscopic systems, although they are yet to investigate bigger setups. It is unlikely to be witnessed in a macroscopic phenomenon, but since entropy plays such a big role in the scientific definition of time, it might be worth keeping an eye on.