Consider a hot cup of coffee, that indispensable fuel for many a physicist. We know that if we leave it alone for long, that hot, steaming cup of coffee cools down, making it easier to drink but less delectable. To prevent or delay this, some of us use special insulating cups, or cover the open top of the cup. Also, consider that one can enjoy a steaming cup of coffee only for a short period of time if one drinks it at an outdoor Paris cafe in the dead of winter, as the cold winter air will cause the coffee to cool down, and ultimately freeze. On the other hand, there is more time to enjoy this same hot cup of coffee in this same outdoor cafe on a warm summer day, due to the air's temperature being closer to that of the coffee. .
We can explain these phenomena by saying that the coffee interacts with the surrounding environment in such a way that its state changes depending on the environment surrounding it. Changing the environment also changes.
the rate at which the coffee's state changes, and minimizing the interaction between the coffee and the environment allows it to change its state much more slowly, giving us more time to enjoy that hot cup of coffee. While the physical system described above may seem familiar to you, we can use it as an analogy to describe open quantum systems. In brief, an open quantum system is a quantum system that interacts with its environment, with this interaction between the system and the environment driving the time evolution of the quantum system, just like how the interaction between the coffee and its surrounding environment drives the change in temperature of the coffee. An open quantum system is much more realistic than a closed quantum system, which is isolated from the environment, because it is almost impossible to completely isolate a quantum system from its surrounding environment. .