In an ultracold chamber, cesium atoms collide with rubidium atoms like billiard balls, exchanging energy. These collisions are the heart of the quantum heat engine: they heat and cool the working fluid. Previously, physicists thought that to speed up the motor, they needed to modify the system itself, but it turned out that simply "tuning" the reservoir, like adjusting a tuning fork that sets the tempo, was enough.
By varying the temperature of the rubidium cloud, the scientists controlled the rate of heat exchange: during heating, cesium atoms gained energy; during cooling, they released it. At a higher reservoir temperature, heating slowed down while cooling sped up. This allowed them to reallocate the cycle time and shorten its overall duration. As a result, at a temperature of 1410 nanokelvin, a billion times colder than room temperature, the engine's power reached its maximum, while its efficiency remained unchanged.
At the optimal temperature of 1410 nanokelvins for power, the thermal rubidium cloud moves at an average speed of about 2 millimeters per second — a thousand times slower than a housefly at room temperature.
In Stanisław Lem's novel 'The Invincible', microscopic machines use the environment for self-replication but face the problem of heat dissipation. Controlling heat exchange through microscopic collisions, as in this work, is a step toward solving such problems at the nanoscale.