World’s smallest engine runs on one atom

German researchers are claiming the world’s smallest heat engine, which runs off just one atom, thanks to quantum effects.

In the past, the classical laws of thermodynamics limited how small engines could get. In particular, the Carnot theorem specified limits on the maximum efficiency any heat engine can obtain, depending on the difference in temperature between the hot and cold reservoirs.  This led to engines which typically weighed over a tonne.

However, with better understanding of quantum effects, the QUANTUM work group at the Institute of Physics of Johannes Gutenberg University Mainz (JGU) in collaboration with theoretical physicists of Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have demonstrated it is possible to build small engines where the limitations of classical thermodynamics no longer apply. Nano engines could have an efficiency greater by at least a factor of two higher than the classical Carnot limit governing conventional heat engines.

The team used a linear Paul trap to trap a single negatively-charged atom of calcium. When the atom is low in energy, it is attracted to one end of the device, where electrically-generated noise heats it up. This miniature “hot reservoir” gives the atom thermal energy, and creates a negative field which repels the atom.

The energised atom is propelled to the larger side of the contraption where another laser, acting as the cold reservoir, cools it through Doppler cooling. The atom, once again low in energy, is propelled back towards the hot end. Repeating this cycle rapidly, the atom moves back and forth within the trap, replicating the stroke of a typical engine. The atom not only acts in the same way as an engine, but also stores the energy.

According to the researchers, it would be possible to harness the mechanical energy of the engine by putting a second ion by the cooler side.

Additionally, as the atom is heated and cooled, its size varies slightly, altering the probability of where the atom exists. However, because the atom is tightly confined within the electrodes, the temperature change forces the atom to vibrate back and forth with the expansions and contractions in its size.

The frequency of the laser which heats and cools the atom is matched to the frequency that the atom naturally vibrates at in order to achieve maximum efficiency. The atoms’ varying size gives the engine a boost, allowing it to surpass the Carnot Theorem’s limits by a large margin.

While the mass of one calcium atom is just 6.3 x 10-23 grams, the engine was able to sustain a power output of 3.4 × 10-22 joules per second. It generates 10-22 watts of power, with an efficiency of 0.3 percent.

Reversing the cycle would allow the use of the device as a single-atom refrigerated, effectively a cooler for nano systems. The power of the heat engine can also be increased by linking it to a quantum heat bath, allowing the construction of new types of engines.

The researchers cautioned that while the engine utilises quantum mechanics to surpass existing limitations, it does not break any laws of physics. The engine still works with energy inputs, particularly for powering the lasers and magnetic fields. And while the engine is the world’s smallest, the equipment needed to sustain it means the engine is unlikely to be seen outside the lab in the short term. However, it does provide much needed insight into thermodynamics at a single-particle level.