Highly efficient nanoengine runs on a single atom and exceeds theoretical Carnot limit

By William Herkewitz

Like the one in your car, Johannes Roßnagel's engine is a four-stroke. In four steps it compresses and heats, then expands and cools. And as with any other engine, this cycle is repeated over and over again—transforming the changing temperature into mechanical energy. 

But Roßnagel's engine is no V-8. And it doesn't use internal combustion. Roßnagel, an experimental physicist at the University of Mainz in Germany, has conceived of and is in the process of building the world's tiniest engine, less than a micrometer in length. It is a machine so small it runs on a single atom. And in a recent paper in the journal Physical Review Letters, its inventors argue that, because of an interesting anomaly of quantum physics, this is also far and away the most efficient engine. 

The nano engine works like this: First, using tiny electrodes, the physicists trap a single atom in a cone of electromagnetic energy. "We're using a calcium-40 ion," Roßnagel says, "but in principle the engine could be built with just about any ion at all." This electromagnetic cone is essentially the engine's housing, and squeezes tightly over the atom. The physicists then focus two lasers on each end of the cone: one at the pointy end, which heats the atom, and another at the base of the cone, which uses a process called Doppler cooling to cool the atom back down. 

Because this heating and cooling slightly changes the size of the atom (more exactly, it alters the fuzzy smear of probability of where the atom exists), and the cone fits the atom so snuggly, the temperature change forces the atom to race back and forth along the length of the cone as the atom expands and contracts. For maximum efficiency, the physicists set the lasers to heat and cool at the same resonance at which the atom naturally vibrates from side to side. 

The result is that, like sound waves that build upon one other, the atom's oscillation between the two ends of the cone "gets accumulated, and becomes stronger and stronger," which can be harnessed, Roßnagel says. "If you imagine that you put a second ion by the cooler side, it could absorb the mechanical energy of our engine, much like a flywheel [in a car engine]." 

And the nano engine has one additional feature, one that, Roßnagel argues, increases the efficiency of the machine so much that it actually surpasses the Carnot Limit—the maximum efficiency any engine can have according to the laws of thermodynamics. 

As the racing atom reaches the hot end to the cone, the researchers slightly contract and expand the sides of the cone a single time. Done at the right frequency, this action puts the moving atom into a quantum mechanical condition called a squeezed state. This means that now, as the atom continues race to the cold end of the cone, it's also slightly pulsating. 

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