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.
Although forcing the atom into a squeezed state doesn't actually transfer any energy, it does mean that the pulsating atom is (because of a quantum mechanical quirk) on average slightly bigger when it hits the cold end of the cone. And while the cooling phase knocks the atom out of this squeezed state, the momentary extra size gives the entire engine a boost. "You can think of it sort of like a supercharger," says Jacob Taylor, a quantum physics researcher at the University of Maryland, who was not involved in the experiment. According to Roßnagel, if you calculate the energy efficiency of this supercharged system, it's four times as efficient as it would be without the squeezing—surpassing the Carnot Limit by a large margin. This would make it the most efficient engine ever built.
However, any claims that an engine can break the laws of thermodynamics deserves extra scrutiny and skepticism. According to Taylor, this ultrahigh efficiency is only a matter of perspective. "There's no free lunch here," he says. Despite the fact that the squeezing process doesn't transfer any energy to the atom's side-to-side movement, "you still have to consider the energy that goes into the squeezing process. You're essentially taking energy from the squeezing process to turbo-boost the engine." And calculating in that squeezing energy, the engine is safely below the Carnot Limit.
Hartmut Häffner, a theoretical physicist at the University of California, Berkeley, who was not involved in the experiment, agrees. "I wouldn't accept this efficiency is just from 'the weirdness of quantum mechanics,'" says Häffner, but he adds that the proposed nano engine itself "is very interesting and very well-described. It's trying to push the boundaries of what we know about thermodynamics into a new regime."
Roßnagels argues that because the squeezing process doesn't actually transfer any energy to the atom's side-to-side movement along the cone, including it in the efficiency calculation for his nano engine is a bit arbitrary. It's like looking at the energy efficiency of a gasoline engine and incorporating in the millions of years of energy it took to create the fossil fuels, he says, or the energy it took to pump the oil out of the ground. He is generally in agreement with Taylor, though, that it all depends on how you look at it. "In general it's kind of a semantic problem," Roßnagel says. "It's where you put your camera and decide what is part of the system and what isn't part of the system."
The sheer amount of laboratory space and equipment these nano engines require means that we won't see them outside a lab anytime soon. (Or perhaps ever. Sorry, nanobots!) But Taylor says the insight we'll gain from this type of experiment can be incredibly helpful in other realms— chiefly, quantum computing. The pursuit of building computers that manipulate the funky physics of quantum mechanics to process information has already captured some of brightest minds in theoretical physics. "And in quantum computation you really need the ability to efficiently move heat around," Taylor says, "and in so far as we can better understand these heat engines, it may improve our ability in developing quantum computers down the road."
