It takes energy to push a ball up a hill. A quantum engine might be able to do similar work by making repeated measurements.

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Quantum measurements could power a tiny, hyperefficient engine

You can’t measure an atom without disturbing it, at least according to quantum mechanics. This effect may seem like a nuisance, but it could power a tiny engine that could run with nearly 100% efficiency—far greater than a car engine, a pair of physicists reports. For the moment, the "measurement engine" is purely hypothetical, but physicists say it might be possible to actually make one.

"It's a very nice idea," says David Herrera Martí, a physicist at ProbaYes, an AI company in Montbonnot-Saint-Martin, France, that invests in quantum computing. "You could imagine having a swarm of little molecular machines that are driven very efficiently with a laser."

An engine is a machine that repeats a cycle of movements to convert energy into useful work—such as pushing your car down the road. Most engines take in heat energy from a "hot bath"—in your car's engine, the hot gases produced by the exploding fuel. To keep chugging, an engine must repeatedly return to its original configuration and in the process, it must lose some heat to a "cold bath"—for your car, the environment. Required by the second law of thermodynamics, that unavoidable waste of energy severely limits the efficiency of a heat engine to below a level set by the temperatures of the baths. A typical car converts around 25% of the energy in gasoline into motion.

However, in the quantum realm of the very small, an engine can feed off a different source of energy—the energy needed to measure the position of a tiny particle—and use it with nearly complete efficiency, say Cyril Elouard and Andrew Jordan of the University of Rochester in New York. The two have devised a scheme that could, for example, raise a particle against the pull of gravity just by trying to measure its position over and over again.

Imagine a bowling ball sitting on the floor of an elevator. No matter how many times you look at it, its position will remain certain. But shrink the bowling ball to, say, a single neutron on tiny movable platform, and quantum mechanics changes the picture dramatically. Because the neutron is so small, its position can no longer be predicted exactly. Instead, the neutron must be described by a diffuse quantum wave that gives the probability of finding it in different places. The quantum wave is a bit like a cloud hovering above the platform, dense near the platform where the neutron is likely to be and thin higher up. It’s not until the measurement occurs that the neutron’s position becomes known.

Exploiting the either-or nature of quantum measurements, Elouard and Jordan envision measuring whether the neutron is hovering within a set distance above the platform or if it is farther up. If the neutron is inside the close zone, they leave the platform alone. If the neutron is outside that close zone, they move up the platform by the same set distance, essentially catching the neutron before gravity pulls it back down. Repeating the process gradually lifts the neutron against gravity, the researchers report in a study published in Physical Review Letters. Curiously, the platform itself never exerts a force to raise the neutron. Instead, the energy to lift the neutron comes from the measurement itself.

There's a catch, however. The measurement also changes the neutron's quantum wave. And measuring whether the neutron is or isn't close to the platform lops off half of the original wave. That hacksaw modification requires a lot of energy. Moreover, to get ready for the next engine cycle, this jagged wave must “relax” back to the original smooth shape, which means it must lose most of the energy it absorbed to its surroundings. Those two effects spoil the engine's efficiency.

To avoid such losses, Elouard and Jordan employ a final key ingredient: less informative measurements. They imagine changing the measurement so that the definition of “out" remains the same—the neutron lies beyond the fixed distance from the platform. But the definition of "in" becomes vaguer: It means only that the particle is within some much greater distance from the surface. If the neutron sits ambiguously between the two bounds, a quantum measurement will randomly yield either "out" or "in."

Weirdly, the less-certain measurement provides a big bonus. Tune things just right, and when the measurement says "in," it leaves the neutron's quantum wave nearly unchanged. When it says "out," the measurement leaves the neutron in a wave almost identical to the original wave, but shifted upward. Crucially, those two overlapping waves so closely resemble the original one that, no matter what the result, very little energy is lost in relaxing and the neutron is ready to start the next cycle. "That's the cleverest part," Herrera Martí says.

The engine can run with up to 99.8% efficiency, Elouard and Jordan calculate. It might be possible to build such an engine, Jordan says. "You wouldn't run a locomotive with it," he says, "but you could run an atom or a molecule." What such a machine would be good for remains to be seen, however.

Of course, there are trade-offs. Employing the less certain measurements requires many more cycles to lift the neutron. So the quantum measurement engine, while efficient, does its job slowly. Ultimately, the engine cannot evade the second law of thermodynamics either, Herrera Martí notes. Although the researchers don't specify their measurement device, it must be a macroscopic machine that will have to waste energy, he says. Still, the measurement engine put a new tool in the quantum mechanic's toolbox.