Heat engines are everywhere—in the power station steam turbines that light up your neighborhood, the internal combustion engine that runs your car, and the engines that push passenger jets through the sky. Typically they weigh a ton or more apiece, heating and cooling some trillion trillion (no, this is not a typo!) molecules to generate power. Now, physicists in Germany have made what could be the world’s smallest heat engine using just a single particle: an ion of calcium.
The new device makes far too little power to supply us with energy anytime soon. But Jacob Taylor, a physicist at the National Institute of Standards and Technology in Gaithersburg, Maryland, says it provides experimental data for "a quiet revolution” in statistical physics, the study of how heat flows both in microscopic systems and on the scale of everyday life.
Any heat engine converts thermal energy (heat) into mechanical energy (motion). A medium of some sort, usually a gas, draws heat from a hot body—or bath—to do mechanical work, such as moving a piston. It then dumps any unused heat into a cold bath. In the latest work, carried out by physicist Kilian Singer of the University of Mainz in Germany and colleagues, the calcium ion doubles as both the working medium and the piston; electrical noise provides the hot bath, and a laser beam the cold bath.
To make their engine, the researchers first enclosed the ion (a calcium atom with one electron removed) inside an 8-millimeter-long funnel-shaped electrical trap created by four electrodes. They then heat it with noise generated by another set of electrodes. The noise—a randomly fluctuating electric field—transfers energy to the ion, causing it to wiggle back and forth inside the trap and move toward the broad end of the funnel, making the engine’s power stroke. The noise is then turned off, and the calcium ion slows down. It cools after colliding with particles of light from a laser beam that constantly shines through the funnel trap. This cooling forces the ion back to the narrow end of the trap, where the electrical field is strongest. Then the cycle begins again.
In the new engine, a calcium ion converts heat to motion when it is hit by noise coming from a set of electrodes. When the noise stops, the ion slides back into its starting position, and the process begins again.
By turning the noise on and off at just the right rate, the researchers can tune the engine so that the frequency of the ion's up-and-down motion exactly matches the trap's natural oscillation frequency. When this happens, the ion travels ever farther along the funnel from one cycle to the next—like a playground swing that gets higher and higher if you push it at just the right moment in each cycle. The result is a sort of flywheel that gradually builds up usable energy.
Currently, the team dumps that energy by letting another laser absorb it. Singer says it could be potentially tapped to drive a tiny electrical generator, although the amount involved—about 10-24 joules per cycle—is so tiny that billions of single-ion engines would be needed to generate useful power. Running all of those engines in parallel would require a radically new kind of integrated circuit in place of the bulky equipment used in the current experiment.
Instead, Singer says, the main aim of the current work is to "prove the validity of thermodynamics in the single-atom regime." He says the values for the power and efficiency of the engine—the former calculated from the number and frequency of the ion's phonons (vibrational mechanical energy)—are very close to what theory says they should be.
Peter Steeneken, a physicist at the Delft University of Technology in the Netherlands, says the German group provides "convincing evidence" for having built a single-ion engine. He adds that the work "provides a way to study the ultimate and fundamental limits of the application of heat engines." But he agrees that practical applications are hard to envisage.
Taylor says it will be interesting to see how the quantum-mechanical behavior of tiny heat engines differs from the “classical” physics that governs familiar engines. The current device takes in about 1000 phonons per cycle from the noise—a minuscule amount of energy, but still "about a factor of 1000" too high for scientists to observe real quantum phenomena, he says. "While the experiment is a beautiful demonstration of how a single atom can be used as a heat engine," Taylor says, "there is still substantial work to go until deviations from classical thermodynamics can be seen."