Physicists led by Johannes Roßnagel at the University of Mainz in Germany, the single atom engine is about as efficient as your car at transforming the changing temperature into mechanical energy.
While the engine itself is tiny, the machinery required to create the electromagnetic cone and suspend the two heating and cooling lasers takes up most of a room.
Scientists have previously created several micro-engines consisting of a mere 10,000 particles, Roßnagel’s new engine blows these out of the water by paring down the machine to a singular atom housed in a nano-sized cone of electromagnetic radiation.
“The engine has the same working principles as the well-known [combustion] car engine,” Roßnagel says. It follows the same four strokes; expanding then cooling, contracting then heating.
step 1. the team traps a single atom in a cone of electromagnetic energy from which the atom cannot escape. You can think of this cone as essentially a tightly-fit engine housing. For this experiment the researchers trapped a lonely calcium-40 atom, but this is an arbitrary detail as pretty much any atom would do.
Step 2. Roßnagel’s team points two lasers toward each end of the cone. The laser pointing at the sharp end of the cone heats the atom, and the laser pointing at the base cools the atom via a process called Doppler cooling.
This laser heating and cooling actually changes the size of the atom. (To be a bit more precise, a physicist would tell you it changes the size of the fuzzy, probabilistic smear were the atom can exist.) Because the cone fits so snugly over the calcium atom, that temperature and size change forces the atom to scoot along the length of the cone. It moves toward the tapered point when cool and contracted, and toward the larger bottom when warm and expanded. To boost the efficiency, Roßnagel’s team set their lasers to cool and heat their calcium atom at the same resonance at which the atom naturally vibrates back and fourth.
This hot and cold oscillation between the two ends of the cone builds up, like a growing sound wave, creating energy that Roßnagel measured (and could theoretically harness)
When Roßnagel’s team of experimental physicists measured the energy output of the motor, they found it produces about 1.5 kilowatts per kilogram—on the same scale as your average car.
Abstract – A single-atom heat engine
Heat engines convert thermal energy into mechanical work and generally involve a large number of particles. We report the experimental realization of a single-atom heat engine. An ion is confined in a linear Paul trap with tapered geometry and driven thermally by coupling it alternately to hot and cold reservoirs. The output power of the engine is used to drive a harmonic oscillation. From direct measurements of the ion dynamics, we were able to determine the thermodynamic cycles for various temperature differences of the reservoirs. We then used these cycles to evaluate the power P and efficiency η of the engine, obtaining values up to P = 3.4 × 10^–22 joules per second and η = 0.28%, consistent with analytical estimations. Our results demonstrate that thermal machines can be reduced to the limit of single atoms.