Project head Kilian Singer, Ph.D. student and first author Johannes Roßnagel, and Ferdinand Schmidt-Kaler, head of the University of Mainz’s QUANTUM group, were part of a team that used ion trapping and laser cooling to create a single-atom heat engine. [Image: © AG QUANTUM]
From steam locomotives to motorcars, thermodynamic heat engines have driven some of the most enduring and consequential outcomes of the industrial revolution. Now, a group of physicists in Germany has used an ion trap and laser cooling to create a heat engine consisting of a single atom (Science, doi: 10.1126/science.aad6320). The tiny engine—the reported output power of which, if scaled up from atomic to macroscopic dimensions, would be equivalent to that of a typical automotive engine—could, according to the researchers, offer a platform for investigating the limits, and applications, of thermodynamics at the nanoscale.
Finding the reservoirs
Heat engines operate by converting thermal energy—usually from a working fluid such as a gas—into work, by cycling between reservoirs of different temperature. To bring that schema to the scale of a single atom, the research team, consisting of scientists from four German institutions (the University of Mainz, the Max Planck Institute for Quantum Optics, the University of Erlangen-Nürnberg, and the University of Kassel), began by capturing a single calcium ion in a tapered, linear radio-frequency (Paul) trap. To create the thermal reservoirs necessary to turn that trapped atom into a heat engine, they used oscillating white electrical noise, switched on and off at precise intervals matching the trap’s resonance frequency, to act as the hot reservoir, and an always-on cooling laser to serve as the cold reservoir.
When the electric field was switched on, the resulting thermalization of the atom caused it to move along the Paul trap’s axial direction; when the field was switched off, the cooling laser beam acted as a “cold bath” that chilled the atom and caused restoring motion in the opposite direction. The result of this thermodynamic cycle was a 1-D oscillation in the atom, analogous to that of a piston in a macroscopic engine.
Stored energy
In a particularly interesting finding, the team discovered that the system could actually “store” this nano-piston’s energy output. Since the oscillations of the hot reservoir (electric field) were carefully timed to match the resonance frequency of the axial trap containing the atom, the work of each cycle was resonantly transferred and stored in the amplitude of the atom’s oscillation—in a manner that the authors compared to the function of a flywheel in a mechanical engine.
The research team used three different techniques to confirm and measure the power output from this tiny engine, based on an analysis of the ion’s thermal state at various points of the cycle, the measured amplitude of the atom’s axial oscillation, and analytic calculations of the system’s work function. They concluded that the single-atom heat engine’s power output was as high as 3.4 × 10–22 watts. As a ratio of power to mass, that equates to some 1.5 kilowatts per kilogram—“comparable,” the authors note, “to that of a typical car engine.”
The paper’s first author, Mainz grad student Johannes Roßnagel, noted in a press statement that the system could conceivably also be run in reverse, like a heat pump, and used as “a single-atom refrigerator … to cool nanosystems coupled to it.” The scientists are also exploring the system as a possible testbed for still more sensitive experiments in the domain of small quantum machines and quantum thermodynamics.