Press the button by the sculpture. The sculpture will move and light up so as to evoke classical and quantum heat-engine cycles. The classical engines are portrayed as powering the lasers that participate in the quantum engine cycle.

As mentioned on the “Components” page, many engines operate by undergoing engine cycles. An engine cycle is a sequence of strokes, or steps. Many famous engine cycles have two or four strokes each. Examples include the Otto cycle, which many car engines undergo, and the Carnot cycle, designed to achieve the greatest efficiency possible (for any engine that interacts with just a hot reservoir and a cold reservoir). During a stroke, the engine may exchange heat with its hot reservoir or cold reservoir. Also, thermodynamic properties of the engine might change. For example, if the engine consists essentially of a classical gas (as do many car engines), its pressure or volume might vary.

An engine cycle is called a cycle because it returns the engine to its initial conditions. For example, a classical-gas engine returns to the temperature, pressure, and volume with which it began the cycle. The cycle changes the engine’s environment, however: the hot reservoir cools a little, and the cold reservoir warms up a little. If one runs enough engine cycles, the reservoirs will end up at the same temperature. No net heat will flow between them spontaneously anymore, and they’ll no longer be able to fuel the engine. (The sculpture doesn’t represent this extreme circumstance.)

Quantum engine cycle

The quantum engine undergoes a cycle described in the paper published here and available for free to all here. (The sculpture doesn’t represent the cycle with scientific accuracy.) The ion (represented by the amber light at the sculpture’s center) sits in a trap (represented by the sculpture’s central gray component) that generates an electromagnetic field. The field is invisible to the naked eye, but the ion senses the field because of having an electric charge. The field acts like a landscape in which the ion moves, similarly to how a bowl can form a landscape in which a marble moves. The electromagnetic landscape is shaped like a funnel. The figure below illustrates the following cycle.

At the beginning of the cycle, the ion sits in the funnel’s narrow neck. The ion exchanges heat with its hot reservoir during the first stroke. This reservoir consists of a randomly buzzing electric field, generated by electrodes used to form the trap. Upon gaining energy from the hot reservoir, the ion behaves similarly to a gas expanding across the funnel. The ion itself can’t expand, because it consists of only one particle (in contrast with a gas, which consists of many particles that can spread out). But, if you measured the ion’s position after the expansion, you’d have a high probability of finding the ion near the funnel’s broad mouth. We say that the ion’s wave function has spread out along the funnel’s long axis and perpendicularly away from that axis. The ion loses energy while expanding through the funnel, similarly to how a gas would lose energy if it were pushing a piston through a funnel. Just as pushing a piston amounts to performing work, the ion performs work while its wave function expands.

Next, the engine is disconnected from the hot reservoir. In other words, the electrodes quit spewing out a random electric field. The ion cools under the influence of a laser, which serves as a cold reservoir.¹ The ion’s wave function contracts back into the funnel’s tip, similarly to a gas undergoing compression. If you measured the ion’s position after the contraction, you’d have a high probability of finding the ion in the funnel tip.

The ion has returned to its initial conditions: localized in the funnel tip, at a low temperature. The next cycle begins by heating the ion back up.

Where does the work performed by the trapped-ion engine go? In the experiment, an extra laser dissipated the work into the surrounding air. That is, the experimentalists wasted the work extracted by their engine. (This wasted energy resembles the energy wasted when water rushes over waterfall uninhibited. If a waterwheel stood below the waterfall, the water’s kinetic energy could turn the wheel, which could power a mill. In the absence of any wheel, the water’s kinetic energy is wasted.) Why did the experimentalists waste the work? Because they simply wanted to show that they could create a single-ion engine; they didn’t care about using the work produced. But one could concoct a scheme for capturing and using the work. For example, an extra laser could couple the engine to other ions, whose electrons could store the energy as quantum batteries.

¹ We often think of lasers as hot, because they emit concentrated light. But imagine a softball flying from left to right, while ping-pong balls fly in the opposite direction. The ping-pong balls reduce the softball’s momentum. The ion resembles the softball and tends to absorb laser photons that move oppositely to it. Just as the ping-pong balls reduce the softball’s momentum, the photons reduce the ion’s momentum. The more momentum the ion loses, the less energy the ion retains, and the lower the ion’s temperature.