Single-ion quantum heat engines

Short answer: A quantum heat engine uses a single trapped ion as its working substance. A 2016 Science paper reports a single calcium-ion engine with power up to 3.4 x 10^-22 joules per second and 0.28% efficiency, and a 2021 theory paper finds that partial thermalization can raise the efficiency of a single-ion Otto engine. These are physics experiments, not energy devices.

How do single-ion engines implement thermodynamic cycles?

A single-ion quantum Otto engine operates by coupling a trapped ion to thermal environments. The always-on thermal environment acts as the hot bath, while the motional degree of freedom of the ion serves as the effective cold bath. The hot isochoric stroke is implemented via the interaction of the ion with its hot environment. Conversely, a projective measurement of the internal state of the ion functions as an equivalent to the cold isochoric stroke. The expansion and compression strokes are realized through suitable changes in the applied magnetic field. This setup allows researchers to study how the finite duration of each stroke affects the engine's performance.

In another experimental realization, a single calcium ion in a tapered ion trap acts as the working substance. A laser-cooling beam plays the part of a cold reservoir for the calcium ion, while electric field noise acts as a hot reservoir. The ion is confined in a linear Paul trap with tapered geometry and driven thermally by coupling it alternately to these hot and cold reservoirs. The output power of the engine is used to drive a harmonic oscillation. Direct measurements of the ion dynamics allow for the determination of thermodynamic cycles for various temperature differences of the reservoirs.

What role does quantum coherence play in efficiency?

Finite-time analysis of the single-ion quantum Otto engine reveals that partial thermalization can enhance the efficiency of the engine. This enhancement is attributed to residual coherence remaining in the system. In contrast, faster expansion and compression strokes increase the inner friction, which therefore reduces the efficiency. These findings highlight how quantum properties interact with thermodynamic limits in finite-time operations.

The presence of quantum coherence and entanglement affects the operation of heat engine cycles in various experimental set-ups, including trapped ions, nitrogen vacancies, and transmon qubits. Models of quantum heat engines are exploited as a platform to investigate the thermodynamics underlying nanoscale energy conversion beyond the weak coupling limit or in the presence of engine–load correlations. These studies examine how the discreteness of energy levels, quantum statistics, and quantum adiabaticity influence performance.

How does the engine interact with a quantum load?

A single-atom energy-conversion device can operate as an engine or a refrigerator coupled to a quantum load. The working fluid consists of the two optical levels of an ion, while the load is one of its vibrational modes, cooled down to the quantum regime. Two important differences from classical engines are the presence of a strong generic coupling interaction between the engine and load, which can induce correlations, and the use of nonthermal baths.

Researchers examine the ergotropy of the load, which indicates the maximum amount of energy of the load extractable using solely unitary operations. It has been shown that ergotropy rises with the number of engine cycles despite an increase in the information entropy of the load. This increase in ergotropy points to the possibility of using the phonon distribution of a single atom as a form of quantum battery.

What performance metrics have been observed experimentally?

Experimental realization of a single-atom heat engine using a calcium ion yielded specific performance values. Researchers obtained power values up to 3.4 x 10^-22 joules per second and an efficiency of 0.28%. The authors present this as showing that heat engines can be built from a single atom.

The thermodynamic cycles were evaluated using direct measurements of the ion dynamics for various temperature differences of the reservoirs. The output power drives a harmonic oscillation, providing a measurable mechanical work output. The authors report the values as consistent with analytical estimations.

FAQ

Can these engines be used for commercial power generation?

No. The papers describe research-stage devices with extremely low power outputs, such as 3.4 x 10^-22 joules per second. They are intended to explore fundamental thermodynamic questions at the nanoscale, not for practical energy production.

Does partial thermalization always improve engine efficiency?

In the specific single-ion Otto engine studied, partial thermalization can enhance efficiency due to residual coherence. However, faster strokes increase inner friction and reduce efficiency. The outcome depends on the specific stroke durations and quantum state dynamics.

What is the role of the quantum load?

The quantum load, such as a vibrational mode, interacts with the engine via strong coupling. This interaction can induce correlations between the engine and load. The load's ergotropy can increase over cycles, suggesting potential use as a quantum battery.

Are these engines based on classical gas laws?

No. They utilize discrete energy levels of a single ion. One of the papers says coherence and entanglement affect heat engine cycles in several experimental set-ups, unlike classical engines that use bulk fluids.

Is the single-atom engine operationally adopted in industry?

No. These are experiments and theory for studying nanoscale thermodynamics. None of the papers describes commercial use.