Miniaturization has lead to the development of increasingly smaller devices. This ongoing size reduction from the macroscale to the nanoscale is approaching the ultimate limit, given by the atomic nature of matter. Prominent macro-devices are heat engines that convert thermal energy into mechanical work, and hence motion. A fundamental question is whether these machines can be scaled down to the single particle level, while retaining the same working principles as, for instance, those of a car engine. It is interesting to note in this context that biological molecular motors are based on completely different mechanisms that exploit the constructive role of thermal fluctuations. At the nanoscale, quantum properties become important and have thus to be fully taken into account. Quantum heat engines have been the subject of extensive theoretical studies in the last fifty years. However, while classical micro heat engines have been fabricated, using optomechanical, micro- electromechanical, and colloidal systems, to date no quantum heat engine has been built.

In this paper, we take a step towards that goal by proposing a single ion heat engine using a linear Paul trap. Specifically, we present a scheme which has the potential to implement a quantum Otto cycle using currently available state-of-the-art ion-trap technology.

—Abah et al.

(a) Energy-frequency diagram of the Otto cycle for the radial degree of freedom of the ion. The continuous line represents the ideal process, while the dots show the results of the Monte Carlo simulations. (b) The pictograms illustrate the four individual strokes of the cycle for the radial thermal state. (c) Geometry of the tapered Paul trap. Obah et al. Click to enlarge.

In their proposal, the researchers simulate the Otto cycle by confining a single ion in a novel trap geometry with an asymmetric electrode configuration and coupling it in an alternating manner to two engineered laser reservoirs of different temperatures that heat and cool the radial state of the ion.

Changes in the temperature the radial thermal state of the ion leads to a modification in the axial component of the repelling force which changes the point of equilibrium. Heating and cooling the radial state thus moves the ion back and forth along the trap axis, (b in the diagram), resulting in the closed Otto cycle (a in the diagram).

This movement corresponds to the mechanically usable movement of a piston of a classical engine, while the radial mode corresponds to the gas in the cylinder.

The energy gained by running the engine cyclically in the radial direction can be stored in the axial degree of freedom, the authors propose.

The authors used semiclassical Monte Carlo simulations, with realistic parameters, to demonstrate the experimental feasibility of such a device.

The operation in the Otto cycle would result in coherent ion motion. Combining analytical and numerical analysis, we have studied the performance of the engine and showed that it can achieve maximum efficiency at maximum power in a wide range of temperatures. We expect a stimulating impact on the development on nano- and micromechanical oscillators which are of fundamental importance for future sensor technologies.

—Abah

Resources

• O. Abah, J. Roßnagel, G. Jacob, S. Deffner, F. Schmidt-Kaler, K. Singer, and E. Lutz (2012) Single-Ion Heat Engine at Maximum Power. Phys. Rev. Lett. 109, 203006 doi: 10.1103/PhysRevLett.109.203006