Researchers Have Developed Powerful Micro Motors To Realize The Otto Cycle in The Quantum World

A classic motor converts a type of energy (such as heat) into mechanical work. Can these laws also be applied to a micro machine with only one cesium atom, thereby improving work efficiency? The Technische Universität Kaiserslautern research team led by Physics professor Dr. Arthur Widla provided evidence. In addition, with the help of the quantum toolbox, scientists are able to run machines stably, even though fluctuations are ubiquitous in the quantum world.

Related research work has now been published in the journal Nature Communications ("A quantum heat engine driven by atomic collisions").

Traditional power engines follow the laws of thermodynamics. For example, when gasoline is ignited, the piston converts heat energy into kinetic energy. Widera's research team, in collaboration with Professor Eric Lutz of the University of Stuttgart, has transferred these basic principles to the quantum world and solved the basic problems of thermodynamics in quantum mechanics.

But how did such a quantum engine be built in the first place? The researchers chose a special experimental device. First, the gas of rubidium atoms was cooled to almost absolute zero to minimize thermal fluctuations. This tiny machine is composed of a single cesium atom. The fuel that drives the machine is the angular momentum of rubidium atoms, which is called spin. When cesium and rubidium atoms collide, they exchange heat and spin at the quantum level.

The study’s first author and doctoral student Jens Nettersheim explained that the spin direction can be changed in two directions, up or down. In our system, this represents heat. And cold, resulting in a thermal difference. When the so-called spin exchange collision occurs, the spin directions of the colliding cesium and rubidium atoms will tilt in opposite directions. At the ultra-low temperature given by our system, we can control the direction of spin change in each collision. In addition, we replaced the piston motion that converts the energy of the system with a changing magnetic field. Using these analogies for heat exchange and piston motion, physicists have successfully realized the Otto cycle of the quantum world.

In the process, the research team overcomes a challenge that was previously considered insurmountable: the properties or states of quantum particles cannot usually be clearly determined, Widera explained. In other words, we can measure them, but we can never predict with certainty the outcome of a single measurement. I can only determine the probability of occurrence of the observed property. It is this "ambiguity" or fluctuation of the measurement results that has so far led the scientific community to doubt whether the quantum heat engine can reliably provide high-efficiency constant power. Widera said that I want to fundamentally rule out the possibility of the engine fluctuating back and forth uncontrollably between different power levels.

In spin exchange collisions, fluctuations occur, but the research team found that over time, the spins of cesium atoms saturate. In other words, they will remain in a state after a period of time, so fluctuations can be controlled. Compared with the "classical" heat engine, the atom reaches a higher excited state in this process. This is the key to the efficient operation of a quantum heat engine. In addition to the advantages of suppressing fluctuations, this quantum technique also allows our quantum machine to convert more energy in one cycle, which is higher than the thermodynamic ability of a quantum machine running at two different temperatures in a traditional classical machine.

The quantum heat engine developed by the researchers releases continuous high power while operating stably, and the efficiency is very high. Widera's research team, with the theoretical support of Professor Lutz, successfully brought thermodynamics to the quantum level, further opening the door to the application of quantum thermodynamics.