The quantum engine is a microscopic device that uses the principles of quantum mechanics to generate energy, instead of the usual way of burning fuel. It works with a single atom and far surpasses the heat engines that today move cars, boats and airplanes. Although it is a possibility for now, it promises to revolutionize many things.
Quantum mechanics is the branch of physics that studies the dynamics of elementary particles, such as electrons and photons. These particles can exist in different states, called overlayswhich are simultaneous combinations of two or more possibilities.
For example, an electron can be in a superposition of spinning up and down at the same time. However, when the state of the electron is measured, it collapses into one of the possibilities, and information about the other is lost.
Quantum engines
The question that has arisen is what if the measurement process could be controlled to extract energy from quantum superpositions? That is the idea behind the quantum engines, microscopic devices that use the principles of quantum mechanics to generate energy, instead of the usual way of burning fuel. In 2021 we already reported on another article of this technology still in its infancy.
Now, an international team of scientists, led by Professor Thomas Busch from the Okinawa Institute of Science and Technology (OIST), has theoretically demonstrated that it is possible to build a quantum engine that works with a single atom.
The motor consists of an atom trapped in an optical cavity, which is a space between two mirrors that reflect light. The atom can be in a superposition of two states: excited and unexcited. When the atom is excited, it emits a photon, which is a particle of light. When the atom is unexcited, it emits nothing.
Four steps
The motor operates through a four-step cycle. In the first step, a laser is applied to the atom to put it in a superposition of excited and unexcited.
In the second step, one of the mirrors is opened to allow the photon to leave the cavity. If the atom was excited, the photon leaves and information about the state of the atom is lost. If the atom was unexcited, nothing happens.
In the third step, the mirror is closed and another laser is applied to the atom to reverse its state. If the atom was excited, it becomes unexcited. If the atom was not excited, it becomes excited.
In the fourth step, the other mirror is opened to allow another photon to leave the cavity. If the atom was excited, the photon leaves and information about the state of the atom is recovered. If the atom was unexcited, nothing happens.
Quantum measurement energy
The net result is that energy has been extracted from the quantum measurement process. The engine produces two photons per cycle, one that is lost and one that is recovered. The difference between the energies of the two photons is the energy generated by the engine.
To better understand this development, we can compare it to a car engine. A car engine burns fuel to produce gases that push a piston into a cylinder. The piston is connected to a connecting rod that rotates a crankshaft. The crankshaft transmits mechanical energy to the wheels of the car.
Piston and cylinder
The quantum engine also has a piston and a cylinder, but they are very different from those of the conventional engine. The quantum engine uses a helium-4 atom as a piston, and takes advantage of its bosonic behavior to extract energy from quantum measurements.
In the quantum engine, the piston is the helium-4 atom and the cylinder is the optical cavity in which it is trapped. The fuel is the laser light that excites the atom.
The gases are the photons that leave the cavity. The connecting rod is an optical device that converts light energy into mechanical energy. The crankshaft is another optical device that transmits mechanical energy to an external load.
Preliminary experiment
For now, all of this is still a proof of concept (that is, a preliminary experiment), so we cannot conclude that we will soon see quantum mechanics powering the engines of our cars, the researchers say in a release.
“While these systems can be very efficient, we have only performed a proof of concept together with our experimental collaborators,” he explains. Keerthy Menon, one of the protagonists of this development. “There are still many challenges in building a useful quantum engine,” he adds.
The main problem is that heat can destroy quantum effects if the temperature rises too high, so researchers must keep their system as cool as possible.
Excellent efficiency
However, this requires a substantial amount of energy to run the experiment at these low temperatures in order to protect the sensitive quantum state.
In this experiment, the researchers achieved a maximum efficiency of 25%, but the team says further improvements could raise it above 50%.
This estimate does not include the energy needed to run the device to cool the atoms and keep them trapped. But it does reflect the work that is put directly into the gas and the work that could be extracted from it, Explain Physicsworld magazine about it.
Outperforms the heat engine
In any case, everything indicates that the quantum engine far surpasses internal combustion engines, the most common form of heat engines used in vehicles, ships, planes and trains.
The main difference between heat engines and quantum engines is that the former depend on heat and entropy to generate energy, while the latter depend on collapse and quantum information to generate energy.
Heat engines are limited by the Carnot efficiency, which is the maximum possible efficiency for a motor operating between two temperatures. Quantum engines are not subject to this limitation, since they do not use heat.
Applications
Although we cannot yet think that quantum motors will power the next generation of motors, it would be more feasible that in the medium term they could be used to charge quantum batteries or power computers and quantum sensors.
According to Professor Busch, this quantum engine could have applications in fields such as quantum computing, quantum metrology and quantum cryptography.
In addition, it could help to better understand the foundations of quantum mechanics and its relationship with thermodynamics.
Quantum engineering
“Engineering is entering the nano world and, at some point, quantum will arrive,” he says. Artur Widera, physicist at the University of Kaiserslautern (Germany) and leader of the collaboration, cited by Physicsworld. “And we better be prepared and know what’s going on and how we can use it.”
The study that recounts this intriguing development was published in the journal Nature and included the collaboration of researchers from the National University of Ireland Galway, the National Autonomous University of Mexico and the National University of Singapore.
Reference
A quantum engine in the BEC–BCS crossover. Jennifer Koch et al. Nature, volume 621, pages 723–727 (2023). DOI:https://doi.org/10.1038/s41586-023-06469-8