New ‘quantum engine’ does work by flipping the identity of atoms

The basic principle is that, at a given temperature, a system of fermions will have more energy than a system of bosons.

Updated - October 15, 2023 12:59 pm IST

Published - October 13, 2023 10:35 am IST - Chennai

A schematic diagram showing the four-step cycle of the quantum engine.

A schematic diagram showing the four-step cycle of the quantum engine. | Photo Credit: Koch, J., Menon, K., Cuestas, E. et al. A quantum engine in the BEC–BCS crossover. Nature621, 723–727 (2023).

Physicists in Germany have come up with a way to convert the energy difference between two quantum states of a group of atoms into work. The device adapts the principles of the familiar classical engine to the subatomic realm, giving physicists a way to study the nascent field of quantum thermodynamics in more detail as well as, possibly, build better quantum computers.

Pauli’s principle

All subatomic particles can be classified as either fermions or bosons. Fermions are the building blocks of matter; bosons are particles that carry the forces acting between them. Now, when a bunch of particles are cooled to very nearly absolute zero, so that their quantum nature comes to the fore, they would all like to have the lowest energy possible – but they can’t. This is known as Pauli’s exclusion principle.

All particles in a system are distinguished by four quantum numbers, sort of like their Aadhaar numbers. The values of the four numbers together tell us something about how much energy a particle has. The exclusion principle states that, in a given system, no two particles can have the same four quantum numbers – that is, they can’t occupy the same energy level. Fermions are particles that are bound by this rule. So they recursively occupy the lowest one available, until all possible energy levels are filled.

Bosons are not bound by the exclusion principle principle: they can all occupy the same lowest energy level at a given low temperature. This is why, for example, superconductivity is possible.

Fermionic energy

So a system of fermions will have more energy at a low temperature than a system of bosons. For this to be the basis of an engine, physicists needed a simple way to convert some particles from being bosons to being fermions. A solution arrived in the early 2000s, when researchers found via multiple studies that if a collection of fermions were cooled almost to absolute zero and then prodded to interact with each other using a magnetic field, they could be made to behave like bosons.

In the new study, researchers with institutes in Germany, Japan, and Argentina did just this with a gas of lithium-6 atoms. (Entire atoms can behave like fermions or bosons if they satisfy a few basic requirements.) The team cooled them to just millionths of a degree above absolute zero, and confined them in a trap of oscillating electric and magnetic fields.

Fermions to bosons and back

Classical engines convert heat into work. For example, the internal combustion engine in a car uses the heat released by the combustion of petrol or diesel to push a piston. Overall the engine has four steps: the fuel is compressed, ignition causes the fuel-air mix to expand and push the piston out, the mix cools and stops expanding, and the piston is brought back to the first step.

The quantum engine, or what the researchers are calling a ‘Pauli engine’, has a similar set of four steps. First, the atoms collected in the trap are compressed and kept in a bosonic state. Second, the strength of a magnetic field applied on the atoms is increased by a small amount. Interactions between the atoms and the field cause the former to slip into a fermionic state: they are forced to move out of the lowest energy level and progressively occupy higher levels. Third, the compression applied in the first step is eased. Fourth: the magnetic field strength is reduced to its original value.

The energy of the atoms increases during the third step and this can be converted to work. The efficiency of the quantum engine is based on how much more energy is released in the third step relative to the energy added to the system in the first step. Currently, according to the researchers’ paper, published in Nature on September 27, their quantum engine is 25% efficient. The researchers expect to be able to increase this to 50% or more in future.

Quantum thermodynamics

“Just observing the development and miniaturisation of engines from macroscopic scales to biological machines and further, potentially, to single- or few-atom engines, it becomes clear that for … particles close to the quantum regime, thermodynamics as we use in classical life will not be sufficient to understand processes or devices,” Artur Widera, the corresponding author of the Nature paper, told The Hindu via email.

There is a branch of physics called quantum thermodynamics, in which scientists study how thermodynamics ‘emerges’ in quantum-physical systems. And “some aspects of how to describe the thermodynamical aspects of quantum processes are even theoretically not fully understood,” Dr. Widera added.

“The first important ‘application’ of our work in my opinion is that we now have a platform where we can study such open questions experimentally, with a high level of control.”

Proof of concept

He also said that another application could be in computing: to, say, cool the particles that make up a quantum computer – like an air-conditioner uses an engine to cool a room. “For this purpose, it would be perfect to have some nanoscopic device that could do the job beyond currently known methods,” according to Dr. Widera.

That said, the quantum engine is still a proof of concept. The researchers have demonstrated that their design can be used to force a bunch of atoms to cyclically release energy as they are switched between bosonic and fermionic states. The researchers need to figure out how this energy can be moved from the inside the trap to a system on the outside.

“There is no definite plan, yet,” Dr. Widera said about achieving this. “But the setup or mechanism will need to be microscopic, just looking at the [amount] of work to be extracted. One option would be coupling some microscopic mechanical object to the gas.”

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