(ORDO NEWS) — How many particles do you need for individual atoms to act collectively? This number is incredibly small, according to new research. Only six atoms will begin to transition into a macroscopic system under the right conditions.
Using a specially designed ultracold laser trap, physicists have observed the quantum precursor of the transition from the normal to the superfluid phase, which has made it possible to study the emergence of collective atomic behavior and the limits of macroscopic systems.
Many-body physics is a field that seeks to describe and understand the systemic behavior of a large number of particles: buckets of water, for example, or a gas cylinder. We can describe these substances in terms of their density or temperature – how the substance acts as a whole.
They are called macroscopic or multi-body systems, and we cannot understand them simply by studying the behavior of individual atoms or molecules. Rather, their behavior arises from interactions between particles that, individually, do not have the same properties as the system as a whole.
Some examples of macroscopic behavior that cannot be described microscopically include collective excitations such as phonons that vibrate atoms in a crystal lattice. Phase transitions are another example – when a substance passes from one phase to another – for example, when ice melts into a liquid, or when a liquid evaporates into a gas.
Physicists have long tried to understand how cooperative behavior arises from the gradual convergence of individual particles – how the macroscopic arises from the microscopic.
So a group of researchers at the University of Heidelberg designed an experiment to try and figure it out.
The experiment consisted of a focused laser beam acting as a “trap” for ultracold atoms of the stable lithium-6 isotope. When cooled in a gas to a fraction of a degree above absolute zero, this fermionic isotope can behave like a superfluid with zero viscosity.
A very small number of lithium atoms could be trapped in a laser trap, which effectively became a simulator of quantum behavior. Within this system, scientists could tune the interactions between atoms using Feshbach resonances.
These resonances occur when the energy of two interacting atoms comes into resonance with the bound state of the molecule, and they can be used to change the force of interaction between the particles.
In each experiment, the researchers injected up to two, six, or 12 lithium-6 atoms into a laser trap, observing when the atoms began to behave collectively.
“On the one hand, the number of particles in the system is small enough to describe the system microscopically,” explained lead researcher Luca Bajha. “On the other hand, the synergies are already evident.”
The researchers tuned the trap from zero attraction to such a strong attraction that the atoms combined into bound pairs. This is a requirement for the formation of a fermionic superfluid – fermionic particles must be bound together as Cooper pairs that act like bosons, a heavier particle that forms a superfluid phase at higher temperatures than fermions.
In each experiment, the team studied when collective behavior occurs, depending on the number of particles and the strength of the interaction between them. They found that the excitation of the particles was due not only to the force of attraction between them, but that they were the precursor of several bodies of quantum phase transition to superfluid Cooper pairs.
“The surprising result of our experiment is that just six atoms show all the signs of the phase transition expected in a many-particle system,” said physicist Marvin Holten.
The degree of control gained by the researchers, the team said, will be useful in the future for other research, such as studying the thermalization process in quantum systems.
They will also be able to conduct fundamental research on fermionic superfluids and investigate the appearance of Cooper pairs in larger systems.
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