How many particles do you need before individual atoms start behaving collectively? According to new research, the number is incredibly low. As few as six atoms will start transitioning into a macroscopic system, under the right conditions.
Using a specially designed ultra-cold laser trap, physicists observed the quantum precursor of the transition from a normal to a superfluid phase – offering a way to study the emergence of collective atomic behaviour and the limits of macroscopic systems.
Many-body physics is the field that seeks to describe and understand the collective behaviour of large numbers of particles: a bucket of water, for example, or a canister of gas. We can describe these substances in terms of their density, or their temperature – the way the substance is acting as a whole.
These are called macroscopic or many-body systems, and we can't understand them by just studying the behaviour of individual atoms or molecules. Rather, their behaviour emerges from the interactions between particles that individually do not have the same properties of the system as a whole.
Some examples of macroscopic behaviours that can't be described microscopically include collective excitations, such as the phonons that oscillate atoms in a crystal lattice. Phase transitions are another example – when a substance transitions from one phase to another – such as when ice melts into liquid, for example, or when liquid evaporates into a gas.
Physicists have long sought to understand how this collective behaviour emerges from individual particles gradually coming together – how the macroscopic emerges from the microscopic.
So a team of researchers from the University of Heidelberg designed an experiment to try to find out.
The experiment consisted of a tightly focused laser beam acting as a 'trap' for ultra-cold atoms of a stable isotope of lithium, called lithium-6. When cooled in a gas to a fraction of a degree above absolute zero, this fermionic isotope can behave as a superfluid, one with zero viscosity.
Within the laser trap, a very small number of lithium atoms could be held, effectively becoming a simulator for quantum behaviour. Within this system, the team could tune the interactions between the atoms using Feshbach resonances.
These resonances occur when the energy of two interacting atoms enters resonance with a molecular bound state, and they can be used to change the interaction strength between the particles.
In each experiment, the team introduced up to two, six or 12 lithium-6 atoms into the laser trap, allowing the researchers to observe when the atoms start to behave collectively.
"On the one hand, the number of particles in the system is small enough to describe the system microscopically," lead researcher Luca Bayha explained. "On the other hand, collective effects are already evident."
With the atoms inside, the researchers tuned the trap, from zero attraction to such a strong attraction that the atoms came together in bound pairs. This is a requirement for forming a fermionic superfluid – the fermionic particles have to become bound together as Cooper pairs that act like bosons, a heavier particle that forms a superfluid phase at higher temperatures than fermions do.
In each experiment, the team studied when the collective behaviour emerged based on the number of particles and the interaction strength between them. They found that the excitations of the particles were not only linked to the strength of the attraction between them, but that they were the few-body precursor of a quantum phase transition to a superfluid of Cooper pairs.
"The surprising result of our experiment is that only six atoms show all the signatures of a phase transition expected for a many-particle system," said physicist Marvin Holten.
The degree of control the researchers obtained will, the team says, be useful in the future for other research, such as studying the process of thermalisation in quantum systems.
They will also be able to conduct probes of fermionic superfluid at a fundamental level, and investigate the emergence of Cooper pairs in larger systems.
The team's research has been published in Nature.