Quantum magnetism involves a subtle effect called the exchange interaction. This is a quantum interaction between pairs of identical fermions such as electrons that tends to prevent neighbouring fermions from having their spin magnetic moments pointing in the same direction. As well as being responsible for the magnetic properties of everyday materials such as iron, quantum magnetism is also believed to play an important role in high-temperature superconductivity and other exotic states of matter such as spin liquids.
Criss-crossing laser beams
Quantum simulations using ultracold atoms allow physicists to create artificial materials in which the atoms play the role of electrons in a solid. However, unlike real materials, where it can be difficult to vary the interactions between electrons, the forces between atoms in a quantum simulator can be fine-tuned by adjusting lasers and magnetic field.
These latest simulations were done by Tilman Esslinger and colleagues at ETH Zόrich and the University of Bordeaux. The team began with an ultracold cloud of potassium-40 atoms, which are fermions. The cloud is a mixture in which half of the atoms are in the 9/2 spin state and the other half in the 7/2 state. This two-state system simulates 1/2 and 1/2 spin states of the electron. The criss-crossing laser beams are then switched on, creating a 2D square lattice wherein each lattice site contains one potassium-40 atom. The exchange interaction is then simulated by applying a magnetic field to the lattice, which makes atoms with the same spin repel each other.
The next experimental step involves solving a thermodynamics problem. Even at the extremely low lattice temperatures there is too much entropy or disorder for quantum magnetism to emerge. To get round this problem, Esslinger and colleagues came up with a way of "stashing" entropy at the edges of the lattice so that quantum magnetism could emerge in the centre.
This is done by tweaking the properties of the optical lattice so that the interactions between nearest-neighbour atoms alternate between strong and weak in the x and y directions. An atom with a strong interaction with a nearest neighbour will form a pair (or dimer) in which the spins point in opposite directions and the lattice of 5000 atoms becomes a collection of antiferromagnetic dimers.