A drop of ink in a glass of water quickly spreads, tinting the whole liquid. Similarly, electrons in a metal move freely, like molecules in a gas. But when a magnetic impurity, an atom with its own magnetic moment, enters this medium, something strange happens. At very low temperatures, electrons seem to stick to the impurity, forming a cloud around it. This cloud screens the magnetic moment, and the metal's resistance increases. The phenomenon was discovered half a century ago and named the Kondo effect.
Reproducing this dance in its pure form, without the mess of a crystal lattice, is a dream for many physicists. The ideal stage is an ultracold gas of strontium atoms. They have two states: a ground state and a long-lived "clock" state (used in the most precise clocks). An atom in the clock state plays the role of the magnetic impurity, while atoms in the ground state act as electrons. Their spins (quantum analogs of magnetic needles) exchange orientation upon collision: this is the quantum analog of the Kondo effect. But the required temperature, below millionths of a degree above absolute zero, is still unattainable.
The authors propose a clever trick: squeeze the gas into a thin tube using a transverse laser beam, and for the clock state, add a longitudinal trap as well, like a spring that gently holds atoms along the tube's axis. Both traps enhance the interaction between atoms through an effect called confinement-induced resonance. When the trap size is tuned so that the energy of a bound state of two atoms matches the energy of their free motion, the interaction sharply increases, as if two musicians hit a note in unison and the sound amplifies.
Calculations show that combining transverse and longitudinal compression can raise the Kondo temperature to values accessible in modern experiments. This is akin to boiling water not at 100 degrees but at room temperature, just by changing the container's shape. This approach turns a strontium gas into a tunable quantum simulator where the Kondo effect can be switched on and studied without interference, step by step unraveling its nature.
The Kondo effect was discovered in 1934, when it was noticed that the resistance of gold with iron impurities does not drop upon cooling but rises — contrary to all expectations.