Electrons in an ultra-clean gallium arsenide film behave like a crowd at a crossroads, where the tilt of the magnetic field sets the direction. At zero tilt, they form a compressible liquid of composite fermions, particles made of an electron and a pair of vortices. This state is known as the fractional quantum Hall effect at filling 3/2, but it has no energy gap and therefore is not considered topological.

When the field is tilted to about 72°, the first road appears at the crossroads: a soft state with a tiny gap of about 30 millikelvins. The resistance drops to nearly zero, and the Hall plateau becomes almost quantized. This state resembles an uncertain step: it exists, but thermal noise can easily destroy it.

As the tilt increases further to 75.3°, the road changes abruptly: the gap closes and then reopens, but now as a hard state. Its gap grows rapidly and reaches a plateau. Simultaneously, a daughter state appears at filling 19/13, like a satellite that follows the parent state and helps reveal its topological order.

The experiment showed that tilting the field acts as a switch: it splits the Fermi contour, the boundary of occupied electron states in momentum space, into two disconnected parts. This turns a single-component system into a two-component one and opens the path to two different quantum states. Previously, no one had observed such a transition in the lowest Landau level at filling 3/2, because it requires ultra-clean samples and precise angle control.

Today this is fundamental knowledge, but it could become the basis for topological qubits, elements of a quantum computer where information is stored in topological states that are resistant to disturbances. The road at the crossroads is laid, and now one can search for a second daughter state to precisely determine which topological order lies behind the hard state.