How integrable defects merge and what happens to entropy

Defects—like domain walls or impurities—in integrable quantum systems have exact mathematical descriptions called g-functions. The researchers computed what happens when two defects collide and merge across three distinct scenarios: purely transmitting defects, defects that partially reflect particles, and defects meeting boundaries. Topological defects simply multiply their transmission properties upon fusion, but non-topological ones generate oscillating quantum corrections and reduce the system's localized entropy. The results pin down an exact dictionary for defect fusion in solvable 2D systems.