Chiral control of quantum states in non-Hermitian spin–orbit-coupled fermions

Spin–orbit coupling is an essential mechanism underlying quantum phenomena such as the spin Hall effect and topological insulators 1 . It has been widely studied in well-isolated Hermitian systems, but much less is known about the role dissipation plays in spin–orbit-coupled systems 2 . Here we implement dissipative spin–orbit-coupled bands filled with ultracold fermions, and observe parity-time symmetry breaking as a result of the competition between the spin–orbit coupling and dissipation. Tunable dissipation, introduced by state-selective atom loss, enables us to tune the energy gap and close it at the critical dissipation value, the so-called exceptional point 3 . In the vicinity of the critical point, the state evolution exhibits a chiral response, which enables us to tune the spin–orbit coupling and dissipation dynamically, revealing topologically robust chiral spin transfer when the quantum state encircles the exceptional point. This demonstrates that we can explore non-Hermitian topological states with spin–orbit coupling.