  Open quantum systems interacting with an environment exhibit dynamics
described by the combination of dissipation and coherent Hamiltonian evolution.
Taken together, these effects are captured by a Liouvillian superoperator. The
degeneracies of the (generically non-Hermitian) Liouvillian are exceptional
points, which are associated with critical dynamics as the system approaches
steady state. We use a superconducting transmon circuit coupled to an
engineered environment to observe two different types of Liouvillian
exceptional points that arise either from the interplay of energy loss and
decoherence or purely due to decoherence. By dynamically tuning the Liouvillian
superoperators in real time we observe a non-Hermiticity-induced chiral state
transfer. Our study motivates a new look at open quantum system dynamics from
the vantage of Liouvillian exceptional points, enabling applications of
non-Hermitian dynamics in the understanding and control of open quantum
systems.
