Asymptotic Exceptional Steady States in Dissipative Dynamics

Abstract: Spectral degeneracies in Liouvillian generators of dissipative dynamics generically occur as exceptional points, where the corresponding non-Hermitian operator becomes non-diagonalizable. Steady states, i.e. zero-modes of Liouvillians, are considered a fundamental exception to this rule since a no-go theorem excludes non-diagonalizable degeneracies there. Here, we demonstrate in the context of dissipative state preparation how a system may asymptotically approach the forbidden scenario of an exceptional steady state in the thermodynamic limit. Building on case studies ranging from NP-complete satisfiability problems encoded in a quantum master equation to the dissipative preparation of a symmetry protected topological phase, we reveal the close relation between the computational complexity of the problem at hand, and the finite size scaling towards the exceptional steady state, exemplifying both exponential and polynomial scaling. Treating the strength $W$ of quantum jumps in the Lindblad master equation as a parameter, we show that exceptional steady states at the physical value $W=1$ may be understood as a critical point hallmarking the onset of dynamical instability.