Monitored quantum dynamics and the Kitaev spin liquid

Quantum circuit dynamics with local projective measurements can realize a rich spectrum of entangled states of quantum matter. Motivated by the physics of the Kitaev quantum spin liquid [A. Kitaev, Ann. Phys. 321, 2 (2006)], we study quantum circuit dynamics in 2 + 1 dimensions involving local projective measurements, in which the monitored trajectories realize (i) a phase with topological quantum order or (ii) a "critical" phase with a logarithmic violation of area-law scaling of the entanglement entropy along with long-range tripartite entanglement. A Majorana parton description of these dynamics, which provides an out of-equilibrium generalization of the parton description of the Kitaev honeycomb model, permits an analytic understanding of the universal properties of these two phases, including the entanglement properties of the steady state, the dynamics of the system on the approach to equilibrium, and the phase transition between these states. In the topologically ordered phase, two logical qubits can be encoded in an initial state and protected for a time which scales exponentially in the linear dimension of the system, while no robust encoding of quantum information persists in the critical phase. Extensive numerical simulations of these monitored dynamics confirm our analytic predictions.