Spin-length Mechanism for Bistability and Entanglement in Driven Superradiance

Collective radiation is the phenomenon of many atoms radiating into a common photonic mode.  We study a driven ensemble of atoms in a cavity where collective radiation competes with individual-atom emission into free space.  This interplay leads to a dissipative phase transition and dynamical switching between two metastable collective quantum states, one of which is entangled, closely resembles the coherently radiating spin state of the idealized system without individual decay, and exhibits spin squeezing [1]. Here we resolve this switching in single quantum trajectories using matrix-product-state simulations, which give direct access to the entanglement entropy along each trajectory. Monitoring collective and individual jumps separately, we find that switching is triggered by bursts of individual jumps that change the total spin length. Collective jumps, which preserve the spin length, generate entanglement within each spin-length sector. As a result, a single trajectory alternates between sectors of distinctly different entanglement entropy, even when the corresponding component of the averaged steady state is not entangled.

[1] Leppenen & Shahmoon, "Quantum correlated steady states under competing collective and individual decay," Phys. Rev. A  114, 013720 (2026)