Boundary time crystals in collective $d$-level systems

TL;DR

This paper explores boundary time crystals in collective d-level quantum systems, revealing how different system sizes and interactions influence the emergence, stability, and complexity of non-equilibrium phases like limit-cycles and chaos.

Contribution

It introduces new models for collective 3- and 4-level systems, analyzing their phase diagrams and stability, and demonstrates the robustness of boundary time crystals under various symmetry-breaking interactions.

Findings

Boundary time crystals appear in collective 2-, 3-, and 4-level systems.

Symmetry-breaking terms can destroy or preserve BTC phases depending on the model.

Complex dynamical behaviors like chaos and multiple limit-cycles are observed.

Abstract

Boundary time crystals (BTC's) are non-equilibrium phases of matter occurring in quantum systems in contact to an environment, for which a macroscopic fraction of the many body system breaks time translation symmetry. We study BTC's in collective $d$-level systems, focusing in the cases with $d=2$, $3$ and $4$. We find that BTC's appear in different forms for the different cases. We first consider the model with collective $d=2$-level systems [presented in Phys. Rev. Lett. $121$, $035301$ ($2018$)], whose dynamics is described by a Lindblad master equation, and perform a throughout analysis of its phase diagram and Jacobian stability for different interacting terms in the coherent Hamiltonian. In particular, using perturbation theory for general (non Hermitian) matrices we obtain analytically how a specific $\mathbb{Z}_2$ symmetry breaking Hamiltonian term destroys the BTC phase in the model. Based on these results we define a $d=4$ model composed of a pair of collective $2$-level systems interacting with each other. We show that this model support richer dynamical phases, ranging from limit-cycles, period-doubling bifurcations and a route to chaotic dynamics. The BTC phase is more robust in this case, not annihilated by the former symmetry breaking Hamiltonian terms. The model with collective $d=3$-level systems is defined similarly, as competing pairs of levels, but sharing a common collective level. The dynamics can deviate significantly from the previous cases, supporting phases with the coexistence of multiple limit-cycles, closed orbits and a full degeneracy of zero Lyapunov exponents.