Bounds on $T\bar {T}$ deformation from entanglement

Motivated by the existence of complex spectrum in $T\bar T$-deformed CFTs, in this paper we revisit the broadly studied topic of (holographic) entanglement entropy in the deformed theory to investigate its complex behaviour. As a concrete example, we show that in case of a 1+1 dimensional holographic CFT at finite temperature $\beta^{-1}$ and chemical potential $\Omega$, the holographic entanglement entropy in the deformed theory remains to be real only within the range $-\frac{\beta^2}{8\pi^2}\frac{(1-\Omega^2)^2}{\Omega^2}< \mu < \frac{\beta^2}{8\pi^2}(1-\Omega^2)$ of the deformation parameter. While the upper bound overlaps with the familiar Hagedorn bound in the deformed theory, the novel lower bound on the negative values of the deformation parameter does not show up in thermodynamic quantities. However, from a holographic perspective we show that this intriguing lower bound is related to a spacelike to null transition of the associated Ryu-Takayanagi surface in the deformed geometry. We also investigate the Quantum Null Energy Condition in the deformed theory, within its regime of validity.