  Temporal interfaces introduced by abrupt switching of the constitutive
parameters of unbounded media enable unusual wave phenomena. So far, their
explorations have been mostly limited to lossless media. Yet, non-Hermitian
phenomena leveraging material loss and gain, and their balanced combination in
parity-time (PT)-symmetric systems, have been opening new vistas in photonics.
Here, we unveil the role that temporal interfaces offer in non-Hermitian
physics, introducing the dual of PT symmetry for temporal boundaries. Our
findings reveal unexplored interference mechanisms enabling extreme energy
manipulation, and open new scenarios for time-switched metamaterials,
connecting them with the broad opportunities offered by non-Hermitian
phenomena.
