  The Hamiltonian interprets how the system evolves, while the scattering
coefficients describe how the system responds to inputs. Recent studies on
non-Hermitian physics have revealed many unconventional effects. However, in
all cases, the non-Hermiticity such as material loss is only considered for the
Hamiltonian, even when studying scattering properties. Another important
component-the scattering channel, is always assumed to be lossless and
time-reversal symmetric. This assumption hinders the exploration on the more
general and fundamental properties of non-Hermitian scattering. Here, we
identify a novel kind of scattering channel that obeys time-reversal
anti-symmetry. Such diffusive scattering channels overturn the conventional
understanding about scattering symmetry by linking the positive and negative
frequencies. By probing non-Hermitian systems with the diffusive channels, we
reveal a hidden anti-parity-time (APT) scattering symmetry, which is distinct
from the APT symmetry of Hamiltonians studied before. The symmetric and
symmetry broken scattering phases are observed for the first time as the
collapse and revival of temperature oscillation. Our work highlights the
overlooked role of scattering channels in the symmetry and phase transition of
non-Hermitian systems, thereby gives the diffusion the new life as a signal
carrier. Our findings can be utilized to the analysis and control of strongly
dissipative phenomena such as heat transfer and charge diffusion.
