  The state of particles injected onto the surface of the Fermi sea depends
essentially on the temperature. The pure state injected at zero temperature
becomes a mixed state if injected at finite temperature. Moreover the electron
source injecting a single-particle state at zero temperature may excite a
multi-particle state if the Fermi sea is at finite temperature. Here I unveil a
symmetry of the scattering amplitude of a source, which is sufficient to
preserve a single-particle emission regime at finite temperatures if such a
regime is achieved at zero temperature. I give an example and analyze the
effect of temperature on time-dependent electrical and heat currents carried by
a single-particle excitation.
