  Recent developments in the coherent manipulation of electrons in ballistic
conductors include the generation of time-periodic electrical currents
involving one to few electronic excitations per period. However, using
individual electrons as carrier of quantum information for flying qubit
computation or quantum metrology applications calls for a general method to
unravel the single-particle excitations embedded in a quantum electrical
current and how quantum information is encoded within it. Here, we propose a
general signal processing algorithm to extract the elementary single-particle
states, called electronic atoms of signal, present in any periodic quantum
electrical current. These excitations and their mutual quantum coherence
describe the excess single-electron coherence in the same way musical notes and
score describe a sound signal emitted by a music instrument. This method, which
is the first step towards the development of signal processing of quantum
electrical currents is illustrated by assessing the quality of experimentally
relevant single electron sources. The example of randomized quantum electrical
currents obtained by regularly clocked but randomly injected unit charge
Lorentzian voltage pulses enables us to discuss how interplay of the coherence
of the applied voltage and of the Pauli principle alter the quantum coherence
between the emitted single particle excitations.
