The quantum theory of $\frac{1}{f}$ noise developed in the preceding Letter is extended by showing that because of interaction with the electromagnetic field, an alternating current carried by a beam of electrons emerging from some scattering process will present phase fluctuations with a $\frac{1}{f}$ spectrum. These fluctuations are expected in any ac current. They have similar power spectra, also determined by infrared radiative corrections, to dc $\frac{1}{f}$ noise.

• Received 16 December 1974

DOI:https://doi.org/10.1103/PhysRevLett.34.1495