  An approach to the problem of 1/f voltage noise in conductors is developed
based on an uncertainty relation for the Fourier-transformed signal. The
quantum indeterminacy caused by non-commutativity of the observables at
different times makes the voltage autocovariance ambiguous, but the power
spectrum of fluctuations remains well-defined. It is shown that a lower bound
on the power spectrum exists, which is related to the antisymmetric part of the
voltage correlation function. Using the Schwinger-Keldysh method, this bound is
calculated explicitly in the case of unpolarized charge carriers with a
parabolic dispersion, and is found to have a 1/f low-frequency asymptotic. A
comparison with the 1/f-noise measurements in InGaAs quantum wells is made
which shows that the observed noise levels are only a few times higher than the
bound established.
