  An approach to the problem of 1/f voltage noise observed in all conducting
media is developed based on an uncertainty relation for the Fourier-transformed
signal. It is shown that the quantum indeterminacy caused by non-commutativity
of observables at different times sets a lower bound on the power spectrum of
voltage fluctuations. Using the Schwinger-Keldysh method, this bound is
calculated explicitly in the case of unpolarized free-like charge carriers, and
is found to have a 1/f low-frequency asymptotic. It is demonstrated also that
account of the charge carrier interaction with phonons results in a shift of
the frequency exponent from unity. A comparison with the experimental data on
1/f noise in InGaAs quantum wells and high-temperature superconductors is made
which shows that the observed noise levels are only a few times as high as the
bound established.
