We show that in disordered $s$-wave superconductors, at sufficiently low frequencies $\omega$, the coupling of two-level systems (TLSs) to external ac electric fields increases dramatically in the presence of a dc supercurrent. This giant enhancement manifests in all ac linear and nonlinear phenomena. In particular, it leads to a parametric enhancement of the real part of the ac conductivity and, consequently, of the equilibrium current fluctuations. If the distribution of TLS relaxation times is broad, the conductivity is inversely proportional to $\omega$, and the spectrum of the equilibrium current fluctuations takes the form of $1/f$ noise.

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• Electrical conductivity
• Impurities in superconductors
• Optical & microwave phenomena
• Quantum interference effects
• Superfluid density
• Superconductivity
• Superconductors