Noise in an ac biased junction. Non-stationary Aharonov-Bohm effect

We study excess noise in a quantum conductor in the presence of constant voltage and alternating external field. Due to a two particle interference effect caused by Fermi correlations the noise is sensitive to the phase of the time dependent transmission amplitude. We compute spectral density and show that at T=0 the noise has singular dependence on the {\rm dc} voltage $V$ and the {\rm ac} frequency $\Omega$ with cusplike singularities at integer $eV/\hbar\Omega$. For a metallic loop with an alternating flux the phase sensitivity leads to an oscillating dependence of the strengths of the cusps on the flux amplitude.