Noise in an ac biased junction: Nonstationary Aharonov-Bohm effect

We study excess noise in a quantum conductor in the presence of constant voltage and alternating external field. Becasue of 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 dc voltage V and the ac frequency {Ω} with cusplike singularities at integer eV/{}{Ω}. For a metallic loop with an alternating flux the phase sensitivity leads to an oscillating dependence of the strength of the cusps on the flux amplitude.