NOISE IN AN AC BIASED JUNCTION - NONSTATIONARY AHARONOV-BOHM EFFECT

We study excess noise in a quantum conductor in the presence of consta nt voltage and alternating external field. Because of a two-particle i nterference 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 depend ence on the dc voltage V and the ac frequency OMEGA with cusplike sing ularities at integer eV/hOMEGABAR. For a metallic loop with an alterna ting flux the phase sensitivity leads to an oscillating dependence of the strength of the cusps on the flux amplitude.