CURRENT NOISE IN BIASED SUPERCONDUCTING WEAK LINKS

Non-equilibrium current noise of a short quasi-one-dimensional constri ction between two superconductors is considered. We derive a general e xpression for the frequency-dependent current-current correlation func tion valid for arbitrary temperatures, transparencies, and bias voltag es. This formalism is then applied to a single current-carrying quantu m mode with perfect transparency, and at zero frequency and temperatur e. Contrary to a transparent channel separating two normal conductors, a weak link between two superconductors exhibits a finite level of no ise. The source of noise is the fractional Andreev scattering of quasi -particles with energies \E\ greater than the half-width Delta of the superconducting gap. For high bias voltages, V >> Delta/e, the zero-fr equency limit of the noise spectrum, S(0), as well as the excess curre nt I-exc, are twice as large than for a normal-superconductor junction , S(0) = (2/5)\e\I-exc.