Fractional charge from auto-correlations and cross-correlations in shot noise experiments

Fractional charge is one of the key features of the fractional quantum Hall effect. Much of the fractional charge information comes from shot noise experiments. It has long been observed that the agreement between theoretical and experimental values of the quasiparticle charge often breaks down at low temperatures in shot-noise experiments with a constriction in a quantum Hall liquid. Recently, surprising evidence has emerged that the agreement only breaks down in experiments that measure current auto-correlations on one side of the constriction. Cross-correlations between the currents on the two sides of the constriction still show the theoretically predicted fractional charge. Yet, theory predicts that excess auto- and cross-correlations are simply opposite to each other in low-frequency experiments in the absence of long-range forces and upstream modes. In this talk, we address possible mechanisms for the observed discrepancy in non-ideal experimental systems. We find that cross-correlations provide a more reliable tool to measure fractional charge.