Universal Conductance Fluctuations of Light

We observed that laser light, multiply scattered through small samples of optically dense colloidal suspensions, builds up anomalous long range correlations in the transmission speckle pattern. The dynamic autocorrelation function of angular integrated light intensity has two contributions, $C_2\left(t\right)$ and $C_3\left(t\right)$, which decay on very different time scales. $C_2$ and $C_3$ are explained by onefold and twofold crossing, respectively, of multiple scattering paths, in quantitative agreement with theory. Our $C_3$ data provide the first evidence for the classical wave analog of universal electronic conductance fluctuations and pinpoint their physical origin as a macroscopic quantum-wave interference effect.