  Quantum states of light have been shown to enhance precision in absorption
estimation over classical strategies. By exploiting interference and resonant
enhancement effects, we show that coherent-state probes in all-pass ring
resonators can outperform any quantum probe single-pass strategy even when
normalized by the mean input photon number. We also find that under optimal
conditions coherent-state probes equal the performance of arbitrarily bright
pure single-mode squeezed probes in all-pass ring resonators.
