  Erasing quantum-mechanical distinguishability is of fundamental interest and
also of practical importance, particularly in subject areas related to quantum
information processing. We demonstrate a method applicable to optical systems
in which single-mode filtering is used with only linear optical instruments to
achieve quantum indistinguishability. Through "heralded" Hong-Ou-Mandel
interference experiments we measure and quantify the improvement of
indistinguishability between single photons generated via spontaneous four-wave
mixing in optical fibers. The experimental results are in excellent agreement
with predictions of a quantum-multimode theory we develop for such systems,
without the need for any fitting parameter.
