  The frequency distinguishability of two single photons was successfully
erased using single photon frequency up-conversion. A frequency non-degenerate
photon pair generated via spontaneous four-wave mixing in a dispersion shifted
fiber was used to emulate two telecom-band single photons that were in the same
temporal mode but in different frequency modes. The frequencies of these
photons were converted to the same frequency by using the sum frequency
generation process in periodically poled lithium niobate waveguides, while
maintaining their temporal indistinguishability. As a result, the two converted
photons exhibited a non-classical dip in a Hong-Ou-Mandel quantum interference
experiment. The present scheme will add flexibility to networking quantum
information systems that use photons with various wavelengths.
