  Interaction of light with media often occurs with a femtosecond response
time. Its measurement by conventional techniques requires the use of
femtosecond lasers and sophisticated time-gated optical detection1-3. Here we
demonstrate that by exploiting quantum interference of entangled photons it is
possible to measure the phase relaxation time of a media on the femtosecond
time scale (down to 100 fs) using accessible continuous wave laser and
single-photon counting. We insert the sample in the Hong-Ou-Mandel
interferometer4 and infer the phase relaxation time from the modification of
the two-photon interference pattern. In addition to its simplicity and ease of
use, the technique does not require compensation of group velocity
dispersion5-8 and does not induce photo-damage of the samples. This technique
will be useful for characterization of ultrafast phase relaxation processes in
material science, chemistry, and biology.
