  We demonstrate and characterize interference between discrete photons emitted
by two separate semiconductor quantum dot states in different samples excited
by a pulsed laser. Their energies are tuned into resonance using strain. The
photons have a total coalescence probability of 18.1% and the coincidence rate
is below the classical limit. Post-selection of coincidences within a narrow
time window increases the coalescence probability to 47%. The probabilities are
reduced from unity because of dephasing and the postselection value is also
reduced by the detector time response.
