Detected HOM dip with different response time. Δ λ A = Δ λ B = 1 nm and λ A 0 = λ B 0 = 702.2 nm . τ c is the coherence time of photons. N is an positive integer. g T ( 2 ) ( 0 ) is the observed second-order coherence function when the response time T of the detection system is taken into consideration. V_T is the visibility of HOM dip for response time T, which is defined as [ 1 − g T ( 2 ) ( 0 ) ] / [ 1 + g T ( 2 ) ( 0 ) ] .