Single photon sources exhibit antibunching. As photons are emitted one at a time, the probability of seeing two photons at the same time for an ideal source is 0. To verify the antibunching of a light source, one can measure the autocorrelation function g ( 2 ) ( τ τ ) . A photon source is antibunched if g ( 2 ) ( 0 ) ≤ g ( 2 ) ( τ τ ) . For an ideal single photon source, g ( 2 ) ( 0 ) = 0 . Experimentally, g ( 2 ) ( τ τ ) is measured using the Hanbury Brown and Twiss effect. Using resonant excitation schemes, experimental values for g ( 2 ) ( 0 ) are typically in the regime of just a few percent. Values down to g ( 2 ) ( 0 ) = 7.5 × × 10 − − 5 have been reached without resonant excitation.