Approaching Single-Photon Pulses

Single-photon pulses cannot be generated on demand, due to incompatible requirements of positive frequencies and times. Resulting states therefore contain small probabilities for multiphotons. We derive upper lower bounds the maximum fidelity realizable that approximate single-photon pulses. The have implications ultrafast optics; is low with few cycles or close onset, but increases rapidly as pulse envelope varies more slowly. also demonstrate strictly localized are single photons.