A technique of depolarization is used to investigate the coherence and
spectral properties of the radiation emitted in the two-photon decay
of metastable atomic deuterium. The results agree with the quantum mec
hanical predictions but may also be interpreted to show that a single
photon of a two-photon pair can be considered to have a very short coh
erence length, a very short coherence time and a broad bandwidth, all
limited, in practice, by the transmission properties of the detection
system.