  Spectrally uncorrelated biphoton state generated from the spontaneous
nonlinear optical process is an important resource for quantum information.
Currently such spectrally uncorrelated biphoton state can only be prepared from
limited kinds of nonlinear media, thus limiting their wavelengths. In order to
explore wider wavelength range, here we theoretically study the generation of
spectrally uncorrelated biphoton state from 14 isomorphs of potassium
dihydrogen phosphate (KDP) crystal. We find that 11 crystals from the `KDP
family' still maintain similar nonlinear optical properties of KDP, such as
KDP, DKDP, ADP, DADP, ADA, DADA, RDA, DRDA, RDP, DRDP and KDA, which satisfy 3
kinds of the group-velocity matching conditions for spectrally uncorrelated
biphoton state generation from near-infrared to telecom wavelengths. Based on
the uncorrelated biphoton state, we investigate the generation of heralded
pure-state single photon by detecting one member of the biphoton state to
herald the output of the other. The purity of the heralded single photon is as
high as 0.98 without using a narrow-band filter; the Hong-Ou-Mandel
interference from independent sources can also achieve a visibility of 98%.
This study may provide more and better single-photon sources for quantum
information processing at near-infrared and telecom wavelengths.

Quantum Physics