Photon source. Laser light is frequency-doubled by type-I SHG in a 1 mm BBO
crystal before pumping collinear type-I SPDC in a second 1 mm BBO crystal.
Horizontally polarized photon pairs are generated at 894.6 and 723.5 nm.
Remaining pump light is filtered out and photon pairs are separated by a 801 nm
long-pass dichroic mirror. The 894.6 nm photon passes through a 5 nm
interference filter, is coupled into a single-mode fibre, and detected on an APD. A
detection heralds the presence of the 723.5 nm photon, which is spatially filtered in
a single-mode fibre and spectrally filtered by an interference filter with bandwidth
of 5 nm (FWHM). The input and write pulses are overlapped using a 750 nm
shortpass dichroic mirror. The input and write pulses are focused into the diamond
by an achromatic lens of focal length 6 cm