An ultra-bright two-photon source with a type-I bulk periodically poled potassium titanyl phosphate

We experimentally demonstrate an ultra-bright two-photon source with a type-I bulk periodically poled potassium titanyl phosphate. With the 390 nm cw laser from our home-made frequency doubler in a ring cavity as the pump, we detect about 340 k/s coincidence counts with a pump power of 0.11 mW in a Hanbury–Brown–Twiss-type experiment, which corresponds to the production rate of 94 MHz/mW with 40 nm bandwidth. With a 3 nm interference filter, we detect about 11.5 k/s coincidence counts with the same pump power, corresponding to the production rate of 4.3 MHz/mW/nm. This is very promising for many applications in quantum information field, such as transferring the information between a photon and an atomic system and so on.