In the experimental setup with cold atoms correlated photon pairs are generated by inducing a diamond scheme of FWM in the 87Rb electronic structure. We are currently exciting the double transitions show below, where the Idler and signal photon pairs are emitted by two decays in a cascade fashion.
In the scheme, p1 is a 780 nm laser stabilized to the transition from the ground state 5S1/2 to the state 5P3/2 with a detuning ∆, which large enough to reduce the spontaneous decay. The second transition, 5P3/2 -> 5D3/2, is induced by the laser beam p2 that has wavelength around 776 nm and a smaller detuning δ. The signal photon (with 762 nm) is emitted whilst atoms decay from the 5P3/2 to the 5D3/2 state; whereas the idler photon (795 nm) is emitted in a second decay from the 5P1/2 to the 5S1/2 state.
Laser beams p1 and p2 are delivered to the main experiment as shown at the left. They are both collimated by a spherical lens with a diameter of 1.1 mm and then mixed by interference filter FI1 that only transmits light with a wavelength of 776 nm and reflects the other two. The three beams are sent collinearly to the MOT with the trapped rubidium atoms. There, the correlated photon pairs are produced in the same direction as the input beams. Two interference filters are added at the output of the MOT: IF3 only transmits the frequency of the signal photons, while IF2 only transmits the frequency of the idler photons. Each filter uses a polarizing beam splitter that sends the photons to their respective APD detectors.
This photon source is bright and narrowband, suitable for its generated light to interact with other atoms. Furthermore, the photons are entanglerd on their quantum variables, such as the polarization or the orbital angular momentum. Our challenge is to be able to control those variables to modulate their interaction with matter, and thereby have the possibility to build quantum networks and/or to realize non-interacting measurements. Building quantum networks with the generated photons might be possible since the rich electronic structure of Rubidium has other diamond schemes in which the signal photons have wavelengths in the telecom region of the optical spectrum, while the idler photons are resonant to their ground state. This brings us the chance to send the photons far away using fiber optics whilst keeping the idler inside an atomic quantum memory.