Generation of telecom-wavelength-compatible entangled photon pairs via diamond-type four-wave mixing in a chip-scale ⁸⁷Rb vapor cell

The generation of entangled photon pairs compatible with quantum communication channels is crucial for long-distance quantum communication. As a preliminary step, we performed a diamond-type four-wave mixing (FWM) experiment in a rubidium vapor cell to identify the optimal conditions for photon-pair generation. To this end, we measured the spectroscopic responses of the vapor cell to the 795 nm pump laser and the 1475 nm coupling laser, respectively, and then tuned the laser frequencies accordingly before carrying out the photon-pair generation experiment. In addition, since a chip-scale vapor cell with a thickness of approximately 2 mm was employed, this work is significant in that it demonstrates the potential for miniaturization of photon-pair sources.

The atomic energy-level structure relevant to the diamond-type FWM process corresponds to the D1 transition of ⁸⁷Rb. The 795 nm pump laser was frequency-stabilized to the corresponding transition using saturated absorption spectroscopy (SAS), while the 1475 nm coupling laser was stabilized by dual-resonance optical pumping (DROP). The photon-pair generation experiment was performed in a co-propagating configuration, in which the pump and coupling beams were combined by a dichroic mirror and sent into the vapor cell. The polarizations of the two beams entering the cell were both set to horizontal using a PBS and a Glan–Thompson polarizer, and achromatic lenses with a focal length of 200 mm were placed before and after the cell to control the beam sizes. After the cell, a dichroic mirror was used to separate the telecom and near-infrared (NIR) wavelengths, and bandpass filters removed residual pump and coupling light, transmitting only the signal and idler photons.