Bright source of polarization-entangled photons using a PPKTP pumped by a broadband multi-mode diode laser
We sure like down-converted photons; they are highly entangled and relatively easy to produce: just pump a nonlinear crystal with a laser and voila, job done! Nevertheless, there is a continuous push to obtain brighter sources of down-converted photons, as these make for faster and better experiments. A possible way of enhancing the generation probability is to use more efficient crystals such as PPKTP over, say, BBO. However, this is not always possible, as they operate at different wavelengths and not everyone can afford to purchase new lasers whenever they would like to. Another possibility is to crank up the laser power; however, this also increases unwanted higher order contributions from the nonlinear terms. Another option is to lengthen the crystal, but this makes the phase matching conditions harder to achieve and also reduces the entanglement strength. This is where Jeong, Hong and Kim join the game and tell us that we don't need to worry about it: even if the coherence length of the pump laser is considerably shorter than the crystal (which degrades entanglement because it makes the photons distinguishable), one can correct for this after the process is over by 'erasing' the unwanted information with a linear optical configuration named "universal Bell state synthesizer". The result is a stream of delightful Bell polarization states, emerging from a garbled incoherent mixture. Specifically, 7000 pairs per mW of pump power if we look only at the Gaussian-Gaussian spatial modes (resulting in 99.2% fidelity with the ideal Bell state), and >90000 pairs per mW if we allow for more spatial modes (still resulting in a 96.8% fidelity).