High rate detection of entangled photon pairs opens the door to quantum applications

Two important parameters of photonic entanglement for many quantum technologies are detected pair rate – or brightness – and photon pair entanglement quality. Lohrmann et al. report a design for a bright, efficient and robust polarization-entangled photon pair source.

“The key ingredient to successfully entangle photon pairs in polarization is to produce photon pair streams with orthogonal polarization and then erase all distinguishing information between these streams, except polarization,” said author Alexander Lohrmann.

The authors revisited an old source design that can spontaneously generate and combine two down-converted, correlated photon pair streams with opposite polarizations inside the same nonlinear crystal. After tailoring the phase of the emitted photons, the authors were able to generate high quality entanglement over a wide spectral range. Next, they used a low-cost, broadband laser diode as the pump for the process, which reduced the operational complexity of the source and provided more power to increase the photon pair rate.

The photon pairs generated by this method can reach rates of over 10 billion pairs per second – high enough for a large number of applications, but too fast for detectors to count. To address this, the researchers propose combining the broad emission spectrum with a multiplexed detector setup in which different wavelength pairs are distributed among multiple detectors.

“One of the limiting factors for entanglement-based quantum key distribution is that the number of photons that can be sent per time interval is limited,” Lohrmann said. “By distributing the photons across multiple channels in the detection paths, this technical limit increases dramatically.”

Entangled photon pair sources based on the presented design may be used as a key component in future quantum networks.

Source: “Broadband pumped polarization entangled photon-pair source in a linear beam displacement interferometer,” by Alexander Lohrmann, Chithrabhanu Perumangatt, Aitor Villar, and Alexander Ling, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5124416 .