An enhanced source of entangled photons ready for quantum imaging

Entangled photons are a key resource for the upcoming quantum technologies. To generate them, one needs Entangled Photon Sources (EPS), which obviously play a fundamental role in the development of quantum sensing, imaging, or communications applications. The majority of EPS’s reported to date operate at long wavelengths ranges (from the Near Infrared Range (NIR) to telecom). However, EPS operating at shorter wavelengths (around 550 nm, in the mid-visible range) with lower diffraction loss and short-wavelength illumination could be advantageous for sensing or imaging applications because of their much higher efficient and less jitter detectors. Although EPSs operating at 532nm have been reported recently, they all seem to still emit at low brightness values.

Now, a team of researchers, which include researchers Adrià Sansa and Fabian Steinlechner from Fraunhofer Institute for Applied Optics and Precision Engineering IOF and partner of the H2020 funded Q-MIC project, Evelyn Ortega, from Institute for Quantum Optics and Quantum Information Vienna, and Markus Gräfe, from Friedrich Schiller University Jena, demonstrate in a new study, recently published in the journal Applied Physics Letters, the development of an enhanced entangled photon source that operates in the visible range (around 532nm) and with high brightness values.
“The EPS pushes the brightness of such a source almost two orders of magnitude to what is was previously reported (135 pairs/s/mW) and this improvement allows it to be used in combination with other setups in experiments for Quantum Key Distribution or Quantum Imaging”, said Adrià Sansa.

By employing a collinear crossed-crystal setup scheme with barium borate crystals and optimized spontaneous parametric down-conversion (SDPC) focus and collection modes, the quantum source is capable of achieving brightness of 9.5k pairs/s/m and a quantum state fidelity pf 98,3%. Since the signal and idler photons were collected in the same single mode fiber, this setup, as researchers highlight in their study, can be integrated in the future in a quantum-enhanced microscope scheme like the one which is being developed in the Q-MIC project.
One of the main challenges that Sansa and his colleagues had to deal with was moving from the usual wavelength of entangled photons in the infrared range (810nm) down to 532nm (visible). Not only did this shift require a pump operating at 266nm, but, as Adrià Sansa recalls, “there are not so many optical components available, and many non-linear crystals cannot be used”. This was the reason why, at the end and after trying different schemes and configurations, the “resulting source was following the scheme of one of the simplest ways of producing entangled photons”.
According to the authors of the study, these results make this source “an ideal candidate for integration in microscopes and make use of the advantages of mid-visible optimized single-photon detection technologies”.
The next steps will be focused on making the EPS more compact and movable. At the moment, the source uses a bulky laser. But, combined with a less powerful and smaller pump to make it more compact and transportable, the EPS ”would be more practical and more likely to be used in microscopy or free space QKD”.

Original article
Sansa, A., Ortega, E., Gräfe, M., and Steinlechner, F. (2022) Visible-wavelength polarization-entangled photon source for quantum communication and imaging. Appl. Phys. Lett. 120, 074001. Doi: 10.1063/5.0069992.