Entangled photon sources I

Engineering optimal performance of sources

The team actively investigates designs and concepts for the generation of entangled photon pairs, and evaluates if these designs are suitable for implementation into portable instruments, such as SPEQS. Contact us if you wish to be involved in research and development of entangled photon pair sources.

The photon pairs are typically generated via the Spontaneous Parametric Downconversion (SPDC) process where an individual pump photon within an appropriate non-linear material is sometimes split into a pair of lower energy photons, while obeying energy and momentum conservation. Entangled photon pairs can be used for quantum communication, but also for clock synchronisation and quantum sensing. One objective of R&D in this field is to understand how the collection of the photon pairs can be optimized.

In 2019, we published a paper that confirmed a theoretical prediction from 1972, that the pump shape matters quite a bit when optimizing the production of SPDC light for collection. This is because the intrinsic walk-off of the pump beam within the material typically leads to an elliptical shape for the output SPDC light. By tailoring the major axis of the pump ellipse to the direction of the walk-off, an improvement can be obtained. See the concept explained in Figure 1.This is important to note, because it is a major departure from the traditional approach to assume the Boyd-Kleinman parameter for all directions, and to use a circular symmetric beam for pump and collection.

We modeled the predicted improvement in performance, and also experimentally confirmed the results. These are seen in Figure 2. The normalised detection efficiency of the source is almost 80,000 observed pairs per second per mW, using a pair of Silicon Geiger-mode avalanche photodiodes (with nominal detection efficiencies of 50%). Even using less than optimal detectors, we have broken the record for pair production using crystals with walk-off in the SPDC process. This was a CQT Highlight article. In this 2019 paper, we used an asymmetric pump and a circular collection beam. Should we use also an asymmetric collection beam as the next step?

The pump shape is not the only thing that matters. The interplay between the pump size, and collection beam size and the walk-off is also important. In 2016, we published a paper describing the interplay of walk-off with beam sizes. The results are available in a paper called "Thick-crystal regime in photon pair sources" from the journal Optica. This paper was also featured on CQT's Highlights page. Is there a way to combine all these observations into a single model? This is an open question, and we are working on it!