SPEQS: Sources of photon pairs

Engineering bright sources

The SPEQS instrument obtains its correlated pairs of photons via spontaneous parametric downconversion (SPDC). The SPDC process is a quantum phenomenon 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. The SPEQS design uses Type-I, non-degenerate SPDC and collects useful photons that are emitted in the same direction (collinear) with the pump beam. A careful arrangement of the crystals used in such a source can be made to generate polarization-entangled photons.

The non-linear material used in SPEQS is called BBO - this is commonly used in many quantum optics labs. However, the trend has been to avoid BBO when building very bright and efficient sources of photon pairs. The reason is because of spatial walk-off: extraordinary polarized light in BBO will wander off at an angle, even if it hit the crystal at normal incidence (see inset of Fig. 1 below). In Type-I SPDC, the pump beam is extraordinary polarized, and walks-off. The SPDC photons are then emitted from an extended region - it was suspected that this prevents efficient collection of the photon pairs into single-mode optical fibers. Efficiency in this context means that when one photon is collected, its partner must also be collected. Another way of thinking about efficiency is the ratio of observed photon pairs to single events at the detectors (aka the pair-to-singles ratio).

We have conducted tests that show single-spatial-mode Type-I BBO sources can show high brightness and collection efficiency despite the presence of spatial walk-off. There is actually a very broad range of pump and collection beam parameters that allow high brightness and efficiency to be achieved. This range of parameters was systematically mapped out (see Fig. 2).

One of the main findings is that the properties of the collected photon pairs depends only on the amount of overlap of the beams within the crystal - this is called the interaction length (see Figure below). We are looking forward to using this data in our next generation SPEQS devices (SPEQS-2) where we hope to increase the brightness and efficiency of the compact entangled photon sources.

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 front page.

Fig. 1. The experimental setup for collecting the emitted photon pairs.

Fig. 2. The collection efficiency and brightness observed for different pump and collection beam parameters.