Spontaneous parametric down-conversion (SPDC) is a workhorse in quantum optics labs as a way to generate entangled pairs of photons. A pump laser beam (shown in blue) is sent through a non-linear crystal to create entangled pairs (red). Most of the pump beam just passes straight through the crystal, but occasionally one pump photon is down-converted into a pair of photons. The original photon’s energy is split between two photons that are born at the same time, so SPDC can be used to create pairs that are time-energy entangled.

In some forms of SPDC the pair evenly splits the pump photon’s momentum, so pairs of photons leave the crystal at equal-but-opposite angles with respect to the pump beam. Every pair spreads apart at a similar opening angle, but oriented randomly around the pump beam. The result is that all the pairs form a cone shape. You can make the cone wider or narrower by rotating the crystal so the pump hits it at different angles.

To make use of the entangled pairs, you need to collect photons from opposite sides of the cone. That way you’ll pick up both members of each pair, which are often referred to as the signal and idler photons. It’s common to collect signals and idlers each into their own piece of fiber optic cable, so they can be routed to additional experimental apparatus, detected and counted, etc.

The interactive demo above contains simplified versions of all these key components. You can try your hand at collecting entangled pairs into fiber by hitting the jitter button to shake up the alignment. Try rotating components to realign the setup and get the power meters back up to ~100%.

I’ve been working with an SPDC setup recently, and aligning the pump, crystal and collection optics can be quite exacting. In real life, the ultraviolet pump laser will blind you if you look at it and the SPDC cone is completely invisible! So aligning an actual setup can be considerably trickier than the demo. I’ll follow up with another article with tips, tricks and an actual procedure for SPDC alignment in the lab.