Spontaneous parametric down-conversion (SPDC) is a workhorse in quantum optics labs as a way to generate entangled pairs of photons. A pump laser (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 to create an entangled pair of photons. The pair is time-energy entangled because they are born at the same time and the original photon’s energy is split between them.

In some forms of SPDC the pair evenly splits the pump photon’s momentum (energy), and each pair leaves 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 collect entangled pairs, you need to capture photons from two points on exact opposite sides of the cone. You’ll miss the pairs everywhere else, but you will pick up both photons from pairs that travel along that particular slice of the cone. It’s common to name the photons in a pair as signals and idlers, and to collect each into their own piece of fiber optic cable. The fiber can route them to additional experimental apparatus so they can be manipulated, detected and counted.

The interactive demo above contains simplified versions of all the key components of SPDC. 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 found that 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 that outlines practical procedures for aligning SPDC in the lab.