This is part 5 in a lengthy series of posts attempting to explain the idea of quantum entanglement to a non-physics audience. Part 1 can be read here, Part 2 can be read here, Part 3 here, and Part 4 here.
So at this point we have an idea of what entanglement is, and some reassurance that it doesn’t ruin all of physics with its existence! Now we turn to a very important question: how, in practice, do we produce entangled quantum particles?
In our discussions so far, we have imagined entanglement arising through the hypothetical decay of a neutral pion into an electron/positron pair, as illustrated below.
This is a fine idea in principle, but it is an utterly impractical method to reliably create entangled particles in a laboratory setting. First, let’s talk about the pion: a pion is a high-energy particle that we typically¹ only see as a product in high-energy particle physics experiments, such as those done at places like CERN. When produced, they are usually moving at relativistic speeds, and in a direction that is more or less random. They decay very quickly, on the order of 10 billionths of a billionth of a second when at rest, which means you can’t store them for future use. Even if you could store them, the direction of their decay products is random as well, which means we should revise our image above to appear as shown below.
In short: even if we managed to get a pion to sit still in one place, we wouldn’t know where to put our pair of detectors to spot the particles. We would miss the vast majority of entangled pairs.
So using pions as a source of entangled particles to do experiments is not practical. Fortunately, it turns out that we have a great source of particles available to us that are relatively easy to produce in an entangled state: photons, i.e. particles of light! By the use of a process known as spontaneous parameteric down conversion, we can reliably and easily produce photons with entangled polarization that appear in very predictable locations. Let’s see how!