A single calcium ion hovers in a vacuum trap, held by electric fields like a pea in an invisible bowl. Around it are two mirrors forming an optical cavity: light bounces between them, amplifying the interaction with the ion. The scientists make the ion emit a photon, and this photon becomes entangled with the ion's internal state, as if a coin in your hand and its reflection in a mirror always showed the same side. Then the ion emits a second photon, and now both photons are tied together by a quantum thread.
Noise is the enemy of such experiments. The slightest jitter of the trap or a random jump in the magnetic field disrupts the phase, and instead of a neat pair of photons you get mush. Here they used a protection scheme: the phase of the entangled state is set not by the emission itself but by an external control field that can be adjusted. It's like tuning a guitar — instead of trying to catch the sound of the string, you turn the peg, and the string comes to the right pitch on its own.
As a result, they obtained a pair of photons entangled in phase, and this phase can be changed at will. Such a source is a ready-made building block for a quantum repeater: a device that extends quantum communication over hundreds of kilometers without letting the signal fade. The ion in the trap works like a tiny factory, stamping out photon pairs on demand — with the right phase, at the right moment.
Calcium-40 is the same element found in milk and bones, but here it serves as a perfect emitter of single photons.
In the series "Dark," quantum entanglement links the fates of characters across time — here it links two photons emitted from the same trap.