A quantum dot is like a tiny lamp that turns on and off at the command of a laser, emitting exactly one photon at a time. But this light has a quirk: the photons don't interact with each other; they fly independently, like random drops from a faucet.
The researchers passed this light through a device made of two mirrors and beam splitters — a Mach-Zehnder interferometer. One arm was made slightly longer, so a photon takes longer to travel through it. At the output, the mirrors almost cancel out single photons, but not completely. As a result, pairs of photons are born, appearing either simultaneously or with a delay.
When such pairs are sent to two detectors and coincidences are measured, the pattern violates Bell's inequality. This is a strict test: classical physics forbids it, but quantum mechanics allows it. So entanglement is truly created.
This method doesn't require complex crystals or traps, just a quantum dot, a laser, and a few mirrors. It opens a simple path to creating entangled photons for quantum communication and computing.
A quantum dot is so small that billions of them could fit on the tip of a needle, yet each one emits photons one at a time.