single-photon source

A single-photon source is a device that, when you ask it to, spits out exactly one photon — one particle of light, no more and no less. That sounds trivial until you try it: a normal laser dimmed way down does not give you one photon at a time, it gives you a random trickle that sometimes sends two photons together and often sends none. For photonic quantum chips, where the photon itself is the qubit, that randomness is fatal. You need a reliable, on-demand supply of clean single photons to feed into the waveguides, beamsplitters, and detectors etched onto the chip, and to send qubits down optical fibers between chips.

There are three main ways people build one. A quantum dot is a tiny man-made speck of semiconductor that traps electrons like an artificial atom; excite it and it relaxes by emitting one photon, then has to be re-excited before it can fire again, which naturally enforces one-at-a-time. A defect center, such as the nitrogen-vacancy color center in diamond, is a single atom-sized flaw in a crystal that behaves the same way. The third route, heralded SPDC (spontaneous parametric down-conversion), is different: a laser passes through a special crystal that occasionally splits one photon into a pair; you detect one of the pair, and that 'click' heralds — announces — that its twin is on its way, so you keep only the photons you have confirmed.

The two metrics that actually matter are efficiency and indistinguishability. Efficiency asks: when I press the button, how often does a usable photon actually arrive at the chip, rather than being lost or absorbed along the way. Indistinguishability asks: are the photons identical enough — same color, same shape, same polarization — that two of them interfere the way quantum optics requires; if they differ even slightly, the quantum logic falls apart. Today's best sources are good but not perfect, and getting high efficiency and high indistinguishability at the same time, repeatably, on a chip you can mass-produce, is still very much an open engineering problem.