How Do Photonic Quantum Computers Work?

How Do Photonic Quantum Computers Work? No frozen chips, no trapped atoms, no artificial molecules. One family of quantum machines computes with light itself, and it works in ways the rest of the industry finds slightly alien. The qubit is a single particle of light. Information rides on a photon’s path, its timing or its polarisation, and, unlike the platforms it competes with, there is nothing to cool to millikelvin and nothing to trap. The program is written in glass. A programmable maze of waveguides and phase shifters performs the operations, so changing the computation means changing the settings on the chip rather than building a new chip. Photons do not interact, so measurement does the work. Two-photon gates are made to happen by interference and detection rather than by force, which is why measurement sits at the centre of every photonic design. Loss is the enemy, not noise. A photon that vanishes takes its information with it, and no amount of shielding brings it back, so every component is judged on how little light it wastes. Squeezed light is the other road. Continuous-variable machines encode in the wave properties of a light field, its amplitude and its phase, rather…