The question of how photonic quantum computers work has a pleasingly literal answer, because almost every part of the machine does exactly what its name says it does. A source fires single particles of light into channels of glass. A programmable maze of splitters and phase shifters interferes them, superconducting counters at the far end record which channels the light came out of, and the pattern of those clicks is the output. Many runs of the machine build that pattern up.
The remarkable part is everything it refuses to need. The photons never require the deep freeze that superconducting processors live in, and they barely notice the electrical noise that plagues every other platform. The chips that guide them come off the same 300 millimetre production lines that make ordinary networking equipment, and photonic firms are betting that the scaling problem is really a manufacturing problem. Light lets them borrow the semiconductor industry wholesale, since a photonic circuit is patterned by the same lithography that makes ordinary optical chips.
The bet comes with strange costs. Photons cannot be made to interact directly, so the logic gates other platforms take for granted become games of chance here, and they cannot be stored either. The whole computer must be choreographed in flight. Everything else in the machine follows from those two facts, from the qubit itself right through to the companies racing to build one worth paying for.
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