Photonic Quantum Computing
Photonic machines encode qubits in single particles of light — for example, which of two paths a photon travels — and compute with beam splitters, phase shifters and detectors. Photons are fast and need no deep-cryogenic chip, but they do not naturally interact and are easily lost, so two-qubit logic is probabilistic and loss dominates the engineering.
How can light be a qubit?
A photon is the smallest indivisible packet of light — light comes in countable lumps, and one lump is a photon. To make a qubit, give one photon a two-way choice. The most common scheme is dual-rail encoding: two waveguides (light-guiding channels etched into a chip, doing the job of optical fibres) run side by side; the photon travelling in the top guide means 0, in the bottom guide means 1, and a superposition means the photon's amplitude is split across both guides at once. Polarisation — the orientation of the light wave's oscillation — is another common choice.
The general word for one distinct channel light can occupy (a path, a polarisation, a time slot) is an optical mode. Photonic spec sheets often count modes and photons separately, which is one reason their numbers cannot be compared with a superconducting qubit count without reading the definitions.
The deepest difference from every platform so far: photons do not sit in a register waiting for pulses. They fly at the speed of light, and the computation happens while they stream through the optical circuit. Gates are physical objects the photon passes through, not signals sent to a stationary qubit.
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