Photonic Qubit

A photonic qubit encodes information in a degree of freedom of a single photon, commonly its polarization, path, or arrival time. Because photons barely interact with their environment or with each other, photonic qubits hold their state well over distance and do not need the millikelvin cooling that superconducting qubits require, which is why photons are also the default carrier for quantum networking links between distant nodes. That same weak interaction is what makes computing hard: two-qubit gates need photons to interact, which normally requires either probabilistic linear-optical schemes or specialized nonlinear materials, and the dominant error source is photon loss rather than decoherence, since a lost photon simply disappears rather than degrading gracefully. PsiQuantum is pursuing a fault-tolerant, loss-tolerant architecture built on this modality, while Xanadu builds continuous-variable photonic processors. Photon loss tolerance, not gate fidelity, is the metric that matters most for this modality's error-correction overhead.