Photonic qubits encode quantum information in the properties of individual photons — polarization (horizontal/vertical), path (which waveguide the photon is in), time-bin (early/late arrival), or spatial mode. Companies like PsiQuantum and Xanadu are developing photonic quantum computers, motivated by photons' natural advantages: they operate at room temperature (no dilution refrigerator needed for the qubits themselves, though single-photon detectors may require cryogenics), travel at the speed of light for natural networking, and are immune to many forms of decoherence that plague matter-based qubits.
The fundamental challenge in photonic quantum computing is that photons do not naturally interact with each other — they pass right through one another. Two-qubit gates therefore require either measurement-based schemes (where entanglement is created probabilistically through photon detection and feed-forward, as in PsiQuantum's fusion-based architecture) or nonlinear optical media (which have extremely weak photon-photon coupling). Measurement-based approaches succeed only a fraction of the time, requiring multiplexing — generating many entangled photon pairs and selecting the successful ones.
PsiQuantum is building a manufacturing-scale photonic quantum computer using silicon photonic chips fabricated in GlobalFoundries' existing semiconductor fabs, arguing that photonics offers a faster path to millions of qubits through established chip manufacturing. Xanadu uses squeezed light states (continuous-variable encoding) with Gaussian boson sampling as a near-term application. Photonic approaches are also uniquely suited for quantum networking and the quantum internet, since photons are the natural carriers of quantum information over optical fiber.