PsiQuantum builds computers on Photonic qubits, encoding information in single photons instead of matter. Photons barely interact with their environment, so they hold onto their quantum state well. That same weak interaction makes it hard to get two photons to interact with each other for a two-qubit gate.
PsiQuantum works around this with linear optics and measurement-induced entanglement, backed by heavy error correction. It fabricates its chips using standard silicon photonics processes at existing semiconductor fabs. The bet is that fabrication maturity, not new physics, is what scales photonics to the million-plus qubits a fault-tolerant machine needs.