Photonic Qubits
Qubits encoded in single particles of light: room-temperature, barely decohering, and the only qubits that travel. Their catch is that photons don't interact.
Photonic qubits encode quantum information in single particles of light, in polarization, timing, or path. Their virtues are unique: they barely decohere, they work at room temperature, and they're the only qubits that travel, which makes them the mandatory currency of quantum communication.
Their vice is equally fundamental: photons don't interact, so two-qubit gates require either measurement-based tricks with heavy overhead or nonlinear optics at the edge of feasibility. Photonic quantum computing companies bet on manufacturing scale (chips from semiconductor fabs) to outrun that overhead.
Even in a state better known for cold atoms, the photonics layer is everywhere: Icarus Quantum (Boulder) builds quantum light sources, and Octave Photonics (Louisville) builds the nanophotonic frequency-comb hardware that links lasers, clocks, and atoms. No photons, no quantum anything.