Photonic quantum computing traditionally encodes information in polarization — a photon's polarization can be horizontal or vertical, giving two states: a qubit. Two states per photon means you need many photons to carry complex information, and each additional photon multiplies the error and loss rates.
A collaboration between TU Wien and Chinese research groups (Nature Photonics, 2026) built a heralded quantum gate that operates on photons in four simultaneous states, using orbital angular momentum — the spatial waveform of the photon — instead of polarization. Orbital angular momentum can take infinitely many values (different amounts of twist in the photon's wavefront), of which four were used here. The gate entangles two four-state photons, creating a quantum operation in a 16-dimensional Hilbert space from just two particles.
The shift from qubits to qudits — from two-state to multi-state encoding — is not just more efficient. It changes what “one particle” means computationally. A single qudit photon carries the information of two qubits. Two qudits access a space that would require four qubits. The information density per particle increases, which means fewer particles carry the same computation, which means fewer loss channels, fewer error sources, and fewer physical resources.
The general principle: when a system is limited by the number of components, increasing the capacity per component can be more effective than adding more components. The traditional approach (more qubits) scales the system. The qudit approach (wider words) scales the alphabet. The photon was always capable of carrying more than two states — polarization was chosen for simplicity, not because it exhausted the photon's state space. The wider encoding was available the entire time; what was missing was the gate that could manipulate it.