A team has demonstrated programmable synthesis of high-dimensional quantum states transmitted through 20 metres of multimode optical fibre. The work, published in Nature, combines photons generated in atoms with long coherence times and control techniques that allow complex states to be prepared and recovered despite the modal dispersion characteristic of this type of fibre.

Multimode fibre is an attractive channel for quantum information because it can carry many spatial modes at once, multiplying capacity compared with single-mode fibre. Its drawback is that modes couple and mix as they propagate, deforming the state. Earlier work had recovered individual modes or controlled classical transmission through long multimode fibres, but active, programmable synthesis of high-dimensional states in a quantum channel remained a challenge.

The method relies on atomic photons with long coherence, which reduce spectral broadening and ease state control. On that basis, the researchers use a modulation and calibration scheme that compensates the fibre's dispersion and allows the desired state to be written into the output modes. The 20-metre length is an order of magnitude longer than previous demonstrations of this kind while maintaining sufficient fidelity to operate with high-dimensional states.

The result opens the door to quantum communication and computing protocols that exploit the internal dimensionality of spatial modes as a resource, rather than limiting themselves to qubits. It also raises questions about scalability: whether control can be maintained in longer fibres, with more modes and in less controlled conditions than in the laboratory. The next phase plans to extend the distance and explore applications in metropolitan quantum networks.