Four-qubit entanglement sent over 29km of optical fiber

Researchers have, for the first time, successfully transmitted a four-qubit cluster state over 29 kilometers of optical fiber, overcoming exponential photon loss that previously limited such transmissions. The team directly generated the entangled state using coherent control of a parametric generation process, eliminating the need for a resource-intensive controlled-phase gate. To process the state, they introduced a novel technique, chirped pulse modulation, enabling beam splitting for time-bins through reconfigurable Fourier-domain pulse shaping; this approach establishes complex quantum resources fully compatible with existing telecommunication infrastructure. Multi-level encoding enabled the first transmission of a cluster state through 29 kilometers of partially deployed…