Entangled in the City: A Quantum Leap for Urban Networks

The dream of an unhackable quantum internet often hits a harsh reality: distance. Quantum states are notoriously fragile, prone to collapsing via decoherence before they can travel meaningful distances. However, a new study has successfully entangled a calcium ion with a photon over a 12-kilometre optical fibre, effectively bridging a metropolitan scale.

The team utilised a chain of ⁴⁰Ca⁺ ions acting as a network node. The innovation lies in a 'hybrid multiplexing' scheme. Rather than relying on a single signal attempt, the system generates up to 44 'time-bin modes'—distinct temporal slots for data transmission—to drastically boost the connection rate. Consequently, they achieved a heralded entanglement success rate of 4.28 per second, a significant pace in the quantum realm.

Crucially, the system’s memory coherence time clocked in at 366 milliseconds. While that sounds fleeting to the human ear, it exceeds the time required to generate the entanglement. This ensures the node retains data long enough to establish a link, a fundamental requirement for functional quantum repeaters. Furthermore, a dual-type framework was employed to shield stored quantum information from the destructive nature of the entangling attempts. This robust approach suggests that urban quantum networks are transitioning from theoretical fancy to engineering reality.