Researchers Swap Entanglement across 40km Fibre with Live Data

Entanglement swapping, an essential process for building future quantum networks, previously required dedicated optical fibres without any concurrent data transmission. For the first time, this key technology operates alongside conventional communications on the same fibre network. A five-node system realised this achievement by transmitting ten gigabits per second of classical data while performing entanglement swapping over long distances; prior demonstrations limited themselves to three connected nodes or used separate fibres.

Standard data transmission now successfully combines with entanglement swapping within existing fibre optic cables, representing a vital advancement for constructing practical quantum networks. This demonstration utilises a five-node network capable of performing both processes concurrently, surpassing previous systems restricted to three connection points or requiring separate fibres. Integrating these quantum operations into current infrastructure enables scaling up future technologies like quantum computing and secure communication methods.

A key milestone in building future quantum networks has been reached as the successful combination of standard data transmission with entanglement swapping within existing fibre optic cables occurred. Entanglement swapping extends the range of entangled particles without physically moving them; consider two people each holding half of a secret message then sharing information, so another can reconstruct it without either revealing their original part directly.

The demonstration used a five-node network transmitting ten gigabits per second alongside these quantum operations over fibres spanning up to forty kilometres, exceeding previous three-node systems or those requiring separate cabling. This work integrates complex processes into current infrastructure but raises questions about how best to mitigate noise in real-world deployments as networks grow even larger.

Five Node Entanglement Achieved Alongside Standard Data Transmission

Entanglement now extends to five connected nodes while simultaneously transmitting data, exceeding previous limits of three. This represents the first demonstration within an operational network. Previous systems required dedicated fibres or limited themselves by node count, hindering progress towards scalable quantum networks capable of long distance communications. The team integrated entanglement swapping with standard ten gigabits per second data transmission over forty kilometres of fibre, utilising dense wavelength division multiplexing to share bandwidth without signal interference.

A strong step beyond laboratory demonstrations and toward practical deployment for applications like secure communication and distributed computing has been achieved. Entanglement swapping confirmed its functionality, an essential process for extending network range via quantum repeaters, while time-bin entangled photons simultaneously transmitted data encoding information into the arrival time of single particles. Characterisation of Raman noise impacts on performance revealed trade-offs between signal quality and power levels; the experiment involved a five-node quantum relay topology connected by four long-distance single mode fibre spools, with total distances of either twenty or forty kilometres.

Coexisting Quantum Key Distribution and Classical Data Transmission via Dense Wavelength Division

Dense wavelength division multiplexing underpinned this team’s success. It functions much like multiple radio stations broadcasting on different frequencies simultaneously, allowing numerous optical signals to share the same fibre without interfering with one another. This technique enabled both quantum information, specifically time-bin entangled photons, and conventional data at ten gigabits per second to be sent down existing fibres concurrently. Careful filtering was required because spurious Raman scattering from high-power classical light sources could degrade delicate quantum states; narrow spectral filters were employed for this purpose.

Entanglement swapping alongside classical data reveals wavelength dependent range limitations

Establishing quantum networks alongside existing infrastructure offers vital prospects for secure communication and distributed computing but isn’t without its challenges. Simulations reveal a trade-off between wavelength choice and performance, despite successful entanglement swapping coexisting with standard data transmission. Shifting quantum signals to the O-band could extend network reach even with increased signal loss. Still, acknowledging that such shifts introduce their own challenges regarding signal weakening, this work remains key as it demonstrably coexists with current fibre optic networks.

Ten gigabits per second data transmission across forty kilometres of optical fibre was integrated by the team with entanglement swapping; dense wavelength division multiplexing enabled simultaneous operation by allocating distinct wavelengths to both quantum information and conventional signals without interference. To maintain quantum fidelity during co-propagation, narrow spectral filters were implemented to mitigate spurious Raman scattering, noise generated from high-power classical light, and automated fibre polarisation controllers compensated for signal drift due to environmental factors.

The researchers successfully demonstrated entanglement swapping alongside the transmission of 10-Gbps classical data over a forty kilometre network comprised of four ten kilometre optical fibres. This coexistence is important because it suggests that quantum networks can be built utilising existing telecommunications infrastructure without requiring entirely new cabling. The study characterised how noise photons impact quantum signals when sharing fibres with conventional communications, revealing performance tradeoffs related to wavelength choice. Authors note simulations suggest shifting quantum signals to the O-band may extend network reach despite increased loss.

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