Physicists at Heriot-Watt University in Edinburgh, Scotland, have made significant strides in quantum communications by successfully routing and teleporting entangled states of light between two four-user quantum networks. This achievement marks a critical milestone in the pursuit of scalable quantum communication systems.
The team, led by Mehul Malik and Natalia Herrera Valencia, developed a novel method that employs light-scattering processes within standard optical fibres to create a programmable circuit. This innovative approach diverges from traditional techniques that rely on photonic chips, enabling the circuit to act as a programmable entanglement router. It can implement various network configurations on demand, demonstrating flexibility in quantum communication.
The experiments utilized commercially available optical fibres, which are multi-mode structures that scatter light through random linear optical processes. As Herrera Valencia elaborates, this means that the light tends to ricochet chaotically through the fibres, taking hundreds of pathways. While this effect can complicate entanglement, researchers from the Institut Langevin in Paris previously established that the scrambling can be understood by examining the light transmission through the fibre.
By leveraging the light-scattering processes, Malik, Herrera Valencia, and their colleagues successfully crafted programmable optical circuits. This “top-down” approach simplifies the circuit architecture, separating the control layer from the mixing layer. By employing waveguides for the transport and manipulation of quantum states, they significantly reduce optical losses.
The outcome is a reconfigurable multi-port device capable of distributing quantum entanglement among numerous users simultaneously, switching between various channels—local, global, or both—as required. Additionally, the channels can be multiplexed, allowing multiple quantum processors to access the system concurrently. This multiplexing is reminiscent of classical telecommunications networks, where vast amounts of data can be transmitted through a single optical fibre using different wavelengths of light.
Despite the promise of controlling and distributing entangled states of light, Malik acknowledges several challenges. Traditional methods that rely on photonic chips are not easily scalable and are highly sensitive to manufacturing imperfections. The waveguide-based strategy devised by the Heriot-Watt team offers significant advantages, providing access to a broader range of modes, which enhances circuit size, quality, and loss characteristics. As Malik shared with Physics World, this approach aligns seamlessly with existing optical fibre infrastructures.
Controlling the complex scattering process within a waveguide posed significant challenges. “The main challenge was the learning curve and understanding how to control quantum states of light inside such a complex medium,” Herrera Valencia said. “It took time and iteration, but we now have the precise and reconfigurable control required for reliable entanglement distribution, and even more so for entanglement swapping, which is essential for scalable networks.”
While the Heriot-Watt team successfully demonstrated flexible quantum networking, they believe this technique could also facilitate the development of large-scale photonic circuits. Such circuits hold potential applications in various fields, including machine learning, quantum computing, and networking.
Looking ahead, the researchers, who published their findings in Nature Photonics, aim to explore larger-scale circuits capable of operating with more photons and light modes. “We would also like to take some of our network technology out of the laboratory and into the real world,” Malik noted, emphasizing that Herrera Valencia is spearheading a commercialization effort to achieve this goal.
