The team says it wants to "overcome the fragility of entangled states in a fibre cable and ensure the efficiency of signal delivery."
For their prototype network, the Qunnect researchers utilised a leased 34-kilometre-long fibre circuit, dubbed the GothamQ loop.
Employing polarisation-entangled photons, they operated the loop continuously for 15 days, achieving an impressive uptime of 99.84 per cent and a compensation fidelity of 99 per cent for entangled photon pairs transmitted at a rate of approximately 20,000 per second. Even at a rate of half a million entangled photon pairs per second, the fidelity remained nearly 90 per cent.
The researchers transmitted 1,324 nm polarization-entangled photon pairs in quantum superpositions through the fibre. One state has both horizontal and vertical polarisations—a two-qubit configuration known as a Bell state. In this superposition, the quantum mechanical photon pairs exist in both states simultaneously.
A report into the project said that while others have transmitted entangled photons before, there has been too much noise and polarisation drift in the fibre environment for entanglement to survive.
To address this, the Qunnect team developed "automated polarisation compensation" devices to correct the polarisation of the entangled pairs.
Their design entangles an infrared photon (with a wavelength of 1,324 nanometres) with a near-infrared photon of 795 nanometres. The latter photon is compatible in wavelength and bandwidth with rubidium atomic systems used in quantum memories and quantum processors.
The team discovered that polarisation drift was both wavelength- and time-dependent, necessitating the design and construction of equipment for active compensation at the same wavelengths.
Qunnect's GothamQ loop demonstration was particularly notable for its duration, hands-off operation, and uptime percentage. The team described it as "progress toward a fully automated practical entanglement network" all this is essential for a quantum internet.
Connect said that since it finished this work, it has already made all the parts rack-mounted, so they can be used everywhere.