NYC Quantum Network Test Shows Promising Results

Engineers face significant challenges in making quantum networks practical, particularly with maintaining the delicate entangled states over fiber cables and ensuring efficient signal delivery. A notable advancement comes from Qunnect Inc. in Brooklyn, New York, where scientists have successfully tested a quantum network beneath New York City streets.

Previous attempts to transmit entangled photons faced issues like excessive noise and polarization drift, which hindered the stability of long-term networks. Qunnect's approach addresses these challenges, as detailed in their study published in PRX Quantum.

The Qunnect team utilized a 34-kilometer fiber circuit, known as the GothamQ loop, for their prototype network. They ran the network continuously for 15 days, achieving an impressive uptime of 99.84% and a compensation fidelity of 99% for entangled photon pairs at a rate of around 20,000 per second. Even at higher speeds of half a million entangled pairs per second, the fidelity remained close to 90%.

Polarization—the direction of a photon's electric field—plays a crucial role. Polarized photons are easily created, manipulated, and measured. They are integral to various quantum technologies, including quantum repeaters, distributed quantum computing, and quantum sensing networks.

In their setup, the researchers entangled an infrared photon (1,324 nanometers) with a near-infrared photon (795 nm), aligning with the rubidium atomic systems used in quantum memory and processors. To manage polarization drift—which varies with wavelength and time—they designed specialized equipment for active compensation.

The researchers generated these entangled photon pairs by directing input beams through a rubidium-78 vapor cell, which caused electron transitions and photon emissions. They transmitted polarization-entangled photons through the fiber, using a two-qubit Bell state, where the photons exist in both states simultaneously.

The challenge in optical cables is that these photons are susceptible to disturbances like vibrations and temperature fluctuations. Qunnect tackled this by developing automated polarization compensation (APC) devices that adjust for polarization drift. They measured drift over different distances and corrected it using APCs, ensuring stable transmission.

The GothamQ loop's success, with its long operational duration and high uptime, represents a significant step toward a fully automated entanglement network, essential for a future quantum internet. Qunnect has already advanced their technology to be more versatile with a combined equipment system called Qu-Val.