Quantum networks have long been plagued by the fragility of entangled states in fiber cables, making it a challenge to ensure efficient signal delivery. However, scientists at Qunnect Inc. in Brooklyn, New York, recently made a significant breakthrough by successfully operating a quantum network under the streets of New York City. While previous attempts at transmitting entangled photons have been hindered by noise and polarization drift in fiber environments, the team at Qunnect managed to achieve stable entanglement over a sustained period.
The researchers at Qunnect utilized a leased 34-kilometer-long fiber circuit, dubbed the GothamQ loop, for their prototype network. By employing polarization-entangled photons, they were able to maintain the loop’s operation for an impressive 15 consecutive days, with an uptime of 99.84% and a compensation fidelity of 99% for entangled photon pairs transmitted at a rate of around 20,000 per second. Even at a higher transmission rate of half a million entangled photon pairs per second, the fidelity remained at nearly 90%.
The Power of Polarization
Polarization plays a crucial role in the success of quantum networks, as it enables the creation, manipulation, and measurement of polarized photons with relative ease. By utilizing polarization-entangled photons, researchers have been able to develop large-scale quantum repeaters, distributed quantum computing systems, and distributed quantum sensing networks. The unique properties of polarization-entangled photons have opened up a world of possibilities in the field of quantum communication.
Quantum entanglement, a phenomenon that earned the 2022 Nobel Prize in Physics, lies at the heart of quantum networks. This peculiar quantum interaction allows particles within a quantum state to be connected in such a way that measuring the properties of one particle automatically determines the properties of another, regardless of the distance between them. In the case of the Qunnect network, infrared photons of wavelength 1,324 nanometers were entangled with near-infrared photons of 795 nm, showcasing the potential for quantum communication on a large scale.
Addressing Polarization Drift
One of the key challenges faced by quantum networks is polarization drift, which can be both wavelength and time-dependent. Qunnect’s innovative approach involved designing and building equipment for active compensation at specific wavelengths to counteract polarization drift. By generating entangled dual-colored photon pairs through the excitation of rubidium atoms in a vapor cell, the researchers were able to create a stable quantum communication system that could withstand external disturbances.
To combat the impact of disturbances on polarization, the Qunnect team developed automated polarization compensation (APC) devices that electronically corrected polarization drift in entangled photon pairs. By sending classical photon pairs down the fiber to measure polarization modifications, the researchers were able to implement corrective measures using APCs. This approach ensured that the entangled photon pairs remained stable and reliable throughout the network.
Qunnect’s groundbreaking demonstration with the GothamQ loop marked a significant step towards the development of a fully automated entanglement network essential for a quantum internet. The team’s commitment to improving stability, efficiency, and operability in quantum networks has paved the way for a future where quantum communication is not only possible but also practical on a large scale. As the team at Qunnect continues to refine their equipment and processes, the vision of a quantum internet moves closer to reality.
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