The development and implementation of quantum networks in the real world pose numerous challenges that engineers must address. One of the main obstacles is the fragility of entangled states in a fiber cable, as well as ensuring the efficiency of signal delivery. Recently, scientists at Qunnect Inc. in Brooklyn, New York, have made significant progress in this area by successfully operating a quantum network under the streets of New York City.
While previous attempts at transmitting entangled photons have been made, there has been an issue with excessive noise and polarization drift in the fiber environment. These factors have made it difficult for entanglement to survive in a long-term stable network. However, the team at Qunnect has made advancements in this area by addressing these challenges.
For their prototype network, the researchers at Qunnect utilized a 34-kilometer-long fiber circuit known as the GothamQ loop. By using polarization-entangled photons, they were able to operate the loop continuously for 15 days, achieving an impressive uptime of 99.84%. The compensation fidelity for entangled photon pairs transmitted at a rate of approximately 20,000 per second was 99%, with the fidelity still holding at nearly 90% when the transmission rate was increased to half a million entangled photon pairs per second.
Polarization plays a crucial role in quantum networks, as it refers to the direction of the electric field of a photon. Polarized photons are valuable due to their ease of creation, manipulation, and measurement. In the case of polarization-entangled photons, they have been used in various applications such as quantum repeaters, distributed quantum computing, and quantum sensing networks.
Quantum entanglement, a phenomenon that won the 2022 Nobel Prize in Physics, involves particles within a quantum state having a connection that determines the properties of others with which they are entangled. In the design implemented by Qunnect, infrared photons with a wavelength of 1,324 nanometers are entangled with near-infrared photons of 795 nm. These entangled photon pairs are crucial for building quantum memories and processors.
One of the challenges faced by Qunnect was the polarization drift, which was found to be wavelength and time-dependent. This required the development of equipment for active compensation at the same wavelengths. By generating entangled dual-colored photon pairs through a vapor cell enriched with rubidium-78, they were able to address this issue.
To address disturbances in optical cables affecting the polarization of entangled photon pairs, Qunnect developed automated polarization compensation (APC) devices. By sending classical photon pairs down the fiber to measure polarization drift, they could use APCs to correct any disturbances and ensure the stability and reliability of the quantum network.
The success of Qunnect’s GothamQ loop demonstration highlights progress towards a fully automated practical entanglement network, which is essential for the development of a quantum internet. The team’s commitment to making their equipment rack-mounted for universal use represents a significant step towards the widespread implementation of quantum networks.
The advancements made by Qunnect in overcoming the fragility of entangled states in a fiber cable and ensuring the efficiency of signal delivery are crucial for the future of quantum networks. By addressing challenges such as noise, polarization drift, and automation, they have paved the way for a more stable and reliable quantum internet.
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