The integration of quantum networks into the market presents a unique set of challenges that engineers must address to ensure success. One of the key obstacles to overcome is the fragility of entangled states within a fiber cable, as well as the need to maintain the efficiency of signal delivery. Researchers at Qunnect Inc., based in Brooklyn, New York, have made significant strides in addressing these challenges by successfully operating a quantum network beneath the bustling streets of New York City.
While previous attempts have been made to transmit entangled photons, the presence of noise and polarization drift in fiber environments has hindered the successful establishment of long-term stable networks. Qunnect’s team, led by co-founder and chief science officer Mehdi Namazi, has made significant advancements in this area. Their innovative network design, methods, and results have been detailed in the publication PRX Quantum.
Prototype Network Success
The Qunnect researchers utilized a 34-kilometer fiber circuit known as the GothamQ loop for their prototype network. Through the use of polarization-entangled photons, they were able to sustain operations for a continuous period of 15 days. This resulted in an impressive uptime of 99.84%, with a compensation fidelity of 99% for transmitted entangled photon pairs at a rate of approximately 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%.
Polarization plays a crucial role in the functioning of quantum networks. The direction of a photon’s electric field, known as its polarization, is essential for creating, manipulating, and measuring photons. By utilizing polarization-entangled photons, researchers have been able to develop advanced technologies such as quantum repeaters, distributed quantum computing, and quantum sensing networks.
The phenomenon of quantum entanglement, which was recognized with the 2022 Nobel Prize in Physics, is central to the functioning of quantum networks. In Qunnect’s design, infrared photons at a wavelength of 1,324 nanometers are entangled with near-infrared photons at 795 nm. This compatibility allows for the seamless integration of these photon pairs with rubidium atomic systems, commonly used in quantum memories and processors.
Polarization drift in quantum networks is a significant concern, as it can impact the stability and performance of the network. Qunnect’s research revealed that polarization drift is both wavelength and time-dependent, necessitating the development of active compensation equipment to address these issues. By generating entangled photon pairs through specific processes, they were able to send these pairs through the fiber while actively compensating for polarization drift.
Automated Polarization Compensation
To mitigate the impact of disturbances on polarization within optical cables, the Qunnect team developed automated polarization compensation (APC) devices. These devices electronically adjust for polarization drift by sending classical photon pairs with known polarizations down the fiber and then utilizing the APCs to correct the polarization of the entangled pairs. This approach enhances the stability and reliability of quantum networks.
Advancements Towards a Quantum Internet
Qunnect’s successful demonstration of the GothamQ loop network represents a significant step towards the realization of a practical entanglement network essential for a quantum internet. The team’s commitment to automation and optimization, as evidenced by their Qu-Val equipment, highlights their dedication to making quantum networks accessible and viable on a larger scale.
The challenges of fragility and efficiency in quantum networks are being effectively addressed through innovative research and technological advancements. The work of the Qunnect team exemplifies the progress being made towards the development of robust and reliable quantum networks that will pave the way for future quantum communication systems.
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