Quantum Internet Prototype Runs For 15 Days Under New York City

Under the streets of New York City, they're testing a "quantum network," reports Phys.org - where engineers from a Brooklyn company named Qunnect Inc are taking steps to "overcome the fragility of entangled states in a fiber cable and ensure the efficiency of signal delivery."For their prototype network, the Qunnect researchers used a leased 34-kilometer-long fiber circuit they called the GothamQ loop. Using polarization-entangled photons, they operated the loop for 15 continuous days, achieving an uptime of 99.84% and a compensation fidelity of 99% for entangled photon pairs transmitted at a rate of about 20,000 per second. At a half-million entangled photon pairs per second, the fidelity was still nearly 90%... They sent 1,324 nm polarization-entangled photon pairs in quantum superpositions through the fiber, one state with both polarizations horizontal and the other with both vertical - a two-qubit configuration more generally known as a Bell state. In such a superposition, the quantum mechanical photon pairs are in both states at the same time. "While others have transmitted entangled photons before, there has been too much noise and polarization drift in the fiber environment for entanglement to survive," the article points out, "particularly in a long-term stable network." So the Qunnect team built "automated polarization compensation" devices to correct the polarization of the entangled pairs:In their design, an infrared photon [with a wavelength of 1,324 nanometers] is entangled with a near-infrared photon of 795 nanometers. The latter photon is compatible in wavelength and bandwidth with the rubidium atomic systems, such as are used in quantum memories and quantum processors. It was found that polarization drift was both wavelength- and time-dependent, requiring Qunnect to design and build equipment for active compensation at the same wavelengths... Qunnect's GothamQ loop demonstration was especially noteworthy for its duration, the hands-off nature of the operation time, and its uptime percentage. It showed, they wrote, "progress toward a fully automated practical entanglement network" that would be required for a quantum internet. And Qunnect's co-founder/chief science officer says "since we finished this work, we have already made all the parts rack-mounted, so they can be used everywhere..." Their network design and results are published in PRX Quantum.

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