Quantum internet prototype to be tested in New York City underground for half a month

To bring quantum networks to market, engineers need to overcome the fragility of quantum entanglement in fiber optic cables to ensure efficient signal transmission. Now, scientists at Qunnect Inc. in Brooklyn, New York, have made a major step forward by operating just such a network underground in New York City.

Although there have been previous examples of transmitting entangled photons, the noise and polarization drift in the fiber environment are too large to maintain entanglement, especially in long-term stable networks.

“This is where our research comes in,” said Mehdi Namazi, co-founder and chief scientific officer at Knecht. The team’s network design, methods and results: PRX Quantum.

For their prototype network, Qunnect researchers used a 34-kilometer dedicated fiber line, called the GothamQ Loop. Using polarization-entangled photons, they operated the loop continuously for 15 days, achieving 99.84% uptime and 99% compensation fidelity for entangled photon pairs transmitted at a rate of about 20,000 per second. Even at 500,000 entangled photon pairs per second, the fidelity remained close to 90%.

The polarization of a photon is the direction of its electric field. (It may be easier to understand if you think of it in terms of a light wave diagram.) You’re probably familiar with the phenomenon of polarized sunglasses: they are filters that allow light from one polarization direction through and block light from other polarization directions, reducing light reflections from water, snow, and glass, for example.

Polarized photons are useful because they are easy to generate, manipulate (using polarizing filters) and measure.

In recent years, polarization-entangled photons have been used to build large-scale quantum repeaters, distributed quantum computing, and distributed quantum sensing networks.

Quantum entanglement, the subject of the 2022 Nobel Prize in Physics, is an unusual quantum phenomenon in which particles in a quantum state have connections, sometimes long-range connections, such that measuring the properties of one particle automatically determines the properties of the other particles it is entangled with.

In their design, infrared photons with a wavelength of 1,324 nanometers are entangled with near-infrared photons with a wavelength of 795 nanometers, the latter photons being wavelength and bandwidth compatible with the rubidium atomic system used in quantum memories and quantum processors. Because the polarization drift turned out to be both wavelength and time dependent, Qunnect had to design and build an instrument that provides active correction at the same wavelength.

To generate these entangled two-color photon pairs, combined input beams of specific wavelengths are sent into a vapor cell enriched in rubidium-78, exciting the rubidium atoms in the cell so that their outer electrons make two transitions from the 5p orbital to the 6s orbital.

From this doubly excited state, a photon at 1,324 nm is occasionally emitted, and subsequent electronic decay produces another photon at 795 nm.

They sent pairs of 1,324 nm polarization-entangled photons down the fiber in quantum superposition states: one state with both polarizations horizontal, and the other state with both polarizations vertical. This is a two-qubit configuration commonly known as a Bell state. In such a superposition, the quantum mechanical photon pair is in both states simultaneously.

But in optical cables, such photonic systems are prone to polarization disruptions caused by vibrations, bending, pressure and temperature fluctuations within the cable, which can require frequent realignment. Because these types of disruptions are nearly impossible to detect or isolate, let alone mitigate, the Qunnect team built an Automatic Polarization Compensation (APC) device to electronically correct them.

By sending 1,324 nm classical, unentangled photon pairs with known polarization down the optical fiber, they were able to measure how much the polarization drifted, or was corrected. Polarization drift was measured over four transmission distances — 0, 34, 69, and 102 km — by sending classical photons 0, 1, 2, or 3 times down the Metropolitan Loop under the streets of Brooklyn and Queens. They then used APC to correct the polarization of the entangled pairs.

Qunnect’s GothamQ Loop demo is particularly notable for its duration, unattended operation time, and uptime. They write that the demo showed “progress toward a fully automated, practical entanglement network” necessary for a quantum internet. Namazi says, “Since completing this work, we have made all the components rack-mountable so they can be used anywhere.” The combined instrument is what they call Qu-Val.

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