Quantum Internet prototype tested under New York for half a month

To bring quantum networks to market, engineers must overcome the fragility of entangled states in a fiber optic cable and ensure efficient signal transmission. Now, scientists at Qunnect Inc. in Brooklyn, New York, have taken a major step forward by operating such a network beneath the streets of New York City.

Although others have already transmitted entangled photons, there has been too much noise and polarization drift in the fiber environment for entanglement to survive, especially in a long-term stable network.

“That’s where our work comes in,” said Mehdi Namazi, co-founder and chief scientific officer of Qunnect. The team’s network design, methods and results are published in PRX Quantum.

For their prototype network, the Qunnect researchers used a 34-kilometer-long leased fiber optic circuit that they called the GothamQ loop. Using polarized photons, they ran the loop for 15 consecutive days, achieving 99.84% uptime and 99% compensation fidelity for entangled photon pairs transmitted at a rate of about 20,000 per second. At half a million entangled photon pairs per second, fidelity was still close to 90%.

The polarization of a photon corresponds to the direction of its electric field. (This may be easier to understand in the wave picture of light.) You may be familiar with the phenomenon of polarized sunglasses, which are filters that let through light from one polarization direction but block others, reducing glare reflected off water, snow, and glass, for example.

Polarized photons are useful because they are easy to create, simple to manipulate (with polarized filters), and simple to measure.

Polarization-entangled photons have been used in recent years to build large-scale quantum repeaters, distributed quantum computations, and distributed quantum sensing networks.

Quantum entanglement, the subject of the 2022 Nobel Prize in Physics, is the special quantum phenomenon in which particles in a quantum state have a connection, sometimes over long distances, such that measuring the property of one automatically determines the properties of the others with which it is entangled.

In their design, an infrared photon with a wavelength of 1,324 nanometers is entangled with a near-infrared photon of 795 nm. The latter photon is compatible in wavelength and bandwidth with rubidium atomic systems, such as those used in quantum memories and processors. The polarization drift was found to be both wavelength and time dependent, requiring Qunnect to design and build active compensation equipment at the same wavelengths.

To generate these entangled two-color photon pairs, coupled input beams of certain wavelengths were sent through a rubidium-78-enriched vapor cell, where they excited the rubidium atoms inside the cell, causing an outer electron to transition twice, through a 5p orbital to a 6s orbital.

From this doubly excited state, a 1324 nm photon was sometimes emitted, and subsequent electron decay produced another 795 nm photon.

They sent pairs of polarized 1,324-nm photons in quantum superpositions through the fiber, one state with both horizontal polarizations and the other with both vertical polarizations—a two-qubit configuration more commonly known as a Bell state. In such a superposition, the quantum-mechanical photon pairs are in both states at the same time.

However, in optical cables, these photonic systems are more susceptible to disturbances in their polarization caused by vibrations, bends, and pressure and temperature fluctuations in the cable and may require frequent recalibrations. Since these types of disturbances can be nearly impossible to detect and isolate, let alone mitigate, the Qunnect team built automated polarization compensation (APC) devices to electronically compensate for them.

By sending pairs of 1324-nm, unentangled classical photons with known polarizations down the fiber, they were able to measure how much their polarization drifted or was changed. Polarization drift was measured at four transmission distances: zero, 34, 69, and 102 km, by sending the classical photons zero, one, two, and three times around the Metropolitan Loop under the streets of Brooklyn and Queens. They then used APCs to correct the polarization of the entangled pairs.

Qunnect’s GothamQ loop demonstration was particularly notable for its length, hands-off nature of its uptime, and its uptime percentage. It showed, they wrote, “progress toward a practical, fully automated entanglement network” that would be needed for a quantum internet. Namazi said that “since we completed this work, we have already racked all the components, so they can be used anywhere”—a combined device they call Qu-Val.

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