Test of a prototype of a quantum internet runs for half a month under New York City

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

While others have succeeded in transmitting entangled photons, there was too much noise and polarization drift in the fiber optic environment for entanglement to survive, especially in a long-term stable network.

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

For their prototype network, the Qunnect researchers used a rented 34-kilometer fiber optic circuit they called the GothamQ Loop. Using polarization-entangled photons, they ran the loop continuously for 15 days, achieving 99.84% availability and 99% compensation accuracy for entangled photon pairs transmitted at a transmission rate of about 20,000 per second. At half a million entangled photon pairs per second, the accuracy was still close to 90%.

The polarization of a photon is the direction of its electric field. (This may be easier to understand by looking at the waveform of light.) You may be familiar with the phenomenon from polarized sunglasses. These are filters that allow light of one polarization direction to pass through but block others, reducing glare from water, snow and glass, for example.

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

Polarization entangled photons have been used in recent years to build large quantum repeaters, distributed quantum computers, and distributed quantum sensor networks.

Quantum entanglement, the subject of the 2022 Nobel Prize in Physics, is a special quantum phenomenon in which particles within a quantum state have a connection, sometimes over great distances, so that measuring the property of one particle can automatically determine the properties of other particles 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 the rubidium atom systems used in quantum memories and quantum processors. The polarization drift was found to be both wavelength and time dependent, so Qunnect had to design and build devices to actively compensate at the same wavelengths.

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

From this doubly excited state, a photon at 1,324 nm was sometimes emitted, and a subsequent electron decay produced another photon at 795 nm.

They sent 1,324 nm polarization-entangled photon pairs through the fiber in quantum superpositions, one state with horizontal polarization and one with vertical polarization—a two-qubit configuration commonly known as a Bell state. In such a superposition, the quantum mechanical photon pairs are in both states simultaneously.

However, in optical cables, such photonic systems are more susceptible to disturbances in their polarization caused by vibrations, bends, and pressure and temperature variations in the cable, and may require frequent recalibration. Since it is almost impossible to detect and isolate this type of disturbance, let alone mitigate it, the Qunnect team has developed automatic polarization compensation (APC) devices to compensate for it electronically.

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

Qunnect’s GothamQ Loop demonstration was particularly notable for its duration, unattended uptime and percent availability. It shows, they wrote, “progress toward a fully automated practical entanglement network” that would be needed for a quantum internet. Namazi said that since completing this work, “we have already assembled all the parts in racks so they can be used anywhere” – a combined equipment they call Qu-Val.

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