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

Although others have sent entangled photons before, there is too much noise and polarization drift in the fiber environment for entanglement to survive, especially in a network that is stable over the long term.

“This is where our work comes into play,” said Mehdi Namazi, co-founder and chief science officer at Qunnect. The team’s network design, methods and results are published in PRX Quantum.

For their prototype network, the Qunnect researchers used a leased 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% uptime and 99% compensation fidelity for entangled photon pairs sent at a rate of about 20,000 per second. At half a million entangled photon pairs per second, the fidelity was still nearly 90%.

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

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

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

Quantum entanglement, the subject of the 2022 Nobel Prize in Physics, is the curious quantum phenomenon in which particles within a quantum state are connected, sometimes over large distances, so that measuring the property of one particle automatically determines the properties of the other particles with which it is entangled.

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

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

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

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

In optical cables, however, such photon systems are more sensitive to perturbations of their polarization by vibration, bending, and fluctuations in pressure and temperature in the cable, and can require frequent recalibrations. Because such perturbations are nearly impossible to detect and isolate, let alone mitigate, the Qunnect team built automated polarization compensation (APC) devices to compensate for them electronically.

By sending classical, unentangled, 1,324 nm photon pairs with known polarizations through the fiber, they could 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, 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, the hands-off nature of its operational time, and its uptime percentage. It demonstrated, they wrote, “progress toward a fully automated practical entanglement network” that would be needed for a quantum internet. Namazi said that “since we completed this work, we have already mounted all of the components in a rack so that they can be deployed anywhere”—a combined device they call Qu-Val.

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