Quantum transmission under the streets of New York – a long-term experiment transmits entangled photons through the city’s fiber optic network for 15 days for the first time

Quantum Post Underground: Quantum Broadcast has passed its first long-term test in New York City. For 15 days, applied photons were sent over a 34-kilometer long telecommunications line under the city – without intermediate calibration or other human assistance. With success: despite vibrations and other disruptive effects typical of cities, the quantum shift achieved a reliability of 99 percent, as physicists report. But how did they do that?

Quantum communication is considered particularly secure and promising. The information is transmitted using photons that physically interact quantumly – instantaneously, unhindered and over great distances. Physicists already have such quantum information in pilot experiments urban fiber optics, via submarine cable​​​​​​through the air and even via satellite drawn.

The transmission of quantum information is prone to interference – especially in fiber optic networks in large cities. © metamorworks/Getty Images

Is quantum booster suitable for daily use?

However, these quantum motions were mostly feasibility tests with very few photons and only a short time. “But for quantum networks that can be used in practice, we need to apply a polarization that is stable over a long period of time and has a high yield rate, high reliability and a long network update,” explained Alexander Craddock and his colleagues from the New York Times Qunec start York. In this context, uptime means uptime excluding downtime for re-calibration and other work.

The problem: “The sensitive nature of the photons involved so far has limited their long-term use in the existing fiber optic infrastructure,” explained the physicist. Especially in cities, any vibration or other movement underground can disturb the photons and cause their quantum state to collapse. Furthermore, the encoding of the quantum information in the polarization of the light particles can be twisted and changed by such disruptive effects.

The involved photons traveled through a 34-kilometer-long fiber optic link under the streets of New York. © Qunect

“GothamQ” – a test track for involved photons

Craddock and his team have now solved these problems and put their system to the bitter test. As a test route – aptly named “GothamQ” – they chose a 34-kilometer section of the telecommunications network in New York City. They would feed photons of the infrared wave of 1,324 nanometers, which is common for optical telecommunications, into this fiber optic cable. The information to be carried was encoded in the polarization of these light particles.

“The transmitted telecommunication photon is involved in a second photon that has a wavelength of 795 nanometers and is measured locally,” explained the physicist. Such near-infrared photons are particularly compatible with atom-based quantum systems such as quantum memories, quantum computers or quantum sensors – so these photons are the bridge between the stationary devices and the trans send

Automatic calibration

The crucial thing, however, is that Craddock and his team used a special technology to compensate for the disruptive effects in the optic fiber. They first sent independent photons of the same polarization through the fiber optic cable and studied how their direction of oscillation changed depending on the path, bandwidth and time. This showed that the shift in polarization depends on time, but also on wavelength.

The team then used this information to take these disruptive effects into account when decoding and reading the photons involved. For this purpose, automatic compensators were used at each end of the transmission path, which independently read the values ​​of the unrelated control photons. These were sent through the optic fiber every four minutes during the 15-day quantum transmission. However, there was no rebalancing or other human intervention during the two weeks of the experiment.

Unlike most pilot experiments, Craddock and his team used a very high data density: they sent between 20,000 and 500,000 photons through the line every second.

Up to 99 percent reliable

The result: Although the quantum transmission under the streets of New York was almost completely automated for two weeks, it was robust and reliable. After the 34 kilometers in the fiber optic cable, almost all of the photons that were involved arrived in a state that was easy to read. At the highest photon density of 500,000 per second, about 90 percent of the quantum information was emitted, and at 20,000 photons it was even 99 percent, as the team reports.

“This shows that the system can operate for 15 days at high performance and without any user support,” said Craddock and his colleagues. Usable network time during the test was 99.84 percent. “This experiment shows that it is possible to work strongly, around the clock for the transmission of information involved – and therefore to use it practically,” said the physicist. They see this as an important step towards the widespread use and commercialization of quantum transmission.

Quantum physicist Michal Hajdušek from Keio University in Japan, who was not involved in the experiment, sees it the same way: “This work represents an important step towards realizing quantum networks in the real world,” he told the American Physical Society. (PRX Quantum, 2024; doi: 10.1103/PRXQuantum.5.030330)

Quelle: PRX Quantum, American Physical Society (APS)

14 August 2024 – Nadja Podbregar