Underground quantum mail: Quantum transmission has successfully passed its first long-term test in New York. For 15 days, entangled photons were sent over a 34-kilometer-long telecommunications line under the city – without intermediate calibration or other human assistance. Successfully: Despite vibrations and other disruptive effects typical of cities, the quantum transfer achieved a reliability of 99 percent, as physicists report. But how did they do it?
Quantum communication is considered particularly secure and promising. Information is transmitted using photons entangled in a quantum physical manner – instantly, tamper-proof and over large distances. Physicists already have such quantum information in pilot tests urban optical fiber, by submarine cablein the air and even by satellite transmitted.
Is Quantum Superiority Suitable for Everyday Use?
However, these quantum transfers were mostly feasibility tests with few photons and short durations. “But for quantum networks to be usable in practice, we need polarization entanglement that is stable over a long period of time and has high throughput, high reliability and long network availability,” Alexander Craddock and his colleagues explain to the New York Times startup Qunnect. In this context, availability means availability without downtime for recalibration and other work.
The problem: “The sensitive nature of entangled photons has so far limited their long-term use in existing fiber optic infrastructure,” the physicists explain. Especially in large cities, any vibration or other movement of the subsurface can disrupt the photons and cause their quantum state to collapse. In addition, the encoding of quantum information in the polarization of light particles can also be distorted and changed by such disruptive effects.
“GothamQ” – Test Track for Entangled Photons
Craddock and his team have now found a solution to these problems and have put their system to a decisive test. As a test route – aptly named “GothamQ” – they chose a 34-kilometer segment of New York City’s telecommunications network. They fed photons with an infrared wavelength of 1,324 nanometers, common for optical telecommunications, into this fiber-optic cable. The information to be transported was encoded in the polarization of these light particles.
“The photon transmitted by telecommunications is entangled with a second photon that has a wavelength of 795 nanometers and is measured locally,” the physicists explain. These near-infrared photons are particularly well compatible with common quantum systems based on atoms such as quantum memories, quantum computers or quantum sensors – these photons thus form the bridge between stationary devices and transmission.
Automatic calibration
The key, however, is that Craddock and his team used special technology to compensate for the disruptive effects of the optical fiber. They first sent unentangled photons of the same polarization through the fiber optic cable and checked how their direction of oscillation changed depending on the route, bandwidth, and time. This showed that the polarization shift depends on time, but also on wavelength.
The team then used this information to account for these disruptive effects when encoding and reading out the entangled photons. To this end, automated compensators were used at both ends of the transmission path, which independently read out the values of the unentangled control photons. These were sent via optical fiber every four minutes during the 15-day quantum transmission. However, there was no recalibration or other human intervention during the two-week test.
Unlike most pilot experiments, Craddock and his team also used a very high data density: they sent between 20,000 and 500,000 entangled photons through the line per second.
Reliable up to 99%
The result: Even though the quantum transmission on the streets of New York was almost completely automatic for two weeks, it proved to be robust and reliable. After the 34 kilometers traveled through the fiber optic cable, almost all of the entangled photons arrived in a readable state. At the highest photon density of 500,000 photons per second, about 90 percent of the quantum information was transmitted, and at 20,000 photons it was even 99 percent, as the team reports.
“This shows that the system can operate for 15 days with high performance and without any user intervention,” say Craddock and his colleagues. The usable network time during the test was 99.84 percent. “This test demonstrates that robust, 24-hour operation for the transmission of entangled information is possible – and therefore practically usable,” the physicists say. They see this as an important step towards the large-scale application and commercialization of quantum transmission.
Quantum physicist Michal Hajdušek of Keio University in Japan, who was not involved in the test, sees it similarly: “This work represents an important step toward realizing quantum networks in the real world,” he told the American Physical Society. (PRX Quantum, 2024; is that i: 10.1103/PRXQuantum.5.030330)
Source: PRX Quantum, American Physical Society (APS)
August 14, 2024 – Nadja Podbregar