“Our equipment was running alongside the fibers that we use for regular communication literally buried underneath the roads and train stations,” said Mirko Pittaluga, a physicist and lead author of the study. Pittaluga and his colleagues at Toshiba Europe sent quantum information from regular computers hooked into the telecommunications network at data centers in the German cities of Kehl and Frankfurt, relaying them through a detector at a third data center roughly midway between them in Kirchfeld. The three-location setup enabled the group to extend the distance the messages were sent more than 150 miles, an uninterrupted distance only ever achieved in a laboratory environment.

Pittaluga said that his team’s work is critical to solving the problem of keeping sensitive data out of the reach of hackers.  One means of fixing this problem, Pittaluga said, is through quantum cryptography, which relies on the physics of quantum mechanics rather than mathematical algorithms to generate encryption keys. But to use quantum encryption keys, you have to successfully distribute them across meaningful distances, a task that has stymied researchers outside the lab for decades.

Quantum data was sent over an ordinary telecom network with fiber-optic cables.© julie sebadelha/Agence France-Presse/Getty Images

Integrating the technology into existing infrastructure using largely off-the-shelf equipment is a key step in expanding the accessibility of quantum communication and its use in encrypting information for more secure transmission of data, according to multiple physicists and engineers who weren’t involved in the study.

“This is about as real-world as one could imagine,” said David Awschalom, a professor of physics and molecular engineering at the University of Chicago who wasn’t a part of the new work. “It’s an impressive, quite beautiful demonstration.”  Working at these types of distances, Awschalom said, means that quantum information could be sent across entire metropolitan areas or between nearby cities, making it useful for hospitals, banks and other institutions, for which secure communications are paramount.

“The likelihood of them being able to reverse engineer a quantum key, which is the number you would need to decrypt your information, is vanishingly small,” according to Awschalom.

Other groups in the U.K. and U.S., including researchers at the University of Pennsylvania, are also working on extending the distances achievable by quantum communication.

Today, bank statements, health records and other data transmitted online are protected using mathematically formulated encryption keys. These keys are the only means of unlocking the data, keeping it secure from cyber thieves. For conventional computers, breaking these keys takes an impractically long time, but quantum computers are up to the task, and as they become more powerful, encryption keys become vulnerable to attack.

“Anything meaningful that’s over the internet can be tapped, recorded and saved for the next decade, and can be decrypted years later,” according to Prem Kumar, a professor of electrical and computer engineering at Northwestern University, who wasn’t a part of the new work. “It’s what’s called harvest now and decrypt later.”

Internet and telecommunications infrastructure are based on optical fibers all over the world that carry pulses of light containing photons. Classical bits of information are sent as a single impulse of light carrying tens of millions of photons.  Quantum information, stored in qubits, is sent in a package of a single photon.

Efficiently detecting single photons usually requires expensive superconducting detectors that cost on the order of hundreds of thousands of dollars. These high-efficiency sensors must be cryogenically cooled, using liquid helium, to super low temperatures below minus 454 degrees Fahrenheit, making the technology expensive and incompatible with existing infrastructure.

Pittaluga and his colleagues at Toshiba got around this by using cheaper detectors known as avalanche photodiodes, which cost just thousands of dollars and can run at or just below room temperature, like today’s traditional internet equipment.

Such detectors hadn’t been used for coherent quantum communication before, as they can be nearly an order of magnitude less efficient at detecting single photons and are affected by what is called the afterpulse effect—when the current detection is frustrated by leftover echoes from an earlier transmission. Superconducting detectors aren’t affected by afterpulsing, Pittaluga said.

To address the effect in the more practical and cost-effective photodiodes, his group employed two separate sets of the detectors, using one to read the signal and the other to remove the environmental noise from that signal.  The goal of this setup is to bring us one step closer to a quantum internet, with incredibly secure information, Pittaluga added.

Yet despite this innovation, the technology remains expensive and difficult to implement compared with current encryption systems and networks—for now. “My personal view is that we’ll be seeing quantum encryption of data sets and metropolitan-scale quantum networks within a decade,” Awschalom added.

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Why quantum computers are faster at solving problems:

Quantum computers are faster than traditional computers for optimization problems, such as finding the more efficient options for supply chains.