If scientists can develop networks based on quantum physics, the potential is huge. It could improve communication security, unite the capabilities of quantum computers, and help create next-generation research instruments.
But a key challenge lies in a quantum phenomenon called entanglement: the way that two objects can be inextricably linked no matter the distance between them.
When one of the paired objects is measured, it determines the results of the measurement made on the other.
So far so quantum, but getting entangled particles across our existing fiber-optic communication channels is really difficult.
Outside of perfect lab conditions, their quantum states are easily distorted.
Now, researchers led by a team at the US National Institute of Standards and Technology (NIST) have managed to successfully transmit particles of light (photons) across 62 kilometers (38.5 miles) of existing network cabling without breaking their entanglement.
Their findings are published in the Journal of Optical Communications and Networking.
"I would call this a stress test of quantum networking systems," says physicist Yicheng Shi, from NIST.
"We put this to an extreme test in an environment that's really noisy. Amazingly, it turned out it still worked."
While this isn't strictly speaking a record distance, it's notable because of the setup: Much of the fiber-optic cabling involved in this experiment was above ground rather than below ground, strung between street-side poles between NIST and the University of Maryland.
These cables are exposed to wind, changes in temperature, and vibrations from everything around them in the environment, including traffic and passing birds.
While the streaming movies and video calls that make up the data of the classical internet aren't affected by these conditions, quantum data definitely is. The environment interferes with the polarization of photons, the direction along which their electric field vibrates, and in which the entanglement information is contained.
"It's about as bad a connection as you can possibly have," says physicist Oliver Slattery, from NIST.
To get around this interference problem, the researchers used a laser light signal as a reference point. By calculating how much twisting this laser light went through, the team could apply the relevant corrections to the entangled photons.
Some alternation between entangled photons and laser light was required, but this only took up 7.2 percent of the operation time in the experiment. The other 92.8 percent could be used to send stabilized quantum information.
"Our results demonstrate the feasibility of distributing polarization-entangled photons over challenging fiber conditions, which is an important step toward the practical deployment of quantum networks," write the researchers in their published paper.
Record-breaking feats of quantum entanglement recently traversed 420 kilometers of optical fiber, but these latest results are from very chaotic real-world settings.
"Although aerial-based optical fibers are typically considered unsuitable quantum channels for transmitting polarization-encoded or polarization-entangled photons, we show that a stable distribution of polarization entanglement is achievable if the fiber link is actively stabilized," the researchers write.
As encouraging as the results of this real-world test are, there's still plenty of work to do. The transmission rate of 200-1,500 entangled photons per second isn't bad, but will need to be improved upon for a quantum internet.
A faster hardware setup and more efficient software algorithms are both potential upgrades the researchers noted.
The study provides more evidence that quantum networks are a technology worth pursuing, though. One way it could be used is to link telescopes at two separate points on Earth, with entangled photons enabling them to essentially work as one giant instrument.
There's also the quantum security upgrade these systems could give the internet. Network nodes large distances apart could share a secret 'code' for data verification that couldn't be intercepted in transit – because the code isn't set until it's checked.
Related: The First Room-Temperature Quantum Material of Its Kind Is Spun From Atoms of Gold
"It's a demonstration that quantum networking protocols can work in real-world environments," says Shi.
The research has been published in the Journal of Optical Communications and Networking.
This article was fact-checked by Clare Watson and edited by Clare Watson. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.