By LUCAS LAURSEN and MARGO ANDERSON

For decades, researchers have tried to squeeze quantum signals alongside classical signals in fiber optic cables. Quantum bits, however, are based on delicate quantum states of individual particles, which can be disrupted by thermal noise and other factors. 

Last month, Northwestern University engineers sent a pair of entangled photons more than 30 kilometers through a fiber that was also carrying a 400 gigabits-per-second classical signal. The entangled states then enabled a quantum data transfer process called teleportation. Quantum teleportation involves transmitting the quantum state of one particle onto another particle at a distant location, effectively allowing the quantum information (a.k.a. the quantum bits or qubits) to be “teleported” across space. 

Read more: Can Qubits Teleport Through Today’s Internet Lines?

Despite the sci-fi connotation of the word teleportation, there’s nothing mystical or other-worldly about it, either. Other than very delicately shunting around extremely fragile neutral atom quantum computers or superconducting circuits, teleportation is one of the main ways quantum information can be physically moved through space. 

“There is lots of new demand for quantum computing,” says co-author Prem Kumar, professor of electrical and computer engineering at Northwestern University in Evanston, Ill. “But designers are realizing that scaling this up will be limited by internal communication.” 

In other words, as the number of qubits in quantum computers scales up, communication between all the qubits—not all of which are necessarily contained within the same physical computer or even in the same building or location—becomes increasingly critical. Therefore, transmitting qubits over existing long-distance fiber optic lines becomes important in the quest to scale up quantum computing. 

So while some research teams today work to build bigger, faster, and better quantum computers, others like Kumar’s team work to expand the range of fiber optic channels that can transmit a qubit from one place to another—which could in turn expand the kinds and levels of complexity of quantum computations that can be performed and the ability the re-use existing infrastructure to do it. 

All of which is far from simple. What is more straightforward are the modes of conveyance of qubits from one place to another. Qubits in motion are very often photons, and photons move well through glass fibers. Existing fiber optics lines—whether dark or even active already with conventional digital data traffic coursing through them—would be the simplest route to send quantum information from point A to point B.

Plus, says Northwestern PhD student and study co-author Jordan Thomas, running qubits through the same fiber optic lines as conventional bits makes a world of practical difference, too. “Coexistence makes quantum networking on a large scale a much more imaginable reality,” he says. 

In 1997, a scientist at British Telecom uncovered a key complicationin the transmission of quantum data alongside classical bits in a conventional fiber optic line. Photonic noise from the conventional data traffic bled over into the delicate quantum signals—like trying to discern a few whispered words over the background din of a bustling dinner party. The first two decades of the 21st century, in fact, saw many attempts at balancing the transmission of faint quantum whispers through the photonic cacophony of gigabits-per-second laser blasts traveling through the same fiber. 

In recent years, Kumar’s group reports in their recent paper in the journal Optica, more and better modes of teleportation of qubitsthrough dark or unused fiber have been explored in recent research reports. Their own group showed in 2023 that they could send entangled particles 48 km through conventional fiber alongside very high-power classical signals, laying the groundwork for the more complex teleportation demonstration last month. 

Is Teleporting the New Transmitting?

Kumar’s team set out to conduct quantum teleportation through conventional fiber optic lines that are also simultaneously conveying conventional many-gigabit-per-second digital communications. “It was an important time to start investigating teleportation and beyond so that this technology would be able to be deployed on a large scale, not limited to dark fibers,” Thomas says.

One step that made this possible now was the improvement in sensitivity of photodetectors. “Over the last 15 years a sort of revolution has taken place in detection technologies,” Kumar says. Their group began working with sensors whose efficiency approached 90 percent in the near-infrared O-band (representing photons in the wavelength range of 1260 to 1360 nanometers). Compare this to the 20 percent efficiency when Kumar’s group embarked on research in the field in 2006.

Combining quantum teleportation and classical communications is still relatively under-researched because of the difficulty sending a reliable signal over any distance, says telecom engineer Arka Mukherjee of the Centre for Development of Telematics in New Delhi, India. For real-world applications, he adds, it’s likely that network engineers will want to include more than just one channel of classical signals in each fiber optic cable. By contrast, he notes, today’s cables can carry dozens of channels, each carrying 100-200 Gbps of traffic in backbone fiber networks. So, he says, the importance of tamping down the noise factor through each fiber optic line will matter even more.

A real-world fiber optic network that conveys qubits from point A to point B will face other kinds of challenges, too. For instance, quantum communications demands precise levels of synchronization for timing precision and entanglement verification. To grapple with this, engineers are developing time protocols and other synchronization methods to allow for peaceful coexistence of classical and quantum signals in the same line.

“Our research group has performed some work on this subject as well,” Thomas says. “So we have some experience with these sync systems and coexisting them in the same fiber, which we will employ in next iterations of the experiments.”

The Kumar demonstration “has the potential to help address these challenges by using a classical signal as a probe signal to compensate for temporal and polarization drift between quantum network nodes,” says physicist Anouar Rahmouni at the National Institute of Standards and Technology in Gathersberg, Md. Kumar’s team’s work is, Rahmouni says, a “pivotal step” towards quantum networking.