Engineers at Northwestern University have achieved quantum teleportation using fiber optic cables already carrying internet traffic. This milestone could simplify the path to secure quantum networks by leveraging existing infrastructure.

“This is incredibly exciting because nobody thought it was possible,” said Prem Kumar, the study’s lead author and professor of electrical and computer engineering at Northwestern. Published in the journal Optica, the research shows how classical and quantum communication can coexist on the same network, paving the way for future applications like quantum computing and advanced sensing technologies.

How Quantum Teleportation Works

At the heart of quantum teleportation lies a phenomenon called entanglement, where two particles remain linked regardless of distance. By manipulating these particles, researchers can transmit information without moving physical matter. This connection is not dependent on traditional methods of information transfer, such as light or sound, but instead seems to operate outside the constraints of space and time. Albert Einstein famously referred to this phenomenon as “spooky action at a distance”.

Imagine two entangled particles created in a laboratory and then separated, with one particle sent to Location A and the other to Location B. When a specific property, such as polarization or spin, is measured on the particle at Location A, the corresponding property of the particle at Location B is instantly known. This connection occurs regardless of the physical distance between the particles, whether they are a few meters or light-years apart.

In quantum communication, this property of entanglement allows for quantum teleportation. Unlike classical communication, where information is transmitted via signals such as electrical pulses or light waves, quantum teleportation transfers the quantum state of a particle from one location to another without physically moving the particle itself. This is achieved through a protocol involving entangled particles and a classical communication channel. For instance, when a quantum state is “measured” in a particular way at one end, the information about this state can be transmitted to the other end of the entangled pair, effectively recreating the original quantum state remotely.

“In optical communications, all signals are converted to light,” Kumar explained. “While conventional signals typically comprise millions of particles of light, quantum information uses single photons.”

The process involves transferring a quantum state from one photon to another through a technique known as destructive measurement. “The photon itself does not have to be sent over long distances,” said Jordan Thomas, the study’s first author. “Its state ends up encoded onto a distant photon.”

This exchange of quantum states over vast distances could pave the way for ultra-secure, ultra-fast data sharing, potentially transforming how we connect and communicate.

The security advantage of this system lies in its inherent properties. If an outsider attempts to intercept or measure the quantum state during transmission, the act of observation disturbs the state due to the principles of quantum mechanics. This disturbance alerts the communicating parties, making quantum entanglement an ideal foundation for secure communication systems, such as quantum key distribution (QKD).

Overcoming Challenges in a Crowded Tunnel

Integrating quantum and classical communication on the same cable posed a unique challenge. Fiber optic cables are already bustling with light signals carrying conventional internet traffic. In this chaotic environment, delicate quantum signals risk being drowned out.

“It would be like a flimsy bicycle trying to navigate through a crowded tunnel of speeding heavy-duty trucks,” Kumar said.

To solve this, Kumar’s team studied how light scatters within the cables. They identified a wavelength with minimal interference and placed their photons there, reducing noise from classical communication with special filters.

The experiment involved a 30-kilometer-long fiber optic cable, with quantum information and high-speed internet traffic sent simultaneously. By executing the teleportation protocol at the midpoint, researchers confirmed successful quantum information transfer despite the heavy traffic.

“Although many groups have investigated the coexistence of quantum and classical communications in fiber, this work is the first to show quantum teleportation in this new scenario,” said Thomas.

Towards a Quantum Internet

Looking ahead, Kumar’s team aims to extend the range of their experiments, explore real-world infrastructure, and advance techniques like entanglement swapping. These steps could enable even more sophisticated quantum applications.

“Quantum teleportation has the ability to provide quantum connectivity securely between geographically distant nodes,” Kumar noted. By integrating quantum capabilities into existing internet cables, researchers could sidestep the costly need for specialized infrastructure.

This achievement signals a future where classical and quantum communications coexist seamlessly, potentially unlocking the full promise of quantum technologies for society.