Building Quantum Networks on Existing Fiber Infrastructure

Image AI Generated

I’ve written here about quantum networks and communications. These systems that connect quantum computers and devices using quantum entanglement hold enormous promise for secure communication and distributed computing. But getting them to work over real-world distances has proven challenging.

The problem comes down to wavelengths. Most quantum systems today use visible or ultraviolet light to create entanglement between atoms. But here's the catch: when you try to send these signals through fiber optic cables over long distances, they degrade rapidly. The sweet spot for fiber optics is the "telecom band" - wavelengths ranging from about 1,260 to 1,675 nanometers or nm (infrared light), with the most efficient transmission around 1,310 nm and 1,550 nm. At these wavelengths, signals can travel hundreds of kilometers with minimal loss.

Converting quantum signals from visible light to telecom wavelengths sounds like an easy fix, but it's not. The conversion process reduces efficiency and introduces errors that corrupt the delicate quantum states you're trying to preserve.

Researchers led by Prof. Jacob P. Covey at the University of Illinois have identified a solution described here: A new scalable approach to realize a quantum communication network based on ytterbium-171 atoms The solution uses ytterbium-171 atoms that naturally emit light at 1,389 nanometers—already in the telecom band. No conversion needed. It's like building a device that speaks the right language from the start.

Ytterbium-171 was chosen strategically. This isotope is already used in ultra-precise atomic clocks because it has an extremely stable internal state. The researchers realized they could exploit this stability for quantum networking while taking advantage of its telecom-compatible light emission.

What makes this work particularly significant is the team's approach to scaling. Instead of just connecting one atom at a time, they created an array of multiple ytterbium-171 atoms held in place by focused laser beams (called optical tweezers). They then aligned this array with standard fiber optic cables - similar to how you might plug multiple ethernet cables into a router. This parallelization means multiple quantum connections can be established simultaneously, like having multiple lanes on a highway instead of a single narrow road. The team demonstrated that all channels maintained high-quality entanglement with virtually no interference between neighboring connections - a critical requirement for practical networks.

The researchers used something called "time-bin encoding" to package their quantum information. Rather than encoding data in properties like light polarization (which can get scrambled in fiber), they encode it in the precise timing of when photons arrive. Think of it as Morse code at the quantum level - the message is in the timing pattern rather than the brightness or color.

One innovation that makes this practical is their "mid-circuit networking protocol." In quantum computing, one of the biggest challenges is that quantum states are fragile—they degrade quickly. This protocol allows the system to establish network connections while keeping other quantum data intact, like being able to download files on your computer without closing all your other programs.

The team demonstrated their system can:

·       Create high-quality entanglement between atoms and photons consistently across all channels

·       Maintain quantum connections after sending photons through 40 meters of fiber optic cable

·       Achieve entanglement fidelity approaching 99% with planned improvements

·       Operate multiple channels simultaneously without crosstalk

The researchers are already designing a second-generation system that will use optical cavities (essentially mirrors that bounce photons back and forth) to dramatically improve collection efficiency. This could increase networking rates by orders of magnitude.

The long-term vision is creating networks where quantum processors at different locations can share entanglement - enabling distributed quantum computing, synchronized arrays of atomic clocks for precision sensing, and fundamentally secure communication channels.

This work shows that quantum networks can be built using existing fiber optic infrastructure while maintaining the high fidelities needed for practical quantum applications. By combining telecom-compatible atoms with scalable parallel architecture, the team has created a roadmap for the quantum networks of the future.