Quantum Network Test: Entangling Atoms Across City Limits

The concept of a quantum internet has long been the subject of science fiction and isolated laboratory experiments. However, a massive shift has occurred recently. Researchers have successfully moved quantum entanglement out of the sterile lab environment and into the chaotic reality of city infrastructure. This breakthrough demonstrates that the un-hackable networks of the future can operate using the fiber optic cables already buried beneath our streets.

The Boston Metro Breakthrough

A team of physicists from Harvard University and Amazon Web Services (AWS) recently achieved a historic milestone in quantum networking. They successfully demonstrated quantum entanglement over a 35-kilometer (22-mile) loop of fiber optic cable in the Boston metropolitan area.

This was not a simulation. The experiment took place across existing telecommunications fiber that runs through Cambridge, Watertown, and Boston. This distinction is vital because real-world environments introduce noise, temperature changes, and vibrations that usually destroy fragile quantum states. By maintaining entanglement in this environment, the team proved that a “quantum internet” is technically feasible using current infrastructure.

The Specific Technology Used

To achieve this, the researchers did not just shoot lasers down a tube. They utilized a specific type of quantum memory node. The core of their technology relies on Silicon-Vacancy (SiV) centers in diamonds.

Here is how the setup functioned:

• The Node: A tiny defect in a diamond crystal (the SiV center) acts as a quantum bit, or qubit.
• The Connection: The team placed these diamond-based nodes at Harvard. They then sent photons (light particles) through the 35km fiber loop.
• The Result: The photons returned to the node entangled. This means the state of the light and the state of the atomic memory in the diamond were linked, despite the long journey through the city.

Why "City Limits" Makes This Difficult

Understanding why this is a big deal requires looking at the obstacles found in a metro area. In a controlled vacuum chamber, maintaining quantum coherence (the stability of the quantum state) is manageable. Under a city street, it is a nightmare.

Environmental Stressors

The fiber optic cables used in the Boston test are subjected to:

• Temperature Swings: Expansion and contraction of cables can alter the phase of light.
• Vibrations: Heavy traffic and subway systems create physical tremors that can disrupt the signal.
• Signal Loss: Photons naturally fade as they travel through glass fiber. In standard internet cables, amplifiers boost the signal. You cannot amplify a quantum signal without destroying it (a rule known as the No-Cloning Theorem).

The Harvard and AWS team overcame these issues by using the SiV diamond nodes as a “quantum repeater.” These nodes can catch, store, and release information, correcting for signal loss without breaking the quantum state.

Global Competition: New York and China

While the Boston test is a headline-grabbing success, it is part of a global race. Other major institutions are conducting similar metro-scale tests.

Stony Brook University and Brookhaven National Laboratory In New York, researchers have established the Long Island Quantum Information Network. They have successfully tested entanglement over a 100-mile (158 km) loop. Their experiments focus on making the technology portable, utilizing room-temperature atomic vapors rather than super-cooled diamonds. This suggests that future quantum modems might not need massive refrigeration units to function.

University of Science and Technology of China (USTC) China remains a leader in this sector. The USTC team in Hefei has demonstrated entanglement over distances exceeding 50 kilometers in urban environments. They utilize a different method involving cold atomic ensembles, pushing the boundaries of how much data these quantum networks can hold.

The Future Implications

The primary goal of these tests is not to make your Netflix stream faster. Quantum networks serve a completely different purpose than the classical internet we use today.

Un-hackable Communication

The most immediate application is Quantum Key Distribution (QKD). Because observing a quantum particle changes its state, any attempt to “tap” a quantum wire is immediately detected. Banks, governments, and defense contractors are investing billions into this technology to secure data against future threats.

The Quantum Internet

Eventually, these metro-scale loops will connect to form a national grid. This will allow powerful quantum computers—which are currently isolated in labs—to talk to one another. A distributed network of quantum computers could simulate complex molecular structures for drug discovery or model climate change with precision that current supercomputers cannot achieve.

Frequently Asked Questions

What is quantum entanglement?

Entanglement is a physical phenomenon where two particles link together in a certain way no matter how far apart they are in space. If you measure the state of one particle, you instantly know the state of the other. Einstein famously called this “spooky action at a distance.”

Will this replace my home Wi-Fi?

No. Quantum networks are not designed to replace the classical internet for browsing or streaming. They will likely exist as a parallel infrastructure used for high-security communication and connecting scientific computing centers.

How long until this technology is available commercially?

While the Boston and New York tests are successful, they are still prototypes. Experts estimate that functional, city-wide quantum networks for commercial banking or government use are likely 10 to 15 years away.

Why do they use diamonds in these experiments?

Diamonds have a rigid crystal lattice structure. When there is a specific defect (like a missing carbon atom replaced by silicon), it creates a stable environment for electrons to spin. This acts as a perfect “trap” to store quantum information, even when the environment around it is noisy.