Yichi Zhang, a doctoral student in Materials Science and Engineering, inspects the source of the quantum signal. (Credit: Sylvia Zhang)
In A Nutshell
- University of Pennsylvania researchers built a chip that lets quantum data “ride along” existing fiber optic cables with classical signals steering the way.
- In campus tests, quantum information stayed intact across real-world fiber links, with routing precision up to 97%.
- A built-in error correction system uses classical light as a stand-in to detect and cancel disturbances every 100 milliseconds.
- While current speeds are modest (10 kbps), switching materials could raise rates to gigahertz levels, making large-scale quantum networks feasible.
PHILADELPHIA — The quantum internet has long felt like science fiction: a future where quantum computers share information instantly and securely across vast distances. The catch? Quantum information is so fragile that it typically collapses when transmitted through ordinary networks. Now, researchers at the University of Pennsylvania have taken a big step toward solving this problem with a clever hybrid approach: let ordinary light signals handle the steering while quantum data tags along as a protected passenger.
Published in Science, the team’s work demonstrates what they call a “classical-decisive quantum internet.” Essentially, it’s a system where traditional internet-style light signals act as managers, ensuring that quantum information knows where to go and how to remain intact as it travels through existing fiber optic cables. Instead of building a whole new quantum-only infrastructure, the researchers showed that quantum information can “piggyback” on the networks already beneath our feet.
It’s sort of like sending fragile package through the mail with an armored escort. The quantum data itself isn’t touched, but the classical signals carry the address labels, direct traffic at every junction, and even warn of bumps in the road.
Part of the equipment used to create a node of the quantum network, roughly one kilometer’s worth of Verizon commercial fiber optic cable away from its source. (Credit: Sylvia Zhang)
Why Classical Signals Make the Difference
Quantum information is unlike anything in our everyday lives. Unlike regular computer bits, which are either 0 or 1, quantum bits (or qubits) can be 0 and 1 at the same time. That’s what makes quantum computers so powerful, and also so delicate. The moment you try to directly measure quantum data, it collapses.
Past efforts to build quantum networks relied on rigid, pre-planned routes and centralized controllers. These setups are too clunky for the real world. The Penn team flipped that model by designing “hybrid packets” that bundle classical headers (like addresses and instructions) with a quantum payload (the actual qubits), similar to the way today’s internet packets carry both metadata and data.
At the heart of the system is a custom silicon nitride chip. A laser at 1550.92 nanometers enters the chip and gets split: one path creates classical header signals, while the other generates pairs of entangled photons (the quantum data) at 1547.72 and 1554.13 nanometers. By keeping these on separate wavelengths, the classical and quantum signals can travel together without interfering.
Campus Test Shows It Works in the Real World
To see if the idea could handle real-world conditions, the researchers tested their system across the University of Pennsylvania’s campus. Underground fiber cables connected two buildings about a kilometer apart, with multiple switching racks along the way. The mimicked the noise, temperature shifts, and vibrations of everyday networks.
The system successfully routed quantum information through setups ranging from simple two-destination paths to multi-router configurations with six possible routes. Depending on the network’s complexity, routing accuracy ranged from 91.6% to 97.1%. Even more importantly, the fragile quantum entanglement survived the journey. The initial states exhibited near-perfect fidelity (98.3%) and purity (97.9%). After transmission with error correction running, entanglement fidelity stayed high at 97.0%. Without error correction, fidelity fell to about 59%.
Yichi Zhang, a doctoral student in Materials Science and Engineering, with the equipment used to generate and send the quantum signal over Verizon fiber optic cables. (Credit: Sylvia Zhang)
Real-Time Error Correction Without Touching Quantum States
One of the trickiest aspects of quantum networking is correcting errors without compromising the data in the process. The Penn team’s solution was simple but powerful: use classical light as a stand-in. Because both classical and quantum signals travel through the same fiber, they experience the same disturbances from heat, vibrations, or bends in the cable. By monitoring how classical signals are distorted, the system can adjust conditions in real-time to keep the quantum data safe.
In practice, the chip sends different classical polarization states through the fiber, detects how they change, and then updates tiny on-chip components to cancel out the disturbance before it affects the quantum payload. Each correction cycle takes about 3.2 milliseconds, repeating every 100 milliseconds. In a five-hour test, the system maintained quantum fidelity above 97% with correction switched on, compared to a steep drop to 59% without it.
When Could a Quantum Internet Arrive?
This research suggests that a practical quantum internet may not necessitate tearing up roads or laying entirely new cables. Instead, it could grow as an upgrade layered on top of today’s fiber optic networks. The Penn team even demonstrated that their hybrid packets could, in principle, integrate with standard IPv4 and IPv6 internet protocols. These are the same ones your laptop uses every day.
A node of the quantum network, roughly one kilometer’s worth of Verizon fiber optic cable away from the quantum signal’s source. (Credit: Sylvia Zhang)
Such a network could one day enable ultra-secure communications, link quantum computers across continents, and create powerful new sensing systems. For now, the experiment remains campus-scale and operates at modest speeds, approximately 10 kilobits per second for the classical headers, limited by the chip’s thermal tuning capabilities. However, the researchers note that using different materials, such as lithium niobate, could boost this to gigahertz speeds, sufficient to support hundreds or even thousands of network nodes.
In short, the building blocks of a quantum internet may already be here — not in some far-off future, but hidden in the same fiber lines carrying our Netflix streams and video calls.
The research team developed a hybrid quantum-classical networking system using a custom silicon nitride photonic chip fabricated with standard nanofabrication techniques. The chip integrates classical transmitters using Mach-Zehnder interferometer modulators with quantum transmitters based on high-quality-factor microring resonators that generate entangled photon pairs through spontaneous four-wave mixing. Testing occurred across the University of Pennsylvania campus using deployed underground fiber optic cables with multiple switching racks to simulate real-world network conditions.
The system demonstrated successful quantum routing across network configurations of increasing difficulty, achieving routing precision rates between 91.6% and 97.1%. Initial quantum states showed fidelity of 98.3% and purity of 97.9%. After transmission through the network with active error correction, quantum entanglement maintained fidelity of 97.0% and purity of 92.5%. Without error correction, quantum state fidelity degraded to 59.3% with purity of 61.5%, demonstrating the critical importance of the classical-decisive error mitigation approach.
Current classical data transmission rates are limited to 10 kilobits per second due to thermal tuning constraints on the photonic chip, though this could potentially reach gigahertz speeds with different materials. The demonstration was confined to a campus-scale network rather than internet-wide deployment. The error correction system requires bidirectional communication between server and nodes, which may introduce latency in larger networks. Routing precision remains below perfect performance due to quantum noise effects inherent to the system.
Research funding came from the Gordon and Betty Moore Foundation (grant GBMF12960), Office of Naval Research (grant N00014-23-1-2882), National Science Foundation (grant DMR-2323468), Olga and Alberico Pompa endowed professorship, and PSC-CUNY award (grant ENHC-54-93). The authors reported no competing financial interests.
“Classical-decisive quantum internet by integrated photonics,” published in Science on August 28, 2025. Authors: Yichi Zhang, Robert Broberg, Alan Zhu, Gushu Li, Li Ge, Jonathan M. Smith, Liang Feng (corresponding author). DOI: 10.1126/science.adx6176