Dounan Du /
A Long-Distance Quantum Network Testbed
Building a quantum networking testbed that connects Stony Brook University and Brookhaven National Laboratory through 158 km of deployed optical fiber.
One of the long-term goals of quantum technology is the Quantum Internet: a network capable of distributing quantum information between distant locations. Such a network could enable secure communication, distributed quantum computing, and entirely new ways of performing precision measurements.
Unlike the classical internet, however, a quantum network cannot simply amplify or retransmit signals along the way. Quantum information is extraordinarily fragile, and even tiny imperfections in timing, polarization, or photon properties can prevent distant quantum nodes from communicating successfully.
This project explores one of the fundamental challenges in building future quantum networks:
Can quantum devices located tens of kilometers apart reliably behave as a single network?
The Challenge
Building a long-distance quantum network is much more difficult than connecting two laboratories with an optical fiber.
Photons traveling through deployed metropolitan fiber experience:
- optical loss
- polarization drift
- environmental fluctuations
- timing uncertainty
Meanwhile, independent quantum nodes must generate photons that are nearly identical despite being produced by completely separate devices.
Even slight differences can prevent the photons from interfering, a key requirement for many quantum networking protocols.
Unlike laboratory demonstrations performed over short fiber spools in carefully controlled environments, a practical quantum network must remain stable while operating over real-world infrastructure.
Building the Testbed
To investigate these challenges, we built a quantum networking testbed connecting Stony Brook University and Brookhaven National Laboratory using 158 kilometers of deployed optical fiber.
The network combines several key components:
- two independent rubidium-based quantum light-matter interfaces
- quantum frequency converters that generate telecom-compatible photons
- long-distance metropolitan optical fiber
- distributed timing synchronization
- active polarization stabilization
- single-photon interference and detection systems
Together, these components form a complete experimental platform for studying long-distance quantum networking under realistic operating conditions.
Making Atomic Quantum Memories Compatible with Optical Fiber
One challenge in quantum networking is that many atomic quantum memories operate using wavelengths that are not well suited for long-distance fiber transmission.
To address this, our testbed uses rubidium-based quantum frequency converters that transform photons generated near atomic transitions into telecom-band photons, where optical fibers exhibit much lower transmission loss.
Importantly, the conversion preserves the narrow spectral properties required for future integration with atomic quantum memories, allowing quantum information to travel efficiently through existing fiber infrastructure.
Keeping the Network Stable
Simply connecting devices together is not enough.
The network must continuously maintain the physical conditions required for quantum interference.
Our testbed includes a layered control architecture that separates quantum operations from the classical systems responsible for controlling and monitoring the experiment.
During operation, the system continuously performs tasks such as:
- synchronizing clocks between distant laboratories
- compensating polarization drift
- balancing photon arrival rates
- coordinating measurements
- verifying experimental conditions
These background processes allow the quantum hardware to operate reliably over long-distance deployed fiber.
Demonstrating Long-Distance Quantum Interference
To validate the testbed, we implemented one of the most fundamental protocols in quantum networking: Hong-Ou-Mandel (HOM) interference.
In this experiment, photons generated independently at two distant quantum nodes travel through the deployed fiber network before meeting at a beam splitter.
If the photons remain sufficiently indistinguishable after their long journey, they interfere with one another, producing a characteristic reduction in coincidence detections.
Although conceptually simple, this experiment is an extremely sensitive test of the entire network.
Successful interference requires:
- precise timing synchronization
- stable polarization
- nearly identical photon spectra
- careful balancing of the optical paths
Achieving all of these simultaneously over metropolitan-scale deployed fiber demonstrates that the network can preserve the delicate properties required for future quantum communication.
Results
Using the complete 158 km testbed, we demonstrated:
- Hong-Ou-Mandel interference over deployed metropolitan fiber
- interference visibilities approaching the theoretical limit for independent weak coherent states
- stable synchronization between distant laboratories
- continuous polarization stabilization during long experiments
- reliable operation of a quantum networking protocol over real-world infrastructure
Together, these results demonstrate that quantum networking experiments can move beyond isolated laboratory setups and operate across geographically separated research facilities.
Why This Matters
Many recent breakthroughs in quantum networking have focused on improving individual devices, such as better quantum memories, photon sources, or detectors.
This project focuses on a different challenge:
Integrating those components into a functioning network.
Future quantum repeaters and distributed quantum computers will require many independent devices to work together reliably over large distances.
Building those systems will depend not only on advances in quantum hardware, but also on robust networking infrastructure capable of coordinating and stabilizing complex experiments in real time.
This testbed represents one step toward that future.
Technical Highlights
- 158 km deployed metropolitan optical fiber
- Stony Brook University to Brookhaven National Laboratory
- Rubidium-based quantum light-matter interfaces
- Telecom-compatible quantum frequency conversion
- Active timing synchronization using White Rabbit
- Automatic polarization stabilization
- Long-distance Hong-Ou-Mandel interference
- Layered quantum-enabled network architecture
Paper
A Long-distance Quantum-enabled Internet Testbed
Authors: Dounan Du, Leonardo Castillo-Veneros, Dillion Cottrill, Guo-Dong Cui, Paul Stankus, Dimitrios Katramatos, Julian Martinez-Rincon, Eden Figueroa