Quantum networks rely on entanglement, one of the most counterintuitive features of quantum mechanics. When two particles become entangled, they share a single quantum state and cannot be fully described as independent objects, even after being separated by long distances. Measuring one particle establishes the corresponding outcome for the other. Although entanglement does not allow information to travel faster than light, it provides correlations that have no classical equivalent and could become the foundation of future quantum communication and distributed computing.

The potential applications are enormous. Entangled telescopes separated by thousands of kilometers could one day combine their observations of distant stars and planets, creating images with far greater resolution than a single telescope could produce. Networks of entangled sensors might detect minute disturbances caused by earthquakes, volcanic activity or underground movement. Quantum computers linked together could share computational tasks that exceed the capacity of any individual machine, opening new possibilities for simulating chemical reactions, designing materials and identifying promising drug candidates.

Before any of those applications become practical, however, researchers must solve a basic problem: quantum states are extraordinarily fragile. The Maryland experiment focused on photons, particles of light whose quantum information was encoded in polarization—the direction in which the electric field oscillates. In a conventional fiber-optic network, small mechanical disturbances are usually insignificant because classical data is transmitted by changing the intensity or power of the light. Quantum information encoded in polarization is much more vulnerable to physical changes in the fiber.

The researchers deliberately used fiber suspended from utility poles, rather than a carefully shielded cable buried underground. During the day, the fiber expands as it warms; at night, it contracts. Wind causes the cable to sway, while birds and other environmental influences add further disturbances. These changes can twist the fiber and transform the polarization of photons traveling through it. If the transformation is not corrected, the relationship between entangled photons can become scrambled, destroying the quantum correlations needed by the network.

“It’s about as bad a connection as you can possibly have,” said NIST physicist Oliver Slattery, one of the study’s authors. The exposed route effectively turned the experiment into a stress test for quantum networking. Instead of building a specialized quantum network from scratch, the team asked whether existing infrastructure—much of it designed for ordinary internet traffic—could be adapted to carry entangled states without requiring constant manual adjustment.

The experiment began with a commercial device that generated pairs of entangled photons. One photon from each pair remained in a NIST laboratory, where its polarization was measured. The other photon traveled through approximately 62 kilometers, or 38.5 miles, of fiber to a second laboratory at the University of Maryland in College Park. The two photons began with linked polarization states, so measurements made at the two locations could later be compared to determine whether their entanglement had survived the journey.

To compensate for the changing fiber, the researchers used a pair of polarization-stabilization devices developed by Qunnect. These systems sent reference light through the same fiber path as the quantum signals. At the receiving end, the devices measured how the fiber had altered the reference light’s polarization. The system then calculated the inverse of that transformation and applied the correction to the quantum photons in real time. In effect, the equipment continuously tracked the fiber’s behavior and attempted to undo its effects before the fragile quantum correlations were lost.

The system transmitted about 1,500 entangled photons per second. Over a 24-hour period, entangled photons were successfully distributed 92.8% of the time, while polarization corrections were required during the remaining 7.2%. Statistical testing confirmed that the photons detected at the two ends retained correlations consistent with entanglement. The result did not surpass the 248-kilometer record set by a European team using underground fiber in 2022, but it addressed a different and highly practical challenge: whether quantum networking can function on infrastructure exposed to the everyday forces of the environment.

Yicheng Shi, the NIST physicist who led the study, described the experiment as an extreme test of quantum networking protocols. The successful transmission suggests that future quantum networks may not require entirely separate fiber systems, potentially reducing the cost and complexity of deployment. The researchers emphasize that major hurdles remain, including improving transmission rates, extending distances, reducing losses and developing quantum repeaters capable of preserving entanglement across much larger networks. Even so, the demonstration shows that the road to a quantum internet may run not through pristine laboratory equipment, but along the same aerial cables already carrying the world’s digital traffic.

Subject of Research: Quantum entanglement and quantum networking

Article Title: Entanglement Distribution Over a Polarization-Stabilized Aerial Fiber

Web References: https://doi.org/10.1364/JOCN.592521

References: Yicheng Shi, Jing Su, Anouar Rahmouni, Pranish Shrestha, Mheni Merzouki, Gabriel Bello Portmann, Anne Lazenby, Mael Flament, Mehdi Namazi, Abdella Battou, Oliver Slattery and Thomas Gerrits, “Entanglement Distribution Over a Polarization-Stabilized Aerial Fiber,” Journal of Optical Communications and Networking, published online July 15, 2026. DOI: 10.1364/JOCN.592521

Image Credits: N. Hanacek/NIST

Keywords

Quantum entanglement, quantum networking, quantum internet, quantum optics, polarization stabilization, entangled photons, aerial fiber, quantum communication, NIST, quantum computing

At the heart of this breakthrough lies a unique fabrication process that employs ultrafast laser inscription to carve intricate waveguide architectures directly into a transparent substrate. This approach allows for unmatched three-dimensional (3D) configurability and miniaturization, overcoming the spatial limitations characteristic of traditional planar photonic integrated circuits. By integrating these reconfigurable waveguides with a high-density SPAD array, the team has effectively merged light manipulation and detection into a compact, scalable module capable of processing single photons with exceptional efficiency and precision.

The laser-writing technique facilitates dynamic, in situ tuning of photonic pathways. Unlike static lithographically defined circuits, whose designs cannot be altered post-fabrication, laser inscription allows multiple reconfiguration cycles through appropriate thermal or optical control mechanisms. This adaptability is a game-changer for applications requiring real-time optical routing, switching, or mode reshaping. Devices based on this method can dramatically enhance the flexibility and functionality of photonic networks, particularly in intricate quantum experiments where photon coherence and path integrity are paramount.

Critically, the direct coupling to a SPAD array addresses a long-standing bottleneck in integrated photonics: the efficient detection of single photons with high temporal and spatial resolution. Single-photon avalanche diodes are semiconductor devices capable of detecting individual photons by amplifying the resultant electron-hole pairs in a controlled avalanche process, producing a measurable electrical pulse. Incorporating the SPAD array directly adjacent to the waveguides reduces coupling losses and timing jitter, thereby boosting overall system performance for photon counting and quantum state measurements.

The research team meticulously optimized the waveguide-to-detector interface, balancing optical mode profiles with electrical constraints to maximize photon detection probability. By engineering the waveguide cross-sections and refractive index profiles through precision laser parameters, the device ensures efficient mode matching with the sensitive detection region of each SPAD pixel. This alignment is critical to achieving low dark count rates and maintaining high signal-to-noise ratios, essential for applications in quantum cryptography and photon-starved imaging environments.

One especially striking feature of this integrated platform is its potential to be fabricated on a compact chip scale using widely accessible laser inscription setups. Unlike conventional semiconductor fabrication processes that require expensive cleanroom facilities and complex lithography steps, ultrafast laser writing offers a more cost-effective, maskless alternative with rapid turnaround times. This democratization of photonic device engineering could accelerate the adoption of sophisticated quantum photonic systems in industry and academia, empowering a new wave of experimental possibilities.

The versatility of the laser-written PIC extends beyond quantum information science. Reconfigurable optical circuits are foundational components in next-generation optical computing architectures, neuromorphic photonic processors, and advanced sensors capable of adaptive spectral and spatial filtering. The seamless integration with SPAD technology further opens new horizons for time-resolved spectroscopy, fluorescence lifetime imaging microscopy (FLIM), and LIDAR systems, where single-photon sensitivity is indispensable.

Moreover, the realized system exhibited impressive scalability potential. The modular nature of the SPAD array, combined with the extensible architecture of the laser-written PIC, enables the construction of large-scale photonic networks that preserve quantum coherence and minimize interconnection losses. This integrated approach is expected to facilitate the development of complex quantum simulators and processors with enhanced fault tolerance and configurability, paving the way towards practical quantum advantage in computation and communication.

Beyond its immediate technological merits, this research embodies a profound shift in how photonic circuits are conceptualized and developed. The co-fabrication of reconfigurable components and detector arrays in a single, cohesive environment represents an important step towards monolithic integration of quantum photonics. This synergy mitigates signal degradation caused by interconnects and external coupling, a persistent challenge in hybrid devices that often impedes system stability and scalability.

The laser-written approach also permits the incorporation of novel material platforms that traditionally pose fabrication challenges for photonic integration. Exotic nonlinear optical materials, biocompatible glasses, or crystalline substrates can be tailored with waveguides and coupled to detectors, broadening the functionality and application scope of the devices. This flexibility encourages interdisciplinary research, bridging photonics with fields such as biophotonics, environmental sensing, and medical diagnostics.

Importantly, the research presents promising results validating the device’s performance under operational conditions relevant to quantum optics experiments. The integrated circuit demonstrated low propagation losses, high extinction ratios for waveguide switches, and temporal resolution compatible with sub-nanosecond photon events. Such performance metrics confirm the practicality of the platform for demanding applications requiring precise photon timing and routing control.

The design also anticipates integration with external electronic control circuitry, enabling active feedback loops and programmable operation modes. Future iterations could incorporate real-time reprogramming of photonic functions through microelectromechanical systems (MEMS), thermal phase tuning, or electro-optic modulation. This would significantly enhance the adaptability and user control of integrated quantum photonic devices, promoting multifunctionality within a single chip footprint.

In the grander scheme, the advent of this laser-written reconfigurable photonic integrated circuit directly coupled to a SPAD array is a vivid illustration of how photonics and quantum technologies are converging to unlock new regimes of information processing and measurement. As quantum communication protocols and quantum sensing schemes mature, the demand for compact, efficient, and tunable photonic platforms will only intensify. This innovation is well-positioned to meet these aspirations, acting as a cornerstone technology for the burgeoning quantum photonics industry.

Looking ahead, the team envisions expanding their work by integrating on-chip sources of single photons, efficient quantum memories, and error-correcting circuits, moving closer to fully integrated quantum photonic processors. Collaborations with material scientists and device engineers will further refine fabrication techniques and device architectures. Such multidisciplinary efforts are poised to catalyze the next generation of photonic technologies with unprecedented performance and versatility.

In summary, this groundbreaking research ushers in a new paradigm where laser-written reconfigurable photonic integrated circuits synergize seamlessly with single-photon detector arrays, culminating in a potent tool for advanced quantum photonics applications. The fusion of manufacturability, configurability, and single-photon sensitivity in a single platform heralds a future where quantum technologies become more accessible, robust, and scalable. The impact of this innovation is poised to resonate across scientific disciplines and industrial sectors, accelerating progress toward practical quantum-enabled devices.

Subject of Research: Reconfigurable photonic integrated circuits and single-photon detection integration

Article Title: Laser-written reconfigurable photonic integrated circuit directly coupled to a single-photon avalanche diode array

Article References:
Gualandi, G., Atzeni, S., Gardina, M. et al. Laser-written reconfigurable photonic integrated circuit directly coupled to a single-photon avalanche diode array. Light Sci Appl 14, 199 (2025). https://doi.org/10.1038/s41377-025-01854-6

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41377-025-01854-6