As quantum computers grow more powerful, many current encryption methods could eventually become vulnerable. One promising solution is quantum cryptography, which relies on the laws of physics rather than mathematical complexity to keep data secure. However, making quantum communication practical requires small, dependable devices that can accurately read delicate quantum signals carried by light.
Researchers from the University of Padua, Politecnico di Milano, and the CNR Institute for Photonics and Nanotechnologies have demonstrated a new approach using an unexpected material: borosilicate glass. Reported in Advanced Photonics, their study describes a high-performance quantum coherent receiver built directly inside glass using femtosecond laser writing. This method delivers low optical loss, stable performance, and compatibility with existing fiber-optic systems, all of which are important for moving quantum technologies beyond lab experiments.
Why Glass Outperforms Silicon in Quantum Devices
Continuous-variable (CV) quantum information processing, used in quantum key distribution (QKD) and quantum random number generation (QRNG), depends on measuring the amplitude and phase of light waves. To do this, a coherent receiver combines a weak quantum signal with a stronger reference beam and analyzes how they interfere.
Most current integrated receivers are made from silicon. While silicon is widely used and supports high integration, it is sensitive to polarization and tends to have higher optical losses, which can limit performance and reliability in quantum systems.
Glass offers several advantages. It is naturally insensitive to polarization, highly stable, and allows waveguides to be written in three dimensions with minimal signal loss. Using femtosecond laser micromachining, researchers can create light-guiding paths directly inside the material, forming compact photonic circuits without the complexity of semiconductor manufacturing.
Inside the Laser-Written Quantum Receiver
The team created a fully tunable heterodyne receiver, a key component for CV-QKD and CV-QRNG, by writing the optical circuit directly inside borosilicate glass. The chip includes:
- Fixed and tunable beam splitters
- Thermo-optic phase shifters for precise electrical control
- Three-dimensional waveguide crossings
- Polarization-independent directional couplers
These features allow the quantum signal and reference beam to interact in a controlled way, enabling simultaneous measurement of two conjugate quadratures. The device also shows:
- Extremely low insertion loss (≈1 dB)
- Polarization-independent operation
- Common-mode rejection ratio above 73 dB, indicating strong suppression of classical noise
- Stable signal-to-noise performance over at least 8 hours
Overall, these results match or exceed the performance of many silicon-based photonic receivers.
One Chip, Two Quantum Technologies
Because the device combines low loss, tunability, and stability, it can handle multiple quantum communication tasks without requiring different hardware. When used as a heterodyne detector, it enabled a source-device-independent QRNG system, meaning it remains secure even if the incoming optical signal cannot be trusted. The chip achieved a secure random bit generation rate of 42.7 Gbit/s, setting a record for this type of system.
The same chip was also used for a QPSK-based CV-QKD protocol, where information is encoded in a four-point constellation of quantum states. In a simulated 9.3-km fiber link, the system reached a 3.2 Mbit/s secret key rate. These results show that a glass-based photonic front end can support advanced CV-QKD without the drawbacks seen in silicon platforms.
Glass Photonics Moves Toward Real-World Use
In addition to strong performance, the study highlights several practical benefits of using glass in integrated quantum photonics:
- Environmental stability: Glass is inert and resistant to temperature and mechanical changes.
- Low-loss fiber coupling: Waveguides closely match standard telecom fiber sizes.
- 3D design flexibility: Circuits can include crossings and complex layouts without added signal loss.
- Scalability and cost-effectiveness: Femtosecond laser writing allows rapid prototyping without expensive semiconductor fabrication.
These qualities support long-term reliability and durability, which are important for real-world deployment and even potential use in space-based quantum communication systems. The researchers note that glass-based photonics could help close the gap between experimental setups and practical quantum networks.
Toward Scalable Quantum Communication Networks
By leveraging these advantages, the team demonstrated two major applications on a single chip: a source-device-independent QRNG with a record-high secure generation rate of 42.7 Gbit/s, and a QPSK-based CV-QKD system achieving a 3.2 Mbit/s secure key rate over a simulated 9.3-kilometer fiber link.
Beyond these results, the work points to glass-based integrated photonics as a durable and versatile platform for future quantum technologies. Glass is stable, cost-effective, and resistant to harsh environments, making it well suited for scalable deployment. This approach could help transition quantum communication from controlled lab settings to real-world infrastructure, marking an important step toward building global quantum networks.