A proven method of distributing secret keys for secure communication between remote participants is quantum key distribution (QKD). Rather than relying on computational complexity as with current communication protocols, its security is based on the quantum qualities of light, which are used to generate secure random keys for encrypting and decrypting data.

The purpose of the QKD is the ability to easily integrate it into a real-world communications network. An essential step towards this goal is the use of integrated photonics.

In a new study, scientists have described a QKD system based on integrated photonics that can transmit secure keys at unprecedented speeds. The proof-of-principle experiments are critical to the practical implementation of this incredibly secure communication technique.

This new QKD system integrates all components into chips except the laser and detectors. This has many advantages, such as compactness, low cost and ease of mass production.

Research team member Rebecka Sax from the University of Geneva in Switzerland said: “This work justifies the technological maturity and helps address the technical aspects surrounding its implementation through optical integrated circuits, which would enable integration into networks and other applications.”

quantum key distribution system (QKD).
Researchers have developed a quantum key distribution system (QKD) based on silicon photonics that can transmit secure keys at unprecedented speeds. The QKD transmitter (pictured) combines a photonic and electrical integrated circuit with an external diode laser. Credits: Rebecka Sax, University of Geneva

In a previous study, the scientists created a three-state QKD protocol that was implemented using standard fiber-based components to achieve QKD transmission at record high speeds. Scientists in this study wanted to implement the same protocol using integrated photonics.

Sax said, “The compactness, robustness and ease of manipulation of an integrated photonic system – with fewer components to verify upon deployment or troubleshoot in a network – enhances QKD’s position as a secure communications technology.”

For this work, scientists teamed up with silicon photonics company Sicoya GmbH in Berlin, Germany, and quantum cybersecurity company ID Quantique in Geneva to develop a silicon photonics transmitter that combines a photonic integrated circuit with an external diode laser.

The silica-based QKD receiver consists of a photonic integrated circuit and two external single-photon detectors. The receiver was created by Roberto Osellame’s team at the CNR Institute of Photonics and Nanotechnology in Milan, Italy, using femtosecond laser micromachining.

The use of an external laser with a photonic and electronic integrated circuit for the transmitter made it possible to accurately produce and encode photons at a record speed of up to 2.5 GHz. For the receiver, a low-loss and polarization-independent photonic integrated circuit and a set of external detectors enabled passive and simple detection of the transmitted photons. By connecting these two components with a standard single-mode fiber optic, secret keys could be produced at high speed.

The integrated transmitter and receiver were thoroughly characterized before performing secure key exchange over 150 km of single-mode fiber and single-photon avalanche photodiodes, which are well suited for real-world applications. They performed tests with single-photon superconducting nanowire detectors, enabling studies with a quantum bit error rate of just 0.8%. In addition to polarization independence, which is difficult to achieve with integrated photonics, the receiver also had a small loss of about 3 dB.

Sax said, “In terms of production of secret key rates and quantum bit error rates, these new experiments yielded results similar to previous experiments performed with fiber-based components. However, the QKD system is much simpler and more practical than the previous experimental setups, thus demonstrating the feasibility of using this protocol with integrated circuits.”

Magazine reference:

  1. R. Sax, A. Boaron, G. Boso, S. Atzeni, A. Crespi, F. Grünenfelder, D. Rusca, A. Al-Saadi, D. Bronzi, S. Kupijai, H. Rhee, R. Osellame, H. Zbinden, “High-speed integrated QKD system”, Photonics Research, 11, 6, 1007-1014 (2023). DOI: 10.1364/PRJ.481475