Quantum key distribution system achieves high transmission speed

Researchers have developed a quantum key distribution (QKD) system based on integrated photonics that can transmit secure keys at high speeds. The proof-of-principle experiments represent a step towards real-world application of this secure communication method. QKD is a well-established method of providing secret keys for secure communication between distant parties. By using the quantum properties of light to generate random secure random keys for encrypting and decrypting data, its security is based on the laws of physics, rather than computational complexity like current communication protocols.

Research team member Rebecka Sax from the University of Geneva said a key goal for QKD technology is the ability to integrate it into a real-world communications network. “An important and necessary step toward this goal is the use of integrated photonics, which allows optical systems to be manufactured using the same semiconductor technology used to make silicon computer chips,” Sax said.

In the journal Photonics Research, researchers led by the University of Geneva’s Hugo Zbinden described the QKD system, in which all components are integrated onto chips except the laser and detectors. This comes with many advantages, such as compactness, low cost and ease of mass production.

“Although QKD can provide security for sensitive applications such as banking, health and defence, it is not yet a widespread technology. This work justifies the technology maturity and helps address the technicalities around implementing it via optical integrated circuits, which would allow integration in networks and in other applications,” Sax said.

Researchers previously developed a three-state time-bin QKD protocol that was carried out with standard fibre-based components to achieve QKD transmission at high speeds. The researchers’ new record is to implement the same protocol using integrated photonics. “The compactness, robustness and ease of manipulation of an integrated photonic system — with less components to verify when implementing or to troubleshoot in a network — improves the position of QKD as a technology for secure communication,” Sax said.

QKD systems use a transmitter to send the encoded photons and a receiver to detect them. The researchers collaborated with silicon photonics company Sicoya in Berlin, 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 QKD receiver was made of silica and consisted of a photonic integrated circuit and two external single-photon detectors. The receiver was fabricated by Roberto Osellame’s group at the CNR Institute of Photonics and Nanotechnology in Italy, using femtosecond laser micromachining.

Sax said that using an external laser with a photonic and electronic integrated circuit made it possible to accurately produce and encode photons at a speed of up to 2.5 GHz. “For the receiver, a low-loss and polarisation-independent photonic integrated circuit and a set of external detectors allowed passive and simple detection of the transmitted photons. Connecting these two components with a standard single-mode fibre enabled high-speed production of secret keys.”

After characterising the integrated transmitter and receiver, the researchers used it to perform a secret key exchange using different simulated fibre distances and with a 150 km-long single-mode fibre and single-photon avalanche photodiodes, which are well-suited for practical implementations. The researchers also used single-photon superconducting nanowire detectors, which enabled a quantum bit error rate as low as 0.8%. The receiver not only featured polarisation independence, which is complicated to achieve using integrated photonics, but also presented low loss, around 3 dB.

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

The researchers are now working to house the system parts in a rack enclosure that will allow QKD to be implemented in a network system.

Image caption: The silica-based QKD receiver shown consists of a photonic integrated circuit and two external single-photon detectors. Image credit: Simone Atzeni, CNR-IFN