Over three decades ago, computer scientists Charles Bennett and Gilles Brassard developed the first quantum cryptography protocol that would allow two remote parties to securely exchange a private key. Since then, secure communication networks enabled by quantum key distribution (QKD) have been implemented in the corporate, financial and medical sectors.

In order to reach the average household, however, current QKD systems must become more affordable without compromising on performance and security. An article published in Applied Physics Letters proposes a compact and cost-effective alternative to the interferometer commonly used to encode qubit and basis information in today’s QKD systems.

The asymmetric Mach-Zehnder interferometer has remained a central component of QKD systems since the early ’90s. As a cheaper and more compact alternative, authors suggest an all-fiber common-path interferometer, based on a highly birefringent optical fiber and built with off-the-shelf parts. Linearly polarized light pulses are coupled at a 45-degree angle with respect to the axis of the birefringent fiber, meaning the light is split into the slow and fast axes of the fiber. This process generates two time-bins, and the pulses are then encoded with bit and basis information using a phase modulator.

To validate the new component, the authors demonstrated comparable performance of the all-fiber, highly birefringent interferometer to the asymmetric Mach-Zehnder interferometer during point-to-point QKD operation. In addition, their interferometer offered a much greater tolerance to length mismatches between interferometers, which is ideal for multiuser applications where the differential temporal delay of numerous interferometers must be matched.

The authors hope that this work is a step toward widespread deployment of QKD technology, with quantum-secured local access networks ultimately reaching small businesses and homes.

Source: “Quantum key distribution using in-line highly birefringent interferometers,” by Amos Martinez, Bernd Fröhlich, James F. Dynes, Andrew W. Sharpe, Winci Tam, Alan Plews, Marco Lucamarini, Zhiliang Yuan, and Andrew J. Shields, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5036827.