Experimental quantum cryptography with qutrits

TLDR

Qutrits offer higher coding density and a larger security margin than conventional binary quantum systems. The authors encode qutrits in the orbital angular momentum of photons using Laguerre–Gaussian modes (l = +1, 0, −1) manipulated by phase holograms, and employ an Ekert‑type protocol that verifies key security through a three‑dimensional Bell‑inequality violation. They successfully generate two identical keys with entangled qutrits, achieving a key error rate of about 10 %.

Abstract

We produce two identical keys using, for the first time, entangled trinary quantum systems (qutrits) for quantum key distribution. The advantage of qutrits over the normally used binary quantum systems is an increased coding density and a higher security margin. The qutrits are encoded into the orbital angular momentum of photons, namely Laguerre–Gaussian modes with azimuthal index l + 1, 0 and −1, respectively. The orbital angular momentum is controlled with phase holograms. In an Ekert-type protocol the violation of a three-dimensional Bell inequality verifies the security of the generated keys. A key is obtained with a qutrit error rate of approximately 10%.