Rotational-invariant quantum key distribution based on a quantum dot source

Quantum Key Distribution (QKD) is a cutting-edge field that leverages the principles of quantum mechanics to enable secure communication between parties involved. Single-photon quantum emitters offer remarkable on-demand photon emission, near-unitary indistinguishability, and low multiphoton generation, thereby enhancing the performance of QKD protocols. Standard approaches in which the polarization degree-of-freedom is exploited are limited by the precise alignment between the communicating parties. To overcome this obstacle, the Orbital Angular Momentum (OAM) of light represents a suitable candidate for encoding the information, as it allows the implementation of rotational-invariant photonic states that remove the need for a fixed physical reference frame between the communicating parties. Here, we demonstrate an alignment-free BB84 implementation using on-demand single photons from a quantum-dot emitter at 925 nm. Time-to-spatial demultiplexing and Q -plates encode rotationally invariant spin–orbit states that we transmit over a 2 m free-space link while rotating the receiver between 0 ∘ and 90 ∘ . The source exhibits g (2) (0)≈0.03; the measured QBER remains (4.04±0.02)% and is essentially independent of the relative angle. A simple security model tailored to sub-Poissonian emission, which explicitly incorporates g (2) (0), yields a positive asymptotic secret fraction and an effective QBER threshold of ∼9.6% under our conditions. These results validate rotationally invariant QKD with an on-demand single-photon source and outline its scalability to long-distance scenarios.