Distribution of high-dimensional entanglement via an intra-city free-space link

TL;DR

This paper demonstrates the successful distribution of high-dimensional hyperentangled quantum states over a 1.2 km free-space link in Vienna, showing potential for robust, long-distance quantum communication.

Contribution

It reports the first distribution of genuine high-dimensional entanglement via an intra-city free-space link using hyperentanglement in polarization and energy-time bases.

Findings

High-visibility interference observed in both degrees of freedom.

Certification of four-dimensional entanglement in the hyperentangled system.

High-fidelity transmission under atmospheric conditions.

Abstract

Quantum entanglement is a fundamental resource in quantum information processing and its distribution between distant parties is a key challenge in quantum communications. Increasing the dimensionality of entanglement has been shown to improve robustness and channel capacities in secure quantum communications. Here we report on the distribution of genuine high-dimensional entanglement via a 1.2-km-long free-space link across Vienna. We exploit hyperentanglement, that is, simultaneous entanglement in polarization and energy-time bases, to encode quantum information, and observe high-visibility interference for successive correlation measurements in each degree of freedom. These visibilities impose lower bounds on entanglement in each subspace individually and certify four-dimensional entanglement for the hyperentangled system. The high-fidelity transmission of high-dimensional entanglement under real-world atmospheric link conditions represents an important step towards long-distance quantum communications with more complex quantum systems and the implementation of advanced quantum experiments with satellite links.