18-qubit entanglement with photon's three degrees of freedom

TL;DR

This paper reports the experimental creation and verification of an 18-qubit GHZ entangled state using three degrees of freedom of six photons, demonstrating advanced control and measurement capabilities in quantum photonics.

Contribution

It introduces a method to generate and verify 18-qubit entanglement exploiting multiple degrees of freedom of photons with high stability and near-unity efficiency.

Findings

Achieved 18-qubit GHZ entanglement with high fidelity.

Developed high-stability interferometers for quantum logic operations.

Enabled simultaneous readout of over 262,000 outcome combinations.

Abstract

A central theme in quantum information science is to coherently control an increasing number of quantum particles as well as their internal and external degrees of freedom (DoFs), meanwhile maintaining a high level of coherence. The ability to create and verify multiparticle entanglement with individual control and measurement of each qubit serves as an important benchmark for quantum technologies. To this end, genuine multipartite entanglement have been reported up to 14 trapped ions, 10 photons, and 10 superconducting qubits. Here, we experimentally demonstrate an 18-qubit Greenberger-Horne-Zeilinger (GHZ) entanglement by simultaneous exploiting three different DoFs of six photons, including their paths, polarization, and orbital angular momentum (OAM). We develop high-stability interferometers for reversible quantum logic operations between the photon's different DoFs with precision and efficiencies close to unity, enabling simultaneous readout of 262,144 outcome combinations of the 18-qubit state. A state fidelity of 0.708(16) is measured, confirming the genuine entanglement of all the 18 qubits.