Środowiskowe Seminarium z Informacji i Technologii Kwantowych

Entanglement is the workhorse of quantum technologies today, ranging from fault-tolerant quantum computation to device-independent quantum communication. The entanglement of more than two quantum particles, commonly known as Greenberger-Horne-Zeilinger (GHZ) entanglement, not only opened the door to the strongest test of local-realism, but also forms a key ingredient of such technologies. Since the discovery of the GHZ theorem, experimental research on multi-particle entanglement has mainly focused on two-dimensional quantum systems with realisations in a diverse range of physical systems including ions, photons, and super-conducting qubits. In all of these systems, a general procedure exists for increasing the number of entangled particles. For example, for photons, a particularly simple experimental scheme uses polarising beam-splitters in combination with post-selection to produce arbitrarily high numbers of photons entangled in a GHZ manner. However, no experiment till date has been able to create a truly high-dimensional and multi-particle entangled state.In this talk, I will discuss the first experimental realisation of a multi-photon entangled state where all photons are genuinely entangled in a high-dimensional manner. Interestingly, our experimental technique was found by using a computer algorithm (MELVIN) but ultimately implemented in the lab by humans. By carefully combining two pairs of high-dimensionally entangled photons in a 27-dimensional multi-port interferometer, we generated a GHZ state consisting of three photons entangled in three dimensions each of their orbital angular momentum. We verified the entanglement through the use of a fidelity-based entanglement witness, and demonstrated three independent violations of the Mermin inequality in three subspaces of our high-dimensional multipartite state. Our results open up a pathway for a further boost to quantum technologies and will enable qualitatively new refutations of local-realistic world views.