Entanglement of the orbital angular momentum states of photons

Abstract

Entangled quantum states are not separable, regardless of the spatial separation of their components. This is a manifestation of an aspect of quantum mechanics known as quantum non-locality1,2. An important consequence of this is that the measurement of the state of one particle in a two-particle entangled state defines the state of the second particle instantaneously, whereas neither particle possesses its own well-defined state before the measurement. Experimental realizations of entanglement have hitherto been restricted to two-state quantum systems3,4,5,6, involving, for example, the two orthogonal polarization states of photons. Here we demonstrate entanglement involving the spatial modes of the electromagnetic field carrying orbital angular momentum. As these modes can be used to define an infinitely dimensional discrete Hilbert space, this approach provides a practical route to entanglement that involves many orthogonal quantum states, rather than just two Multi-dimensional entangled states could be of considerable importance in the field of quantum information7,8, enabling, for example, more efficient use of communication channels in quantum cryptography9,10,11.

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Figure 1: The wave front (top) and the intensity pattern (bottom) of the simplest Laguerre–gaussian (LGlp) or ‘doughnut’ mode.
Figure 2: Experimental set-up for single-photon mode detection.
Figure 3: Conservation of orbital angular momentum.
Figure 4: Experimental evidence (left; right, simulation) of entanglement of photon states with phase singularities.

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Acknowledgements

This work was supported by the Austrian Fonds zur Förderung der wissenschaftlichen Forschung (FWF).

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Correspondence to Anton Zeilinger.

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Mair, A., Vaziri, A., Weihs, G. et al. Entanglement of the orbital angular momentum states of photons. Nature 412, 313–316 (2001). https://doi.org/10.1038/35085529

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