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Abstract
Classically, one photon can transport one bit of information. But more is possible when quantum entanglement comes into play, and a record ‘channel capacity’ of 1.63 bits per photon has now been demonstrated, using a method that overcomes fundamental limitations of earlier approaches to ‘superdense coding’. Dense coding is arguably the protocol that launched the field of quantum communication1. Today, however, more than a decade after its initial experimental realization2, the channel capacity remains fundamentally limited as conceived for photons using linear elements. Bob can only send to Alice three of four potential messages owing to the impossibility of carrying out the deterministic discrimination of all four Bell states with linear optics3,4, reducing the attainable channel capacity from 2 to log23≈1.585 bits. However, entanglement in an extra degree of freedom enables the complete and deterministic discrimination of all Bell states5,6,7. Using pairs of photons simultaneously entangled in spin and orbital angular momentum8,9, we demonstrate the quantum advantage of the ancillary entanglement. In particular, we describe a dense-coding experiment with the largest reported channel capacity and, to our knowledge, the first to break the conventional linear-optics threshold. Our encoding is suited for quantum communication without alignment10 and satellite communication.Cite this article
Barreiro, J., Wei, TC. & Kwiat, P. Beating the channel capacity limit for linear photonic superdense coding. Nature Phys 4, 282–286 (2008). https://doi.org/10.1038/nphys919 