Beating the channel capacity limit for linear photonic superdense coding

TL;DR

This paper demonstrates a quantum dense coding protocol that surpasses the traditional linear optics capacity limit by using entanglement in an additional degree of freedom, achieving the largest reported channel capacity.

Contribution

It introduces a method using spin and orbital angular momentum entanglement to fully discriminate Bell states, breaking the linear optics capacity limit in dense coding.

Findings

Achieved the largest reported channel capacity in dense coding

Demonstrated deterministic discrimination of all Bell states with linear optics

Enabled quantum communication without alignment and for satellite applications

Abstract

Dense coding is arguably the protocol that launched the field of quantum communication. Today, however, more than a decade after its initial experimental realization, 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 optics, reducing the attainable channel capacity from 2 to log_2 3 \approx 1.585 bits. However, entanglement in an extra degree of freedom enables the complete and deterministic discrimination of all Bell states. Using pairs of photons simultaneously entangled in spin and orbital angular momentum, 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 alignment and satellite communication.