Experimental Demonstration of Entanglement-Enhanced Classical Communication over a Quantum Channel with Correlated Noise

We present an experiment demonstrating the entanglement enhanced capacity of a quantum channel with correlated noise, modeled by a fiber optic link exhibiting fluctuating birefringence. In this setting, introducing entanglement between two photons is required to maximize the amount of information that can be encoded into their joint polarization degree of freedom. We demonstrated this effect using a fiber-coupled source of entagled photon pairs based on spontaneous parametric down-conversion, and a linear-optics Bell state measurement. The obtained experimental classical capacity with entangled states is equal to $0.82\pm 0.04$ per a photon pair, and it exceeds approximately 2.5 times the theoretical upper limit when no quantum correlations are allowed.

Figure 1

Experimental setup. HWP1, HWP2, half-wave plates; SC, Soleil compensator; BG, blue BG39 filter; X1, X2, down-conversion crystals; RG, red RG665 filters; IF1, IF2, interference filters; L1, L2, aspheric lenses; PC1, PC2, manual polarization controllers introducing additional 6 ns delay. BS1, . . . , BS5 single-mode $50/50$ fiber optic couplers; L3, L4; collimators; D1, . . . , D4, photon counting modules. Singlet events are coincidences between D1 and D3, D1 and D4, D2 and D3, or D2 and D4, whereas triplet events are defined as concidences between D1 and D2 or D3 and D4.

Figure 2

Singlet (○) and triplet (▲) coincidences as a function of the optical delay for separable states of (a) parallel and (b) orthogonal polarizations, and for entangled triplet (c) and singlet (d) polarization states. The solid lines are fitted Gaussian functions.