Experimental simulation of a polarization-dispersion-fluctuating channel with photon pairs

We report on an experimental implementation of a tunable source of mixed two-photon entangled states. In contrast to previous implementations there is no correlation between the phase shift and spectral property of photons which would in principle allow local spectral filtration. The source provides genuinely mixed states that mimic various statistics of polarization-dispersion fluctuations. We believe that our source may be useful for testing robustness of quantum-information protocols against this specific form of noise. We demonstrate this function on a simple case of filtration by means of a balanced beam splitter.

Figure 1

Experimental setup as described in detail in the text. HWP, half-wave plate; QWP, quarter-wave plate; BBO, two $\beta$-BaB${}_2$O${}_4$ nonlinear crystals; NF, narrow (10 nm) spectral filter; FPC, fiber polarization controller; MT, motorized translation; BS, balanced beam splitter; PBS, polarizing beam splitter; DET, detector; and C, coincidence electronics.

Figure 2

Purity as a function of the distribution width for both constant distribution (a) and Gaussian distribution (b). Lines represents theoretical prediction for the purity of mixed two-photon states before (dashed line) and after filtration (full and dotted lines), while the markers display the experimentally determined values. The full line (equals to 1) is the theoretical purity of filtered states on a perfectly balanced beam splitter, and the dotted line is the theoretical prediction taking into account unbalanced splitting ratio 40:60.

Figure 3

Success probability of filtration as a function of the distribution width for both constant distribution (a) and Gaussian distribution (b). The full line represents theoretical prediction, while the dots display the experimentally determined value. The dashed line corresponds to the theoretical prediction multiplied by factor 0.9.