Eliminating spectral distinguishability in ultrafast spontaneous parametric down-conversion

Generation of polarization-entangled photon pairs with a precise timing through down-conversion of femtosecond pulses is often faced with a degraded polarization entanglement quality. In a previous experiment, we have shown that this degradation is induced by spectral distinguishability between the two decay paths, in accordance with theoretical predictions. Here, we present an experimental study of the spectral compensation scheme proposed and first implemented by Kim and Grice [J. Mod. Opt. 49, 2309 (2002)]. By measuring the joint spectral properties of the polarization correlations of the photon pairs, we show that the spectral distinguishability between the down-converted components is eliminated. This scheme results in a polarization visibility of 97.9 +/- 0.5% without any spectral filtering.