Experimental verification of high spectral entanglement for pulsed waveguided spontaneous parametric down-conversion

We study the spectral properties of spontaneous parametric down-conversion (SPDC) in a periodically poled waveguided structure of potassium-titanyl-phosphate (KTP) crystal pumped by ultrashort pulses. Our theoretical analysis reveals a strongly entangled and asymmetric structure of the two-photon spectral amplitude for type-II SPDC. We confirm these predictions experimentally by measuring single-photon spectra, on one hand, and the dependence of Hong-Ou-Mandel interference visibility on the width of spectral filtering, on the other hand.

Figure 1

(Color online) Schematic shape of the two-photon spectral intensity (TPSI). The HOM interference visibility depends on the relative overlap between TPSI and its reflection with respect to the ${\omega }_i={\omega }_s$ line. A small relative overlap always indicates that TPSI is stretched, and then the TPSI tilt can be experimentally found from the widths of single-photon spectra $\Delta {\omega }_s,\Delta {\omega }_i$. By measuring the overlap versus the filtering bandwidth, one can find the TPSI aspect ratio.

Figure 2

(Color online) TPSI on a large scale and after a zoom into its center (top right inset). Bottom left inset shows the Schmidt decomposition.

Figure 3

Experimental setup: O1,2, coupling and decoupling objectives; UVM, highly reflective UV mirror; HWP, half-wave plate; PBS, polarizing beam splitter; APD1,2, single-photon counting modules (SPCMs); FP, Fabry-Pérot interferometer; OSA, optical spectrum analyzer; IF, interference filters; SPR CCD, single-photon-resolving charge-coupled device (CCD) camera combined with a grating spectrometer.

Figure 4

(Color online) Single-photon spectra of signal and idler radiations.

Figure 5

(Color online) Experimental dependencies of the coincidence counting rate on the HWP orientation for the filtering bandwidths of (a) 0.45 and (b) 0.15 nm. Solid lines show the fit of experimental data.

Figure 6

(Color online) TPSA overlap measured (points) and calculated (solid line),versus the filter bandwidth. Dashed line shows the same dependence calculated for a two-photon state twice less entangled and dotted line shows that for a state twice more entangled.