Figure 1: Experimental setup.

A frequency-doubled vertically polarized laser pulse with a pulse width of ~200 fs (central wavelength: 390 nm, average power: 120 mW) from a mode-locked Ti:Sa laser is focused onto a type-I β-barium-borate (BBO, thickness: 2 mm) crystal by a lens (not shown) with a focal length of 300 mm. Horizontally polarized signal (s) and idler (i) photons are generated via non-collinear frequency-degenerate spontaneous parametric down-conversion (SPDC) and filtered by a set of narrow bandpass filters (F). They are centred at 780 nm with FWHM of 4 nm. The signal and idler photons are then coupled into single-mode fibres (SMF) for cleaning up the spatial modes. The outputs of the SMFs are collimated and compensated for polarization rotation, induced by the propagation through the SMFs, with a set of half-wave and quarter-wave plates (not shown). The photons are then combined at a 50:50 beam splitter (BS). The temporal overlap of the photons at the BS is controlled by adjusting the arrival time delay τ between the signal and the idler photons with a fibre collimator attached to a motorized stage located at the input mode b of the BS. Photons are then detected at the output modes (c and d) of the BS with single-photon detectors D₁~D₄ (Perkin-Elmer SPCM-AQ4C) coupled to single-mode fibres and 50:50 fibre beam splitters (FB), the coincidence window is less than 8 ns. Different detector arrangements permit to detect m and n photons at the output modes c and d, respectively, denoted by (m,n)-detection. Detection of (2,0) and (4,0) can be observed in a and detection of (1,1) and (3,1) can be observed in b.