Fig. 4: Quantum-state tomography.

a Experimental setup for quantum polarization-state tomography. A short-pass dichroic mirror (DM) with cut-on wavelength at λ = 1600 nm splits the signal (orange, upper arm) and idler (red, lower arm) frequency modes. Quantum-state tomography is performed with a set of quarter-wave plate (QWP), half-wave plate (HWP) and linear polarizer in each arm, then temporal correlations are measured as with the experiment described in Fig. 2a. b Measured spectra of the signal (orange) and idler (red) frequency modes. The dip around the degenerate wavelength (dashed green line) is caused by the slight detuning of the cut-on wavelength of the dichroic mirror from the degenerate SPDC wavelength. The dark-shaded areas mark the statistical uncertainty. The light blue lines show the level and standard deviation of the noise floor. The dashed, dark blue line is the reflection curve of the dichroic mirror. c and d Experimentally measured polarization density matrices \(\hat{\rho }\) for (c) for y-, and (d) x-polarized pump. For both cases, the real and imaginary parts \({\rm {Re}}(\hat{\rho })\) and \({\rm {Im}}(\hat{\rho })\), respectively, are shown. e and f Theoretically expected polarization density matrices obtained from fully vectorial Green’s function calculations for y- and x-polarized excitations of 3R-MoS₂, respectively.