Fig. 3: Experimentally measured biphoton properties in polarization and frequency DoFs.
From: Entangling entanglement: coupling frequency and polarization of biphotons on demand
In Case 1 (no polarization rotation) the two DoFs are decoupled: a The interference fringes are observed in the spectrum intensity. The experimental data are obtained from the setup depicted in Fig. 2b, and the simulation curve is obtained through a function proportional to \(| \phi ({\omega }_{{{{\rm{A}}}}},{\omega }_{{{{\rm{B}}}}}){| }^{2}{\cos }^{2}\alpha ({\omega }_{{{{\rm{A}}}}},{\omega }_{{{{\rm{B}}}}})\). b The high degree of polarization entanglement [concurrence \({{{{\mathcal{C}}}}}^{{{{\rm{Pol}}}}}=0.95(2)\) and fidelity F = 97.2%]. In Case 2 (a polarization rotation of π/4) the two DoFs are coupled: c Tracing over the polarization DoF destroys the interference in the spectrum. The measured spectrum of the biphoton state at the output of the NLI is identical to that from the SPDC source. d Tracing over frequency DoF leads to the depletion of polarization entanglement [\({{{{\mathcal{C}}}}}^{{{{\rm{Pol}}}}}=0.10(4)\)]. The cut-off wavelengths of the DCM constrain the range of the spectrum.
