The supplementary materials for the study "Quantum entanglement and interference at 3 μm" describe the experimental setup and methods used to achieve quantum entanglement and interference at mid-infrared wavelengths. The half-wave plate (HWP) is used to adjust the phase of photons, and it is mounted on a rotating translation stage. The HWP's fast axis remains aligned with the 0-degree direction of the HWP holder, ensuring no change in polarization. As the HWP rotates, the optical path length changes, affecting the phase of the photons. The PZT is used to stabilize the interferometer using an auxiliary laser beam at 1077 nm. The phase difference introduced by the HWP rotation is calculated based on the optical path difference of the mid-infrared photons and the reference light at 1077 nm. The total phase difference is given by a formula involving the refractive indices and wavelengths of the photons. The CAR (Coincidence Rate) and interference visibility are calculated using formulas involving coincidence counts and accidental counts. The HOM interference visibility is defined as the ratio of maximum and minimum coincidence counts. The photon pair bandwidth is calculated using the central wavelength and coherence length. The coincidence window is chosen to ensure sufficient time resolution and avoid multiphoton recombination events. The MIR laser at 3082 nm is generated through a difference frequency generation (DFG) process, using a Ti:sapphire laser and a Yb-doped fiber laser. The MIR laser is used to pump the SPDC crystal to measure the second harmonic spectrum. The generated MIR laser has a power of 11.2 mW and a bandwidth of 0.03 nm. The MIR beam is tested in the UCD module to evaluate conversion and coupling efficiencies. The study also describes the difference frequency generation and up-conversion detection process.The supplementary materials for the study "Quantum entanglement and interference at 3 μm" describe the experimental setup and methods used to achieve quantum entanglement and interference at mid-infrared wavelengths. The half-wave plate (HWP) is used to adjust the phase of photons, and it is mounted on a rotating translation stage. The HWP's fast axis remains aligned with the 0-degree direction of the HWP holder, ensuring no change in polarization. As the HWP rotates, the optical path length changes, affecting the phase of the photons. The PZT is used to stabilize the interferometer using an auxiliary laser beam at 1077 nm. The phase difference introduced by the HWP rotation is calculated based on the optical path difference of the mid-infrared photons and the reference light at 1077 nm. The total phase difference is given by a formula involving the refractive indices and wavelengths of the photons. The CAR (Coincidence Rate) and interference visibility are calculated using formulas involving coincidence counts and accidental counts. The HOM interference visibility is defined as the ratio of maximum and minimum coincidence counts. The photon pair bandwidth is calculated using the central wavelength and coherence length. The coincidence window is chosen to ensure sufficient time resolution and avoid multiphoton recombination events. The MIR laser at 3082 nm is generated through a difference frequency generation (DFG) process, using a Ti:sapphire laser and a Yb-doped fiber laser. The MIR laser is used to pump the SPDC crystal to measure the second harmonic spectrum. The generated MIR laser has a power of 11.2 mW and a bandwidth of 0.03 nm. The MIR beam is tested in the UCD module to evaluate conversion and coupling efficiencies. The study also describes the difference frequency generation and up-conversion detection process.