First technique to measure all spatial modes of entangled photon pairs

Entangled photon pairs from spontaneous parametric down-conversion carry information across thousands of spatial modes, but until now experimenters could only measure how energy is distributed across those modes if they already knew what the modes looked like. This technique determines the complete spatial Schmidt basis from scratch, with no prior knowledge required. Applied to real SPDC sources, it successfully characterizes states containing over 3000 Schmidt modes at up to 98% fidelity — the largest number reported. The advance removes a key bottleneck for using spatial entanglement in high-dimensional quantum communication and computing protocols.