Spectral filtering and crystal length as control parameters for conditional correlations in quantum imaging

Abstract: We establish spectral filtering as a control parameter for the conditional momentum and position correlations of SPDC biphotons used in quantum imaging. The conditional momentum uncertainty in spontaneous parametric down-conversion (SPDC) is strongly crystal-length and spectral-filter dependent along the walk-off axis; therefore, the effect is observed only in critically phase-matched (CPM) crystals such as $β$-barium borate (BBO) crystals, while quasi-phase-matched (QPM) crystals and the non-walk-off axis of BBO remain scaled strictly according to the standard pump waist size ($w_0$) dependent scaling law $1/w_0$-independent of the filter. In position space, the spectral-filter effect is universal and produces a flat-dip-rise (FDR) profile in every crystal class examined. Although this FDR profile was previously demonstrated in BBO only in the nondegenerate regime, our results establish its generality: the FDR dip is also present at exact degeneracy in QPM crystals, an unexpected feature that was previously thought to deliver a resolution advantage in the nondegenerate regime alone. Our treatment applies to all SPDC-based quantum-imaging regimes (including CPM and QPM crystals, walk-off and non-walk-off axes, degenerate and nondegenerate emission, and signal and idler filtering) and offers enhanced quantum-imaging resolution via the design rules presented-most prominently along the walk-off axis of CPM crystals (in far field) and across all transverse axes in the near field.