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spontaneous parametric down-conversion uses a second-order nonlinear polarization to convert pump energy into correlated signal and idler photons. With a classical undepleted pump, a single-mode interaction-picture model isThe creation term acts on the signal/idler vacuum, so no classical signal seed is required. The pump supplies the emitted energy; vacuum fluctuations are not an extra energy source. In the small-gain limit the SPDC pair state iswith normalization corrections understood at second order. More generally it is a two-mode squeezed vacuum with correlated pair numbers. A classical calculation with both generated fields exactly zero would miss the spontaneous start.

Efficient generation requires ${\omega }_p={\omega }_s+{\omega }_i$ and approximate wave-vector matching ${\mathbf{k}}_p={\mathbf{k}}_s+{\mathbf{k}}_i$. birefringence or a reciprocal grating vector in quasi-phase matching can compensate dispersion. In a continuum-mode treatment the pair amplitude includesThe pump spectrum constrains the sum frequency, while phase matching constrains the allowed frequency combinations and emission directions. type-I phase matching produces daughters in the same polarization class; type-II phase matching produces orthogonal ordinary/extraordinary classes. Correlation of pair numbers or orthogonal polarizations does not alone guarantee a polarization-entangled state in two chosen output paths.

One application is a heralded single-photon source: detect one partner and condition measurements on the other. Low gain suppresses multiple-pair contamination, whose probability begins at fourth order in the gain. Efficient collection and low detector noise matter. Spectral entanglement with an unresolved herald can leave the partner mixed; engineering or filtering the joint spectrum improves purity at a possible cost in brightness. Heralding does not make a photon appear in every pump pulse.

A second application is entangled-pair generation for quantum communication and tests of Bell inequalities. If the assignments $H_s{V}_i$ and $V_s{H}_i$ are indistinguishable except for polarization, their amplitudes can form $(|H_s{V}_i⟩+e^{i\phi }|V_s{H}_i⟩)/\sqrt{2}$. Walk-off, group delay or frequency labels can reveal which assignment occurred and destroy polarization interference. Their compensation is part of preparing the state, not merely an improvement in count rate.

A primary example pumped a 3-mm beta-barium-borate crystal with a $351.1\mathrm{nm}$ argon-ion laser. Collecting the two intersections of its type-II emission cones selected paired output directions. Half-wave rotation and compensating birefringent crystals reduced temporal walk-off, and polarization analysers with avalanche photodiodes measured coincidence fringes. Varying analyser settings tested the correlations; accidentals, mode distinguishability and detector/collection assumptions limited interpretation. The reported visibility exceeded 97 percent and supported a Bell violation under the stated auxiliary assumptions, rather than a loophole-free claim.^[ref]

Other uses include preparing interfering photons for quantum-information experiments, correlation-based imaging, and pair-based detector calibration. All rely on specified spatial and spectral modes and coincidence statistics. A pump-blocked background run, delayed-coincidence estimate, gain dependence and polarization/temporal-overlap scans distinguish correlated pairs from fluorescence, detector dark counts and accidental coincidences. The useful resource is the controlled joint quantum state, not simply two bright output beams.