Spontaneous Parametric Down-Conversion (SPDC)
Spontaneous Parametric Down-Conversion (SPDC)
One-line summary: The standard lab method for making entangled photon pairs — a pump photon splits inside a nonlinear crystal into two lower-energy photons that can be polarization-entangled.
The insight
SPDC is how most entangled photons used in Bell tests and quantum networks are generated. A high-energy pump photon passing through a nonlinear, birefringent crystal occasionally converts into two lower-energy photons (signal and idler), conserving energy and momentum — the phase-matching condition that only specific frequency/direction combinations satisfy.
Evidence
- From 2026-05-30-autoresearch-quantum-entanglement: SPDC "converts one photon of higher energy into a pair of photons of lower energy, in accordance with the laws of energy conservation and momentum conservation"; phase matching requires birefringent materials (Wikipedia: SPDC).
- From 2026-05-30-autoresearch-quantum-entanglement: Type-I — signal and idler share polarization (orthogonal to pump); Type-II — signal and idler are mutually perpendicular, and where their emission cones intersect the pair is polarization-entangled in a superposition (Wikipedia: SPDC).
- From 2026-05-30-autoresearch-quantum-entanglement: conversion is very inefficient (≈4×10⁻⁶ for optimized PPLN waveguides), but detecting one photon "heralds" its partner (Wikipedia: SPDC).
- Primary papers (from 2026-05-30-academic-research-quantum-entanglement): telecom entanglement swapping from PPLN-waveguide SPDC sources, fidelity to 80.5% after accidental-coincidence subtraction (Xue et al. 2012, PRA); asynchronous-source swapping at 0.84 fidelity, also yielding GHZ states (Tsujimoto et al. 2016, Sci. Reports); "flat-optics" GaP-film SPDC with pump-tunable entanglement (Sultanov et al. 2022, Optics Letters).
Why it matters
SPDC is the practical engine behind entangled-photon experiments and the source feeding entanglement swapping (Bell-state measurement on photons from two independent pairs entangles the two leftover photons that never interacted) — the primitive behind quantum repeaters and network fusion (see quantum-internet).
Open questions
- Brightness/efficiency and on-demand single-pair generation remain engineering limits; solid-state sources (quantum dots) are an alternative being pushed for repeaters (see quantum-internet).