Physics / mechanism
Quantum imaging exploits correlations between photon pairs—typically generated via spontaneous parametric down-conversion (SPDC) in nonlinear crystals (BBO, KTP, PPKTP)—to extract image information beyond classical limits. Ghost imaging uses one photon to illuminate the object while its entangled twin, never touching the object, carries the spatial information via coincidence detection; the image emerges from correlation measurements, not direct detection. Sub-shot-noise imaging suppresses photon-number variance below the Poisson limit using squeezed or N00N states, enabling SNR improvements scaling as √N over classical bounds. Current lab benchmarks: ~15 dB squeezing (PTB, 2023), ghost imaging at cm-scale resolution with <1 μW illumination. Key materials chokepoint is low-loss integrated nonlinear waveguides; free-space systems dominate but chip-scale PPLN and AlGaAs platforms are closing in. Entanglement rates in fiber-coupled sources now exceed 10⁸ pairs/sec.
Competitive landscape
Classical low-light imaging (EMCCD, SPAD arrays, InGaAs APDs) competes directly on sensitivity without the complexity overhead. Structured illumination and computational ghost imaging (single-pixel, classical speckle correlations) replicate some ghost-imaging advantages without entanglement. LIDAR with Geiger-mode APDs addresses range-gated low-flux scenarios.
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Frontier (open questions)
- To be added.