Dispersion-Cancelled Quantum OCT

Source Mode

Interferometer

Mirror delay τ 0 fs

Tissue dispersion (GDD) 200 fs²

Source bandwidth σ₀ 15 fs

Coincidence Interferogram

Quantum (dispersion-cancelled) Classical (dispersion-broadened)

Live Readouts

Coincidence rate3%

Dip visibility97%

Axial resolution3.8 µm

How it works

A nonlinear crystal converts a pump photon into a frequency-anticorrelated signal/idler pair (spontaneous parametric down-conversion, ω_s+ω_i = ω_pump). Idler goes to a reference mirror, signal probes the tissue sample, and both recombine on a beamsplitter feeding two detectors that register coincidences.

Coincidence rate:  C(τ) = R₀ [1 − V · exp(−τ² / 2σ²)]
Classical width:   σ_cl(GDD) = √(σ₀² + (GDD/σ₀)²)
Quantum width:     σ_q = σ₀            (even-order dispersion cancels)
Classical visibility: V_cl(GDD) = V₀ / √(1 + (GDD/σ₀²)²)

Mirror delay τ — scans the reference arm's path length; the coincidence rate dips to a minimum at zero relative delay (a Hong-Ou-Mandel-type dip), giving the depth marker.
Tissue dispersion (GDD) — group-delay dispersion the sample imprints on the light. For frequency-anticorrelated pairs, the two photons' opposite-sign detunings make their even-order dispersion terms cancel in the joint (coincidence) detection, so the quantum dip stays narrow; a classical (unentangled) measurement has no such cancellation and its interferogram broadens and shallows as GDD grows.
Source bandwidth σ₀ — sets the transform-limited coherence time and hence the best achievable axial resolution, Δz ≈ c·σ₀·2.355 / (2n).

This dispersion-cancellation effect (Steinberg, Kwiat & Chiao, 1992) underlies quantum-optical coherence tomography: it lets a scan keep micron-scale depth resolution through dispersive biological tissue where a classical OCT interferogram would blur.

Idler → reference mirror Signal → tissue sample Coincidence detectors

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