Quantum Dot Re-excitation — Pulsed g²(τ) Comb

Pulsed Excitation

Pulse period T_rep 25 ns

Exciton lifetime τ 1.2 ns

Re-excitation probability 8%

Background / dark counts 2%

HBT Coincidence Readout

g²(0) estimate

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Single-photon purity

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Pulses fired / coincidences binned

0 / 0

How it works

A pulsed laser excites a quantum dot's exciton state once per period T_rep. The exciton decays radiatively with lifetime τ, emitting at most one photon per pulse in the ideal case — the hallmark of a true single-photon source. Each photon hits a 50/50 beamsplitter and is routed at random to detector D1 or D2 (a Hanbury Brown–Twiss setup), and every detector pair click is binned by delay into a coincidence histogram:

g²(τ) = ⟨n₁(t) n₂(t+τ)⟩ / (⟨n₁⟩⟨n₂⟩)

For an ideal pulsed single-photon emitter this histogram is a comb of peaks spaced by T_rep, with the central peak at τ = 0 vanishing — a photon cannot be detected at both arms from the same emission event. Two real mechanisms fill that dip back in:

Re-excitation — after emitting, the dot can be driven back up and emit a second photon within the same pulse, before the next pulse arrives. This is the dominant purity-limiting effect in above-band-pumped quantum dot sources.
Background / dark counts — detector dark counts or stray scattered light add uncorrelated clicks that also raise the central peak.

The simulator estimates g²(0) as the central-peak area divided by the mean area of the neighbouring side peaks (which are set to 1 by construction for a Poissonian source), and reports purity as 1 − g²(0). Real quantum dot sources report g²(0) below 0.01–0.05; g²(0) < 0.5 is the accepted threshold for calling a source "single-photon".

Signal photon Background click Side peak (τ=nT_rep) Central peak (τ=0)

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