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Harald Weinfurter coauthored the preparation and observation of three-photon GHZ states. The experiment combined two entangled photon pairs and selected appropriate coincidence detections, producing a conditional three-photon quantum state with polarization correlations beyond those of independent pairs. Its value was to make a genuinely multipartite entangled state available to direct measurement.^[ref]

A physicist who holds an experimental quantum-physics chair at LMU Munich.^[ref]

By Dik Bouwmeester, Jian-Wei Pan, Matthew Daniell, Harald Weinfurter and Anton Zeilinger. Published in Physical Review Letters 82, pp. 1345–1349.

Published article: doi.org/10.1103/PhysRevLett.82.1345

Open preprint: arxiv.org/abs/quant-ph/9810035

In 1998, Jian-Wei Pan, Dik Bouwmeester, Harald Weinfurter and Anton Zeilinger created two photon pairs by two passages of a pulsed pump through a crystal using spontaneous parametric down-conversion. Interfering the inner photons at a nonpolarizing beam splitter implemented a partial Bell-basis measurement. Narrow optical filters and spatial filtering removed distinguishing information; changing the outer polarizer bases checked the conditional correlations. Fourfold detections selected the entanglement swapping events. The reported interference supported conditional quantum entanglement, but the paper reserved a statistically significant Bell inequality violation for improved visibility; it was not a demonstration closing the detection loophole.^[ref]

The ideal calculation makes the distinction between conditional and unconditional quantum states explicit. Start with spin singlet states $|{\psi }^{-}{⟩}_{12}|{\psi }^{-}{⟩}_{34}$, where $|{\psi }^{-}⟩=(|HV⟩-|VH⟩)/\sqrt{2}$ and $H,V$ denote orthogonal photon polarizations. Taking the inner product with $⟨{\psi }^{-}{|}_{23}$ gives $-|{\psi }^{-}{⟩}_{14}/2$: expanding the four terms leaves precisely the two antisymmetric outer terms. That Bell-basis measurement outcome has probability $1/4$ and leaves the outer pair in a spin singlet state. Ignoring the inner outcome instead leaves the outer density matrix $I_4/4$, a maximally mixed state. A conditional preparation flag is useful for later Bell tests only with the timing and setting conditions in setting-independent heralding preserves Bell locality.