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Anton Zeilinger coauthored an experiment preparing GHZ states of three spatially separated photons. The distinguishing feature is the quantum coherence between the two three-photon alternatives: recording the same populations alone would also be compatible with an incoherent mixture. Measurements in additional polarization bases tested this distinction, making three-party entanglement an experimental resource for investigating local realism.^[ref]

A physicist researching quantum optics and quantum foundations.^[ref]

Alain Aspect, John F. Clauser and Anton Zeilinger shared the award equally for experiments with entangled photons, violations of Bell inequalities and pioneering quantum information science.^[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 2015, Marissa Giustina, Anton Zeilinger and collaborators combined spontaneous parametric down-conversion with rapid random analyzer choices and efficient transition-edge sensors. Local records included settings and detector signals; their Bell inequality used detected and undetected trials, avoiding fair sampling. Locally fixed time slots prevented coincidence-based trial selection. Calibrated optical and electrical delays checked spacelike separation of each choice from the remote readout. The violation addressed detection and locality together. Measurement independence still requires assumptions about the hidden state's origin; the experiment cannot exclude every possible common cause of source and settings.^[ref]

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.