Concept of our quantum eraser under Einstein locality conditions. Hybrid entangled photon-pair source, labeled as S, emits path-polarization entangled photon pairs. System photons are propagating through an interferometer (Right) and the environment photons are subject to polarization measurements (Left). Choices to acquire welcher-weg (which-path) information or to obtain interference of the system photons are made under Einstein locality so that there are no causal influences between the system photons and the environment photons. Copyright © PNAS, doi:10.1073/pnas.1213201110

Of Einstein and entanglement: Quantum erasure deconstructs wave-particle duality

January 29, 2013
by Stuart Mason Dambrot

(—Quantum physics presents several counterintuitive features, including entanglement, tunneling and – as demonstrated in double-slit experiments – wave-particle duality. When studying wave-particle duality, however, so-called interferometric quantum eraser experiments – in which wave-like behavior can be restored by erasing path information – allow researchers to perform differential measurements on each of two entangled quantum systems. (Double-slit experiments not involving quantum erasure utilize superposition of single particles, while in quantum eraser experiments two particles are entangled.) Specifically, the particle feature’s welcher-weg (which-path) information is erased (or not) from one system, and interference-based measurements in the other system are used to observe (or not, as the case may be) its wave feature.

While previous quantum eraser experiments made the erasure choice before or (in delayed-choice experiments) after the interference – thereby allowing communications between erasure and interference in the two systems, respectively – scientists in Prof. Anton Zeilinger’s group at the Austrian Academy of Sciences and the University of Vienna recently reported a quantum eraser experiment in which they prevented this communications possibility by enforcing Einstein locality. They accomplished this using hybrid path-polarization entangled photon pairs distributed over an optical fiber link of 55 meters in one experiment and over a free-space link of 144 kilometers in another. Choosing the polarization measurement for one photon decided whether its entangled partner followed a definite path as a particle, or whether this path-information information was erased and wave-like interference appeared. They concluded that since the two entangled systems are causally disconnected in terms of the erasure choice, wave-particle duality is an irreducible feature of quantum systems with no naïve realistic explanation. The world view that a photon always behaves either definitely as a wave or definitely as a particle would require faster-than-light communication, and should therefore be abandoned as a description of quantum behavior.

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