If single quantum particles can exist in two places at once, and interfere with themselves in predictable patterns, what happens when there are two quantum particles? Can they interfere with each other? Prof. Mordehai Heiblum of the Weizmann Institute's Condensed Matter Physics Department and his research team have been experimenting with electrons fired across special semiconductor devices.

Quantum mechanics predicts that two electrons can indeed cause the same sort of interference as that of a single electron - on one condition: that the two are identical to the point of being indistinguishable. Heiblum and his team showed that, because of such interference, these two particles are entangled - the actions of one are inextricably tied to the actions of the other - even though they come from completely different sources and never interact with each other. The team's findings recently appeared in the journal Nature.

Dr. Izhar Neder and Nissim Ofek, together with Drs. Yunchul Chung, Diana Mahalu, and Vladimir Umansky, fired such identical electron pairs from opposite sides of their device, toward detectors that were placed two to a side of the device. In other words, each pair of detectors could detect the two particles arriving in one of two ways: particle 1 in detector 1 and particle 2 in detector 2, or, alternatively, particle 2 in detector 1 and particle 1 in detector 2. Since these two "choices" are indistinguishable, the choices interfere with each other in the same way as the two possible paths of a single quantum particle interfere.

The scientists then investigated how the choice of one particle affected the pathway taken by the other, and found strong correlations between them. These correlations could be affected by changing, for example, the length of the path taken by one particle. This is the first time an oscillating interference pattern between two identical particles has been observed, proving, once again, the success of quantum theory.

Prof. Mordehai Heiblum's research is supported by the Joseph H. and Belle R. Braun Center for Submicron Research; the Wolfson Family Charitable Trust; Hermann Mayer and Dan Mayer; and Mr. Roberto Kaminitz, Sao Paulo, Brazil. Prof. Heiblum is the incumbent of the Alex and Ida Sussman Professorial Chair in Submicron Electronics.

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