This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Physicists at the University of Twente have improved the standard test for the quality of individual particles of light. By letting three photons interfere at the same time instead of two, they draw more information from every measurement.
Their experiment outperforms even a perfect, noise-free run of the old method. The work appeared in Physical Review Letters. Quantum technology based on light works only if the photons involved are as close to identical as possible.
In practice, they never quite are. Laboratories therefore measure in advance how well their photons match. Since 1987, that has been done with the Hong-Ou-Mandel experiment, in which two photons meet at a beam splitter.
If the two are indistinguishable, they always leave the splitter together. "Hong-Ou-Mandel is one of the first experiments you learn about in quantum optics," says Stefan van den Hoven, Ph.D. candidate and first author of the study. "It is usually also the first benchmark that quantum researchers use for their experiments." Anyone building a photonic quantum computer has to check not one pair of photons but hundreds.
The old test compares two photons at a time, so with 10 photons there are already 45 pairs to work through. On top of that, a single measurement never gives an answer. A quantum experiment produces one outcome at a time, and only after thousands of repetitions does it become clear how often each outcome occurs.
That distribution reveals how alike the photons are. The Twente researchers placed a second beam splitter behind the first so that a third photon can join in. That addition uses the very thing that gives photonic quantum technology its power: interference between photons.
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