Quantum reality more complex than previously thought

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Imagine you order a delivery of several glass vases in different colors. Each vase is sent as a separate parcel. What would you think of the courier if the parcels arrive apparently undamaged, yet when you open them, it turns out that all the red vases are intact and all the green ones are smashed to pieces? Physicists from the University of Warsaw and the Gdansk University of Technology have demonstrated that when quantum information is transmitted, nature can be as whimsical as this crazy delivery man.

Experiments on individual photons, conducted by physicists from the Faculty of Physics at the University of Warsaw (FUW) and the Faculty of Applied Physics and Mathematics at the Gdansk University of Technology (PG), have revealed yet another counterintuitive feature of the quantum world. When a quantum object is transmitted, its quantum property – whether it behaves as a wave or as a particle – appears to depend on other properties that at first glance have nothing to do with the transmission. These surprising results were published in the research journal Nature Communications.

Wave-interference experiments are some of the simplest and most elegant, and can be conducted by almost anyone. When a laser beam is directed at a plate with two slits, we observe a sequence of light and dark fringes. It has long been known that the fringes are visible even when just individual particles – single electrons or photons – pass through the slits. Physicists assume that every individual particle exhibits wave properties, passing through both slits at once and interfering with itself.

The situation is very different when it is possible to detect the path taken by a given photon or electron and determine which slit the particle has passed through, at least in principle. When information about the particle path leaks from the system to the observer, the interference disappears and instead of interference fringes no pattern is observed.

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