Superposition of Orders of Quantum Measurements. Applying the superposition principle to a photon’s motion can lead it in two different directions at the same time. If a different order of operations is applied in each path, this can be used to create a genuinely indefinite order of operations (Copyright: Jonas Schmöle, Faculty of Physics, University of Vienna).
Our understanding of the world is mostly built on basic perceptions, such as that events follow each other in a well-defined order. Such definite orders are required in the macroscopic world, for which the laws of classical physics apply. However, in the quantum world orders can be 'scrambled'. It is possible for different orders of quantum operations to coexist in a superposition. The current work by a team of physicists from the University of Vienna and the Austrian Academy of Sciences is the first experimental quantification of such a superposition. It will be published in an upcoming issue of "Science Advances". When describing nature using physical laws, scientists often start from everyday experiences. However, our usual intuition does not apply to the quantum world. Physicists have recently realized that quantum theory even forces us to question innate concepts, such as the order in which things occur. Imagine, for example, a race between two friends, Alice and Bob. In everyday life, the winner is the first to cross the finish line. Thus, common sense says that either Alice wins, Bob wins, or they tie. This reasoning, however, is not always applicable in the quantum world. In fact, quantum mechanics allows each runner to win and lose in one race: Alice could reach the finish line both before and after Bob in quantum superposition. However, even if we held such a quantum race, how could we verify that both racers won in superposition? Part of the problem is that quantum mechanics says when we observe the race it "collapses". This means that we only see either Alice win or lose the race: we can't see the superposition. Witnessing scrambled orders of operations
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