Over the past century, we have mostly gotten used to the concept of quantum superposition: A particle can exist as a combination of multiple states at once, as famously dramatized by Schrödinger’s cat. But quantum mechanics may allow for an even more mind-boggling type of superposition, called “indefinite causal order,” where the superposition is not between states but between a sequence of events. A few experiments have delivered hints of indefinite causal order, but they all had several “loopholes”—assumptions that leave room for classical explanations.
Now Carla Richter of the University of Vienna and colleagues have devised and performed a new experiment intended to reduce the number of assumptions involved [1]. While some loopholes remain open, the experiment is a useful step toward showing that “we actually have this superposition of orders that cannot be explained classically,” Richter says.
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