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https://physics.aps.org/articles/v19/38
INTRO: 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.
The ideas behind indefinite causal order emerged in the mid-2000s and early 2010s in pioneering work by Lucien Hardy, Giulio Chiribella, Ognyan Oreshkov, and others, says Lee Rozema of the University of Vienna. In this framework, two events can occur in a coherent superposition of one happening before the other and vice versa, so that no single causal order is well-defined—defying the roots of our classical intuition.
The concept evokes baffling thought experiments that echo Schrödinger’s cat. Imagine making a cake. In the classical world, “I mix the ingredients, I bake them, I let the cake cool down, and then I eat it,” says Krister Shalm of the National Institute of Standards and Technology in Colorado, who was not involved in the work. “In quantum mechanics, in some cases, you can still be mixing the ingredients and already be eating [the fully baked cake].” (MORE - details)
INTRO: 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.
The ideas behind indefinite causal order emerged in the mid-2000s and early 2010s in pioneering work by Lucien Hardy, Giulio Chiribella, Ognyan Oreshkov, and others, says Lee Rozema of the University of Vienna. In this framework, two events can occur in a coherent superposition of one happening before the other and vice versa, so that no single causal order is well-defined—defying the roots of our classical intuition.
The concept evokes baffling thought experiments that echo Schrödinger’s cat. Imagine making a cake. In the classical world, “I mix the ingredients, I bake them, I let the cake cool down, and then I eat it,” says Krister Shalm of the National Institute of Standards and Technology in Colorado, who was not involved in the work. “In quantum mechanics, in some cases, you can still be mixing the ingredients and already be eating [the fully baked cake].” (MORE - details)