Cats and other everyday things we can see don’t exist in two mutually exclusive states at the same time. But Erwin Schrödinger’s proposal in 1935—that a cat could be in a superposition of being dead and alive simultaneously—and subsequent quantum theory say that a superposition of two states should be observable even at macroscopic scales. Does the lack of quantum behavior in large objects mean that the theory breaks down between the macroscopic and microscopic, despite its superlative predictions of phenomena at the smallest scale of matter? (For more on the classical–quantum boundary, see Physics Today, May 2004, page 25.)
A cat state, named after Schrödinger’s thought experiment, is defined as a quantum state that is in two classically distinct states or conditions at the same time. Various explanations have been put forth for why macroscopic cat states have never been observed. Perhaps macroscopic objects interact with their environments in such complex ways that no quantum state can survive for any measurable coherence time. Or such objects may have intrinsic sources of noise that interfere with the generation of quantum states.
Nevertheless, numerous experiments have demonstrated progressively more macroscopic Schrödinger cat states in trapped-ion quantum computers, superconducting quantum interference devices, Bose–Einstein condensates, matter–wave interferometers, and other systems. Now Marius Bild, Matteo Fadel, Yu Yang, and colleagues—all part of ETH Zürich's Hybrid Quantum Systems Group, led by Yiwen Chu—have created a cat state in a mechanical resonator made of 1017 atoms, which is among the most massive demonstrations to date.
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