Cats and other everyday things we see don’t appear to exist in two mutually exclusive states at the same time. But Erwin Schrödinger’s thought experiment in 1935—that a cat could be dead and alive simultaneously—and subsequent quantum theory specify that a superposition of two states should be observable even for macroscopic objects. But despite its superlative predictions of phenomena at the smallest scale of matter, large objects don’t appear to exhibit quantum behavior. Does that mean the theory breaks down at some microscopic–macroscopic transition, beyond which quantum mechanics ceases to apply? (For more on the classical–quantum boundary, see Physics Today, May 2004, page 25.)
One way to test whether quantum mechanics is valid at macroscopic scales is to experimentally generate a cat state, named after Schrödinger’s thought experiment. It’s a quantum state that is a superposition of two classically distinct states at the same time. In phase space, the two states correspond to well-separated probability distributions.
When it comes to determining what counts as macroscopic, researchers don’t entirely agree on a formal definition. But there are two generally accepted criteria that most would say qualify a cat state as macroscopic: The system being studied should be large—often, although not exclusively, measured by its number of atoms—and the two superposed states should have a distinguishable difference, such as being dead and alive or being separated by a long distance.
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 (see the article by Wojciech Zurek, Physics Today, October 2014, page 44). Or such objects may have intrinsic sources of noise that interfere with the generation of quantum states.
Nevertheless, numerous experiments have demonstrated Schrödinger cat states at sizes that are approaching macroscopic scales, in trapped-ion quantum computers, superconducting quantum interference devices, Bose–Einstein condensates, and matter–wave interferometers. 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 the most massive demonstration to date.1
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