Macroscopic objects tend to behave classically rather than quantum mechanically—unless they can be cooled to ultralow temperatures. With a macroscopic quantum system, physicists could elucidate both the transition from classical to quantum and the causes of decoherence, including the role of gravity. Such systems would also have potential sensor applications for small forces (see Physics Today, October 2018, page 19).

A suite of laser-cooling techniques has successfully pushed the temperature of atomic gases down to within a fraction of a degree above absolute zero, where quantum mechanics dictates their behavior. But the gases are not in macroscopic superposition states. Another potential route to macroscopic quantum systems is quantum optomechanics, in which an optical cavity is paired with a mechanical resonator of dimensions up to hundreds of millimeters (see the article by Markus Aspelmeyer, Pierre Meystre, and Keith Schwab, Physics Today, July 2012, page 29). Although researchers...

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