Will macroscopic quantum superposition get off the ground?

Quantum superposition is one of the fundamental principles of quantum mechanics. The principle affirms that valid quantum states can be added together and the result will be another valid quantum state. The curious interference pattern of photons in the famous double-slit experiment is an observable consequence of superposition.

In popular culture, though, when we talk about superposition, we think of Schrödinger's cat. A cat in a closed box with a poison – which is activated by a certain unpredictable quantum process – is both alive and dead until the box is open. The cat and its atoms are not in two different states, but since the system is isolated and governed by quantum mechanics, the description dead and alive is perfectly valid.

Macroscopic systems are usually too complex to “superpose,” so the Stanford team created a Bose-Einstein condensate (BEC) cloud with 10,000 supercooled rubidium atoms. When a substance is in a BEC state, all its atoms are in the same state, and quantum phenomena become apparent at the macroscopic level.

The scientists used a laser to send the BEC cloud about 10 meters high inside a super-chilled chamber. This caused the atoms to enter one of two possible states: a ground state (the minimum possible energy for the atoms) or a more excited state. The scientists observed that when the cloud reached the top, the two states were perfectly mixed and separated by 54 centimeters. The cloud was then allowed to reform and fall back to the bottom of the chamber, where the team observed that the cloud was in superposition; every atom appeared to be falling from both 10 meters and nine meters and 46 centimeters.

Although this achievement is incredible, it is still not clear if macroscopic objects can be put in superposition. Still, the ability to superpose large groups of atoms could allow for better tests of general relativity and gravity, and even future gravitational wave detectors.