In this month’s Scientific American article “Gravitational-Wave Detectors Get Ready to Hunt for the Big Bang,” Ross Andersen writes about an audacious plan to use exquisitely precise measuring tools called atom interferometers to map out what the universe looked like when it was just a trillionth of a second old.
Atom interferometers work by cooling down a cloud of atoms to just a few degrees above absolute zero. These atoms are so cold that all the atoms enter the same atomic state, becoming essentially indistinguishable. The cloud can then be treated as a single object.
If you want to use the atom interferometer to detect faint gravitational waves from just after the big bang, you would have to put the interferometers in spacecraft (two or more) flying in precise formation around the sun. Laser beams would excite each cloud of atoms, putting it into a superposition of two parts, with two different velocities. After 10 seconds another laser reverses the process, so that the two parts start coming back together. As the atom clouds overlap again, more lasers measure them. If during the 20 seconds it takes for the process to run its course a gravitational wave rolls through the space between the spacecraft, it will shift the distance between the two individual cloud pairs by a tiny amount, imparting a measurable change in the final state of the atoms.