Atom interferometers have existed for 30 years, but none could accomplish what Folman and his colleagues wanted to do. So, they built a new one called the Quantum Galileo Interferometer (QGI).

Atoms under a chip

The QGI is rather tiny and relies on microwave and magnetic pulses. It uses about 20,000 rubidium atoms maintained as a Bose-Einstein condensate, held by currents in microscopic wires on a chip that hangs upside-down roughly 113 micrometers above them. A microwave pulse puts every atom into superposition of two states, where one state feels magnetic forces, and the other is blind to them. A magnetic pulse then plays the role of the cannon, kicking any magnetically sensitive atoms upward.

This converts the superposition of states into a superposition of trajectories.

Immediately afterward, a second microwave pulse flips the atom into the opposite state, so the superposition that’s flying upward changes into a magnetically blind one and starts falling under gravity, with no other forces influencing it. At the same time, its stationary twin becomes sensitive to magnetism, but the field is tuned so its upward force exactly cancels the atom’s weight, effectively making it hover motionless with respect to the Earth.

As one of the trajectories falls back, a second magnetic pulse acts as a parachute, killing its speed just as it returns to where it started. In Folman’s experiment, at their farthest apart, the two trajectories were about 7.5 micrometers apart, and the longest flight lasted two-thousandths of a second.

At the end of each run, the atoms that landed in one of the interferometer’s two exits were sorted by the phase accumulated by the falling path. As the researchers lengthened the fall, this phase increased—a behavior in line with the theory.

“When somebody falls in free fall, he doesn’t feel his weight. He has no weight. Sometimes we feel [a bit of this] in an elevator that goes down very fast,” Folman says. In the quantum world, the falling path from the atom’s point of view experiences no gravity or any other force, and its wave should not pick up any extra phase. At the same time, from the laboratory’s point of view, gravity is pulling the same falling phase down, so its wave should pick up a specific amount of phase predicted by the theory. That phase difference did show up in the data.