Atom interferometry is the most sensitive known technique for measuring gravitational forces and inertial forces such as acceleration and rotation. It’s a mainstay of scientific research and is being commercialized as a means of location-tracking in environments where GPS is unavailable. It’s also extremely sensitive to electric fields and has been used to make minute measurements of elements’ fundamental electrical properties.
The most sensitive atom interferometers use exotic states of matter called Bose-Einstein condensates. In the latest issue of Physical Review Letters, MIT researchers present a way to make atom interferometry with Bose-Einstein condensates even more precise, by eliminating a source of error endemic to earlier designs.
Interferometers using the new design could help resolve some fundamental questions in physics, such as the nature of the intermediate states between the quantum description of matter, which prevails at very small scales, and the Newtonian description that everyday engineering depends on.
A laser beam standing wave divides the condensate into approximately equal-sized clusters of atoms, each its own condensate. In the MIT researchers’ experiment, for instance, the standing wave divides about 20,000 rubidium atoms into 10 groups of about 2,000, each suspended in a “well” between two zero points of the standing wave.
This technique has yielded highly accurate measurements of gravitational and inertial forces. But it has one problem: The division of the condensate into separate clusters is not perfectly even. One well of the standing wave might contain, say, 1,950 atoms, and the one next to it 2,050. This imbalance yields differences in energy between wells that introduce errors into the final energy measurement, limiting its precision.
To solve this problem, Burton, Ketterle, and their colleagues use not one but two condensates as the starting point for their interferometer. In addition to trapping the condensates with a laser, they also subject them to a magnetic field.
Both condensates consist of rubidium atoms, but they have different “spins,” a quantum property that describes their magnetic orientation. The standing wave segregates both groups of atoms, but only one of them — the spin-down atoms — feels the magnetic field. That means that the atoms in the other group — the spin-zero atoms — are free to move from well to well of the standing wave.
Since a relative excess of spin-down atoms in one well gives it a slight boost in energy, it will knock some of its spin-zero atoms into the neighboring wells. The spin-up atoms shuffle themselves around the standing wave until every well has the exact same number of atoms. At the end of the process, when the energies of the atoms are read out, the spin-zero atoms correct the imbalances between spin-down atoms.