In a paper published in Physical Review Letters, a research team from the MIT-Harvard Center for Ultracold Atoms presents a method to increase the precision of atom interferometry with Bose-Einstein condensates, a result, the group says, that eliminates a source of error endemic to earlier designs. Interferometers using the new concept, for example, could help resolve some fundamental questions in physics, such as the nature of the intermediate states between the quantum description of matter and the Newtonian description upon which everyday engineering depends.
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 companies have commercialized it 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, clusters of atoms that, when cooled almost to absolute zero, all inhabit the same quantum state. In this state, the atoms display some unusual properties, including extreme sensitivity to perturbation by outside forces.
A standard approach to building a Bose-Einstein condensate interferometer involves suspending a cloud of atoms—the condensate—in a chamber and then firing a laser beam into it to produce a “standing wave.” If a wave is thought of as a squiggle with regular troughs and crests, then a standing wave is produced when a wave is precisely aligned with its reflection. The zero points—the points of transition between trough and crest—of the wave and its reflection are identical.
The standing wave divides the condensate into approximately equal-sized clusters of atoms, each its own condensate. In the MIT 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.
When outside forces act on the condensates, the laser trap keeps them from moving. But when the laser is turned off, the condensates expand, and their quantum state reflects the forces to which they were subjected. Shining a light through the cloud of atoms produces a visual pattern from which those forces can be calculated.
This technique has yielded highly accurate measurements of gravitational and inertial forces but 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, the MIT method uses not one but two condensates as the starting point for its interferometer. And in addition to trapping the condensates with a laser, the researchers also subject them to a magnetic field. Both condensates consist of rubidium atoms, but with different “spins,” a quantum property that describes magnetic orientation. The standing wave segregates both groups of atoms, but only one of them—the spin-down atoms—feels the magnetic field. As a result, atoms in the other group—the spin-zero atoms—are free to move from well to well in the standing wave.
Since a relative excess of spin-down atoms in one well gives it a slight boost in energy, it knocks 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 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.
A Bose-Einstein condensate is interesting because it exhibits relatively large-scale quantum effects, and quantum descriptions of physical systems generally reflect wave-particle duality—the fact that, at small enough scales, matter will exhibit behaviors characteristic of both particles and waves. The condensates in the MIT experiments can thus be thought of as waves, with their own wavelengths, amplitudes and phases.
In atom interferometry, the clusters of atoms trapped by the laser must all be in phase, meaning that the troughs and crests of their waves are aligned. The MIT team showed that its “shielding” method kept the condensates in phase much longer than was previously possible, a result that should improve the accuracy of atom interferometry, according to the researchers.
“One of the great expectations for Bose-Einstein condensates [BECs], which was highlighted in the Nobel citation, was that they would lead to applications,” says Dominik Schneble, an associate professor of physics at Stony Brook University. “And one of those applications is atom interferometry. But interactions between BECs basically give rise to de-phasing, which cannot be very well-controlled. One approach has been to turn the interactions off. In certain elements, one can do this very well. But it’s not a universal property. What they are doing in this paper is they’re saying, ‘We accept the fact that the interactions are there, but we are using interactions such that it’s not only not a problem but also solves other problems.’ It’s very elegant and very clever. It fits the situation like a natural glove.”
William Burton, an MIT graduate student in physics, is first author on the paper. He is joined by his advisor, professor of physics Wolfgang Ketterle, who won the 2001 Nobel Prize in physics for his pioneering work on Bose-Einstein condensates; Colin Kennedy and Woo Chang Chung, both graduate students in physics; Wenlan Chen, a postdoc at MIT’s Research Laboratory of Electronics; and Samarth Vadia, an undergraduate physics major. All are members of the MIT-Harvard Center for Ultracold Atoms, which Ketterle directs.