Focus: Just Another Tool
Just as lasers evolved from lab curiosities to lab tools to commercial devices, physicists have expected the creation of Bose-Einstein condensates (BECs) of atoms a decade ago to lead to new types of measurements. Now, in the September Physical Review A, researchers report harnessing this ultracold, purely quantum state to measure, more precisely than ever before, an elusive force that bears witness to the quantum-mechanical churn of empty space.
According to quantum mechanics, although there is no net electric field in a vacuum, there are tiny fields everywhere, and they constantly change in strength and direction from one moment to the next. These fluctuations become stronger when they interact with a surface, which creates an adjacent region of increased fields, although the average remains zero. A neutral atom floating nearby would be pulled toward the surface, just as a paper clip is pulled toward the pole of a magnet, where the magnetic field is strongest. This so-called Casimir-Polder force was first derived in 1948  but it wasn’t measured definitively until 1993 . (A different and better known attraction between two parallel plates is called the Casimir effect.)
To measure this tiny force with much higher precision, Eric Cornell and his colleagues at the University of Colorado in Boulder and JILA, a research institute on the Boulder campus, employed the BEC, in which a cloud of ultracold atoms is coerced to join into a single quantum-mechanical state. “It’s very easy to look at the BEC,” says team member John Obrecht, whereas “it’s difficult to isolate the force on one atom” due to a surface.
The researchers created a horizontal cigar-shaped BEC of rubidium atoms several microns in diameter and gently set it oscillating vertically within a magnetic trap just below a silica plate. Then they measured the oscillation frequency. Because the Casimir-Polder force pulled more strongly on the upper edge of the cloud than on the lower edge, the frequency was altered by a few parts in 10,000 compared with its value when the BEC was moved far from the surface, where the force had no effect. The team determined the force from the frequency measurements and repeated the experiment at a range of distances between 6 and 10 microns.
Other research teams haven’t been able to detect the exceedingly small force on the atoms beyond 2 microns from the surface. With the BEC, the Boulder team measured forces hundreds of times weaker than others had measured at closer range. The results agree well with recent theoretical calculations .
The team also hopes to detect another effect: When the atoms are more than about 5 microns from a room temperature surface, infrared and other thermal radiation from the surface should overwhelm the quantum vacuum fluctuations and provide a force stronger than the Casimir-Polder effect. Even with the BEC technique the researchers can’t quite see this “thermal” effect, but Obrecht is hopeful that heating the silica plate will allow them to distinguish it clearly.
Steve Lamoreaux of Los Alamos National Lab in New Mexico, who first cleanly measured the Casimir force, calls the new experiment a “marvel of modern science.” He adds that “this is one of the first direct applications [of BEC] to a measurement where the condensate itself wasn’t the focus.”
- H. B. G. Casimir and D. Polder, “The Influence of Retardation on the London-van der Waals Forces,” Phys. Rev. 73, 360 (1948)
- C.I. Sukenik et al., “Measurement of the Casimir-Polder Force,” Phys. Rev. Lett. 70, 560 (1993)
- M. Antezza, L. P. Pitaevskii, and S. Stringari, “Effect of the Casimir-Polder Force on the Collective Oscillations of a Trapped Bose-Einstein Condensate,” Phys. Rev. A 70, 053619 (2004)