Researchers from the Massachusetts Institute of Technology (MIT) and Draper Laboratory (both in Cambridge, MA) have come up with a new approach to atomic timekeeping that may enable more stable and accurate portable atomic clocks, potentially the size of a Rubik's cube instead of a room.1
While chip-sized atomic clocks (CSACs) are commercially available, the researchers say these low-power devices, about the size of a matchbox, drift over time, and are less accurate than fountain clocks, the much larger atomic clocks that set the world's standard. However, while fountain clocks are the most precise timekeepers, they can't be made portable without losing stability.
"You could put one in a pickup truck or a trailer and drive it around with you, but I'm guessing it won't deal very well with the bumps on the road," says Krish Kotru, a graduate student in MIT's Department of Aeronautics and Astronautics, and a Draper Lab Fellow. "We have a path toward making a compact, robust clock that's better than CSACs by a couple of orders of magnitude, and more stable over longer periods of time."
In the new clock, alkali-metal atoms are interrogated in a certain sequence based on stimulated Raman transitions with photons. The researchers' implementation of so-called Raman adiabatic rapid passage (ARP), a route to atom optics, is what enables the two-orders-of-magnitude improvement.
Kotru says such portable, stable atomic clocks could be useful in environments where GPS signals can get lost, such as underwater or indoors, as well as in militarily hostile environments, where signal jamming can block traditional navigation systems.
Laser instead of microwave probing
The most accurate atomic clocks today use cesium atoms as a reference. Since the 1960s, one second has been defined as 9,192,631,770 oscillations of a cesium atom between two energy levels. To measure this frequency, fountain clocks toss small clouds of slow-moving cesium atoms a few feet high and measure their oscillations as they pass up, and then down, through a microwave beam.
Instead of a microwave beam, the group chose to probe the atom's oscillations using laser beams, which are easier to control spatially and require less space. While some atomic clocks also employ laser beams, they often suffer from "AC Stark shift," in which exposure to an electric field, such as that produced by a laser, can shift an atom's resonant frequency. This shift can throw off the accuracy of atomic clocks. Kotru and his team looked for ways to use laser beams while avoiding AC Stark shift.
In laser-based atomic clocks, the laser beam is delivered at a fixed frequency and intensity. But Raman ARP applies laser pulses of changing intensity and frequency.
"For our approach, we turn on the laser pulse and modulate its intensity, gradually turning it on and then off, and we take the frequency of the laser and sweep it over a narrow range," says Kotru. "Just by doing those two things, you become a lot less sensitive to these systematic effects like the Stark shift."
The stability and accuracy of the system, he says, should be comparable to that of microwave-based atomic clocks on today's GPS satellites, which are bulky and expensive.
The team is now working to reduce the size of other components of the system, including the vacuum chamber and electronics.
1. Krish Kotru et al.,Physical Review A (2014); doi: 10.1103/PhysRevA.90.053611