Sandia Trapped Atoms on a Glass Fiber 200 Times Thinner Than a Hair Using 5 Milliwatts. The Goal Is a Chip That Navigates When GPS Is Jammed.

ALBUQUERQUE — Jamming GPS is easy. You broadcast noise loud enough to drown out the faint signals from positioning satellites, and every aircraft in range is thrown back on its own accelerometers and gyroscopes, which drift off course within minutes to hours. Sandia National Laboratories has been working on a quantum replacement that would hold a heading far longer, and a paper in AVS Quantum Science reports the team has cleared one of the obstacles that kept the idea on paper for decades.

The device is an atom interferometer, a motion sensor that uses the wave nature of ultracold atoms to measure acceleration with a precision no mechanical instrument can match. Laboratory versions work by dropping a cloud of atoms through a vacuum chamber and reading their motion with laser pulses. The trouble is that a hard jolt can knock the atoms out of the lasers' view mid-measurement. Sandia's Jongmin Lee is pursuing a different geometry: instead of free fall, the atoms are held in halos of light hugging an optical fiber only 420 nanometers across, and guided along it like marbles through a pipe. Rock the fiber and the atoms shift with it. They do not fall off.

The new results show the team trapped cesium atoms on that nanofiber with just 5 milliwatts of optical power, roughly 2,000 times less than an LED bulb draws and one-sixth to one-fourth the power previous approaches needed. Using 150 nanowatts, they were able to take measurements of atomic coherence that mimic atom interferometry. "Our nanofiber results show a clear potential path toward chip-scale quantum inertial sensing," Lee said.

Power matters because of heat. The lasers that create the guiding halos also warm the guide, and a structure 200 times thinner than a human hair, sitting in vacuum with nowhere to dump heat, can crack like a burned-out light-bulb filament. For years engineers had to choose between fragile suspended waveguides that load atoms well but shatter, and rugged substrate-mounted ones that survive but load atoms poorly. "Both heat dissipation and efficient atom loading are really important," Lee said. "This idea has not been fully realized by the community for decades, due to challenges in dissipating heat from photonic devices in vacuum and in efficiently loading atoms around them."

The paper's second contribution is a membrane-waveguide that splits the difference. The nanothin membrane is anchored at each end by small pins of silicon that look enormous under a microscope and act as heat sinks, pulling warmth away from the laser-lit section. Cold, slow atoms produced by laser cooling drift into a hole in the membrane or a gap between two silicon needles, and the waveguide spanning that gap does the nanofiber's job of forming light halos to guide them. Lee tested his measurement protocols on the nanofiber because the membrane still has kinks to work out, but he said the approach "is feasible on the membrane-waveguide photonic integrated circuit platforms developed at Sandia."

What remains is to gather trapping and power data on the new guide, add the momentum kicks to the atoms that a real interferometer needs, and integrate everything on a photonic chip. "Our concept is not fully demonstrated yet, but we're very close," Lee said. The payoff would be an inertial sensor small and frugal enough to ride in a military vehicle or aircraft, and precise enough to keep it on course through turbulence or over rough roads while the satellites are silent.