Ultra-precise motion sensor could enable GPS-free navigation

Peel apart a smartphone, fitness tracker or virtual reality headset, and inside you’ll find a tiny motion sensor tracking its position and movement. Bigger, more expensive versions of the same technology help navigate ships, aeroplanes and other vehicles with GPS assistance. Now, scientists are attempting to make a motion sensor so precise that it could minimise our reliance on global positioning satellites.

Until recently, such a sensor — a thousand times more sensitive than today’s navigation-grade devices — would have filled a moving truck, but advancements are dramatically shrinking the size and cost of this technology. Researchers from Sandia National Laboratories have now used silicon photonic microchip components to perform a quantum sensing technique called atom interferometry — an ultra-precise way of measuring acceleration — with their results published in the journal Science Advances.

As explained by Sandia scientist Jongmin Lee, “Accurate navigation becomes a challenge in real-world scenarios when GPS signals are unavailable.” For example, in a war zone, these challenges pose national security risks, as electronic warfare units can jam or spoof satellite signals to disrupt troop movements and operations. Quantum sensing offers a solution.

“By harnessing the principles of quantum mechanics, these advanced sensors provide unparalleled accuracy in measuring acceleration and angular velocity, enabling precise navigation even in GPS-denied areas,” Lee said.

Typically, an atom interferometer is a sensor system that fills a small room. A complete quantum compass — more precisely called a quantum inertial measurement unit — would require six atom interferometers. But Lee and his team have been finding ways to reduce its size, weight and power needs — they already have replaced a large, power-hungry vacuum pump with an avocado-sized vacuum chamber and consolidated several components usually delicately arranged across an optical table into a single, rigid apparatus.

The team’s new high-performance silicon photonic modulator — a device that controls light on a microchip — is the centrepiece of a laser system on a microchip. Rugged enough to handle heavy vibrations, it would replace a conventional laser system typically the size of a refrigerator.

Lasers perform several jobs in an atom interferometer, and the Sandia team uses four modulators to shift the frequency of a single laser to perform different functions. However, modulators often create unwanted echoes called sidebands that need to be mitigated.

Sandia’s suppressed-carrier, single-sideband modulator reduces these sidebands by an impressive 47.8 dB — a measure often used to describe sound intensity but also applicable to light intensity — resulting in a nearly 100,000-fold drop. As noted by Sandia scientist Ashok Kodigala, “We have drastically improved the performance compared to what’s out there.”

Besides size, cost has been a major obstacle to deploying quantum navigation devices. Every atom interferometer needs a laser system, and laser systems need modulators. As noted by Lee, “Just one full-size single-sideband modulator, a commercially available one, is more than $10,000.”

Miniaturising bulky, expensive components into silicon photonic chips helps to drive down these costs. According to Kodigala, “We can make hundreds of modulators on a single 8″ wafer and even more on a 12″ wafer.”

Lee added that, since they can be manufactured using the same process as virtually all computer chips, “This sophisticated four-channel component, including additional custom features, can be mass-produced at a much lower cost compared to today’s commercial alternatives, enabling the production of quantum inertial measurement units at a reduced cost.”

As the technology gets closer to field deployment, the team is exploring other uses beyond navigation. Researchers are investigating whether it could help locate underground cavities and resources by detecting the tiny changes these make to Earth’s gravitational force. They also see potential for the optical components they invented, including the modulator, in LiDAR, quantum computing and optical communications.

“I think it’s really exciting,” Kodigala said. “We’re making a lot of progress in miniaturisation for a lot of different applications.”

The team’s grand plan — to turn atom interferometers into a compact quantum compass — bridges the gap between basic research at academic institutions and commercial development at tech companies. An atom interferometer is a proven technology that could be an excellent tool for GPS-denied navigation, and Sandia’s ongoing efforts aim to make it more stable, fieldable and commercially viable.

Image caption: Sandia National Laboratories’ four-channel, silicon photonic single-sideband modulator chip, measuring 8 mm on each side, sits inside packaging that incorporates optical fibres, wire bonds and ceramic pins. Image credit: Craig Fritz.