The Mother of All Motion Sensors

August 14, 2024

(Nanowerk News) Unfold a smartphone, fitness tracker, or virtual reality headset, and inside you’ll find a tiny motion sensor tracking its position and movement. Larger, more expensive versions of the same technology, the size of a grapefruit and a thousand times more accurate, help navigate ships, planes, and other vehicles with GPS.

Now scientists are trying to create a motion sensor so precise that it could minimize the country’s dependence on GPS satellites. Until recently, such a sensor—a thousand times more sensitive than today’s navigation devices—would fill a moving truck. But advances in the field are drastically reducing the size and cost of the technology.

For the first time, Sandia National Laboratories scientists have used silicon photonic microchip components to perform a quantum-sensing technique called atom interferometry, an ultraprecise way to measure acceleration, the latest milestone in the development of a kind of quantum compass for navigation when GPS signals are unavailable.

The team published their findings and presented a new, efficient silicon photonic modulator — a device that controls light on a microchip — as a cover story in the journal Advances in science (“High-performance single-sideband silicon photonic modulators for cold atom interferometry”). The four-channel, single-sideband silicon Sandia National Laboratories photonic modulator, measuring 8 millimeters on each side and marked with the green Sandia Thunderbird logo, is housed in a package containing optical fibers, wire bonds and ceramic pins. (Photo: Craig Fritz)

The research was supported by Sandia’s Laboratory Directed Research and Development program and was conducted in part at the National Security Photonics Center, a research facility that develops integrated photonics solutions to complex national security problems.

GPS-free navigation a national security issue

“Accurate navigation becomes a challenge in real-world situations where GPS signals are unavailable,” said Jongmin Lee, a scientist at Sandia.

In war zones, these challenges pose a threat to national security because electronic warfare units can jam or spoof satellite signals, thereby disrupting military movements and activities.

The solution is a quantum sensor.

“By leveraging the principles of quantum mechanics, these advanced sensors provide unparalleled accuracy in measuring acceleration and angular velocity, enabling precise navigation even in areas without GPS,” Lee said. Modulator at the heart of chip-scale laser system

Typically, an atom interferometer is a sensor system that fills a small room. A complete quantum compass—more accurately called a quantum inertial measurement unit—would require six atom interferometers.

But Lee and his team have found ways to reduce its size, weight, and power requirements. They’ve already replaced a large, power-hungry vacuum pump with an avocado-sized vacuum chamber and consolidated several components, usually delicately arranged on an optical table, into a single, rigid device.

The new modulator is the centerpiece of a laser system on a microchip. Rugged enough to handle high vibrations, it would replace a conventional laser system, typically the size of a refrigerator.

Lasers perform several tasks in an atom interferometer, and the Sandia team uses four modulators to change the frequency of a single laser to perform different functions.

However, modulators often produce unwanted echoes, called sidebands, that must be mitigated.

Sandia’s suppressed-carrier single-sideband modulator reduces these sidebands by an unprecedented 47.8 decibels—a value often used to describe sound intensity, but also applies to light intensity—resulting in a nearly 100,000-fold reduction.

“We have significantly improved performance compared to what is available commercially,” said Sandia scientist Ashok Kodigala.

Silicon device available in bulk and more affordable

In addition to size, cost has been a major obstacle to deploying quantum navigation devices. Every atom interferometer needs a laser system, and laser systems need modulators.

“Just one full-size single-sideband modulator available commercially costs more than $10,000,” Lee said.

Miniaturizing large, expensive components and placing them in silicon photonic systems can reduce these costs.

“We can produce hundreds of modulators on a single 8-inch wafer, and even more on a 12-inch wafer,” Kodigala said.

Because they can be manufactured using the same process as virtually all computer chips, “this advanced quad-channel component, including additional custom functionality, can be mass-produced at a much lower cost than today’s commercial alternatives, enabling the production of quantum sensing units at lower cost,” Lee said.

As the technology approaches field deployment, the team is exploring applications beyond navigation. The researchers are investigating whether it could help locate underground cavities and resources by detecting the tiny changes they make to Earth’s gravitational pull. They also see potential for the optical components they’ve invented, including a modulator, in LIDAR, quantum computers, and optical communications.

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

Interdisciplinary team puts quantum compass concept into practice

Lee and Kodigala represent two halves of an interdisciplinary team. One half, including Lee, is made up of experts in quantum mechanics and atomic physics. The other half, like Kodigala, are experts in silicon photonics—think of a microchip, but instead of electric current flowing through its circuits, it has beams of light.

These teams collaborate at Sandia’s Microsystems Engineering, Science and Applications Complex, where scientists design, manufacture and test integrated circuits for national security applications.

“We have colleagues we can talk to about this and work together to figure out how to solve the key problems with this technology and make it usable,” said Peter Schwindt, a quantum sensor scientist at Sandia.

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 technology companies. The atom interferometer is a proven technology that could be a great tool for navigation without GPS. Sandia’s ongoing efforts are aimed at making it more stable, field-usable, and commercially viable.

The National Security Photonics Center works with industry, small businesses, academia, and government agencies to develop new technologies and help introduce new products. Sandia has hundreds of issued patents and dozens more pending that support its mission.

“I’m passionate about seeing these technologies put into practice,” Schwindt said.

Michael Gehl, a Sandia scientist who works with silicon photonics, shares that same passion. “It’s great to see our photonic chips being used in real-world applications,” he said.