Scientists are attempting to make a motion sensor so precise it could minimize the nation’s reliance on global positioning satellites. Until recently, a sensor of this caliber — 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 used silicon photonic microchip components to perform a quantum sensing technique called atom interferometry, an ultra-precise way of measuring acceleration. It’s the latest milestone toward developing a kind of quantum compass for navigation when GPS signals are unavailable. 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. Sandia National Laboratories scientist Jongmin Lee (left) prepares a rubidium cold-atom cell for an atom interferometry experiment while scientists Ashok Kodigala, (right) and Michael Gehl initialize the controls for a packaged single-sideband modulator chip. Courtesy of Sandia National Laboratories/Craig Fritz. Sandia scientist Jongmin Lee and his team have been finding ways to reduce its size, weight, and power needs. They already 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 new modulator is the centerpiece 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 unprecedented 47.8 decibels — a measure often used to describe sound intensity but also applicable to light intensity — resulting in a nearly 100,000-fold drop. “We have drastically improved the performance compared to what’s out there,” said Sandia scientist Ashok Kodigala. 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. Just one full-size commercially available single-sideband modulator costs more than $10,000, Lee said. Miniaturizing bulky, expensive components into silicon photonic chips helps drive down these costs. “We can make hundreds of modulators on a single 8-inch wafer and even more on a 12-inch wafer,” Kodigala said. And because 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,” Lee said. 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. The research was published in Science Advances (www.doi.org/10.1126/sciadv.ade4454).
