Silicon Photonic Modulator Enables Atom Interferometry

Researchers are working to develop a motion sensor precise enough to reduce reliance on global positioning satellites. Until recently, a sensor like this would have filled a moving truck, but advancements are dramatically shrinking the size and cost of this technology.

At Sandia National Laboratories, researchers have reported the use of silicon photonic microchip components to perform a quantum sensing technique called atom interferometry — a precise way to measure acceleration. According to the researchers, this is the first time the technique has been achieved in this way, and the advance marks a milestone in the development of a quantum compass for navigation when GPS signals are unavailable.

“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,” said Sandia scientist Jongmin Lee.

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’ four-channel, silicon photonic single-sideband modulator chip, measuring 8 mm on each side and marked with a green Sandia thunderbird logo, sits inside packaging that incorporates optical fibers, wire bonds, and ceramic pins. Courtesy of Sandia National Laboratories/Craig Fritz.

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 newly developed high-performance silicon photonic 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 single-sideband modulator, a commercially available one, is 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.

The research was published in Science Advances (www.doi.org/10.1126/sciadv.ade4454).