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Entanglement Could Step in Where GPS Is Denied

Tapping quantum weirdness to make an ultra-accelerometer

3 min read
green boxes with green laser and pink pulses in middle

Each membrane in this array of membranes is probed by laser beams that are quantum mechanically entangled with one another.

Ehsan Faridi & Ehsan Keshavarzi/Inmywork Studio

Using the strange quantum phenomenon known as entanglement, which can link particles together anywhere in the universe, sensors can become significantly more accurate and faster at detecting motion, a new study reveals. The findings may help augment navigation systems that do not rely on GPS, scientists say.

In the new study, researchers experimented with optomechanical sensors, which use beams of light to analyze how their components move in response to disturbances. The sensors serve as accelerometers, which smartphones use to detect motions. Accelerometers can find use in inertial navigation systems in situations where GPS performs badly, such as underground, underwater, inside buildings, remote locations, and places where radio signal jamming is in use.

To boost the performance of optomechanical sensing, researchers experimented with using entanglement, which Einstein dubbed “spooky action at a distance.” Entangled particles essentially act in sync regardless of how far apart they are.

The researchers expect to have a prototype entanglement accelerometer chip within the next two years.

However, quantum entanglement is also incredibly vulnerable to outside interference. Quantum sensors capitalize on this sensitivity to help detect the slightest disturbances in their surroundings.

“Previous research in quantum-enhanced optomechanical sensing has primarily focused on improving sensitivity at a single sensor,” says study lead author Yi Xia, a quantum physicist at the University of Arizona at Tucson. “However, recent theoretical and experimental studies have shown that entanglement can significantly improve sensitivity among multiple sensors, an approach known as distributed quantum sensing.”

Optomechanical sensors depend on two synchronized laser beams. One beam gets reflected off a component known as an oscillator, and any movement of the oscillator changes the distance the light travels on its way to a detector. Any such difference in distance traveled shows up when the second beam overlaps with the first. If the sensor is still, the two beams are perfectly aligned. If the sensor moves, the overlapping light waves generate interference patterns that reveal the size and speed of the sensor’s motions.

In the new study, the sensors from Dal Wilson’s group at University of Arizona at Tucson used membranes as oscillators. These acted much like drumheads that vibrate after getting struck.

Instead of having one beam illuminate one oscillator, the researchers split one infrared laser beam into two entangled beams, which they bounced off two oscillators onto two detectors. The entangled nature of this light essentially let two sensors analyze one beam, altogether leading to improvements in speed and precision.

“The vision is to deploy such sensors in autonomous vehicles and spacecraft to enable precise navigation in the absence of GPS.”
—Zhenshen Zhang, University of Michigan

“Entanglement can be leveraged to enhanced the performance of force sensing undertaken by multiple optomechanical sensors,” says study senior author Zheshen Zhang, a quantum physicist at the University of Michigan at Ann Arbor.

In addition, to boost the precision of the device, the researchers employed “squeezed light.” Squeezed light takes advantage of a key tenet of quantum physics: Heisenberg’s uncertainty principle, which states that one cannot measure a feature of a particle, such as its position, with certainty without measuring another feature of that particle, such as its momentum, with less certainty. Squeezed light takes advantage of this trade-off to “squeeze” or reduce the uncertainty in the measurements of a given variable—in this case, the phase of the waves making up the laser beams—while increasing the uncertainty in the measurement of another variable the researchers can ignore.

“We are one of the few groups who can build squeezed-light sources and are currently exploring its power as the basis for the next-generation precision measurement technology,” Zhang says.

All in all, the scientists were able to collect measurements that were 40 percent more precise than with two unentangled beams and do it 60 percent faster. In addition, the precision and speed of this method is expected to rise in proportion to the number of sensors, they say.

“The implication of these findings would be that we can further push the performance of ultraprecise force sensing to an unprecedented level,” Zhang says.

He adds that improving optomechanical sensors may not only lead to better inertial navigation systems but also help detect enigmatic phenomena such as dark matter and gravitational waves. Dark matter is the invisible substance thought to make up five-sixths of all matter in the universe, and detecting the gravitational effects it might have could help scientists figure out its nature. Gravitational waves are ripples in the fabric of space and time that could help shed light on mysteries from black holes to the Big Bang.

The scientists now plan to miniaturize their system. They can already put a squeezed-light source on a chip just a half centimeter wide. They expect to have a prototype chip in the next year or two that includes a squeezed-light source, beam splitters, waveguides, and inertial sensors. “This would make this technology much more practical, affordable, and accessible,” Zhang says.

In addition, “we are currently working with Honeywell, JPL, NIST, and a few other universities in a different program to develop chip-scale quantum-enhanced inertial measurement units,” Zhang says. “The vision is to deploy such integrated sensors in autonomous vehicles and spacecraft to enable precise navigation in the absence of GPS signals.”

The scientists detailed their findings online 20 April in the journal Nature Photonics.

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