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Wayfinding Without GPS

planets orbit around two compasses, illustration

Credit: CyberMagician

It is incredibly easy to overlook how Global Positioning System (GPS) satellites have revolutionized the way people navigate using airplanes, trains, boats, and automobiles. Widespread satellite coverage has rendered paper maps mostly obsolete and has completely automated the task of getting from Point A to Point B.

Yet GPS systems at times suffer from a fatal flaw: They don't work in some places and in certain situations, including in caves, under thick canopies of trees, underground, underwater, and in other places where signals cannot reach, such as outer space. It also is possible to jam or hack GPS. The result is a user experience that can range from mildly frustrating to outright dangerous.

As a result, researchers are pointing navigational research in a new direction: They are exploring ways to navigate without GPS signals. One method that has appeared on the radar is quantum entanglement. Although the field dates back to research first conducted by Albert Einstein in the 1930s, researchers have studied quantum entanglement over the greater part of the last century, and it is emerging at the center of quantum computing and other technologies.

"Quantum entanglement is a valuable resource that will likely introduce new and more powerful systems related to geospatial sensing and navigation," says Zheshen Zhang, an associate professor in the electrical engineering and computer science department of the University of Michigan. "The technique can improve both measurement sensitivity and how fast a measurement takes place."

Zhang, along with a group of researchers from the University of Arizona (where he was formerly a faculty member), are studying the viability of quantum entanglement for wayfinding—and other spatially related tasks. Their April 2023 paper, Entanglement-enhanced Optomechanical Sensing, reported on a method that uses optomechanical sensors and entangled light to deliver data about location—without a GPS signal.a

"The field of optomechanics has been advancing for years and it has reached a point where we're able to measure the quantum limited measurements of a single mechanical device. Entangled light would allow us to extend this capability to an array of devices, opening the door to a broad class of distributed force sensing applications, such as gravity-based subterranean imaging, seismometry, and magnetic resonance imaging," says Dalziel Wilson, an assistant professor in the College of Optical Sciences at the University of Arizona, Tucson, and a member of the research team.

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A Quantum Compass

The subatomic world of quantum entanglement has long fascinated physicists. Under appropriate conditions—natural or engineered—two particles, such as photons or electrons, become entangled. When this event takes place, certain properties of the particles (say, the color of the photons) stay connected, even when they are physically separated over distances as great as 100 kilometers.b Scientists refer to this as an emergent resource; Einstein dubbed it "spooky action at a distance."

In the physical world, entanglement occurs among millions and even hundreds of millions of particles. The phenomenon is common in nature, including in atoms and molecules residing within metals, minerals, and animals. "Quantum entanglement is a fundamental part of how nature works," says Jon R. Pratt, a mechanical engineer and former chief of the Quantum Measurement Division of the Physical Measurement Laboratory (PML) at the U.S. National Institute of Standards and Technology (NIST).

Nevertheless, scientists are still struggling to understand the nature and properties of quantum entanglement in environments as diverse as wet biological spaces and outer space. What scientists do understand is that once hundreds of particles become entangled, they display remarkable qualities. For instance, they can behave as a single unified object—for nanoseconds or microseconds—until they become unentangled. Some scientists postulate this phenomenon is what makes it possible for a flock of migrating birds to act as a singular group—even when the birds are not in direct contact with one another.

The value of this naturally occurring behavior has not escaped scientists, who increasingly plug quantum mechanics into a variety of technology frameworks and systems, including quantum computers, quantum networks, and new types of encrypted communication. "Quantum entanglement has been used for several years already, mostly in the area of precision measurements," says Felipe Guzman, an associate professor in the Department of Aerospace Engineering (with a joint appointment in the department of Physics and Astronomy) at Texas A&M University.

For example, he is examining how to use quantum entanglement to enhance the sensitivity of gravitational wave observatories that incorporate laser interferometer systems and inertial sensors. This could lead to any array of applications, including the use of lasers for ultra-precise sensing within nano systems and better environmental sensing techniques based on spatial relationships and data. "This could greatly increase today's resolution levels," he says.

"Quantum entanglement has been used for several years already, mostly in the area of precision measurement."

The field is certainly attracting growing attention. In 2022, John Francis Clauser, Alain Aspect, and Anton Zeilinger (working independently) captured a Nobel prize for their multi-year research into entangled photons.c Guzman says that while a great deal of research currently revolves around entangled light, there's also interest in entangled matter. "This could open the door to many remarkable applications," he says.

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Untangling Spatial Relationships

The study conducted by the University of Arizona group aimed to take sensing where it had not gone before. "In quantum mechanics, there's a special type of measurement noise, called back action, which dictates that the act of measuring something disturbs the outcome of that measurement," Wilson says. "Entanglement allows you to engineer correlations between the back action of a set of measurements, so that when you combine them appropriately, the net disturbance is smaller."

More specifically, there are two types of noise in optical position measurement: apparent motion due to the random arrival of photons on the object being measured (scientists call this "imprecision noise") and physical motion due to those photons imparting their momentum on the object (scientists refer to this as "backaction noise"). "Entanglement ensures that, for a set of measurements on physically separate objects, both forms of noise are correlated," Wilson explains. 100-nm-thick sheet of glass, supporting tiny drum modes—to produce an accelerometer. A pair of entangled laser beams probe the sensors by sending "squeezed" light though a beam splitter. Each beam reflects off its membrane and encodes its vibration—and therefore its acceleration—as a time delay. "Since a laser beam is a wave, this time delay corresponds to a phase shift," Wilson explains. "The intensity can be measured by overlapping the beam with another reference laser beam, called a local oscillator."

An algorithm sums the phase of the two reflected fields and returns information about their collective motion, with the level of uncertainty reduced through entanglement. However, with a collection of sensors in place, the team also can extract information about their relative motion, something akin to a topographical map, Wilson explains. Adds Zhang: "A sensor network allows you to map out the geometry of the object you are interested in and understand its spatial relationships."

Entanglement-enhanced sensing is extraordinarily challenging because any loss of photons leads to diminished performance. It is crucial to engineer optomechanical sensors that preserve this low-loss condition. "To some extent, we had to sacrifice sensitivity in order to reflect back as many photons as possible," Wilson explains. So that they could maximize the light, the team integrated a highly reflective mirror behind the membrane, forming an optical cavity. "That way, the system measures any light that enters and senses the vibrations," he says.

In the end, the researchers were able to produce measurements that were 40% more precise and 60% faster than systems that use two unentangled beams. "It was a significant performance boost and a step forward," Zhang says.

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Evolving Beyond GPS

Quantum entanglement is not the only technology that could revolutionize geospatial sensing and inertial navigation. Other researchers are currently exploring a variety of approaches, including the use of low Earth orbit (LEO), satellites that calculate ground location within several meters, and more advanced inertial navigation systems that rely on acceleration, rotation, and other variables to detect the direction in which a device or vehicle is moving.

NIST's Pratt views entanglement as "the last knob that can be turned in the quest to gain the ultimate measurement resolution."

Yet, quantum entanglement is an intense area of scientific interest. As a complement to GPS, quantum-enhanced inertial navigation could fill in the blank spots due to weak or nonexistent signals. At the same time, the technology could introduce new use cases, such as finding oil or spotting geothermal energy sources deep beneath the earth. It could also be used for autonomous vehicles and in outer space. "When you introduce, say, hundreds of sensors and hundreds of different paths between them, you can plug this into a more complex algorithm that delivers a sharper image of the signal landscape," Wilson says.

"Many types of mapping and sensing systems, such as radar and LiDAR, could utilize entanglement," Zhang says. "It's also possible to interconnect quantum-based systems, and perhaps build a quantum Internet of Things. This framework would support distributed entanglement over great distances and possibly introduce new and entirely different sensing and navigation."

NIST's Pratt views entanglement as "the last knob that can be turned in the quest to gain the ultimate measurement resolution." He believes the breakthrough Zhang and Wilson reported is significant. "One of the things we come up against in precision measurement is that averaging a signal to mitigate noise only gets you so far so fast. If we can entangle multiple sensors, there is the promise of getting to the same resolution at much faster time scales, with much greater bandwidth."

For now, quantum-enhanced inertial navigation remains at a somewhat nascent stage. However, the project's researchers say they are optimistic about shrinking current system components and developing a functional entanglement-accelerometer on a single chip. The system would combine a squeezed-light source, beam splitters, waveguides, and inertial sensors. The ultimate goal is a practical and affordable quantum technology, Zhang says. Combined with more advanced algorithms, it will be possible to arrive at a commercially viable product.

Concludes Guzman, "Over the next decade or two, we are likely to witness an explosion in quantum technologies. Many of the ideas and research currently taking place in labs will lead to an array of real-world applications, including in areas such as sensing and navigation. Networks of inertial sensors could significantly change the way we observe, measure, and navigate many physical events and phenomena."

* Further Reading

Agrawal, A.R., Pluchar, C.M., Brady, A.J., Liu, Z., Zhuang, Q., Wilson, D.J., Zhang, Z., and Xia, Y.
Entanglement-enhanced Optomechanical Sensing, Nature Photonics, Vol. 17. Pages 470–477, April 20, 2023.;

Tang, J., Cai, Q., Cheng, Z., Xu, N., Peng, G., Chen, P., Wang, D., Xia, Z., Wang, Y., Song, H., Zhou, Q., and Deng, G.
A perspective on quantum entanglement in optomechanical systems, Physics Letters A, Vol. 429, Page 127966, March 30, 2022.;

Richardson, L., Hines, A., Schaffer, A., Anderson, B.P., and Guzman, F.
Quantum hybrid optomechanical inertial sensing, Applied Optics, Vol. 59, Issue 22, pp. G160-G166, June 2020.;

Erhard, M., Krenn, M., and Zeilinger, A.
Advances in high-dimensional quantum entanglement, Nature Reviews Physics, Vol. 2, pages 365–381, June 2020.;

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Samuel Greengard is an author and journalist based in West Linn, OR, USA.

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