What quantum sensors can be used as GPS replacements?

There’s a variety of quantum sensors available for use as a global positioning system (GPS) replacement including quantum compasses, atomic gyroscopes, and atom interferometers. Several options for a quantum GPS replacement, also called quantum position sensing (QPS), are being developed.

Three examples include quantum active navigation that overlays and supplements existing satellite-based GPS systems, quantum passive navigation that uses quantum inertial sensor systems to provide location information independent of traditional GPS, and hybrid quantum accelerometers that combine the features of quantum and classical accelerometer technologies.

QPS is expected to provide highly secure and precise systems without the vulnerabilities of today’s GPS signals. While GPS signals can be disrupted or spoofed, QPS uses the Earth’s magnetic field to provide robust operation. QPS can be divided into three categories, quantum active navigation, quantum passive navigation, and a hybrid approach.

Active quantum navigation
Active QPS uses a system architecture like conventional GPS. One proposed architecture for active GPS uses three baselines, each using two low-earth-orbiting (LEO) satellites with the center of the Earth as the coordinate origin (Figure 1). The baselines form a three-axis coordinate system perpendicular to each other.

Each baseline consists of a semiconductor laser, a delay filter, a beam splitter, and two-photon detectors. The laser sends a beam to the two satellites. Satellite-based QPS requires many entangled photons during the ranging process. By adjusting the delay time to minimize the number of entangled photons, the paths have the same propagation time. By calculating the delay generated by the delay filter and the distance between the satellites, the position of the target can be calculated.

Passive quantum navigation
Like active QPS, passive quantum navigation is still aspirational, but it’s closer to realization. Passive quantum navigation relies on quantum principles to create an inertial navigation system. Compared with conventional inertial navigation systems, quantum systems will experience less drift and provide higher sensitivity and accuracy. Passive quantum navigation systems will consist of four elements, a three-dimensional atomic gyroscope, quantum accelerometer, atomic clock, and signal processing module.

Quantum atomic clocks are highly developed and use single ions that are laser-cooled in an electromagnetic ion trap. They are several orders of magnitude more accurate than today’s Cesium atomic clocks. The signal processing element needed for passive quantum navigation can be fabricated with existing technology.

Quantum accelerometers and gyroscopes are a different story; they operate based on the wave properties of matter. Lasers are used to cool clouds of atoms to near absolute zero, where they act like waves of light, creating interference patterns. Another set of lases is used to measure changes in the interference patterns and that information can be used to precisely track the movement of the accelerometer through space and the angular velocity in a gyroscope.

Quantum accelerometers and gyroscopes are relatively large structures and efforts are underway to shrink and ruggedize them for use on ships and submarines. In the future, cold-atom accelerometers and gyroscopes may shrink to fit in a package several millimeters on a side.

Hybrid approach
Hybrid quantum navigation is based on the combination of classical inertial measurement units (IMUs) with quantum sensors based on atom interferometry described above. The ideal inertial sensor must provide continuous signals at a high rate and maintain precision and sensitivity over long periods of time. Classical IMUs can provide continuous signals but can drift over time. Atom interferometers can provide drift-free measurements at high sensitivities, but the measurement process has significant dead times when no new signal is available.

By combining both approaches, a hybrid inertial navigation system has been demonstrated that combines the benefits of classical IMUs and atom interferometry. The classical IMU is the primary sensor, and the atom interferometer is used to provide continuous recalibration to overcome the problem of drift. The combined device produces a steady signal at the same rate as a classical IMU but with 50 times the precision (Figure 2).

Summary
Quantum sensors are envisioned that can enable active and passive QPS architectures. However, quantum sensors, especially quantum IMUs have performance limitations. Combining classical IMUs with quantum atom interferometry in a hybrid approach may provide a higher-performance solution.

References
A Quantum of Sensing—Atomic Scale Bolsters New Sensor Boom, IEEE Spectrum accelerators and clocks
High-Tech Quantum Sensors: Navigating When GPS Goes Dark, SciTech Daily
Quantum sensor for a future navigation system tested aboard Royal Navy ship, Imperial College London
Quantum sensors for GPS-free orientation, Physics.org
Review of Quantum navigation, IOP Conference Series: Earth and Environmental Science
Tracking the vector acceleration with a hybrid quantum accelerometer triad, Science Advances