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Solid State Quantum Magnetometers—Seeking out water worlds from the quantum world

“Follow the water!” The solar system is full of water in different states, from the Sun’s water vapor to the ice of Pluto and beyond. Water is not only linked to the possibility to sustain life, it is also interesting for its own geological properties and potential uses. For example, ice on the Moon and Mars could support human exploration. Comets that hit Earth may have deposited water on our planet. The icy comets and rings of Saturn reveal how solar systems change over time.

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Jun 5, 2024 - 00:00

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Solid State Quantum Magnetometers—Seeking out water worlds from the quantum world

On the left side, a white moon covered in ice with reddish ridges. One-eighth of the moon has been cut out to show spherical layers below the moon’s surface, from outside to center: Ice Shell-Ocean-Interior Core. On the right side, the same moon with straight magnetic field lines from bottom left to top right. Close to the moon, the field lines are bent.

Left: Jupiter’s moon Europa and its presumed interior. A thick ice shell covers a planetary saltwater ocean, presumed to hold twice as much water as Earth’s oceans. Right: Simulation of the ocean bending the magnetic field lines emitted by Jupiter that are close to Europa

Image credit: C. Cochrane/ NASA/JPL-Caltech

“Follow the water!” The solar system is full of water in different states, from the Sun’s water vapor to the ice of Pluto and beyond. Water is not only linked to the possibility to sustain life, it is also interesting for its own geological properties and potential uses. For example, ice on the Moon and Mars could support human exploration. Comets that hit Earth may have deposited water on our planet. The icy comets and rings of Saturn reveal how solar systems change over time.

Liquid water, however, has a special role in enabling life. Scientists have discovered indications that liquid water might exist on a number of moons orbiting our solar system’s gas and ice giants. The mantra of the astrobiology community is to “Follow the Water” to find life, so subsurface oceans on Jupiter’s Europa, Saturn’s Enceladus, and other moons are compelling targets for future missions.

However, looking beneath the miles-thick ice crusts of these planetary bodies with conventional remote-sensing instruments, like cameras and radar, is challenging. Until we can send landers or rovers that drill or melt through the ice, we can use other techniques to track down these enormous, but elusive, water bodies. One method—Magnetometry—stands out since magnetic fields penetrate solid material and can therefore provide information about the interior of planet-sized bodies.

Briny water conducts electricity; therefore, a saltwater ocean can function as a planet-sized electric circuit. The strong rotating magnetic field of the parent planet of an ocean world can induce an electric current in this “circuit,” which in turn disturbs and modifies the magnetic field near the ocean world under investigation. These magnetic field disturbances can be observed from a spacecraft and may indicate the presence of liquid water. For example, a distortion of Jupiter’s magnetic field in the vicinity of Europa was measured by the magnetometer on NASA’s Galileo mission, providing further evidence for the initial suspicions of a water ocean under that moon’s icy crust.

A bright green laser beam is hitting a tiny glassy cube. The bottom photo shows the same setup, but with a large circular filter added in front of the glassy cube. The laser beam is invisible behind the filter, the glassy cube emits a reddish glow.

The heart of optically pumped quantum magnetometers: a diamond crystal enriched with color centers. Unlike many other quantum systems, diamond and SiC solid state quantum color centers operate at room temperature and can be readily accessed electrically or optically. The bottom photo, filtering the laser light for the observer, shows the red-shifted emission response of the quantum system. This response is encoded with quantum spin information, and can be used to read environmental influences, such as temperature, pressure, electric and, most importantly for us, magnetic field properties.

Image credit A. Gottscholl/ NASA/JPL-Caltech

Solid-state quantum magnetometers are an upcoming instrument class promising to measure magnetic fields at competitive sensitivities, while offering lower size, weight, and power footprints. In addition, these instruments offer quantum benefits like self-calibration on spin-nuclear quantum interaction, which means that the magnetometer can compensate for drifts over time. This capability is especially important for decades-long missions to the outer ice-giants. Other solid-state quantum advantages include radiation resilience and an inherent ability to withstand very high/low temperatures.

Solid-state quantum magnetometers leverage quantum color centers located in semiconductors such as diamond and silicon carbide. Color centers are defects in the crystal lattice—for example, a missing atom or a different atom replacing a crystal atom. In everyday life, color centers give crystals their color, but they can also be probed on the quantum level using modulated light. Due to their quantum spin properties these color centers are sensitive to environmental magnetic fields. As these color centers are exposed to varying magnetic fields, the changing quantum spin properties can be read electrically and/or optically, providing insight into the magnetic field properties and enabling us to detect the presence of water.

Research teams at NASA’s Jet Propulsion Laboratory are developing two magnetometers to measure spin properties from space. The incredibly simple but elegant SiCMAG (Silicon Carbide Magnetometer, Lead Dr. Corey J. Cochrane) instrument reads spin properties electrically, while the OPuS-MAGNM (optically pumped solid state quantum magnetometer, Lead Dr. Hannes Kraus) promises access to higher sensitivities through the addition of optics. Optically pumped here means that the quantum system is pumped with green (diamond) or deep red (silicon carbide) laser light, and the system’s response is read with a light detector.

According to Dr. Kraus, “Novel quantum sensors not only enable new science, but also offer the chance to downscale former flagship-class instrumentation to a size and cost allowing flagship-class science on CubeSat-class platforms.”

NASA has been funding solid state quantum magnetometer sensor research through its PICASSO (Planetary Instrument Concepts for the Advancement of Solar System Observations) program since 2016. A variety of domestic partners from industry and academia support this research, including NASA’s Glenn Research Center in Cleveland, the University of Iowa, Q-Cat LLC and QuantCAD LLC, as well as international partners such as Japan’s National Institutes for Quantum Science and Technology (QST Japan) and ETH Zurich, a public research university in Zurich, Switzerland.

Two smiling team members stand on either side of an optical table with multiple instruments and a barrel-shaped open chamber.

PI Dr. Kraus (left) and postdoctoral researcher Dr. Andreas Gottscholl (right) in the JPL Quantum Magnetometer lab, with the optically detected magnetic resonance (ODMR) spectrometer apparatus—a larger-scale stepping stone towards a miniaturized integrated magnetometer instrument—built by Dr. Gottscholl in the background. The optically pumped quantum sensor crystals (not visible here, as the sensor itself is only millimeters in size) are located in the concentric barrel-shaped four-layer µ-metal chamber, which is capable of shielding the Earth’s and other magnetic field disturbances by a factor of 100,000.

Image Credit H. Kraus/ NASA/JPL-Caltech

Acknowledgment: The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).