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2-D Materials

Supersensitive crystal may help find dark matter

Ultracold ions overcome quantum limit to detect weak magnetic fields

by Neil Savage, special to C&EN
August 16, 2021

An illustration of a self-assembled quantum crystal.
Credit: S. Burrows/JILA
A self-assembled quantum crystal made of beryllium ions (red) detects weak electric fields and could help in the search for dark matter.

A quantum crystal made of ultracold ions has pushed past a limit on detector sensitivity, making it a candidate for finding elusive evidence of dark matter (Science 2021, DOI: 10.1126/science.abi5226).

Astronomers observing the motion of distant galaxies see effects of gravity that are greater than can be explained by the amount of matter they can find, suggesting there’s something else producing the extra gravitational pull. In fact, about 85% of the mass that would be needed to explain their observations is missing, leading to speculation about dark matter that barely interacts with the rest of the universe.

The new crystal can detect an electrical field with a strength as low as 240 nanovolts/m/s, less than a tenth the lower limit of previous sensors. If a certain type of dark matter exists, scientists believe it should generate a small electrical field when it interacts with normal matter. Scientists have been looking for evidence of such electrical fields for about a decade, using superconducting circuits that operate at a higher frequency than this new sensor, says John J. Bollinger, a physicist at the National Institute of Standards and Technology, who performed the research with Ana Maria Rey, Kevin A. Gilmore, and colleagues from NIST and the University of Colorado, Boulder. “This opens up a window for detecting these dark matter particles in a different frequency range,” he says.

To make the crystal, the researchers cooled a collection of about 150 beryllium ions to within a few millikelvins of absolute zero and collected them in a magnetic trap, where they naturally arranged themselves into a 2D triangular lattice. The magnetic field from the trap causes the lattice to oscillate up and down at a frequency of 1.6 MHz, like the head of a drum vibrating when it’s been struck. If another electrical field is present, that will increase the amplitude of the oscillation, so measuring the motion the system can detect that electrical field.

To detect the motion, the scientists turn to another quantum property, the spin of the ions, which can be thought of as a bar magnet with its north pole pointing up, down, or any direction in between. Shining a laser through the crystal causes it to fluoresce, and the direction of the spin makes the output brighter or dimmer. By tuning the system just right and subtracting the intrinsic motion of the crystal from the signal, the researchers end up with an electric field detector more sensitive than the Standard Quantum Limit, which is a cap on sensitivity imposed by the natural fluctuations of any quantum mechanical system. The researchers applied their own electric field as a test and were able to measure it below that limit.

The sensor has a strong “quantum advantage,” meaning it’s more sensitive than a classical sensor could possibly be, says Holger Müller, a professor of physics at the University of California, Berkeley, who was not involved with the research. “This is an extremely important advance in the worldwide quest to use the non-classical properties of quantum systems for next-generation sensors,” he says.


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