Quantum crystals with “time reversal” could become new sensors for dark matter

Physicists at the National Institute of Standards and Technology (NIST) have correlated or “involved” the mechanical motion and electronic properties of tiny blue crystals, providing a quantum advantage in measuring electric fields with record sensitivity that is the understanding of the universe.

The quantum sensor consists of 150 beryllium ions (electrically charged atoms) locked in a magnetic field so that they arrange themselves into a flat 2D crystal with a diameter of only 200 millionths of a meter. Quantum sensors like these have the potential to detect signals from dark matter – a mysterious substance that, among other theories, can turn into subatomic particles that interact with normal matter through weak electromagnetic fields. The presence of dark matter can cause crystals to sway in harmful ways, expressed by a collective change between crystal ions in one of their electronic properties called spin.

As described in the August 6, 2021 issue scienceresearchers were able to measure the excitation of the crystal vibrations – a flat plane that moves up and down like an eardrum – by monitoring changes in the collective spin. The rotation measurement indicates the degree of vibration excitation, which is referred to as displacement.

Illustration of a quantum crystal from NIST. Photo credit: Burrows / JILA

This sensor can measure an external electric field that has the same frequency of oscillation as the crystal, with a sensitivity more than 10 times that of all the atomic sensors previously shown. (Technically, the sensor can measure 240 nanovolts per meter in one second.) In the experiment, the researchers applied a weak electric field to excite and test the crystal sensor. The search for dark matter will look for such signals.

“Ion crystals can detect certain types of dark matter — examples include axions and hidden photons — that interact with normal matter through weak electric fields,” said John Bollinger, senior author of NIST. “Dark matter forms a background signal with an oscillating frequency that depends on the mass of the dark matter particles. Experiments to find this type of dark matter have been going on with superconducting circuits for more than a decade. The movement of the trapped ions ensures sensitivity over different frequency ranges.”

The Bollinger group has been working with ion crystals for more than a decade. What’s new is the use of certain types of laser beams to ensnare the collective motion and spin of large numbers of ions, as well as what the researchers call a “time reversal” strategy to detect the results.

Quantum crystal

Illustration of a quantum crystal from NIST. Photo credit: Burrows / JILA

The experiment benefits from a collaboration with NIST theorist Ana Maria Rey, who works at JILA, a joint institute of NIST and the University of Colorado Boulder. The theoretical work is critical in understanding the limitations of laboratory settings, providing new models for understanding experiments that are valid for large numbers of trapped ions, and demonstrating that quantum benefits result from entanglement in spin and motion, Bollinger said.

Rey noted that entanglement is useful in canceling out the intrinsic quantum noise of ions., However, it is difficult to measure the entangled quantum state without destroying the information shared between spin and motion.

“To avoid this problem, John was able to reverse the dynamics and parse the spin and motion after the shift was applied,” says Rey. “This time the reversal separates the spin from the motion, and now the collective spin itself has stored the displacement information, and if we measure the spin, we can determine the displacement very accurately. That’s neat! “

The researchers used microwaves to generate the desired value for the spin. Ions can be spin-up (often referred to as up arrow), spin-down, or other angles, including both at the same time, special quantum states. In this experiment, all the ions have the same spin – first up and then horizontally – so when excited they rotate together in a pattern characteristic of gyroscopes.

Crossed laser beams with nearly the same frequency difference as the motion are used to entangle the collective spin with the motion. The crystal is then vibrated. The same laser and microwave are used to break the bond. To determine how much the crystal moved, the researchers measured the degree of ion fluorescence spin (spin-up scatters light, spin-down dark).

In the future, increasing the number of ions to 100,000 by creating 3D crystals is expected to increase the detection capability thirty-fold. In addition, the stability of the excited motion of the crystal can be improved, which will improve the time reversal process and the accuracy of the results.

“If this aspect can be improved, this experiment could become a fundamental resource for detecting dark matter,” Rey said. “We know that 85% of the matter in the universe is made up of dark matter, but to this day we don’t know what dark matter is. This experiment could allow us to uncover this secret in the future.”

Co-authors include researchers from the University of Oklahoma. This work was supported in part by the US Department of Energy, Air Force Office of Scientific Research, Defense Advanced Research Projects Agency, Office of Army Research, and the National Science Foundation.

Reference: “Quantum-enhanced electric field and displacement sensors with two-dimensional trapped ion crystals” by KA Gilmore, M. Affolter, RJ Lewis-Swan, D. Barberena, E. Jordan, AM Rey and JJ Bollinger., August 5, 2021, science.
DOI: 10.1126 / science.abi5226

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